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NBS TECHNICAL NOTE 630

Developments in Cryoelectronics
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3,

1 901 . The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

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THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader

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•Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

' Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.
NATIONAL Wmm OF STANDARDS

JAN 2 4 13f3

.—*_>-£ et^e

><3o Developments in Cryoelectronics

R.A. Kamper

D.B. Sullivan

Cryogenics Division

Institute for Basic Standards

[)'$>• National Bureau of Standards

' Boulder, Colorado 80302

'fjuX^L&fl -%AjJks~ -vu» • t.30

NBS Technical Notes are designed to supplement the

Bureau's regular publications program. They provide

a means for making available scientific data that are of transient or limited interest. Technical Notes

may be listed or referred to in the open literature.

U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secretary

NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, Acting Director

Issued November 1972
National Bureau of Standards Technical Note 630

Nat. Bur. Stand. (U.S.), Tech. Note 630, 73 pages (November 1972)

CODEN:NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. 03.46:630.)
CONTENTS

Page

SUMMARY 1

1. BACKGROUND: SUPERCONDUCTIVITY, JOSEPHSON

JUNCTIONS AND SQUIDS 3

2. MAGNETOMETRY 18

3. COMPUTERS 24

4. FAR INFRARED TECHNOLOGY 26

5. PRECISE METROLOGY 31

6. SPECIALIZED SCIENTIFIC INSTRUMENTS 44

7. TRANSMISSION LINES FOR INFORMATION 51

8. ACCELEROMETERS AND GYROS 53

9. CONCLUSION 55

References 56

List of Figures

Figure 1. Practical Josephson junctions 5 Figure 2. Current versus voltage characteristic of a thin film

Josephson junction 8

Figure 3. Dependence of I (fig. 2) on applied magnetic field. . 9 c Figure 4. The quantum interferometer 10

Figure 5. Variants of the SQUID 12

Figure 6. Current versus voltage characteristic of a Josephson

junction irradiated at a frequency / = 2eV/h. ... 13

Figure 7. Josephson oscillation 14

Figure 8. A magnetocardiogram obtained with a SQUID

gradiometer 20

in
List of Figures (continued)

Page

Figure 9. Comparison of measurements of e/h (1971). ... 30

Figure 10. Meservey's proposed absolute ammeter 34

Figure 11. Circuit for precise dc measurements of current or

voltage by counting flux quanta 36

Figure 12. Circuit for rf measurements using the flux quantum

as a reference 40

Figure 13. Circuit for noise thermometry 48

IV
Developments in Cryoelectronics

Robert A. Kamper and Donald B. Sullivan

SUMMARY

This is a survey of progress to date in the development of new- electronic instruments taking advantage of the unique properties of

superconductors. This has received copious attention in universities,

and government and industrial laboratories, largely because of its

strong appeal to physicists. Commercial activity has been in the nature

of an investment in the future: very few devices have actually been sold

yet. Computers, information transmission lines., and magnetocardiography may become commercially significant applications of superconductivity. Many other devices perform specialized functions of

measurement and detection uniquely well, and will repay the cost of

development without generating a large market. Josephson Junctions

(weak but stable electrical contacts between superconductors), and the

Superconducting QUantum Interference Devices (SQUIDs) derived from

them, will probably have a significant impact on the following activities:

Magnetometry: including magnetocardiography , ELF communication,

geomagnetism, and submarine detection. The SQUID is the world's

most sensitive magnetic sensor. The first few commercial superconducting instruments have been SQUID magnetometers.

Computers : the tunneling cryotron is cheap, compact, suitable for

mass production, and capable of sub -nanosecond switching times with

extremely small dissipation of power.

Electrical Metrology : including maintaining the legal volt and measurement of current, voltage, power, and attenuation ratio, from dc through

the microwave range of frequency.

Far Infrared: and millimeter wave receivers, harmonic mixers, and

spectrum analyzers.

Galvanometers : null detectors, amplifiers, thermometers, phonon generators and detectors, etc. Specialized instruments for specialized

scientific measurements.

Contribution of the National Bureau of Standards, not subject to copyright.
In addition, the simple properties of superconducting materials,

such as the Meissner effect and the drastic reduction of electrical dissipation, can be used to make very significant improvements in the performance of the following devices:

Information Transmission Lines : superconducting coaxial lines combine

physical compactness with a wide bandwidth and low attenuation.

Pulse Delay Lines : require the same qualities as for transmission of

information.

Stable Oscillators : recent developments in the stabilization of microwave oscillators, with superconducting resonant cavities, have created

the world's most stable oscillator over periods of time up to a few

seconds. Combined with the available long-term stability of the cesium

beam resonator, this will become the best oscillator over any period of

time.

Accelerometers : magnetic levitation using the Meissner effect, and

superconducting magnets operating in the persistent current mode, give

a very stable and almost frictionless suspension. This has proven

worth for the measurement of long-term variations in gravitation, and

possible usefulness for gyros.

The remainder of this report will discuss these topics in greater

detail.

Key Words: Electronics; Josephson effect; precise measurements;

superconductivity.
1. BACKGROUND: SUPERCONDUCTIVITY,

JOSEPHSON JUNCTIONS AND SQUIDS

Superconductors comprise a class of metals which undergo a reversible, thermodynamic phase transition at a low critical temperature T which is characteristic of the individual material. At tem- c peratures above T they have no common characteristic to set them

apart from other metals. Below T they enter a state characterized

c by the vanishing of electrical resistance to small direct currents. The

highest known value of T is 21 K for an alloy of niobium, aluminum

c and germanium. The superconductors commonly used in devices have

transition temperatures ranging from 3 K to 18 K. These include the

elements tin (3.7 K) , lead (7. 2 K) , and niobium (9.4 K); the alloys

niobium-titanium (6 K - 10 K), and lead-tin (7 K - 8 K); and the intermetallic compounds Nb Sn (18 K) and NbN (16 K). Superconducting

devices normally operate in a bath of liquid helium, to maintain the

low temperature required for the superconducting state.

Although the electrical resistance of superconductors is strictly

zero only for direct current, it remains exceptionally low for frequencies from zero through the microwave range. At the higher frequencies

it becomes a sensitive function of the preparation of the surface of the

superconductor

.

Another striking feature of the superconducting state, discovered

by Meissner and Ochsenfeld in 1933, is known as the Meissner Effect.

It would occur in its simplest form in a perfect, defect-free, single

crystal of a superconducting metal with a smooth, regular shape. If such a specimen were to be cooled through its transition temperature

in the presence of a small magnetic field, the magnetic flux would be

completely expelled from its interior, to return again on warming
through T . This is a thermodynamically reversible effect which cannot be explained without considerations that go beyond the vanishing of

electrical resistance.

The situation in real specimens of superconducting metals is

complicated by the tendency of irregularities of structure or composition

to impede the motion of the flux lines, so that in practice one usually

observes only a partial exclusion of flux. Another complication is the

concentration of magnetic flux at sharp corners of the specimen, leading to very large flux densities in small regions. These are accommodated by partial penetration of magnetic flux into a structure of finely

divided superconducting and normal state domains, known as the intermediate state. For this reason experiments are usually performed

with rod- shaped specimens oriented parallel to the applied magnetic

field.

The Meissner effect is the consequence of electric currents

which flow spontaneously on the surface of the specimen or on the boundary between the superconducting and normal state phases. The current

density falls off approximately exponentially with depth below the surface

of the superconductor, with a characteristic length X known as the penetration depth. The magnitude of \ depends upon the composition of

the superconductor, and the temperature and magnetic field at the sur-

-7 -8

face. It is typically of the order of 10 to 10 m. The magnetic flux

also penetrates to this depth below the surface of the superconductor.

Josephson junctions are weak, stable electrical contacts between

superconductors [ 1-3] . Several practical ways of making them are

illustrated in figure 1. An ideal junction might consist of two superFigures in brackets indicate the literature references at the end of

this paper.
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conductors separated by a vacuum gap about 10 A (10 cm) wide and

perhaps a square millimeter in area. Electrons would cross this gap

by the quantum mechanical tunnel effect [4] . The nearest practical

approach (fig. la) consists of a strip of thin film of one superconductor

deposited on an insulating substrate such as glass, quartz, or sapphire.

Its surface is covered with an insulating barrier of the desired thickness (usually by oxidation of the metal), and then a strip of thin film of

the second superconductor is deposited across the first to complete the

junction. The barrier does not need to be an insulator. A layer of

o normal metal such as copper, about 10,000 A thick, will work instead

[5] . Another approach (fig. lb) is to use a continuous strip of superconducting thin film with a very narrow constriction [6] (a few micrometers wide). The electrical characteristics of this microbridge structure differ from those of crossed film junctions, but the similarities

are sufficient for many practical purposes. An intermediate approach

(fig. lc,d) is to press a slender point on one superconductor lightly

against the other to form a point contact junction [ 7] . It is not quite

clear whether a surface layer formed by atmospheric corrosion forms

an insulating barrier, or if the narrow contact area functions as a microbridge. However, point contacts have the great advantage that

they can be adjusted while in operation and they work very well. The

more permanent structures such as crossed film junctions and microbridges have the compensating advantage of greater mechanical stability.

The relative merits of tunnel junctions, microbridges, and mechanical

point contacts (insofar as they are understood at this writing) were

spelled out at length at the 1972 Applied Superconductivity Conference

[8-11].

Superconductors commonly used to make Josephson junctions are

lead, tin, indium, niobium, tantalum, etc. It is not important whether

the same or two different superconductors are used to form a junction.
The electrical characteristics of these junctions depend on how

they are made [ 12-14] , but one feature they all have in common is the

ability to pass a limited current (in the range from a few microamperes

to a few milliamperes) without any voltage appearing. A somewhat

idealized I-V curve for a crossed film junction is shown in figure 2. If the current passing through the junction exceeds I (fig. 2) a voltage

c appears and there is often an unstable region of negative dynamic resistance. There is a characteristic voltage of the order of a millivolt,

determined by the energy gaps of the superconductors, at which the current rises sharply before settling down to an ohmic response at high

voltage. Microbridges differ from crossed film junctions mainly by the

absence of the low current region at intermediate voltage.

