US 10,641,655 B2

13

as approximately 11000 photons per second (or -117 dBm
at 895 nm). We attenuated the optical signal from the tunable
laser under test to -105 dBm, -110 dBm and -115 dBm, and
their spectrum measurement results are shown in FIG. 10.
The results show the spectrum peak of the -115 dBm optical
signal is clearly above the noise deviation level. It is worth
pointing out that current superconducting single photon
detectors, such as supercomputing nanowire single-photon
detectors or superconducting transition-edge sensor detec-
tors, can achieve close to 100% detection efficiency and a
lower dark count rate compared to the Si-APD. If such type
of single photon detector replaced the Si-APD in this set-up,
the spectrometer sensitivity may be further enhanced by
about a factor of three.

Wavelength accuracy of the spectrometer is determined
by the wavelength accuracy of the coupling beam. The
coupling beam is from a diode laser and shifted by an AOM.
The frequency shift of the AOM is controlled by a function
generator, which has Hz level accuracy. Therefore, the
accuracy is primarily determined by the wavelength accu-
racy of the coupling beam laser. The laser wavelength is
locked to the peak of Doppler free hyperfine line of 6°S,,5
F=46°P,,. F'=4 transition based on Cs atom absorption
saturation spectroscopy. The bandwidth of the natural line
width of Cs D1 line (Doppler free hyperfine line) is approxi-
mately 5 MHz. With suitable parameters in Pound-Drever-
Hall and proportional-integral-differential control settings,
the wavelength can be accurately locked to within 1% of the
bandwidth of the natural linewidth. Therefore, the wave-

length reading of the spectrometer can be calibrated by the 3

Doppler free hyperfine line of Cs atoms with an accuracy of
better than 50 kHz. To demonstrate wavelength accuracy of
our measurement, two optical signals with 1.4 MHz fre-
quency difference separated by an AOM are generated and

then measured by the spectrometer. FIG. 11 shows the 3

measured frequency separation for the two signals is very
close to 1.4 MHz.

Direct measurement range of the EIT spectrometer
involves the absorption area around the atomic transitions,
where EIT can be observed. The Doppler broadened absorp-
tion range is temperature dependent, and is a few hundred
MHz for Cs atom. The spectrum of single-photon sources in
this range can be characterized directly using this approach.
Single photon sources whose wavelength is not close to a
suitable atomic transition line can be frequency-converted to
this wavelength range and then measured using this
approach. Since single photon frequency conversion was
first proposed in 1990, single photon frequency conversion
has involved nonlinear optical processes, such as sum-
frequency generation in bulk and waveguided periodically-
poled lithium niobate and four-wave mixing in fiber. Single
photon conversion can occur in telecom wavelength (around
1550 nm or 1310 nm) to atomic transition wavelengths. It
has been demonstrated that the quantum states of these
photons are preserved during the conversion process. The
wavelength of the converted photons is dependent on the
wavelengths of both the original photon and the pump light.
Therefore, using a narrow-linewidth and wavelength-stable
pump laser, the spectral information of the original photon
source can be obtained by measuring the converted photon.
Such a frequency conversion based spectrometer has been
demonstrated to measure the spectrum in the infrared range.
The same technology can also extend the measurement
range of this EIT-based spectrometer to other wavelengths,
such as the telecom region.

Moreover, correlated biphoton spectroscopy is another
method for extending the measurement wavelength range.

S

2

40

45

6:

14

Correlated biphoton spectroscopy is a technique used to
measure the spectral characteristics of an object, such as an
optical filter or an absorber, by monitoring the coincidence
counts from correlated signal and idler photon pairs gener-
ated by SPDC. Like frequency-conversion technology, if the
pump wavelength in correlated biphoton spectroscopy is
stable and has a narrow linewidth, an accurate spectral
measurement of single photons in one beam can be obtained
by measuring its correlated twin photons in another beam.
We have generated signal photons at a telecom (near 1310
nm) wavelength and idler photons at an atomic (cesium near
895 nm) transition wavelength. By applying the EIT spec-
trometer for the spectral measurement of 895 nm photons,
the 1310 nm telecom spectrum can be obtained by coinci-
dence counting. With regard to direct measurement range of
the EIT spectrometer, single photon frequency conversion
and correlated biphoton spectroscopy technologies extend
the technology to any other wavelength ranges while main-
taining the performance, including high spectral resolution,
high wavelength accuracy, and high detection sensitivity.

