Apparatus and method for quantum mechanical encryption for the transmission of secure communications

An apparatus and method that permit the transmission of secure communications. The invention uses quantum mechanical effects to establish nonlocal correlations between a pair of photons. This is analogous to an automatic encryption code that exists at only one location and is immediately destroyed after either of the photons is detected. This latter feature also provides a means for detecting any unauthorized tap on the transmission line.

BACKGROUND OF THE INTENTION 
The invention consists of an apparatus and method that use 
quantum-mechanical effects to permit the transmission of secure 
communications without the need for encryption codes. 
Classically, the transmission of a secure/classified message requires 
encoding the message before transmission and decoding the message after 
its receipt. The necessary equipment and the distribution and use of the 
necessary codes is expensive and inconvenient and tends to limit the use 
of encrypted communications. 
More importantly, there have been instances in which topsecret crypto codes 
have been divulged to the Soviet Union over a period of many years. The 
damage caused to the national security by the disclosure of these codes is 
a very real problem, and any method to eliminate the need for such codes 
would be of considerable importance. 
SUMMARY OF THE INVENTION 
The quantum-mechanical effects used in the present invention allow the 
transmission of secure communications without the need for encryption 
codes. Roughly speaking, quantum mechanics establishes nonlocal 
correlations between a pair of photons which are somewhat analogous to an 
automatic encryption code that exists at only one location and is 
immediately destroyed after either of the photons is detected. The latter 
feature also provides a means of determining whether or not there has been 
any unauthorized attempt to tap into the transmission line. 
For a more complete appreciation of the invention, attention is invited to 
the following detailed description of a preferred embodiment of the 
invention taken with the figures of the drawings. The scope of the 
invention, however, is limited only through the claims appended hereto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An illustrative embodiment of the invention is shown in FIG. 1 of the 
drawings. 
A light source 10, (typically a nonlinear crystal) emits a pair of photons 
.gamma..sub.1 and .gamma..sub.2 that are very nearly coincident, although 
the time at which they are emitted remains uncertain in the 
quantum-mechanical sense. After traveling a large distance D, the photons 
encounter two identical interferometers 12, 14 that have a shorter path of 
length S and a longer path of length L. The path-length difference 
.DELTA.l=L-S is chosen to be much larger than the first-order coherence 
length. Phase shifts .phi..sub.1 and .phi..sub.2 are introduced in the two 
longer paths. For experimental purposes, two mirrors (not shown) can be 
used to reflect the photons back towards the source, so that the optical 
path lengths, D, can be much larger than the actual separation between the 
two detectors. 
In the case in which coincident photons are observed in detectors D.sub.1 
and D.sub.2 with a time resolution much better than the difference in 
travel times .DELTA.T=.DELTA.I/c, where c is the speed of light, then both 
photons must have traveled over the shorter paths or both photons must 
have traveled over the longer paths. Interference between the 
quantum-mechanical probability amplitudes for those two possibilities 
results in a coincidence rate given by 
##EQU1## 
where .omega..sub.0 is the frequency of the pump laser, .phi..sub.1 
=.phi..sub.1 +.phi..sub.2, and R.sub.c0 is the coincidence rate with the 
beam splitters removed. 
Although Equation (Eq.) (1) violates Bell's inequality, the time resolution 
of the coincidence circuits is often much worse than .DELTA.T, in which 
case there is also an incoherent contribution to the coincidence rate 
corresponding to situations in which one of the photons traveled along the 
longer path while the other photon traveled along the shorter path. The 
coincidence rate is then given by 
##EQU2## 
In an experiment, the resolution of the coincidence circuit used in the 
invention was 3 nanoseconds while .DELTA.T was 132 picoseconds, which 
corresponds to the conditions of Eq. (2) rather than Eq. (1). The 
visibility of the corresponding interference pattern was therefore 
insufficient to violate Bell's inequality but will be seen to violate the 
classical inequality discussed in J. D. Franson, Phys. Rev. Lett. 67, 
290(1991). 
The design of the light source 10 is illustrated in FIG. 2. The light from 
a helium-cadmium laser operating at a wavelength of 325 nanometers is 
incident upon a half-wave plate used to rotate the polarization of the 
light from the vertical plane to the horizontal plane. Filter F.sub.1 
passes the ultraviolet light from the laser but attenuates any visible 
light produced in the gas discharge tube. 
A nonlinear crystal of lithium iodate converts a small fraction of the pump 
photons into coincident pairs of photons .gamma..sub.1 and .gamma..sub.2 
via parametric down conversion. The pump power is 5 milliwatts and the 
crystal is 1 cm thick. The orientation of the crystal is automatically 
adjusted by a personal computer to satisfy the phase-matching conditions 
necessary to produce downconverted photons traveling in very nearly the 
same direction as the incident pump beam (degenerate type-I process). 