The magnitude of I is very sensitive to a magnetic field applied

in the plane of the junction. Figure 3 shows a plot of I versus magnetic flux density for an ideal crossed film junction. Similar curves

have been observed in the laboratory [ 15] . The vertical scale depends

critically on the thickness of the insulating barrier, while the horizontal

scale is controlled by the width of the junction and the properties (penetration depths) of the superconductors. The periodicity is caused by

quantum interference between electrons crossing different parts of the

barrier. It is analogous to optical diffraction at a narrow slit. The

analog of optical interference between two slits can be created by connecting two Josephson junctions in parallel in a superconducting circuit

as shown in figure 4. The curve shown in figure 3 then becomes the

envelope of an interference pattern of much shorter period. This period

-15

corresponds to the quantum of magnetic flux h/2e « 2 x 10 weber

(h is Planck's constant and e is the electron charge). Each maximum

in the interference pattern occurs when a whole number of magnetic
Figure 2. Current versus voltage characteristic of a thin film

Josephson junction.
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Figure 4. The quantum interferometer.
flux quanta link the superconducting loop. This arrangement has become

known as a quantum interferometer [ 16] . It has obvious applications to

magnetometry and related fields.

It is possible to observe the effects of magnetic flux quantization

with a single Josephson junction closing a superconducting loop, by monitoring the impedance of the loop at a convenient radio frequency. The

resulting device, known as the rf -biased Superconducting QUantum

Interference Device (SQUID) has received intensive development as a magnetic sensor [ 17-21] . Several versions of the SQUID are illustrated

in figure 5. The most advanced design is the one at lower right. It consists of 12 superconducting loops (with very small mutual inductance)

connected in parallel across a single point-contact Josephson junction

[22] . This gives the device great sensitivity to variations in magnetic

flux and also a strong readout signal. This latter can also be improved

by raising the readout frequency [ 17,23] . Silver and Zimmerman have

written an exhaustive discussion of all aspects of SQUIDs [24] .

If a radio frequency or microwave signal is addressed to a bare

Josephson junction, the direct current versus voltage relationship

becomes profoundly modified as shown in figure 6. It develops a series

of steps at equal intervals of voltage [25] . The voltage interval V is

related to the frequency / by h/ = 2eV. This corresponds to approximately 484 MHz/i^V.

Finally, if a receiver is connected to the junction a coherent

oscillation [25-27] can be detected at the same frequency / = 2eV/h.

The waveform is usually nonsinusoidal, so harmonics of this frequency

can also be detected at submultiples of the fundamental voltage. These

are illustrated by an actual recorder chart shown in figure 7. The

receiver was tuned to 43 MHz and the odd signal shape was a result of

11
RESISTIVE

RF COIL COUNTERSUNK HOLE

for CONTACT SCREW

R F COIL

MIDPLANE

(insulated)

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Figure 5. Variants of the SQUID.

12
V VOLTAGE

Figure 6. Current versus voltage characteristic of a Josephson

junction irradiated at a frequency / = 2eV/h.

13
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14
lock-in detection. Frequencies ranging from a fraction of 1 Hz [28] to

270 GHz [ 29] have been observed. The presence of oscillation at frequencies up to 8 THz has been inferred indirectly [30] .

The Josephson effect is a quantum mechanical phenomenon on an

unusually" large scale. It happens because the superconducting state is

one in which the conduction electrons of the metal have a high degree of

order. The wave functions describing their motion have a coherence of

phase extending over large distances --greater than a meter in one observation [31] . Variations in the phase of these wave functions control the

movements of the electrons and hence the currents which flow in the

superconductor. A Josephson junction is a place where the wave function

is sufficiently weak to permit a discontinuity of phase, but not weak

enough to interrupt communication between the superconductors or either

side completely. Thus a "supercurrent" (a dissipationless current) can

flow across the junction. The magnitude J of the current density depends upon the quantum mechanical phase difference 6 thus

J = J sin (1) o

where J is a constant depending on the properties of an individual

o junction. If a voltage V is applied across the junction, the phase difference 6 advances so that its rate of change is

d6/dt = 4TTeV/h. (2)

Equations (1) and (2) were originally derived from fundamental quantum

mechanics by Josephson [ l] . Together they are known as the Josephson

equations. Taken together they account for all the bizarre properties of

point contact Josephson junctions and microb ridges . Combining them,

we find that the supercurrent I flowing through a single point contact

obeys the equation

15
dl/dt = (2eV/h) VI

2

- I 2

(3) c

where I is defined in figure 2. In addition to the supercurrent obeying

equation (3), there is an ohmic leakage current and a displacement current through the capacitance of the junction which modify its electrical

characteristics. This behavior leads directly to such functions as microwave mixing, harmonic generation, oscillation, and the constant voltage

steps induced in the dc characteristics by irradiation with microwaves.

These last come about because of the combined functions of harmonic

generation and self -oscillation. Whenever the voltage is such that the

frequency of self-oscillation coincides with one of the harmonics of the

incoming microwaves, the two signals phase lock together to generate a

stable zero -frequency beat which contributes to the steady current flowing through the junction.

The phenomenon of interference which is observed in extended or

multiple junctions in the presence of a magnetic field is a consequence

of the fact that a magnetic field induces a phase shift in the wave function.

The total phase shift © integrated around a closed loop of superconductor linked by a magnetic flux cp is

9 = 4TTecp/h. (4)

If the loop contains two Josephson junctions this phase difference will

appear across them. When it is equal to tt, 3tt, 5tt, etc. , the phase differences across the two junctions must be equal and opposite. Hence,

according to equation (1), the currents flowing through the two junctions

must be equal and opposite also. This occurs when cp = h/4e, 3h/4e,

5h/4e, etc. For intermediate values of magnetic flux (and hence phase

difference) the two currents add. This is closely analogous to optical

16
interference of light transmitted by a pair of slits. Inspection of equations (1) and (4) will also show that the response of a single extended

Josephson junction to a magnetic field, shown in figure 3, is analogous

in the same way to optical diffraction by a single slit. In a. solid superconducting ring, with no Josephson junctions, the total phase shift must

be 0, 2tt, 4tt, etc. , for the wave function to be single valued. Hence cp

is constrained to be an integral multiple of h/2e. This is the phenomenon of magnetic flux quantization.

17
2. MAGNETOMETRY

The most sensitive magnetometers existing today consist of

SQUIDs coupled to the field to be sensed via superconducting flux transformers. It is possible to dispense with the SQUID by mechanically

modulating the inductance of a superconducting circuit to generate an

audio -frequency signal proportional to the linked magnetic flux. However, none of the instruments which have been built on this principle

can compete with SQUIDs in sensitivity, so we dismiss them with a reference to reviews by Opfer [32] and Deaver and Goree [33] .

One of the earliest descriptions of a properly engineered SQUID

magnetometer was published by Forgacs and Warnick [34] . Taking

advantage of the periodic nature of the basic response of the SQUID,

they used two modes of operation. In the first, or analog mode, the

magnetic flux linking the SQUID was modulated at an audio frequency,

and the output of the readout circuit was detected with a lock-in detector

with reference derived from the modulation. The output of the lock-in

detector was amplified and supplied to a coil coupled to the SQUID, so as to servo the flux linking the SQUID to remain constant in the face of

variations in the external field. The current required to do this is linearly related to the external field, and was used to drive the output indicator of the instrument. In the second, or digital mode, an up-down

counter was used to count the flux quanta driven through the SQUID by

variations in the external field. The counter was driven by two lock-in

detectors, one operating at the modulation frequency and the other at the

second harmonic thereof. This combination supplied sufficient information to give the counter a sense of direction. This early instrument did

not use a flux transformer, relying instead on the direct coupling of the

(double junction) SQUID to the external field.

18
Subsequent improvements in magnetometry have been directed

towards improvements in the SQUID to increase the signal-to-noise

ratio, and hence the sensitivity and bandwidth (permitting faster counting). Also, rf -biased SQUIDs appear to have inherently greater dynamic

range than the double junction type, which suffers from the need for an

adjustable (dc) bias level to cover a range of more than a few hundred

flux quanta. Many millions of quanta can be sensed by an rf -biased

single junction SQUID with no adjustment of the operating conditions.

Sensitivity of the order of 10 T/VHz is now available with an instrument developed by Zimmerman and Frederick [35] . This instrument

has a multiple-loop SQUID operating at a readout frequency of 300 MHz.

Using this sensitivity to the best advantage is another problem.

The natural fluctuations in the Earth's magnetic field are of the order of

-11

10 T in the range of frequency used in magnetometry. Two alternatives are open: the full sensitivity of a SQUID magnetometer may be

used in a heavily shielded enclosure, or the external coils of the flux

transformer may be arranged to convert the instrument to a magnetic

gradiometer. This is done by using two equal coils wound in opposition

and placed a finite distance apart. With careful attention to balancing

the device the magnetic flux coupled to the SQUID by the pair of coils in

response to a uniform external field can be nulled out. A gradient in the

external field will permit coupling of the difference in flux linking the

coils to the SQUID. The result is to make the instrument insensitive to

disturbances from distant sources, but to retain its full sensitivity to

signals from sources at a distance comparable to the separation of the

coils.

The development of gradiometers has occupied the attention of

several groups of researchers, because they are the most useful form

of ultrasensitive magnetometers. The major problem has been the

19
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surprising difficulty in attaining a stable balance. Typically gradiometers have appeared to be unbalanced by as much as several percent,

and often the unbalance changes unpredictably between measurements.

Usually it is greater than expected from the precision with which the

coils were made. Although it is not fully understood, the origin of this

unbalance appears to be the movement of trapped magnetic flux in the

superconducting metal of the flux transformer. The cure was discovered by Zimmerman and Frederick [35] . It consists of the simple

strategy of making the flux transformer with many turns of very fine

superconducting wire, to severely limit the freedom of the trapped flux

to migrate. This enabled them to achieve a stable balance of one part

5

in 10 (both axial and transverse). This is just sufficient to enable them

to use the full sensitivity of their best instrument while exposed to the

natural fluctuations of the Earth's field, away from magnetic objects

which generate local gradients.

The most promising applications of ultrasensitive magnetometers

(at the time of writing) are the following:

Magnetocardiography

The electrical activity associated with the action of a beating

heart drives small electric currents in the torso which can be detected

with a very sensitive magnetometer. The resulting record bears a strong resemblance to an electrocardiogram (fig. 8). Whether it contains more or less information is an open question at present, to be

answered by research which is now in progress. The main operational

advantage of magnetocardiography is freedom from the need to attach

electrodes to the patient. The main inconvenience is the need for heavy

magnetic shielding to isolate the weak cardiac signal from the background

noise of a hospital. This need may be softened by further sophistication

in signal processing.

21
The pioneering work in magnetocardiography was done by Baule

and McFee [36] without the aid of an ultrasensitive magnetometer.