For a single photon source to efficiently interact with
quantum memories, the spectrometer characterizes the
single photon source’s spectral properties including wave-
length and linewidth. Based on the principle of electromag-
netically-induced transparency (EIT), the spectrometer pre-
cisely characterizes the spectral properties of narrow-
linewidth single-photon sources using an atomic vapor cell.
The method can be used for spectrum measurement of
narrow linewidth weak optical signals, such as for the
characterization of the narrow linewidth single photon
sources for quantum memories. By using a Cs atomic vapor
cell and Si-APD, we experimentally demonstrated this
method with a spectral resolution of better than 150 kHz, a
wavelength reading that is accurate to within 50 kHz and a
sensitivity suitable for optical signals as weak as -117 dBm.
By using superconducting single photon detectors, the sen-
sitivity can be increased further. This method is demon-
strated with Cs atoms, but it can also be applied to other
atomic systems commonly used as storage media for quan-
tum memories, such as Rubidium. In addition, by integrating
single-photon frequency conversion or correlated biphoton
spectroscopy, this technology can also be used for the
characterization of single-photon sources at wavelengths
that are not close to resonant transitions in atoms.

While one or more embodiments have been shown and
described, modifications and substitutions may be made
thereto without departing from the spirit and scope of the
invention. Accordingly, it is to be understood that the present
invention has been described by way of illustrations and not
limitation. Embodiments herein can be used independently
or can be combined.

Reference throughout this specification to “one embodi-
ment,” “particular embodiment,” “certain embodiment,” “an
embodiment,” or the like means that a particular feature,
structure, or characteristic described in connection with the
embodiment is included in at least one embodiment. Thus,
appearances of these phrases (e.g., “in one embodiment” or
“in an embodiment”) throughout this specification are not
necessarily all referring to the same embodiment, but may.
Furthermore, particular features, structures, or characteris-
tics may be combined in any suitable manner, as would be
apparent to one of ordinary skill in the art from this
disclosure, in one or more embodiments.

All ranges disclosed herein are inclusive of the endpoints,
and the endpoints are independently combinable with each
other. The ranges are continuous and thus contain every
value and subset thereof in the range. Unless otherwise


US 10,641,655 B2

1
DIRECT ABSOLUTE SPECTROMETER FOR
DIRECT ABSOLUTE SPECTROMETRY

CROSS REFERENCE TO RELATED
APPLICATIONS

The application claims priority to U.S. Provisional Patent
Application Ser. No. 62/683,291 filed Jun. 11, 2018, the
disclosure of which is incorporated herein by reference in its
entirety.

STATEMENT REGARDING FEDERALLY
SPONSORED RESEARCH

This invention was made with United States Government
support from the National Institute of Standards and Tech-
nology (NIST), an agency of the United States Department
of Commerce. The Government has certain rights in the
invention. Licensing inquiries may be directed to the Tech-
nology Partnerships Office, NIST, Gaithersburg, Md.,
20899; voice (301) 975-2573; email tpo@nist.gov; refer-
ence NIST Docket Number 18-018US1.

BRIEF DESCRIPTION

Disclosed is a direct absolute spectrometer to perform
direct absolute spectrometry, the direct absolute spectrom-
eter comprising: a first light source that provides single
photon light; a second light source that provides pump light;
an optical combiner in optical communication with the first
light source and in optical communication with the second
light source and that: receives the single photon light from
the first light source: receives the pump light from the
second light source; combines the single photon light and

the pump light; and produces dual light that comprises the 3

single photon light and the pump light; an optical cell that
receives the dual light from the optical combiner; an elec-
tromagnetically induced transparent (EIT) medium disposed
in the optical cell, the EIT medium being: optically trans-
parent to the single photon light in a presence of the pump
light, such that the EIT medium produces output light
comprising: the pump light; and an EIT light; and optically
‘opaque to the single photon light in an absence of the pump
light, such that the EIT medium does not produce the output
light; and a filter in optical communication with the EIT
medium and that: receives the output light from the optical
cell when output light is produced by EIT medium; and
filters the pump light from the output light; and provides EIT
light free from pump light, wherein EIT light is a direct and
absolute metric of a linewidth and a wavelength of the single
photon light such that direct absolute spectrometer provides
direct and absolute determination of the linewidth of the
single photon light and direct and absolute determination of
wavelength at a maximum of intensity of the single photon
light.