Filter F.sub.2 then absorbs the ultraviolet pump photons with very little 
fluorescence and passes the visible down-converted photons, both of which 
have a wavelength centered around 650 nanometers. The singles counting 
rate is equal to the detector dark count whenever the crystal is rotated 
away from the phasematching angle, which demonstrates that the ultraviolet 
pump photons are completely eliminated by filter F.sub.2 and that any 
fluorescence is negligible. 
Lens L.sub.1, a microscope objective, is mounted on a three-axis translator 
whose position can be controlled with a resolution of 0.1 .mu.m by the 
computer. Proper positioning of the lens allows both photons to be focused 
through pinhole P with a 25 .mu.m diameter. Achromatic lens L.sub.2 then 
produces a collimated (&lt;10.sup.-4 rad) beam with a diameter of 
approximately 2.5 cm. Filter F.sub.3 is an interference filter with a 
bandwidth of 10 nanometers centered on 650 nanometers (nm). Beam splitter 
BS.sub.2 separates the two photons onto different paths toward the 
detectors; those events in which both photons travel along the same path 
produces no coincidence counts and can be neglected. 
The light from a helium-neon laser is attenuated by neutral density filter 
F.sub.4 before being imaged onto pinhole P by means of mirror M.sub.1 and 
beam splitter BS.sub.1. This is used in the alignment of the apparatus and 
in stabilizing the interferometers against thermal drift, as will be 
described below. When not in use the HeNe beam can be blocked off by a 
shutter under the control of the computer. 
The large distances between the source and the interferometers require that 
both light beams be very well collimated. That, in turn, results in a 
relatively low coincidence rate typically on the order of one event every 
3 min. Because of the low counting rates, the interferometers have to be 
extremely stable over long periods of time. In addition, it is desirable 
that the interferometers be constructed in such a way that their alignment 
can not change when they are moved from one location to another. 
Each interferometer is therefore constructed from a solid plate of fused 
silica, as illustrated in FIG. 3. One of the collimated light beams from 
the source is incident upon the fused silica plate at very nearly a normal 
angle of incidence. Reflections from the front and back surfaces of the 
plate, both of which are coated with a dielectric giving 17% reflectance, 
produce a uniform interference pattern. The phase of the interference 
pattern can be controlled by the computer by rotating the plate through a 
small angle .theta.. Most of the incident light is transmitted and 
absorbed by a black surface behind the plate. Beam splitter BS.sub.3 is 
necessary to extract the reflected beam from the incident beam, although 
it does not form part of the interferometer itself. In an experiment, all 
the data was collected within one fringe of .theta.=0. 
The front and back surfaces of the fused silica plates are within 1/20 
wavelength, giving a total wave front distortion of less than 1/10 
wavelength upon reflection or transmission. The visibility of the usual 
(first-order) interference pattern observed using the HeNe laser and a 
single detector is typically 90% and is limited primarily by the 
reflective coatings. The fused silica plates used in the two 
interferometers have equal thicknesses (13.6 mm) to within one wavelength, 
having been cut from a single polished plate. 
The main advantage of this kind of interferometer is its long-term 
stability. The phase shifts of both interferometers are periodically 
monitored by the computer using the HeNe laser and adjusted as needed by 
changing the angle .theta.. The observed phase shifts at .theta.=0 varied 
by roughly one fringe throughout the course of an entire experiment as the 
result of temperature changes in the laboratory. The wavelength of the 
down-converted photons (650 nm) is sufficiently close to that of the HeNe 
laser (632.8 nm) that the corresponding phase shifts differed by an 
unknown constant that depends upon the exact thicknesses of the plates. 
Since the path-length difference never varies by more than one wavelength, 
the small difference in wavelengths is negligible over that range. Thus, 
the phase shifts measured with the HeNe laser provide a direct measurement 
of .phi..sub.1 and .phi..sub.2, aside from a fixed but unknown offset. 
It would have been desirable in the experiment, mentioned above, to have 
the two photons travel in opposite directions toward the two detectors. 
That was not possible due to the limited size of the laboratory and, 
instead, both beams traveled side down a long (-25 m) metal enclosure to 
two mirrors which reflected the beams back towards the source. The optical 
path length D was 51 m while the actual separation of the two beams was 20 
cm. 
The data was collected by setting the two interferometers to a series of 
phase shifts (as measured with the HeNe laser) ranging from 0.degree. to 
360.degree. in 45.degree. increments. These phase settings were changed 
roughly every 2 min. for one interferometer and every 3 min. for the 
other, so that there was no synchronization between the two settings. The 
number of coincidence counts obtained for each such pair of settings was 
recorded by the computer, along with the difference in detection times. 