They used a large coil of wire connected to a conventional amplifier to

pick up the weak signal. It was necessary to use an electrocardiac

signal for a phase reference to extract the wanted magnetic signal from

the background noise. Subsequently, Cohen [37] , at the National Magnet

Laboratory, built an elaborate magnetic shield and used a SQUID magnetometer to obtain unassisted magnetocardiograms of similar quality to

standard electrocardiograms. He is now the acknowledged leader in

this field of research. Two reports of magnetocardiograms obtained

with gradiometers instead of magnetic shielding have appeared in the

literature [23,38] . Both required "quiet" locations away from electrical machinery. In the light of Cohen's most recent work [39] , the

future of magnetocardiography looks very bright if it wins acceptance by

the medical profession.

Magnetoencephalography

There is also magnetic activity in the brain. The expected signal

levels are even lower than those generated by the heart. A signal at the

frequency of the <2-rhythm(~ 10 Hz) has been detected by Cohen [40] , with a coil of wire and conventional amplifier, using a signal from electrodes on the subject's head to supply the phase information required for

signal averaging. Several groups intend to explore this field with SQUID

magnetometers, but not much has been accomplished yet.

Magnetic Anomaly Detection

There is some activity and speculation on the detection of magnetic anomalies (e.g. , submarines) with sensitive magnetometers. For

this, SQUIDs would be used in the gradiometer configuration. The work

has not yet proceeded far enough to demonstrate its value.

22
ELF Communication

Matching requirements force antennas, for radio reception at

extremely- low frequencies, to become inconveniently large if they are

to retain acceptable efficiency with normal resistive materials. Below

a frequency of about 100 Hz it is generally conceded that a sensitive

magnetometer would be a better receiver than a conventional loop antenna and radio receiver. Once again SQUIDs would offer the extra possibility of the gradiometer configuration.

ELF communication is useful in situations where the signal must

traverse a path of significant length through water or rock. The conductivity of these preferentially attenuates electromagnetic fields at high

frequencies. One suggested need for ELF communication is the location

of trapped miners after an underground disaster, and another is for communication with submarines.

Commercial Activity

We are aware of a few companies, mostly small, who are selling

SQUID magnetometers with various degrees of sophistication. Total

sales probably amount to about 250 at the time of writing. It is possible

to buy the basic SQUID and electronic readout equipment plus flux transformers, etc. , on a custom basis, or a combined magnetometer and

superconducting shield for magnetic measurements on materials, particularly rock specimens. Equipment suitable for magnetocardiography is

also under commercial development.

23
3. COMPUTERS

Both the logic and the memory of a computer are built up with

switches, which are required in enormous numbers. The desirable

qualities of these switches are:

1) they must be cheap,

2) they must be suitable for simultaneous fabrication in compact

arrays with a high yield,

3) they must dissipate very little heat,

4) they must switch very quickly, and

5) their electrical characteristics must be compatible with a

physically realizeable network to convey the pulses which

actuate them from one to another.

The story of superconducting computers began with Buck's invention of the Cryotron in 1956 [41] . This was a switch in which a superconducting "gate" (originally a fine wire, later a strip of thin film) was

switched from the superconductive to the resistive state by the magnetic

field from a current in the "control": a wire winding or strip of thin

film in close proximity to the gate. The control was also made of superconducting material. The gain of the cryotron was defined as the ratio

of the critical current of the gate to the control current required to

switch the gate to the resistive state. If this is greater than unity cryotrons can be cascaded in complex logic circuits without the need for

intermediate amplifiers.

The invention of the cryotron created a flurry of activity to solve

some of the problems it presented, notably in the technology of thin

evaporated films (it quickly became apparent that a version made of thin

films would be superior to the original wire-wound device). This in

turn stimulated the development of other thin film devices, both for logic

24
and for memory [42] . However, judged by the criteria listed at the

beginning of this section, none of these devices (including the cryotron

itself) could compete successfully with the advancing semiconductor and

magnetic core devices. The development of all of them ha-s been abandoned, with the exception of a single survivor.

The survivor is the Tunneling Cryotron, invented by Matisoo at

IBM in 1967 [43] . It differs from the thin film version of Buck's cryotron in that the gate is replaced by a Josephson tunnel junction, which

has much more desirable switching characteristics than any plain strip

of superconducting film. Its principle of operation may be understood

with reference to figure 2, which shows the current versus voltage characteristics typical of the gate of this device. If it is supplied with a stabilized current less than I there are two possible operating points:

the first at zero voltage and the second on the steeply rising part of the

curve at a voltage of the order of a millivolt. A very small magnetic

field in the plane of the junction, generated by the control strip, will

suffice to lower I sufficiently to switch the device from the first

c operating point to the second (see fig. 3). A temporary interruption of

the gate current will switch it back again. An individual junction will

-12 -13

switch in 85 x 10 sec [44] , requiring a pulse of energy of only 10

-12

Joules [45] . Cycle times of 550 x 10 * sec have been achieved for

steering current from one limb of a network to another [44] .

The tunneling cryotron is under active development. Its remaining problems appear to be soluble, and the chances are good that it will

finally lead to the realization of a commercially viable superconducting

computer.

25
4. FAR INFRARED TECHNOLOGY

One of the present frontier areas in radio technology is the part

of the electromagnetic spectrum at frequencies above 100 GHz. The

development of infrared lasers has provided powerful, coherent sources

of radiation throughout this range: for example at 981 GHz (HCN); 2. 5,

3. 8, and 10. 7 THz (HO); and at 28. 3 and 32. 2 THz (CO ). Receivers

and other signal processing equipment are not yet available to take full

advantage of these lasers. For example, the best available receiver at

300 GHz has a noise temperature of 2000 K, and things get worse at

higher frequencies. There is a clear need for new devices which the

Josephson effect may be able to supply, at least in the first two decades

of this range of frequency. The first significant work in this field was

started by Grimes, Richards, and Shapiro [46,47] at the Bell Telephone

Laboratories. The two topics which have attracted the greatest attention so far are detectors and harmonic mixers. Putting together the hardware that has been developed to make a proper receiver is probably one

or two years away.

Far infrared detection is unusual in that even the best nonsuperconducting detectors operate at very low temperatures. The reason

is that at frequencies below 10 THz the quantum of radiation has less

energy than kT at ambient temperature, so any process which would

respond to such small photons would be subject to noisy thermal excitation. A good example of the semiconductor detectors available in this

spectral range is Low's germanium bolometer [48] . It has a noise equiv-

-13

alent power of 5 x 10 W /VHz when operating at 2 K.

An early superconducting device was a thin-film bolometer reported by Martin and Bloor in 1961 [49] . This consisted of a strip of

thin superconducting film, on a very light substrate, connected to the

helium bath via a thermal resistance. The temperature of the film

26
would therefore be increased slightly by power absorbed from the incident radiation. When the equilibrium temperature of the film was adjusted to coincide with its transition to the resistive state, its resistance

would be very sensitive to small changes in temperature caused by small

changes in the level of irradiation. In its final form, this device had a

-12

noise equivalent power of 10 W with a time constant of 1 . 25 sec. It became the workhorse of a productive group of infrared spectroscopists

at Queen Mary College in London [50] . At the low frequency end of the

infrared spectral range, it is possible to do even better with a Josephson

junction for a detector.

Radiation detection with a Josephson junction is based on the

properties of the current versus voltage characteristic illustrated in

figure 6. The constant-voltage steps in the curve appear under the influence of irradiation, so the current flowing through the junction when

it is biased to a voltage near one of the steps is clearly a sensitive function of the intensity of the incoming radiation. The width I (in current)

of the nth step varies with the amplitude of the voltage V , induced in

the junction by the radiation, approximately according to function

I = I J (2eV /hf) (5) n c n o

where f is the frequency of the radiation and J is the nth order

Bessel function. Since the spacing of the steps (in voltage) is proportional to the frequency of the radiation, the Josephson junction can be used

as a tunable detector. Its sensitivity is enhanced by coupling it to a resonant cavity, also tuned to the frequency of the incoming radiation.

The resulting detector was reported by Richards and Sterling [51] to

- 14

have a noise equivalent power of less than 10 W/a/Hz at 190 GHz.

This sensitivity remains close to the best that has been attained. The

27
question of the high frequency limit of radiation detection by Josephson

junctions has been addressed by McDonald, et al. [30,52-54]. They

have observed the steps in the current versus voltage characteristic to

extend to 17 mV, implying the presence of harmonically generated currents at frequencies up to 8.2 THz. They actually observed harmonic

mixing (and hence detection) with a signal at 3. 8 THz from a water

vapor laser.

Ulrich [55] has taken the first steps towards astronomy with a detector based on a Josephson junction. He has made some observations

of the sun, moon, and Venus at a frequency of 250 GHz.

Harmonic generation and mixing is another function for which the

Josephson junction is uniquely well adapted. It has the feature that the

power converted into the harmonics of an incoming signal diminishes

only slowly with increasing harmonic number, compared with other

devices. McDonald, et al. , have observed the generation of the 401st

harmonic of a signal at 9 GHz from a klystron, and the difference signal

created by mixing it with a signal from a water vapor laser at 3. 8 THz

[54] . This performance is out of reach of any other known device.

Another mode of operation of a Josephson junction is to use its

self oscillation for a local oscillator [ 56] . Often the output is taken at

zero frequency, and this is essentially the mode in which Richards and

Sterling's resonant detector operated [51] . With suitable electronics

to locate incipient steps in the current versus voltage characteristics,

a Josephson junction can be used as a quick-reading spectrometer

throughout the range up to its high frequency limit [30,57] .

The first applications of far infrared and sub -millimeter wave

receivers will probably be to astronomy. This spectral range is rich

in molecular resonances which could give abundant information about

28
conditions in various parts of the solar system. Coming closer to home,

the same resonances can be used to detect pollutants in our atmosphere.

Another, more distant, prospect is communication, over short distances

in the atmosphere (because of attenuation) and over long distances in

outer space.

29
.0035 .0036 .0037 .0038

2e/h-483.5900THz/VNBs

Figure 9. Comparison of measurements of e/h (1971),

30
5. PRECISE METROLOGY

In this section we collect several applications of superconductivity

to precise measurements and measuring instruments. Taken together,

they show that this is one area where it does have a natural place. The

commercial significance of these applications is the same as for all

precise metrology: they exert an important secondary influence, but do

not represent a large investment in manufactured hardware.