‘Also disclosed is a process for performing direct absolute
spectrometry, the process comprising: combining a single
photon light with a pump light; producing a dual light in
response to combining the single photon light and the pump.
light; subjecting an EIT medium to the dual light; producing,
in the EIT medium, interacting light as the dual light
propagates through the EIT medium; producing, from the
EIT medium, output light from the interacting light when the
EIT medium is optically transparent to the single photon
light in a presence of the pump light, the output light
comprising: the pump light; and an EIT light; and absorbing
the single photon light when the single photon light propa-

30

40

0

3!

0

2

gates in the EIT medium in an absence of the pump light,
such that the EIT medium does not produce the output light;
filtering the output light to remove the pump light; and
producing, from filtering the output light, EIT light in an
absence of the pump light to perform direct absolute spec-
trometry.

BRIEF DESCRIPTION OF THE DRAWINGS.

The following description should not be considered lim-
iting in any way. With reference to the accompanying
drawings, like elements are numbered alike

FIG. 1 shows a direct absolute spectrometer;

FIG. 2 shows a second light source;

FIG. 3 shows a graph of intensity versus wavelength;

FIG. 4 shows a graph of intensity versus wavelength in
panel A and a graph of intensity versus wavelength in panel
B;

FIG. 5 shows a lambda electromagnetically induced trans-
parency (EIT) energy level configuration;

FIG. 6 shows a graph of an imaginary part of suscepti-
bility ? that represents absorption of a medium versus
frequency detuning in an EIT medium in panel A, and panel
B shows a graph of a real part of x“ that represents

refractive index of the medium versus frequency detuning in

an EIT medium;

FIG. 7 shows a direct absolute spectrometer for (box a) a
set-up. with (box b) calibration probe light generation, and
(box c) photon source under test for (box d) an energy
structure of Cs atom, wherein acronyms and abbreviations
are as follows EOM: electro-optic modulator; AOM:
acousto-optic modulator, PBS: polarizing beam splitter;
FM: flip mirror; VOA: variable optical attenuator; Pol.:
Glan-Thompson polarizer; Det.: single photon detector;

FIG. 8 shows a graph of bandwidth versus light power for
a bandwidth of EIT transparent window with calibartion
probe light;

FIG. 9 shows, in panel A, a graph of counts versus
detuning for a measured result and measured background
noise, and panel B shows a graph of counts versus detuning
for a measured result after subtracting background noise;

FIG. 10 shows a graph of counts versus detuning for
optical signals at -105 dBm, -110 dBm, and -115 dBm after
background noise subtraction; and

FIG. 11 shows a graph of counts verus detuning for two
optical signals at different wavelengths after background
noise subtraction.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is
presented herein by way of exemplification and not limita-
tion.

It has been discovered that a direct absolute spectrometer
has ultra-high resolution, is accurate and sensitive and
provides a determination of a narrow-linewidth single pho-
ton source. Electromagnetically-induced transparency (EIT)
is an optical phenomenon around atomic transition absorp-
tion that provides a very narrow transparent window. The
direct absolute spectrometer includes an EIT medium. Ben-
eficially and unexpectedly, the direct absolute spectrometer
provides a spectral resolution of better than 150 kHz, a
wavelength determination that is accurate to within 50 kHz,
and is sensitive to an optical signal that is as weak as -117

5 dBm. Advantageously, the direct absolute spectrometer

overcomes problems with convention technologies that are
based on dispersive optical elements, which limits their


US 10,641,655 B2

Sheet 3 of 11

May 5, 2020

U.S. Patent

<< Apsusquy

Lmax L2

Li

Wavelength ———»

FIG. 3


US 10,641,655 B2

15

stated or contextually inapplicable, all percentages, when
expressing a quantity, are weight percentages. The suflix
“(s)” as used herein is intended to include both the singular
and the plural of the term that it modifies, thereby including
at least one of that term (e.g., the colorant(s) inchides at least
‘one colorants). “Optional” or “optionally” means that the
subsequently described event or circumstance can or cannot
occur, and that the description includes instances where the
event occurs and instances where it does not. As used herein,
“combination” is inclusive of blends, mixtures, alloys, reac-
tion products, and the like.

As used herein, “a combination thereof” refers to a
combination comprising at least one of the named constitu-
ents, components, compounds, or elements, optionally
together with one or more of the same class of constituents,
components, compounds, or elements.

All references are incorporated herein by reference.