Photon counts differing by more than 4.5 ns were rejected as accidental. 
The accidental rate was simultaneously determined from the events outside 
that range and subtracted off; the accidental rate was typically a factor 
of 10 less than the true coincidence rate. The coincident events were also 
put into bins based on the sum of the phases of the two interferometers. 
All data collection was performed automatically by the computer over a 
total time interval of 98 hours and no data editing or selection was 
performed. 
The coincidence data obtained at a large optical path length (D=51 m) are 
shown in FIG. 4. Here .phi..sub.1 is the sum of the phases of the 
interferometers as measured with the HeNe laser, which differs from the 
corresponding phase at the wavelength of the down-converted photons by an 
unknown constant, as discussed above. A peak in the coincidence rate can 
be seen at approximately -45.degree., indicating that the thicknesses of 
the two plates were such that the difference between the phases at the two 
wavelengths happened to have that value. Measurements made at a shorter 
distance (D=0.55 m) gave the same offset in phase. 
The single-photon counting rates in both detectors were recorded in a 
similar fashion and it should be emphasized that they showed no measurable 
modulation or interference, as expected. The only interference observed 
was in the coincidence rate and it depended only on the nonlocal sum of 
the two phases. 
The visibility of an interference pattern is defined by 
##EQU3## 
where R.sub.max and R.sub.min are the maximum and minimum counting rates. 
The data of FIG. 4 give a value of .mu.=0.31.+-.0.04. Although Eq. (2) 
predicts a visibility of 0.5, the expected visibility can be shown to be 
reduced to 0.336 by the finite coherence length (10 cm) of the pump 
laser, given a difference in optical path lengths of .DELTA.1=3.97 cm as 
determined from the thickness of the plates and the index of refraction. 
The expected visibility should be further reduced by roughly 10% due to 
the limited perfection of the optics, giving an overall expected 
visibility of 0.30. Thus, the observed visibility was in good agreement 
with the quantumthoery prediction. 
Data were also collected with the two interferometers located as close as 
possible to the light source, which corresponded to D=0.55 m. The results 
obtained were similar to that shown in FIG. 4 and gave a visibility of 
0.27.+-.0.04, which is consistent with the visbility measured at the 
larger distance to within the experimental uncertainty. Thus, there is no 
indication that the nonlocal correlations are reduced due to a collapse of 
the wave function over larger distances. 
The visibility of the interference pattern of FIG. 4 is not sufficiently 
high to violate Bell's inequality. Nevertheless, it was recently shown 
that the predictions of any classical or semiclassical field theory for 
two-photon interferometer experiments of this kind must satisfy the 
following inequality: 
##EQU4## 
Here R.sub.c0 (.DELTA.T) is the coincidence rate that would be obtained 
with the beam splitters removed and at a delay time of .DELTA.T using 
coincidence electronics with unlimited time resolution. When the 
interferometer measurements are performed using relatively large 
coincidence windows, as was the case in this experiment, Eq (4) can be 
generalized to 
##EQU5## 
It is well known that the pairs of photons emitted in parametric down 
conversion are highly coincident. Direct timing measurements have shown 
that the photons are coincident to within at least 91 picoseconds (one 
standard deviation), while indirect measurements suggest that they are 
coincident to within a few femtoseconds. More recent direct measurements 
have shown that the photon pairs from parametric down conversion are 
coincident to within 50 picoseconds. Only the results of the direct timing 
measurements will be used here, since it is conceivable that there may be 
a classical model that can account for the indirect measurements without 
giving coincident pulses. In that case, the inequality of Eq. (5) limits 
the visibility in any semiclassical theory to 0.139 for the conditions of 
the present experiment. The experimentally observed visibility violates 
this limit by 4 standard deviations. Thus, there is no semiclassical 
theory consistent with all the experimental observations and these effects 
must be viewed as quantum mechanical in nature. 
In summary, the coincidence rates in the two-photon interferometer 
experiment discussed above were measured with an optical path length of 
102 m between the two interferometers. The coincidence rate showed an 
interference pattern that depended on the sum of the phases of the two 
interferometers, with a visibility that was in good agreement with the 
predictions of the quantum theory. The singles counting rates showed no 
modulation at all. The visibility exceeded that achievable in any 
semiclassical field theory, which indicated that the effects observed were 
quantum-mechanical in nature. No significant difference was observed 
between the visibilities obtained at optical path separations of 102 and 
1.1 m, which provides some indication that the collapse of the wave 
function is not dependent upon the distance over which it occurs and that 
the quantum theory remains valid when extrapolated to distances many 
orders of magnitude larger than atomic dimensions. 