The Measurement of e/h

The constant-voltage steps in the current versus voltage characteristic of a Josephson junction, exposed to microwave radiation at

frequency f, are spaced out at equal intervals V, where V = (h/2e)f.

Thus a simultaneous measurement of V and f will yield the value of

the fundamental constant e/h. The constant -voltage steps can be made

to extend to about 10 mV, permitting a precise measurement with the

exercise of some care, while the measurement of the frequency of the

microwave source is straightforward.

Finnegan, Denenstein, and Langenberg [58] have performed

Q

these measurements to a precision of 3 parts in 10 . Because of difficulties in relating their maintained standard to the NBS volt, their accuo racy was limited to 12 parts in 10 . While some improvement in accuracy might be worthwhile, it is clear that drift in the primary standard

will limit the value of more refined measurements. The measurement

of e/h has been accomplished at several standards laboratories including those of the United States [59] , Great Britain [60] , Australia [61] , and West Germany [62] . Results of those measurements, with recent

voltage intercomparisons as reference [63] , are shown in figure 9.

31
Over the course of these measurements the accuracy of the

frequency-voltage relation has been probed. Some early concern about

frequency pulling effects [64,65] were settled with an article by

McCumber [66] . Clark [67] showed experimentally that the frequencyvoltage relation is independent of junction material to at least one part

o in 10 . Langenberg and Schrieffer [68] and Hartle, et al. [69] have

argued that the relation should be free of quantum electrodynamic corrections. The full impact of the measurement was elucidated by Taylor,

Parker, and Langenberg [70] in their monumental reevaluation of the

physical constants which was based primarily on the new value for e/h.

A New Voltage Standard

The logical extension of this work is the replacement of the

chemical cell by the Josephson effect as our standard emf. Hopefully,

within the year, the NBS Electricity Division will base the U. S. legal

volt on the Josephson effect rather than the mean value of a group of

chemical cells. In practice this will probably be accomplished by using

the Josephson effect to monitor and correct the emf of a group of chemical cells at something like two -week intervals.

The agreement on e/h between standards labs of involved nations

is quite good and we should expect to eventually see the international

volt redefined in terms of the Josephson effect. This would place the

volt on the same footing as length and time, both of which are defined

in terms of atomic processes.

Absolute Ammeter

The quantization of magnetic flux in a superconducting ring offers

an interesting parallel to the Josephson frequency- voltage relation. If n flux quanta are contained in the ring then the magnetic flux density,

B , in the ring is

32
B = ncp /s (6) o

where S is the ring area and cp =h/2e is the quantum of magnetic

flux. Both effects are a result of the macroscopic phase coherence of

the superconducting electrons and the fundamental constant ratio e/h

enters in a similar fashion. In principle, we need measure only B and

S to obtain a value for h/e.

Meservey [71] has proposed the construction of an absolute

ammeter based upon the above relation. In his proposal, the superconducting ring is replaced by a thin-film SQUID which measures the number of flux quanta enclosed. The field is provided by a carefully characterized solenoid through which the test current is passed. The current

is thus measured in terms of the flux enclosed by the SQUID loop, the

solenoid geometrical factors, and the area of the SQUID loop. The diagram of figure 10 shows the arrangement.

The concept is sound, but technological problems stand in the

way of realization of its full potential. The most severe of these problems is the measurement of the SQUID area and of the solenoid geometrical factors. Present technology places an accuracy limit of the order

of one ppm on such measurement. There may be merit in using the

scheme to check the consistency of our fundamental constants, but the

gains promised as an absolute ammeter are insufficient to overcome the

investment of significant effort on such a project. Be mindful that the

limitations are largely due to present technology limitations. These

limitations may gradually be surmounted and make Meservey' s proposal

more attractive. The precision of the ammeter far surpasses its accuracy and a later section is devoted to its value as a ratio device.

33
Figure 10. Meservey's proposed absolute ammeter, showing the

SQUID (A), and its leads on the quartz rod (B), inside

the solenoid (C), supported on the quartz cylinder (D).

34
The Measurement of h/m

e

If a hollow superconducting cylinder is spun about its longitudinal

axis, a field is generated which is just a result of conservation of the

canonical momentum of the system. The flux enclosed by the hollow

cylinder is [72]

cp = cp (n-4Su)m /h) (7) o e

where S is the area of the cylinder, m is the electron mass, h is

Planck's constant, cjd is the rotation rate, and n is an integer representing the number of flux quanta, cp , trapped at zero rotation rate.

Parker and Simmonds [72] have used a thin-film SQUID in place of the

cylinder to measure the flux generated by rotation and have thus obtained

a value for h/m . Their value has an uncertainty of several hundred

ppm and is consistent with accepted values of the physical constants.

At an uncertainty of 5 ppm, this measurement will provide some input

to the adjustment of the constants. As with the absolute ammeter,

present technology limits the measurement to an uncertainty of the order

of one ppm. Field uniformity and alignment problems also place severe

restrictions on the measurement.

Frequency Stabilized Oscillators

The ultrahigh Q cavities developed at Stanford [73] for the

superconducting linear accelerator are now finding application as stabilizing elements in microwave oscillators. Benard, et al. [74] at

9 Or say have used a cavity with an unloaded Q of 10 to stabilize an X-band

12

klystron to 3 parts in 10 for a period of 10s. Turneaure and Stein [75]

13

at Stanford have succeeded in obtaining a stability of one part in 1 at

10s and they expect considerable improvement when they achieve better

35
Flux

Counter DVM

OOOOOO

JTL

0000

I

Interpolation

Supply

In Lock-In

Amp

Ref.

LF

Osc.

Out

Ns

Figure 11. Circuit for precise dc measurements of current or

voltage by counting flux quanta. The current supply

may be regulated by closing the servo loop at the

switch S. The instrument basically compares

currents, but current ratios may be transformed

to voltage ratios by the stable cryogenic resistor R.

36
temperature stability. If such improvements are realized, these oscillators may well surpass the performance of the temperature-controlled

quartz oscillators which are currently used to stabilize klystrons for

cesium beam frequency standards. Turneaure suggests that frequency

stabilities approaching one part in 1 may be feasible.

dc Ratio Measurements

Meservey's ammeter proposal offers an attractive means of

obtaining precise current ratios. Towards this end we have been developing a modified version of the ammeter which will be described below.

The device should provide an elegant and precise means of measuring

current and voltage ratios and furthermore has been demonstrated to be

an excellent current regulator. This work is in progress at NBS.

Figure 11 shows a schematic diagram of the system. The interference device is an rf -biased multihole SQUID developed by Zimmerman [ 22] . The flux state of the SQUID is detected by measuring its

radio frequency impedance [ 18] . The signal detected by the rf circuitry

is a periodic function (period of cp ) of the field applied to the SQUID.

The current to be measured is passed through the dc coil (solenoid)

which is wound about the SQUID. It is clear that the geometrical factors

are so complicated that an absolute measurement is prohibited, but for

ratio measurements the only requirement is dimensional stability.

The SQUID can be operated in an analog mode [34] by phase

detecting the output (modulated by an appropriate signal at the SQUID)

and feeding the detected signal back to the rf coil. The system locks

onto a peak or valley of the periodic impedance function and the feedback

current is then a linear function of the dc field applied to the SQUID. In

our system we feed the phase detected signal back to the current supply

to be added to the main current delivered by batteries. The SQUID

37
then acts to regulate the current through the dc coil. One can thus open

the feedback loop and move to any desired flux state and close the loop

again to obtain a known current ratio. The ratio of currents in the two

states, 1 and 2, is just n~/n where n and n are the flux quantum

numbers for each of the states (assuming that zero current corresponds

with an integral flux state, a condition which can be preset). The flux

counter is used to count the number of flux quanta in each state and a small dc current in the rf coil (which produces zero to one flux quantum)

facilitates interpolation between flux states.

We can readily interpolate between flux quanta to better than one

part in 1 . If we use 10 quanta this gives a dynamic range of 10

parts. Reversible or nonreversible motion of flux into the SQUID walls

would be the most probable cause of breakdown of the linearity of the

device. The same sort of problems are encountered with losses in

niobium cavities [75] and these have been handled successfully by careful surface preparation and a high temperature vacuum anneal. Presumably the SQUIDs could be similarly treated should such problems

become apparent. So far, we have tested the linearity of this device to

10 ppm over a range of 10 quanta. We have regulated a current of

9

3 mA to one part in 10 . We have investigated flux trapping and found

6 a SQUID which appears to be free of this problem at flux level up to 10

quanta. More stringent tests are in progress.

This instrument can probably be adapted to a number of other

applications. Current ratios could be transformed to voltage ratios

through a stable cryogenic resistor (shown in fig. 11). A cryogenic

resistor which we have developed [76,77] should allow voltage ratio

measurements with a precision commensurate with current ratio measurements. Two coils could be wound in opposition about the SQUID to

produce a more sensitive version of the conventional dual arm current

38
comparator. This device would be capable of precise current ratio

measurements even in the submilliampere range. We plan to investigate such applications as soon as we complete development of the basic

instrument.

rf Measurements

The same functions that we have described for dc measurements

with a SQUID can be extended to rf measurements also. The first requirement is that the SQUID itself should respond to variations in magnetic flux at the required radio frequency. We have developed such a SQUID [21]. It is an rf -biased device, operating at a frequency of 9 GHz.

The source of the magnetic flux it senses is the current flowing in a coaxial line inductively coupled to the SQUID. A simple microwave

readout system monitors the variation in reflection coefficient of the

device in response to variations in current in the coaxial input line.

This prototype device has a flat response to input currents at

frequencies from zero to over 1 GHz. The readout signal has the functional form J (2ttI/I ), where J is the zero order Bessel function,

o o o

I is the amplitude of an rf current flowing in the input line, and I is o the current required to drive one flux quantum into the SQUID. Figure

12 shows an arrangement to locate the zeros of the Bessel function with

a lock-in detector. These provide a natural scale for rf measurements,

which can be found in published tables of Bessel functions. The scaling

factor I can be determined by a single measurement with dc. oWe have tested the system in a measurement of attenuation at

30 MHz [78] . Over a dynamic range of 40 dB it was in agreement with

the national standard of attenuation in coaxial line to within ± 0. 004 dB.

This is well within the uncertainty normally expressed on calibration

39
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certificates. The overwhelming advantage of the new system is that it

is capable of this level of performance at any frequency from zero to

1 GHz, whereas the piston attenuator on which the national standard is

based is a narrow-band device.