The use of the terms “a” and “an” and “the” and similar
referents in the context of describing the invention (espe-
cially in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by con-
text. “Or” means “and/or.” Further, the conjunction “or” is
used to link objects of a list or alternatives and is not
disjunctive; rather the elements can be used separately or
can be combined together under appropriate circumstances.
It should further be noted that the terms “first,” “second,”
“primary,” “secondary,” and the like herein do not denote
any order, quantity, or importance, but rather are used to

distinguish one element from another. The modifier “about” 3

used in connection with a quantity is inclusive of the stated
value and has the meaning dictated by the context (e.g., it
includes the degree of error associated with measurement of
the particular quantity).

What is claimed is:

10

0

1. A direct absolute spectrometer to perform direct abso-

lute spectrometry, the direct absolute spectrometer compris-
ing:
a first light source that provides single photon light:
a second light source that provides pump light;
an optical combiner in optical communication with the
first light source and in optical communication with the
second light source and that:
receives the single photon light from the first light
source;
receives the pump light from the second light source:
combines the single photon light and the pump light;
and
produces dual light that comprises the single photon
light and the pump light;
an optical cell that receives the dual light from the optical
combiner;
an electromagnetically induced transparent (EIT) medium
disposed in the optical cell, the EIT medium being:
optically transparent to the single photon light in a
presence of the pump light, such that the EIT
medium produces output light comprising:
the pump light; and
an EIT light; and
optically opaque to the single photon light in an
absence of the pump light,
such that the EIT medium does not produce the output
light; and
a filter in optical communication with the EIT medium
and that:
receives the output light from the optical cell when
output light is produced by the EIT medium;

40

45

50

0

16

filters the pump light from the output light; and
provides the EIT light free from pump light,

wherein the EIT light is a direct and absolute metric of a

linewidth and a wavelength of the single photon light
such that the direct absolute spectrometer provides
direct and absolute determination of the linewidth of
the single photon light and direct and absolute deter-
mination of wavelength at a maximum of intensity of
the single photon light.

2. The direct absolute spectrometer of claim 1, further
comprising a magnetic shield disposed on the optical cell to
shield the EIT medium from a magnetic field external to the
optical cell,

3. The direct absolute spectrometer of claim 1, further
comprising a detector in optical communication with the
filter and that:

receives the EIT light from the filter; and

produces EIT signal, based on EIT light.

4. The direct absolute spectrometer of claim 1, wherein
the second light source comprises:

a laser to produce laser light;

a wavelength tuner in optical communication and that

receives laser light from laser and produces pump light.

5. The direct absolute spectrometer of claim 4, wherein
the second light source further comprises:

a wavelength locker in optical communication with the

laser and that:

receives the laser light from laser; and

produces laser lock that is communicated to laser to
lock a wavelength of laser light.

6. The direct absolute spectrometer of claim 5, wherein
the wavelength locker comprises:

a reference cell in optical communication with the laser

and that:
receives the laser light from laser; and
produces reference signal, based on laser light; and
a feedback unit in communication with reference cell and
that:
receives the reference signal from the reference cell;
and
produces the laser lock, based on the reference signal.

7. The direct absolute spectrometer of claim 4, wherein
the second light source further comprises:

a tuner controller in electrical communication with the

wavelength tuner and that:

produces a control signal that is communicated to
wavelength tuner to control production of pump light
by wavelength tuner.

8. The direct absolute spectrometer of claim 1, wherein
the filter comprises an etalon, a polarizer, an atomic filter, or
a combination comprising at least one of the foregoing
optical elements.

9. The direct absolute spectrometer of claim 1, wherein a
wavelength of single photon light is less than a wavelength
of pump light.

10. The direct absolute spectrometer of claim 1, wherein
a wavelength of EIT light is less than a wavelength of pump
light.

11. The direct absolute spectrometer of claim 1, wherein
the EIT medium comprises an atomic vapor.

12. A process for performing direct absolute spectrometry,
the process comprising:

combining a single photon light with a pump light;

producing a dual light in response to combining the single

photon light and the pump light;

subjecting an electromagnetically induced transparent

(EIT) medium to the dual light;


US 10,641,655 B2

9

based on naturally broad (THz) spontaneous processes, such
as spontaneous parametric down-conversion (SPDC) or
spontaneous four wave mixing (SFWM), suitable linewidths
for quantum memory integration have been implemented
using extremely narrow filters or cavity enhancement.