It has occasionally been suggested that the collapse of the wave function 
may be a dynamic process, presumably nonlinear in nature, in which case it 
would be confined to those regions of space where the wave function is 
nonvanishing. In that case, the collapse of the wave function would 
propagate only along the optical paths of the two photons and the fact 
that the spatial separation between them was relatively small in this 
experiment would be of no significance. The results of this experiment are 
relevant to theories of that kind but do not rule out the more general 
situation. 
In the above embodiment secure communications can be sent by establishing a 
code based on emitting a certain number of photons and measuring their 
coincidence rate. Once established, the code can then be used to transmit 
secure communications with the same inability to tap in and "listen" as 
described below. This is due to the fact that the two photons shown in 
FIG. 1 are totally correlated when .phi..sub.1 =.phi..sub.2 in the sense 
that one will be detected in detector D.sub.1 and the other in D.sub.2, or 
both will travel along the direction at the arrows to two other detectors 
D.sub.1 ' and D'.sub.2 (not shown). If detection in the unprimed detectors 
is taken to be a bit "o" while detection in the primed detectors is taken 
to be a bit "1", then these correlations establish a shared secret key or 
code. That can then be used to encode and decode messages sent over an 
open communication link. The experiment described above is the first 
demonstration of this ability over large separations. 
Another embodiment of the invention is illustrated in FIG. 5. Assume that 
an observer at location P.sub.B intendes to transmits a secure message to 
an observer at location P.sub.A, as shown by the direction of the message 
arrow. P.sub.A will be equipped with a laser of frequency .omega..sub.0 
whose output passes through a nonlinear crystal (such as KDP), thereby 
producing coincident pairs of photons .gamma..sub.1 and .gamma..sub.2 with 
frequencies .omega..sub.1 +.omega..sub.2 =.omega..sub.0 via parametric 
down-conversion. These photon pairs are emitted at random times, with 
.gamma..sub.1 and .gamma..sub.2 correlated in both time and energy. 
P.sub.A then transmits .gamma..sub.1 down an optical fiber toward P.sub.s, 
while retaining .gamma..sub.2 in an optical fiber delay line of the same 
length contained in his own secure area; .gamma..sub.2 will later serve as 
the decoding "key". P.sub.B then transmits a bit of information by either 
reflecting .gamma..sub.1 back to P.sub.A down a second optical fiber with 
no additional delay (bit "1"), or delaying .gamma..sub.1 by a few 
nanoseconds before sending it back (bit "0"). P.sub.A can then "decode" 
the message by comparing .gamma..sub.1 and .gamma..sub.2 ; if the photons 
are coincident, the message bit is "1", whereas the message bit is "0" if 
they are not coincident. Note that the stream of .gamma..sub.1 photons 
returning to P.sub.A would appear to be a random sequence (white noise) 
with or without a delay added. 
Now consider the information that can be obtained by an unauthorized set of 
taps as shown in FIG. 5. If classical pulses of light were being 
transmitted down the optical fibers instead of single photons, then one 
tap could be used to monitor the pulses going into P.sub.B while the other 
tap could be used to monitor the pulses leaving P.sub.B, thus determining 
the time delay introduced by P.sub.B and reading the bit transmitted. But 
no such possibility exists for single photons, which can be detected only 
once; any photons observed by the first of the taps will be eliminated and 
therefore not modulated by P.sub.B. Thus does the quantum-mechanical 
difference between single photons and classical pulses of light allow for 
secure communications. 
The quantum-mechanical correlations used by the invention would be 
destroyed if an unauthorized person made any kind of measurements or 
observations on .gamma..sub.1 via a tap. This allows P.sub.A to 
continuously check for any unauthorized attempts at tapping the line. This 
capability is a result of the general property of quantum mechanics that 
any measurement on a particle or system unavoidably destroys the initial 
state of that system. The two-photon interference effects allow a 
practical means of making these checks for unauthorized taps. 
Another embodiment of the invention consists of using photon polarizations. 
This approach uses a pulsed laser which produces a single photon by 
attenuating a high-intensity pulse. Using pockels cells, one observer 
rotates the polarization of the photon through an angle .theta.. A second 
observer measures the polarization and will obtain total correlation (same 
polarization). Any interception of the photon will destroy the 
correlation. As shown in FIG. 6, the design incorporates four pockels 
cells, polarization preserving optical fiber, and feed-back loop for phase 
drift compensation. The advantages of this embodiment include high data 
rates compact, inexpensive laser; and good coupling to optical fibers.