We are presently working to extend the dynamic range of the

system by automatic flux quantum counting and by adapting it to calibratable linear detection of very weak signal levels. We have already

demonstrated its ability to stabilize an rf current level by using the output of the lock-in detector to control the current through a modulator.

Taking advantage of its broadband nature, this system could be used to

stabilize the current from a generator with swept frequency. This will

support the strong present trend towards radio measurements with swept

frequency. In addition to our first measurement of attenuation, the

SQUID can be used to measure power, current, and phase difference. It may ultimately replace several of our national rf electrical standards.

Secondary Standards of EMF

There is little question about the value of the Josephson standard

of EMF. However, the equipment may not meet the portability and

simplicity requirements for laboratory intercomparisons of standards.

For chemical cells during shipment, fractional voltage changes as large

as 0. 5 ppm are not unusual [79] . For this reason it may be desirable

to develop alternate means for transferring and maintaining voltage

standards.

Finnegan and Denenstein [79] have suggested such a standard

which is based on quasiparticle tunneling between superconductors.

The cur rent -voltage characteristic for such a junction if shown in

figure 2. The sharp rise in current at the energy gap provides a simple

reference EMF. The stability of the bias current on the step need not

41
be commensurate with the desired stability in voltage. To operate at

an EMF of one volt, approximately 400 such junctions would have to be

connected in series. Two distinct advantages which may be realized

with this device are: reversibility of the voltage and immunity from

adverse effects upon shorting the output.

The SQUID could be used to regulate the current through the

tunnel junctions (see the section on dc ratio measurements). This

would provide a stability well beyond that necessary for the purpose.

In fact, with the current stability offered by this regulator, a stable

cryogenic resistor L76, 77] might provide an EMF of sufficient stability.

A Low Temperature Voltage Divider

Perhaps the simplest means of obtaining a precise fixed voltage

division with a large ratio is with the series -parallel resistance divid- er [80J. Switching N matched resistors from series to parallel gives a

2

resistance ratio equal to N with an error that is independent, to first

order, of the deviations of the individual resistors from their mean

r i 9 value L81J. To obtain a division which is precise to one part in 10 , 5

the individual resistors need be trimmed to only three parts in 10

of the mean value of the set. Because of the resistance of interconnections of the resistors at room temperature, current and voltage compensating resistors must be added if the full value of the concept is to

be realized L82J. Such compensated dividers are capable of voltage

o division with an accuracy of several parts in 10 [58, 83]. The use of

superconductors for the resistor interconnections eliminates the need

for the compensating resistors. Besides this simplification, cryogenic

operation offers the advantages of: low Johnson noise in the resistors

(smaller by more than an order of magnitude), negligible thermal EMF' s

42
and the large thermal conductance of ja. superfluid helium bath (temperature less than 2.2 K) in which the resistors are immersed.

Using a recently developed resistance material [76] we have

constructed just such a voltage divider [77]. The instrument was cono structed with a conservative design uncertainty of one part in 10 , although direct tests have only verified this uncertainty to the one ppm

level. The resistors (and the ratio of resistance) appear to have at

o least a short term stability of one part in 10 .

Null Detectors

In conjunction with the divider described above, a SQUID null

detector has also been developed [77]. This is essentially the same

device as is used for the dc ratio measurements. At 2 K the null de-

-12

tector has a sensitivity of 5 x 10 " AA/Hz with an input resistance of

0.1 Q. A number of other devices may be suitable as null detectors,

although the input impedance is quite low in some cases. Clarke used

a SLUG (Superconducting Low-inductance Undulating Galvanometer) to

establish that the Josephson frequency-voltage relation is independent of

Q

material to one part in 10 [67] Ries and Satterthwaite [84] have developed a superconducting galvanometer based on vibrating coils, while

Gifford, et al. [85] and Lukens, et al. [20] describe other variations

of the rf -biased SQUID as a galvanometer.

Conclusions

The foregoing discussions indicate that low temperature devices

are making deep inroads into electrical metrology. Unique phenomena

as well as low thermal noise appear to offer significant advantages for

further applications. The field is one in which a relatively small number

of well engineered devices can be expected to have significant impact

43
(e. g. , the Josephson volt work). The new concepts which are beginning

to emerge are perhaps just a hint of a new approach to measurement

science.

6. SPECIALIZED SCIENTIFIC INSTRUMENTS

This is the "miscellaneous" category required by any honest

catalogue. The distinction between this and the previous section is

blurred. In general we include here both instruments designed for

rather specialized purposes, and working principles which may become

instruments with development but are at an early stage yet.

Galvanometers

A large part of the early work on superconducting instruments

was devoted to making very sensitive voltmeters. An early example

was Templeton' s superconducting chopper amplifier [86] . Despite later

refinements, this retained enough unsolved problems to prevent its

widespread use. One of its descendents was a modulator amplifier developed by Ries and Satterthwaite [84J. This almost came to the stage

of being marketed as a picovoltmeter several years ago. It

failed because its low input impedance (about 10 Q) limited its usefulness. This was a common weakness of many early superconducting

instruments, shared by Clarke's "SLUG" [87], which must be among

the simplest of them all. It consisted of a blob of solder frozen onto

a niobium wire. At a suitable bias point the contact resistance between

the two (with the metals superconducting) is a periodic function of the

current in the niobium wire, apparently as the result of a quantum

interference effect. The basic sensitivity of this device to current

(about 10 A/a/Hz) is not very impressive: its advantage lies in the

extremely low input impedance presented by the short piece of superconducting niobium wire. Making full use of this requires transformer

44
coupling L88], but even then it does not appear to compete with null

detectors which have been engineered using SQUIDs [20, 77].

To summarize: using an rf -biased SQUID it is possible to make

a galvanometer with any reasonable desired impedance, and with sensitivity limited by thermal noise (at 4 K) generated by the circuit on

which measurements are to be made [20, 77]. With this flexibility,

elegant measurements of the electrical and magnetic properties of

materials at very low temperatures can be accomplished [85]. The

earlier devices with which we opened this section do not offer the

same convenience and flexibility.

RF Amplifiers

Many superconducting amplifiers operate on the parametric

principle [89J. In essence, the power added to the signal by the amplifier is not supplied from a dc power supply, but rather from a "pump"

supplying power at some frequency higher than that of the signal, with

which it interacts in some non-linear device. The output can be taken

either at the same frequency as the input (in a negative resistance

parametric amplifier) or at the difference between the pump and signal

frequencies (in a parametric up converter). In both cases a substantial

power gain is possible with the addition of very little noise. Some

remarkably simple devices are possible. Clorfeine [90] and Bura [91]

have reported successful parametric amplifiers based on the non-linear

penetration of electromagnetic fields through superconducting films.

Josephson junctions also work well as the required non-linear devicesL92],

The action of the rf -biased SQUID may be described as parametric upconversion, especially when the magnetic flux being sensed varies with

time. Zimmerman and Silver [28,93] have explored many of the ramifications of this. Recently, Kanter and Silver [94] have reported early

45
results with a parametric (negative resistance) microwave amplifier in

which the self oscillation of a Josephson junction under the influence

of a voltage bias is used for the pump. The work on noise thermometry

which we describe later on in this section demonstrated that parametric

amplifiers based on the Josephson effect can have effective noise tem-

-3

peratures less than 10 K.

In contrast with these parametric amplifiers, Newhouse and

Edwards L95] have reported what appears to be a well engineered amplifier based on a cryotron, biased to a point where it is halfway switched.

In this condition the gate resistance is a very -sensitive function of small

variations in control current. It is necessary to use a servo loop to

maintain the desired operating point, and the device amplifies at frequencies outside the bandwidth of this servo.

This is an area where spectacular developments could happen

very quickly in response to a demonstrated need.

Ultrasonic Acoustics

Tunnel junctions can be used to generate and detect ultrasonic

vibrations in the crystal lattice of the substrate L96J. This comes about

because of the disturbance of thermal equilibrium during the tunneling

process. The major part of the tunneling current at a finite voltage

consists of "quasi-particles", or excited electrons not taking part in the

superconducting ground state. The sharp rise in current at a voltage

of the order of a millivolt reflects the threshold energy required to create

these quasi particles. If a junction is biased to a voltage below this

threshold, the current can be increased with the assistance of ultrasound

to supply the necessary energy. A tunnel junction in this condition therefore becomes a detector of ultrasound.

46
When a junction is biased to a voltage over the threshold, a copious current flows. This injects an anomalously large concentration

of quasi particles into the positive side of the junction. Equilibrium is

restored by the extra quasi particles pairing up and collapsing into the

ground state. In this process they radiate most of their energy in the

form of ultrasonic waves.

Thus tunnel junctions between superconductors can be used to

generate and detect ultrasound at frequencies towards the upper limit of

the spectrum of lattice waves in a solid. This is another working principle

awaiting a creative application.

Noise Thermometers

The Josephson effect has been used to extend noise thermometry

down to the millikelvin range of temperature L97-10l]. The strategy

was to use an oscillating (voltage biased) Josephson junction to convert

the variations in voltage to be detected into variations in frequency,

which can be measured absolutely. A straightforward application of

the theory of frequency modulation also eliminated the need to measure

the bandwidth of the system. The two major problems of noise thermometry were therefore avoided, and a system was developed which is

capable of realizing the absolute scale of temperature with an uncertainty

of less than 10 K [lOl], Tests so far have been conducted at temperatures down to 0. 02 K.

The principle is illustrated in figure 13. A Josephson junction is

-9

connected in a structure of very low inductance (< 10 H) with a shunt

_5

with much lower resistance than the junction (10 Q is a typical value. ) A direct current of the order of 10 A (derived from a battery and

resistor) through this shunt maintains a bias voltage of about 10 V,

47
L.O

N

•M Tl

LL. L. ^

C* Q>C

O

Q> ro O

C *-» o c o 3

*« ^C * CO

CO

Q)

C

NC

'O o

iCO1 I oo

cu

-P -^

03 J* 3 S o o ' - M n fl « 9 O m

.H CU ts a a u

ft

cu

M o

PI 03 o X ^

<U d) -P M 03 03] g M 1 s o

Cl •-, !3 -P

CO

flO

.i-i -pa

+->o o

COO T3

h o u

CU &1

fn CU 0) 3 J

CD -P

0)03oso

CDU

•r-l p^ > CU -p 5 <u

O +jO

Jh J-i 03

CO1—1

cdu

48
which causes the junction to oscillate at a frequency of about 5 kHz.