To characterize a ~MHz bandwidth source, the spectral
resolution of the measuring instrument should have at least
one order of magnitude better, ie. ~100 kHz. This may
beyond the ability of many gratings and etalons except for

some type of very high-Q (greater than 10°) cavity. It is still

technically challenging to implement a high spectral stabil-
ity cavity with such a high Q value. Therefore, the spectral
measurement of single photons with linewidths of the order
of MHz is difficult for instruments based on traditional
optical dispersive elements. Without a suitable spectrometer,
conventional single photon linewidths can be estimated
indirectly from cavity lifetime measurements. The precise
wavelength is uncertain and further tuning is involved for
integration with a quantum memory system. In addition, the
spectral characterization of such sources requires single-
photon level sensitivity.

This Example shows measurement of spectra of narrow-
linewidth single photons. The method herein is based on EIT
in an atomic vapor cell. It provides high spectral resolution,
high wavelength accuracy, and high detection sensitivity for
photons at a wavelength close to an atomic transition line.
The spectral measurement range can be extended beyond
immediate atomic transition lines by including single-pho-

ton frequency conversion and correlated biphoton spectros- °

copy. Cs atoms were studied and their performance in terms
of resolution, accuracy, and sensitivity have been deter-
mined.

EIT is a destructive interference of two light fields in a

three-level atomic energy structure. The optical response of °

an atomic medium is modified when laser beams lead to
quantum interference between the excitation pathways. This
quantum interference can eliminate absorption and refrac-
tion (linear susceptibility) at the resonant frequency of the
atomic transition.

In the lambda EIT configuration show in FIG. 5, the
transition between two lower energy states, |1) and |2), is
dipole-forbidden and atoms can remain in either of the two
states for a long time. Both energy states have allowed
transitions to an excited state, |3) . The optical fields that are
resonant with the two allowed transitions of the atoms are
absorbed by the atoms if they enter the atomic medium
alone. However, when the two fields are applied to the
atomic medium simultaneously, they interfere destructively,
and therefore no atoms are excited to the state 13) . Conse-
quently, the field will not be absorbed.

A strong field between 11) and 13) is called the coupling
field and a weak field between |1) and |2) is called the
probe field, When the control beam is strong and its intensity
is constant in time, the response of the atomic ensemble can
be described in terms of the linear susceptibility spectrum
¥(e), per Eq. (1):

yg tio ay

Ow) = ex N > —___.,
OS ENG tials + ia) + 1OP

where y,, and y,, are the decoherence rates of 11) 12) and
11) 13) transitions respectively, @ is the Rabi frequency
for the coupling field, N is the total number of atoms in the
interaction area, g is the atom-field coupling constant, and

&

10
is the frequency detuning of the resonance of the two optical
fields (the coupling and probe fields). When «=0, the two
optical fields are resonant.

In an ideal EIT medium, due to the dipole-forbidden
transition between |1) and 12), the coherence relaxation
rate is very small, i.e. y,,>0. Based on Eq. (1), the linear
susceptibility spectrum of the EIT medium for a signal beam
is shown in FIG. 6. The linear response of an atom to the
light can be described by the linear susceptibility y. The
imaginary part of x“ determines the dissipation of the field
by the atomic gas (i.e. absorption), while the real part of x
determines the refractive index. From panel A of FIG. 6,
when =0, the imaginary part of x“? goes to zero, which
indicates no absorption at the atomic resonance. In other
words, the medium is transparent. The transparent frequency
is determined by the frequency detuning of two optical fields
and the transparency window is usually very narrow. There-
fore, the spectrum of the probe light can be obtained by
scanning the frequency of the coupling light.

To accurately measure the spectrum and linewidth of
single photons, we developed an experimental spectrometer
based on EIT ina warm Cs atomic cell. The configuration of
the spectrometer is shown in FIG. 7 (Box a). A tunable CW
diode laser around 895 nm is locked to the Cs D1 transition
6S, F=3-+6°P,, F'=4. The beam was modulated by an
acousto-optic modulator (AOM). The frequency of the first
order diffraction output of the AOM is determined by Eq.
(2):

Vou Vintec 2)

where the v,,, and v,,,, are the frequency of input and output
optical field of an AOM and v,.. is the AOM driving
frequency from an RF function generator that can be
scanned and controlled by a computer.

The coupling beam is combined with the probe beam (the
light under test) using a polarizing beam splitter (PBS) and
is then sent into the Cs cell. The Cs cell is shielded in a
3-layer 1-metal chamber to block external stray magnetic
fields. The vapor cell is heated by a temperature controller.

40 After the beam passes through the cell, the coupling light is

removed by a Glan-Thompson polarizer and a Fabry Perot
(EP) Etalon. The probe light is transmitted and detected by
a single-photon detector. The output photon-count signal
from the Si-APD is sent to the same computer. While the
computer scans the RF signal (i.e., AOM driving frequency),
it simultaneously collects the counts from the single photon
detector. A spectrum based on the scanning frequency and
corresponding photon counts is recorded and analyzed by a
specifically built computer program.