The frequency of oscillation / is related to the bias voltage V by

h/ = 2eV (8)

where h is Planck's constant and e is the electron charge. The

fluctuations in voltage due to thermal noise in the shunt resistor cause

variations in the frequency /, which may be measured either by using

a spectrum analyzer to determine the average linewidth A/ (between the

points at half peak power) or by using a frequency counter to repeatedly

measure the frequency. The quantity of interest is then the variance (or

mean square deviation) a (/) of the fluctuations. The absolute temperature T of the shunt resistor R may then be calculated from the formulae

A/ = l6TTe

2kTR/h2

(9)

a

2

(/) = 8e

2 kTR/Th2

(10)

where T is the gate -time of the frequency counter.

Equation (9) was verified experimentally [98] in 1967, and our

more recent efforts have been directed towards making a practical

thermometer based on equation (10). A schematic diagram of this

instrument is shown in figure 13. Because of the highly nonlinear

characteristics of a Josephson junction, its rf impedance is modulated

strongly at the frequency of its self -oscillation. This modulation may

be detected at some convenient radio frequency (e. g. , 30 MHz) with the

circuit shown. The advantage of this procedure is the substantial power

gain available by parametric up-conversion L89J- The readings of the

frequency counter are punched onto paper tape and analyzed by a simple

computer program. The only additional information required to

49
determine absolute temperature is the bias current and the average

frequency of oscillation (and hence the bias voltage), from which the

value of R in equation 10 may be computed.

This is now at the stage of a working instrument. Soulen LlOl]

is presently engaged in stringent tests and further refinements which

will determine the value of noise thermometry as a tool for realizing

the Kelvin scale at very low temperatures. Because of the rather long

times required for measurements at the lowest temperatures, it probably

will not replace the more common secondary thermometers, which require calibration but are capable of the quick readings which are required by routine experimental work. Rival absolute thermometers are

based on the temperature dependence of the magnetic susceptibility of

nucleii. This can be monitored either by measuring the anisotropy of

Y-ray emission from decaying nuclei L102J or by direct magnetic measurement with a SQUID [l03, 104]. Recently, Wheatley et al [85] have

developed a variant of the noise thermometer, in which the resistor

is coupled to an rf -biased SQUID via a transformer. The advantage of

this system is that it can use a wider bandwidth than the version we have

described here, and hence make measurements with equal precision in a shorter time. However, in doing this the directness of absolute thermometry is sacrificed. Wheatley' s thermometer will probably be used

in practice with a calibration at a reference temperature determined by

some other means. This is common practice in noise thermometry at

higher temperatures with conventional techniques L105J.

50
7. TRANSMISSION LINES FOR INFORMATION

The most efficient way to use the capacity of a given electrical

channel for communication is by Pulse Code Modulation (PCM). Thus

the merits of transmission lines for information are determined by their

behavior with very short pulses. The historical sequence of development started with the investigation of miniature superconducting coaxial

lines for delaying pulses. As a result of this work their virtues for the

transmission of information were recognized, and this has motivated a further effort to develop them for that purpose. It now appears that they

will probably be the most economical way of constructing systems with

very large capacity.

The leading pioneers in this field were Nahman and his coworkers.

At an early stage they constructed a fairly crude 50 Q coaxial line with a niobium inner conductor, teflon dielectric (about 2 mm OD), and lead

outer conductor. With this they demonstrated the possibilities of superconducting lines for transmitting pulses Ll06, 107]. They were able to

-10

preserve a risetime of less than 5x10 sec for a pulse travelling

several hundred meters (a delay of several |~l sec). Encouraged by this

early work, they undertook a detailed analysis of the theoretically attainable performance of superconducting lines [l08, 109]. This showed that

there was considerable scope for improvement in the prototype lines.

The factors which limit the transmission of pulses are losses in

the conductors and the dielectric, and reflections caused by irregularities

in the line. The reason for the superiority of superconducting lines is

that the surface resistance of all normal metals, at the frequencies

excited by the passage of a pulse, is high because of the anomalous

skin effect, regardless of the quality of the metal. Nahman' s first

crude lines reaped a large part of the available benefit of superconductors

51
in this respect. The aspect where most improvement is possible is

in the elimination of spurious reflections. This requires manufacture

to very tight tolerances. Vigorous development is now in progress in

Japan. Hoshiko and Chiba have sketched out their ultimate hopes for

an information system using superconducting lines [lio]. They envision

a cable of ZOO individual coaxial lines (each about 1 mm in diameter),

requiring repeaters at intervals of 25 to 50 km. With a bandwidth of

3 GHz, the capacity of such a system for transmitting information will

be about 300 G bits /sec. In private communications (to Nahman), they

report considerable progress towards solving the problem of manufacturing the cable with the required tolerances.

The nearest conventional rival is a cylindrical waveguide with

helical walls to prevent mode mixing. Made with normal metal it could

have a bandwidth of 40 GHz - wider than miniature coaxial line. However

the attenuation would be greater, and repeaters would be required at

intervals of 16 km. These would be the most expensive parts of the

system.

This appears to be yet another area where superconductors will

have a clear superiority over other technologies when the world requires

systems with sufficiently large capacity.

52
8. ACCELEROMETERS AND GYROS

The combination of the Meissner effect and an inhomogeneous

magnetic field, generated by superconducting coils carrying persistent

currents, has obvious appeal as a stable bearing with very little friction.

The two applications of this principle which have received attention so

far are gyros and gravimeters. At the time of writing, the latter appear

to be the nearer to acceptance as valuable instruments.

Gyros

The earliest public discussion of the development of a practical

superconducting gyro was by Harding and Tuffias [ill] in I960. They

listed most of the expected problems and laid out a program of research

to solve them. Four years later, Harding reported significant progress

but not the completion of the project [ 1 1ZJ . One of the major discoveries

of the intervening years had been the full extent of ac loss in superconductors, even with well prepared surfaces exposed to modest magnetic

fields. This proved to be a severe but not insurmountable problem:

superconducting gyros could become very good, but they would never be

frictionless. Other groups were working in this field at the same time,

but very little of their work was published. The observed conclusion is

that superconducting gyros are not in general use for navigation today.

One situation where the problems of superconducting (or any

other)gyros are drastically reduced is on board a satellite in orbit.

The absence of a strong force of gravitation reduces the restoring forces

required of the suspension, and hence gives greater freedom to reduce

friction. Fairbank and his students are in the process of developing a gyro with that in mind [ll3]. Their objective is to detect a small precession (from 0. 05 to 7 seconds of arc per year) predicted by the general

theory of relativity. They have two possible designs in mind. The first

is similar to the levitated spheres that other groups have been working

53
with. The second relies on the very long relaxation time of the spinning

3

nuclei of liquid He. In the absence of external perturbation (which can

be excluded by a superconducting shield) the magnetization of these nuclei

would persist for several years. Both these designs expect to use a superconducting magnetometer for readout of the direction in which the

gyro points. The accuracy required for the experiment is 0.001 seconds.

of arc.

Thus some promise remains for the future of superconducting gyros,

Meanwhile inertial navigation continues to tolerate the uncertainties of

mechanical suspensions.

Gravimeters

Measurement of long-term variations in the Earth's gravitation

requires an elastic suspension which is not subject to the creep that

affects metal springs. Prothero and Goodkind have developed an instrument, based on a levitated superconducting body, which satisfies this

requirement [ll4j. It consists of a superconducting sphere levitated by

an inhomogeneous magnetic field, maintained by persistent currents in a

set of superconducting coils. The position of the sphere is sensed by two

independent systems: a magnetic flux detector and a detector which senses

the capacitance between the sphere and a set of nearby plates. A signal

from the capacitance detector drives a servo system to maintain the

sphere in a constant position, by applying a small additional restoring

force. By monitoring both position detectors it is possible to discriminate

between variations in gravitation and variations in the magnetic field.

The stability of the system is at least 10 per hour, and runs of many

months are routinely in progress. This instrument has been in operation

for several years now, generating information of real geophysical

importance [l 15].

54
9. CONCLUSION

The general impression left by this sketch of the present state of

cryoelectronics is one of vigorous development, which is beginning to be

guided towards the solution of real problems. Shortage of space has

precluded doing full justice to a large amount of early work which has

since been eclipsed by the research it helped to stimulate.

55
References

L l] Josephson, B. D. , Possible new effects in superconductive

tunneling, Phys. Letters 1_, 251 -253 (July, 1962).

[ 2] Josephson, B. D. , Coupled superconductors, Rev. Mod. Phys.

36, 216-220 (January, 1964).

[ 3J Josephson, B. D. , Supercurrents through barriers, Advances in

Physics 14, 419-451 (October, 1965).

L 4] Giaever, I. , Energy gap in superconductors measured by electron

tunneling, Phys. Rev. Letters 5_, 147-148 (August, I960).

L 5] Clarke, J. , Experimental comparison of the Josephson voltagefrequency relation in different superconductors, Phys. Rev.

Letters 21_, 1566-1569 (December, 1968).

[ 6] Anderson, P. W. , and Dayem, A. H. , Radio frequency effects

in superconducting thin film bridges, Phys. Rev. Letters 13,

195-197 (August, 1964).

[ 7j Zimmerman, J. E. , Cowen, J. A. , and Silver, A. H. , Coherent

radiation from voltage biased weakly connected superconductors,

Appl. Phys. Letters 9, 353-355 (November, 1966).

L 8J Nisenoff, M. , Thin film superconducting weak link devices,

Applied Superconductivity Conference, Annapolis, Maryland

(May, 1972). Proceedings to be published by IEEE.

L 9J Matisoo, J. , Josephson tunnel junctions, Applied Superconductivity Conference, Annapolis, Maryland (May, 1972). Proceedings

to be published by IEEE.

L 1 J Zimmerman, J. E. , Properties and applications of superconducting point contacts, Applied Superconductivity Conference,

Annapolis, Maryland (May, 1972). Proceedings to be published

by IEEE.

56
Lll] Clarke, J. , Low frequency measurements with Josephson

devices, Applied Superconductivity Conference, Annapolis,

Maryland (May, 1972). Proceedings to be published by IEEE.

L 12] McCumber, D. E. , Effect of ac impedance on dc voltage -current

characteristics of superconductor weak link junctions, J. Appl.

Phys. 39, 3113-3118 (June, 1968).

Ll3] Stewart, W. C. , Current-voltage characteristics of Josephson

junctions, Appl. Phys. Letters J_2, 277-280 (April, 1968).

[l4] Hansma, P. K. , Rochlin, G. I., and Sweet, J. N. , Externally

shunted Josephson junctions: generalized weak links, Phys. Rev.

B 4 , 3003-3014 (November, 1971).

[l5j Rowell, J. M. , Magnetic field dependence of the Josephson tunnel

current, Phys. Rev. Letters 11_, 200-202 (September, 1963).

[l6J Zimmerman, J. E. , and Silver, A. H. , Macroscopic quantum

interference effects through superconducting point contacts,

Phys. Rev. 141 , 367-375 (January, 1966).

[l7] Silver, A. H. , and Zimmerman, J. E. , Quantum states and

transitions in weakly connected superconducting rings, Phys.

Rev. 157 , 317-341 (May, 1967).

[l8] Zimmerman, J. E. , Thiene, P., and Harding, J. T. , Design and

operation of stable rf -biased superconducting point-contact

quantum devices, and a note on the properties of perfectly clean

metal contacts, J. Appl. Phys. 41_, 1572-1580 (March, 1970).

[19] Mercereau, J. E. , Superconducting magnetometers, Rev. Phys.

Appl. 5_, 13-20 (February, 1970).

[20] Lukens, J. E. , Warburton, R. J., and Webb, W. W. , Versatile

superconducting femtovolt amplifier and multimeter, J. Appl.

Phys. 42_, 27-30 (January, 1971).

57
[2l] Hamper, R. A., and Simmonds, M. B. , Broadband superconducting quantum magnetometer, Appl. Phys. Letters 20, 270-272

(April, 1972).

l22J Zimmerman, J. E. , Sensitivity enhancement of superconducting

quantum interference devices through the use of fractional-turn

loops, J. Appl. Phys. 42_, 4483-4487 (October, 1971).

[23] Zimmerman, J. E. , and Frederick, N. V., Miniature ultrasensitive superconducting magnetic gradiometer and its use in cardiography and other applications, Appl. Phys. Letters 19, 16-19

(July, 1971).

[24] Silver, A. H. , and Zimmerman, J. E. , Josephson weak link

devices, Air Force Report SAMSO-TR-71 -1 8 (November, 1970).

1.25] Shapiro, S. , Josephson currents in superconducting tunneling:

the effect of microwaves and other observations, Phys. Rev.

Letters 11_, 80-82 (July, 1963).

L26] Langenberg, D. N. Scalapino, D. J. , Taylor, B. N. , and Eck, R. , Investigation of microwave radiation emitted by Josephson junctions,

Phys. Rev. Letters 15, 294-297 (August, 1965).

L27j Yanson, I. K. , Svistunov, V. M. , and Dmitrenko, I. M. , Experimental observation of the tunnel effect for Cooper pairs with the

emission of photons, Soviet Physics JETP 21, 650-652 (September,

1965).

L28] Zimmerman, J. E. , and Silver, A. H. , A high sensitivity superconducting detector, J. Appl. Phys. 39, 2679-2682 (May, 1968).

[29] Ulrich, B. T. , and Kluth, E. O. , Josephson junction one millimeter microwave source: coupling outside a dewar, Applied

Superconductivity Conference, Annapolis, Maryland (may, 1972).

Proceedings to be published by IEEE. (See also a post-deadline

paper by R. K. Elsley at the same conference. )

58
[30] McDonald, D. G. , Kose, V. E. , Evenson, K. M. , Wells, J. S. , and Cupp, J. D. , Harmonic generation and submillimeter wave

mixing with the Josephson effect, Appl. Phys. Letters 15,

121-122 (August, 1969).

L3l] Vant-Hull, L. L. , and Mercereau, J. E. , Normal Josephson

junctions and quantum coherence, Phys. Rev. Letters 17,

629-631 (September, 1966).

[32j Opfer, J. E. , Modulated inductance magnetometers, Rev. Phys.

Appl. 5, 37-40 (February, 1970).

L 33] Deaver, B. S. , and Goree, W. S. , Some techniques for sensitive

magnetic measurements using superconducting circuits and

magnetic shields, Rev. Sci. Instr. 3_8, 311-318 (March, 1967).

[34J Forgacs, R. L. , and Warnick, A. , Digital -analog magnetometer

utilizing superconducting sensor, Rev. Sci. Instr. 38, 214-220

(February, 19 67).

[35] Zimmerman, J. E. , and Frederick, N. V. , Ultrasensitive superconducting magnetic gradiometer, NBS Report 10 736 (March, 1972),

[36] Baule, G. M. , and McFee, R. , Detection of the magnetic field of

the heart, Amer. Heart J. 66_, 95-96 (July, 1963). See also

McFee, R. , and Baule, G. M. , Research in electrocardiography

and magnetocardiography, Proc. IEEE 60, 290-321 (March, 1972).

[37] Cohen, D. , Edelsack, E. A., and Zimmerman, J. E. , Magnetocardiograms taken inside a shielded room with a superconducting

point-contact magnetometer, Appl. Phys. Letters l_6, 278-280

(April, 1970).

[38] Rosen, A. , Inouye, G. T. , Morse, A. L. , and Judge, D. L. , Magnetic recordings of the heart's electrical activity with a cryogenic magnetometer, J. Appl. Phys. 42_, 3682-3684

(September, 1971).

59
[39] Cohen, D. , Norman, J. C. , Molokhia, F. , and Hood, W. , Jr. , Magnetocardiography of direct currents. S-T segment and

baseline shifts during experimental mycardial infraction, Science

172 , 1329-1333 (June, 1971).

L40J Cohen, D. , Magnetoencephalography: evidence of magnetic fields

produced by alpha-rhythm currents, Science 1 61, 784-786 (August,

1968).

L4l] Buck, D. A. , The cryotron--a superconductive computer component, Proc. IRE 44, 482-493 (April, 1956).

L42J Proc. IEEE 52, no. 10, special issue on cryogenic electronics

(October, 1964).

L43j Matisoo, J. , The tunneling cryotron--a superconductive logic

element based on electron tunneling, Proc. IEEE 55, 172-180

(February, 1967).

L44] Zappe, H. H. , and Grebe, K. R. , Ultra high speed operation of

Josephson tunneling devices, IBM Journal of Research 15,

405-407 (September, 1971).

[45j Anacker, W. , Potential of superconductive Josephson tunneling

technology for ultrahigh performance memories and processors,

IEEE Trans. Magnetics MAG-5, 968-975 (December, 1969).

[46] Grimes, C. C. , Richards, P. L. , and Shapiro, S. , Josephson

effect far infrared detector, J. Appl. Phys. 3% 3905-3912

(July, 1968).

[47] Grimes, C. C. , and Shapiro, S. , Millimeter wave mixing with

Josephson junctions, Phys. Rev. 169, 397-406 (May, 1968).

L48j Low, F. J. , Low temperature germanium bolometer, J. Opt.

Soc. Amer. 51, 1300-1304 (November, 1961).

60
[49] Martin, D. H. , and Bloor, D. , The application of superconductivity

to the detection of radiant energy, Cryogenics 1, 159-165 (March,

1961).

[50] Bloor, D. , Dean, J. J., Jones, G. O. , Martin, D. H. , Mawer,

P. A., and Perry, C. H. , Spectroscopy at extreme infrared

wavelengths, Proc. Roy. Soc. (London) A260 , 510-522 (March,

1961).

L5l] Richards, P. L. , and Sterling, S. A., Regenerative Josephson

effect detector for far infrared radiation, Appl. Phys. Letters

14, 394-396 (June, 1969).

[52] McDonald, D. G. , Evenson, K. M. , Wells, J. S. , and Cupp, J. D. , High frequency limit of the Josephson effect, J. Appl. Phys.

42, 179-181 (January, 1971).

[53] McDonald, D. G. , Risley, A. S. , Cupp, J. D. , and Evenson,

K. M. , Harmonic mixing of microwave and far infrared laser

radiation using a Josephson junction, Appl. Phys. Letters 18,

162-164 (February, 1971).

[54J McDonald, D. G. , Risley, A. S. , Cupp, J. D. , Evenson, K. M. , and Ashley, J. R. , Four hundredth order harmonic mixing of

microwave and infrared laser radiation using a Josephson

junction and a maser, Appl. Phys. Letters 20 , 296-299 (April,

1972).

[55] Ulrich, B. T. , Astronomical observations with Josephson junction

detectors, unpublished invited talk at the Applied Superconductivity Conference, Boulder, Colorado (June, 1970).

[56] Zimmerman, J. E. , Heterodyne detection with superconducting

point contacts and enhanced heterodyne signals from tightly

coupled contacts, J. Appl. Phys. 4]_, 1589-1593 (March, 1970).

61
[57] Blaney, T. G. , and Bradley, C. C. , The Josephson junction as a frequency analyzer and mixer of submillimeter radiation sources,

J. Phys: D 5, 180-184 (January, 1972).

[58] Finnegan, T. F. , Denenstein, A. , and Langenberg, D. N. , ac Josephson effect determination of e/h: a standard of electrochemical potential based on macroscopic quantum phase coherence

in superconductors, Phys. Rev. B 4_, 1487-1522 (September, 1971).

[59] Finnegan, T. F. , Witt, T. J., Field, B. F. , and Toots, J.,

2e/h via the ac Josephson effect, Proceedings of the 4th International Conference on Atomic Masses and Fundamental Constants

(September, 1971). To be published.

L60] Petley, B. W. , and Morris, K. , A measurement of 2e/h by the ac Josephson effect, Metrologia 6, 46-51 (April, 1970).

[6l] Harvey, I. K. , Macfarlane, J. C. , and Frenkel, R. B. , Determination of 2e/h based on the ac Josephson effect, Phys. Rev.

Letters 25_, 853-856 (September, 1970).

L62] Kose, V. , Melchert, F. , Fack, H. , and Schrader, H. -J. , Die

bestimmung von e/h mit hilfe des Josephson-effektes, PTB

Mitteilungen 81, 8-10 (January, 1971).

[63] Finnegan, T. F. , Witt, T. J., Field, B. F. , and Toots, J.,

Superconductivity in dc voltage metrology, The Science and

Technology of Superconductivity, ed. by W. D. Gregory and

W. N. Matthews, 1972, to be published.

[64] Stephen, M. J., Theory of a Josephson oscillator, Phys. Rev.

Letters 21, 1629-1632 (December, 1968).

[65] Scully, M. O. , and Lee, P. A., Frequency-pulling effects in

Josephson radiation, Phys. Rev. Letters 22, 23-26 (January,

1969).