To calibrate the spectrometer, we generated a probe field
from the coupling light directly (Box b in FIG. 7). An
electro-optic phase modulator (EOM) is used to generate
sidebands at about a 9.2 GHz separation corresponding to
the hyperfine splitting of the two ground states of Cs. An
etalon selects the red-shifted sideband light, which corre-
sponds to the transition 6°S,,, F=46?P,,. F'=4. Another
AOM is used to provide a similar detuning from the atomic
transition and a variable optical attenuator is used to attenu-
ate the light to single-photon level for system calibration. To
experimentally study the performance of the spectrometer,
we use another tunable laser as an independent photon
source (Box c in FIG. 7). Box d in FIG. 7 shows the energy
structure for the experiment.

High spectral resolution is provided by this spectrometer.
The spectral resolution is determined by the linewidth of the
coupling field and the bandwidth of EIT transparent window.
The linewidth of coupling field is determined by the laser


U.S. Patent

800

May 5, 2020 Sheet 8 of 11

US 10,641,655 B2

450 |

400}

measured data (50 ©) |
measured data (60 C)_

350

300

250 +

200 -

EIT spectrum bandwidth (FWHM kHz)

480) «

400

: L : L
45 2 25 3 3.8
Coupling light power (mW)

FIG. 8


U.S. Patent May 5, 2020 Sheet 5 of 11 US 10,641,655 B2

Coupling

@

1A

A: frequency detuning from atomic transition
w: frequency detuning of the resonance of two optical fields

FIG. 5


US 10,641,655 B2

Sheet 4 of 11

May 5, 2020

U.S. Patent

204

210

(A)

Imax +—------------------3

<—

Aqisuequy

cooneoon=- Led

210

conoo---% Liong
Lblue «----------

“1
Hi
t
'
t
'
t
'
t
'
r

Lshort <-----

T T
LPL | LP2
LP

T
Li

L3
Wavelength ———>

L4

<—— Apsuoequy

(B)

FIG. 4


US 10,641,655 B2

11

linewidth, which is about 100 kHz in our experiment. The
bandwidth of EIT transparent window for a Cs gas cell is
influenced by the magnetic shield, cell temperature, cou-
pling light power and angle between probe and coupling
light. To ensure a narrow bandwidth, the cell is magnetically
shielded by a 3-layer j1-metal chamber and the probe and
coupling light are aligned in the same direction. The rela-
tionship between the bandwidth of EIT transparent window
and coupling beam power and the cell temperature has been
extensively studied. The full width half maximum (FWHM)
of EIT bandwidth can be estimated by Eq. (3):

lop 1 3)

ys VoL"

ElTewny = 2

where EIT yz, is the FWHM of the EIT transparent
window bandwidth and aL is the optical depth. The |QI? is
linearly proportional to the intensity of the coupling field, so
the bandwidth is linearly proportional to the intensity of the
coupling beam. The optical depth (aL) increases with the
cell temperature, so the higher the temperature, the narrower
the bandwidth.

Although a narrower bandwidth can be obtained with
lower intensity of coupling beam or higher cell temperature,
there is a tradeoff between bandwidth and transmittance of
EIT transparent window. The transmittance directly influ-
ences the spectrometer’s sensitivity. The output probe field
intensity can be estimated by Eq. (4):

ys7i
210F

4

[Blow = VElexe| el

where IEI,,,2 and IEI,,;
probe beam. When y,> is not zero, the probe field will be
attenuated through the EIT transparent window, and the
attenuation is proportional to optical depth (aL) and
inversely proportional to IQI?. Therefore, a narrower band-
width of the EIT transparent window will increase the loss
for the probe field and reduce the spectrometer sensitivity.

‘A special calibration probe beam is generated (box b in
FIG. 7) and then used to calibrate the EIT spectrometer. The
calibration probe beam is a small portion split from the
coupling beam and then shifted 9.2 GHz (the hyperfine
splitting in the ground level), so any wavelength instability
will not influence the measurement in the calibration pro-
cedure. FIG. 8 shows the measured bandwidth of the EIT
signal of the calibration probe beam as a function of cou-
pling beam power and cell temperature. The bandwidth of
the transparent window decreases with lower coupling beam
intensity and higher temperature. When the coupling beam
power is reduced to 0.7 mW, the bandwidth reaches approxi-
mately 120 kHz, which is already close to the linewidth of
the laser beam itself. However, the transmittance is already
low at such a power level. As a compromise between
bandwidth and transmittance, we set the cell temperate to be
60 C and coupling beam power to be 1.3 mW. In this setting,
we can get a resolution of approximately 150 kHz with a
40% transmittance at the center of the EIT window. The
resolution and the transmittance can be optimized by vary-
ing the cell temperature or coupling beam power to accom-
modate a variety of measurements.