62
[66] McCumber, D. E. , Frequency-pulling effects in Josephson

radiation, Phys. Rev. Letters 2_3, 1228-1231 (November, 1969).

[67] Clarke, J. , Experimental comparison of the Josephson voltage

frequency relation in different superconductors, Phys. Rev.

Letters 2J_, 1566-1569 (December, 1968).

[68] Langenberg, D. N. , and Schrieffer, J. R. , Comments on quantumelectrodynamic corrections to the electron charge in metals, Phys.

Rev. B3, 1776-1778 (March, 1971).

[69] Hartle, J. B. , Scalapino, D. J. , and Sugar, R. L. , Absence of

quantum-electrodynamic corrections to the charge of the electron

as measured in Josephson junction experiments, Phys. Rev. B 3_, 1778-1781 (March, 1971).

[70] Taylor, B. N. , Parker, W. H. , and Langenberg, D. N. , The

fundamental constants and quantum electrodynamics, Rev. Mod.

Phys. 41_, 375-496 (July, 1969).

[7l] Meservey, R. , Proposed ammeter using flux quantization, J. Appl. Phys. 39_, 2598-2605 (May, 1968).

[72] Parker, W. H. , and Simmonds, M. B. , Measurement of h/m

using rotating superconductors, Precision Measurement and

Fundamental Constants, NBS Special Publication No. 343,

(U.S. Government Printing Office, 1971), pp. 293-248.

[73] Turneaure, J. P., and Viet, N. T. , Superconducting Nb TM

Ql0

mode electron-beam welded cavities, Appl. Phys. Letters 1_6,

333-335 (May, 1970).

[74] Benard, J., Jimenez, J. J., Sudraud, P., and Viet, N. T. , Frequency stability improvement in a klystron stabilized by

a superconducting cavity, submitted to Electronics Letters

(February, 1972).

63
[75] Turneaure, J. P. , The status of superconductivity for rf applications, Applied Superconductivity Conference, Annapolis,

Maryland (May, 1972). Proceedings to be published by IEEE.

L76] Sullivan, D. B. , Resistance of a silicon bronze at low temperatures, Rev. Sci. Instr. 42_, 612-613 (May, 1971).

L77j Sullivan, D. B. , A low temperature voltage divider and null

detector, Rev. Sci. Instr. 43, 499-505 (March, 1972).

[78] Kamper, R. A. , Simmonds, M. B. , Adair, R. T. , and Hoer,

C. A. , Quantum mechanical measurement of rf attenuation,

Applied Superconductivity Conference, Annapolis, Maryland

(May, 1972). Proceedings to be published by IEEE.

L79J Finnegan, T. F. , and Denenstein, A. , A new transfer and

maintenance voltage standard using superconducting tunnel

junctions, Metrologia 7, 167 (October, 1971).

L80] Lord Rayleigh, Experiments to determine the value of the

British Association Unit of Resistance in absolute measure, Phil.

Trans. Roy. Soc. (London) 173, 661-697 (March, 1882).

l81] Wenner, F. , Methods, apparatus and procedures for the comparison of precision standard resistors, J. Res. Nat. Bur.

Standards 25, 229-294 (August, 1940).

[82] Hamon, B. V., A 1-100 Q build-up resistor for the calibration

of standard resistors, J. Sci. Instr. 31, 450-453 (December,

1954).

[83] Finnegan, T. F. , ac Josephson effect determination of e/h: A

standard of electrochemical potential based on macroscopic

phase coherence in superconductors, PH.D. Thesis, U. of

Pennsylvania, 1971.

64
[84] Ries, R. P., and Satterthwaite, C. B. , Superconducting parametric amplifier for the measurement of small dc voltages, Rev.

Sci. Instr. 38_, 1203-1209 (September, 1967).

[85] Gifford, R. P., Webb, R. A., and Wheatley, J. C, Principles

and methods of low frequency electric and magnetic measurements

using an rf -biased point contact superconducting device, J. Low

Temp. Phys. 6_, 533-610 (March, 1972).

[86] Templeton, I. M. , A superconducting modulator, J. Sci. Instr.

32_, 314-315 (August, 1955).

[87] Clarke, J. , A superconducting galvanometer employing Josephson

tunneling, Phil. Mag. 13 , 115-127 (January, 1966).

[88] Clarke, J. , Tennant, W. E. , and Woody, D. , Impedance matching

a Josephson galvanometer by means of a superconducting transformer, J. Appl. Phys. 42_, 3859-3865 (September, 1971).

[89] Louisell, W. H. , Coupled Mode and Parametric Electronics

(John Wiley and Sons, Inc. , New York, I960).

[90] Clorfeine, A. S. , Microwave amplification with superconductors,

Proc. IEEE 52_, 844-845 (July, 1964).

[91] Bura, P., Parametric amplification with superconducting films,

Proc. IEEE 54, 687-688 (April, 1966).

[92] Di Nardo, A. J. , and Sard, E. , Superconducting microwave

mixers utilizing Josephson junctions, (abstract only) J. Appl.

Phys. 42_, 105 (January, 1971).

[93] Silver, A. H. , and Zimmerman, J. E. , Multiple quantum resonance spectroscopy through weakly connected superconductors,

Appl. Phys. Letters 10, 142-145 (March, 1967).

[94] Kanter, H. , and Silver, A. H. , Self pumped Josephson parametric

amplification, Appl. Phys. Letters 19, 515-51 7 (December, 1971).

65
[ 95] Newhouse, V. L. , and Edwards, H. H. , An ultrasensitive linear

cryotron amplifier, Proc. IEEE 52, 1191-1206 (October, 1964).

[ 96] Dayem, A. H. , Miller, B. I., and Wiegand, J. J., Phonon

generation and detection in superconducting lead diodes, Phys.

Rev. B3, 2949-2961 (May, 1971).

L 97] Hamper, R. A., Millidegree noise thermometry, Symposium

on the Physics of Superconducting Devices, Charlottesville,

Virginia (April, 1967). Proceedings published as an ONR report.

L 98] Silver, A. H. , Zimmerman, J. E. , and Kamper, R. A. , Contribution of thermal noise to the line width of Josephson radiation

from superconducting point contacts, Appl. Phys. Letters 11,

209-211 (September, 1967).

L 99] Kamper, R. A. , and Zimmerman, J. E. , Noise thermometry

with the Josephson effect, J. Appl. Phys. 42, 132-136 (January,

1971).

LlOO] Kamper, R. A. , Siegwarth, J. D. , Radebaugh, R. , and

Zimmerman, J. E. , Observation of noise temperature in the

millikelvin range, Proc. IEEE 59., 1368-1369 (September, 1971).

LlOl] Soulen, R. J., and Marshak, H. , The use of Josephson junctions

for noise thermometry below 1 Kelvin, Applied Superconductivity

Conference, Annapolis, Maryland (May, 1972). Proceedings to

be published by IEEE.

[l02] Thorp, T. L. , Triplett, B. B. , Brewer, W. D. , Cohen, M. L. , Phillips, N. E. , Shirley, D. A., Templeton, J. E. , Starky, R.

W. , and Schmidt, P. H. , Search for superconductivity in lithium

and magnesium, J. Low Temp. Phys. 3_, 589-602 (December,

1970).

66
[l03] Burhman, R. A., Halperin, W. P., Schwenterly, S. W. , Reppy, J., Richardson, R. C. , and Webb, W. W. , Static

nuclear magnetization thermometry in the millikelvin region,

Proceedings of the 12th International Conference on Low Temperature Physics, Kyoto, Japan, 831-832 (September, 1970).

[l04] Goodkind, J. M. , and S to Ifa, D. L. , The superconducting magnetic flux detector, Rev. Sci. Instr. 41_, 799-807 (June, 1970).

L 105] Kamper, R. A. , Survey of noise thermometry, 5th Symposium

on Temperature, Washington, D. C. (June, 1971).

[l06] Allen, R. J., and Nahman, N. S. , Analysis and performance

of superconductive coaxial transmission lines, Proc. IEEE 52,

1147-1154 (October, 1964).

[107] Nahman, N. S. , The measurement of baseband pulse rise times

-9

of less than 10 second, Proc. IEEE 55_, 855-864 (June, 1967).

[l08] McCaa, W. D. , Jr., and Nahman, N. S. , Frequency and time

domain analysis of a superconductive coaxial line using the two

fluid model, J. Appl. Phys. 3% 2542-2596 (May, 1968).

[l09] McCaa, W. D. , Jr., and Nahman, N. S. , Frequency and time

domain analysis of a superconductive coaxial line using the

BCS theory, J. Appl. Phys. 40, 2098-2100 (April, 1969).

[llO] Hoshiko, S. , and Chiba, N. , Superconductive communication

cables (in Japanese), Oyo Buturi 40, 905-906 (August, 1971).

[ill] Harding, J. T. , and Tuffias, R. H. , The cryogenic gyro,

Advances in Cryogenic Engineering 6, 94-105 (conference in

August, I960).

[ll2] Harding, J. T. , Drift data for the cryogenic gyro, Advances in

Cryogenic Engineering 1_0, 137-143 (conference in August, 1964).

67
[ 1 1 3] Fairbank, W. M. , A superconducting linear accelerator and the

use of superconductivity in some fundamental experiments in

physics, Symposium on the Physics of Superconducting Devices,

Charlottesville, Virginia (April, 1967). Proceedings published

as an ONR report.

Lll4j Prothero, W. A., Jr., and Goodkind, J. M. , A superconducting

gravimeter, Rev. Sci. Instr. 39, 1257-1262 (September, 1968).

L115] Prothero, W. A., Jr., and Goodkind, J. M. , Earth tide measurements with the superconducting gravimeter, J. Geophys.

Res. 77, 926-937 (February, 1972).

68
orm NBS-114A (1-7D

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

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1. PUBLICATION OR REPORT NO.

NBS-TN-630

2. Gov t Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Developments in Cryoelectronics

5. Publication Date November 1972

6. Performing Organization Code

7. AUTHOR(S)

Robert A. Kamper and Donald B. Sullivan

8. Performing Organization

10. Project/Task/Work Unit No. 2750187

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs,

DEPARTMENT OF COMMERCE

Boulder, Co. 80302

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Same as Item 9,

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Survey - 1969 to 1972

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p. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) This is a survey of progress to date in the development of new electronic

instruments taking advantage of the unique properties of superconductors.

7. KEY WORDS (Alphabetical order, separated by semicolons)

Electronics; Josephson Effect; Precise Measurements; Superconductivity

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