The spectrometer measures signals at a single-photon
power level, and its sensitivity is a performance parameter.
The sensitivity is determined in part by transmittance, detec-
tion efficiency, and noise.

re the output and input intensity of 3

8

40

&

12

Transmittance is determined by the transmittance of both
the EIT transparent window and the filtering system. The
transmittance of the EIT transparent window is determined
by the coupling beam power and cell temperature. However,
the higher the transmittance. the lower the spectral resolu-
tion. In our experiment, we balance the two parameters with
a 150 kHz resolution and a 40% transmittance. The filtering
system is designed to significantly suppress the coupling
beam, but it also causes losses in the probe beam. The
measured transmittance of the filtering system for the probe
beam is approximately 80%. The detection efficiency of the
single photon detector is approximately 38% near 895 nm.
Therefore, the overall efficiency is approximately 12%

The noise of the spectrometer is due to intrinsic dark
counts of the single photon detector, fluorescence photons
generated in the cell, and residual photons from coupling
beam. The intrinsic dark count rate is a constant, approxi-
mately 100 counts per second in our case. The fluorescence
in the cell is mainly caused by collisions between atoms and
collisions between the atoms and the cell wall. A buffer-
gas-free and paraffin-coated cell can reduce the collision-
induced fluorescence, and this kind of cell is used in this
experiment.

In the spectrometer, a strong coupling beam comes out of

5 the cell together with the weak probe beam at a single-

photon power level. The residual coupling beam becomes a
significant source of noise in the system. Since the two
beams should propagate collinearly through the cell to
achieve an optimal resolution, a spatial filter is not suitable.
Therefore, only polarization and spectral filters can be used
to suppress the coupling beam. A Glan-Thompson polarizer
is used as the polarization filter providing more than a 60-dB
extinction in our experiment. For spectral filtering, since the
frequency separation between the probe and the coupling
beams is small (only 9.2 GHz in our experiment), it is not
feasible to use dispersion-based spectral filters. Custom
designed FP etalons are used as spectral filters in this
experiment. The FP etalon is designed with a free spectral
range of 18.4 GHz (9.2 GHzx2) and a finesse of 17, which
optimizes the extinction ratio of the transmittance for the
probe to the coupling beams. Two FP etalons in series can
provide more than 40 dB suppression to the coupling beam
and approximately 80% transmittance for the probe beam.
Combining the polarization filter and spectral filters, the
coupling beams is suppressed by over 100 dB.

These noise sources are spectrally independent and nearly
constant within the measurement range. Therefore, noise can
be measured separately, and easily subtracted from the
measurement results. Panel A of FIG. 9 shows a measured
result and a corresponding measured background noise
under the same conditions. Panel B of FIG. 9 shows a clean
result after the background noise is subtracted. In this case,
the measurement sensitivity is limited only by the noise
deviation.

The spectrometer sensitivity is determined by detection
efficiency and deviation of noise. The noise counts have a
shot noise behavior whose deviation is equal to the square
root of the average number of counts. The noise count rate
in the measurement range is approximately 20000 counts per
second, so the noise count deviation is approximately 140
counts per second. Because we need to perform measure-
ments two times (one for signal and another for back-
ground), the total noise deviation after subtracting the back-
ground noise is approximately 280 counts per second. To get
a clear spectrum, the signal counts should be 5 times greater
than the deviation. When we take the total efficiency of 12%
into account, the spectrometer can measure signals as week


US 10,641,655 B2

Sheet 7 of 11

May 5, 2020

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a2 United States Patent

US010641655B2

(10) Patent No.: US 10,641,655 B2

Ma et al. (45) Date of Patent: May 5, 2020
(54) DIRECT ABSOLUTE SPECTROMETER FOR —_(58)_ Field of Classification Search
DIRECT ABSOLUTE SPECTROMETRY CPC .... GOLJ 3/0213; GOLJ 3/10; GOLJ 3/42; GOL
3/0237
(71) Applicant: Government of the United States of See application file for complete search history.
America, as Represented by the
Secretary of Commerce, Gaithersburg, (56) References Cited
MD (US)
U.S. PATENT DOCUMENTS
(72) Inventors: Lijun Ma, Rockville, MD (US); Xiao .
Tang, Gaithersburg, MD (US); Oliver 6,028,873 A 2/2000 Yamamoto G02F ee
Slattery, Gaithersburg, MD (US) 2005/0175358 Al* 8/2005. TIchenko G02F WoL
(73) Assignee: GOVERNMENT OF THE UNITED Continued 398/198
STATES OF AMERICA, AS (Continued)
REPRESENTED BY THE
SECRETARY OF COMMERCE, OTHER PUBLICATIONS
Gaithersburg, MD (US) Whiteneck, B., et al., “Electromagnetically Induced Transparency
, : _
(#) Notice: Subject to any disclaimer, the term of this (11) Amplitude Nolss Spsetroscony”, 2018
patent is extended or adjusted under 35 (Continued)
USC. 154(b) by 0 days Primary Examiner — Hina F Ayub
(74) Aitorney, Agent, or Firm — Office of Chief Counsel
(21) Appl. No.: 16/039,391 for National Institute of Standards and Technology
(22) Filed: Jul. 19, 2018 on ABSTRACT
(65) Prior Publication Data A direct absolute spectrometer includes: a first light source:
a second light source; an optical combiner that produces
US 2019/0376846 Al Dec. 12, 2019 dual light; an optical cell that receives the dual light; an
Related U.S. Application Data electromagnetically induced transparent medium that is
; —_ optically transparent to single photon light in a presence of
(60) Provisional application No. 62/683,291, filed on Jun. pump light, such that output light is produced; and a filter
11, 2018. that filters output light and provides EIT light free from,
wherein electromagnetically induced transparent EIT light is
(51) Int. Cl. a direct and absolute metric of a linewidth and a wavelength
GO1J 3/02 (2006.01) of single photon light such that the direct absolute spec-
GOLS 3/42 (2006.01) trometer provides direct and absolute determination of the
GOLT 3/10 (2006.01) linewidth of the single photon light and direct and absolute
(52) determination of wavelength at a maximum of intensity of

eevee GOLF 3/0213 (2013.01); GOLF 3/0237
(2013.01); GOLF 3/10 (2013.01); GOLT 3/42
(2013.01)

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the single photon light.

18 Claims, 11 Drawing Sheets

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U.S. Patent May 5, 2020 Sheet 10 of 11 US 10,641,655 B2

-105 dBm
-110 dBm

Counts (1/s)

bs Ra A od’ Tu, Ags &,
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99 992 994 996 998 100 100.2 1004 1006 1008 101
Detuning from the Cs transition peak (MHz)

FIG. 10


U.S. Patent May 5, 2020 Sheet 9 of 11 US 10,641,655 B2

(A)

(B)

Counts (is)

Counts (4/8)

3
Pad

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Measured background noise

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Detuning from the Cs transition peak (MHz}

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Detuning from the Cs transition peak (MHz)

FIG. 9


U.S. Patent May 5, 2020 Sheet 6 of 11 US 10,641,655 B2

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FIG. 6


US 10,641,655 B2

17

producing, in the EIT medium, interacting light as the

dual light propagates through the EIT medium;

producing, from the EIT medium, output light from the
interacting light when the EIT medium is optically
transparent to the single photon light in a presence of 5
the pump light, the output light comprising:

the pump light; and

an EIT light; and

absorbing the single photon light when the single photon
light propagates in the EIT medium in an absence of the
pump light, such that the EIT medium does not produce
the output light;

filtering the output light to remove the pump light; and

producing, from filtering the output light, EIT light in an
absence of the pump light to perform direct absolute
spectrometry.

13. The process for performing direct absolute spectrom-

etry of claim 12, further comprising:

producing an EIT signal from the EIT light after filtering

the output light to remove the pump light.

18

14. The process for performing direct absolute spectrom-
etry of claim 12, further comprising:

adjusting a wavelength of the pump light; and

changing a wavelength of the EIT light in response to

adjusting the wavelength of the pump light.

15. The process for performing direct absolute spectrom-
etry of claim 14, further comprising:

directly determining a linewidth of the single photon light

based on adjusting the wavelength of the pump light.

16. The process for performing direct absolute spectrom-
etry of claim 14, further comprising:

absolutely determining a peak wavelength of the single

photon light based on adjusting the wavelength of the
pump light.

17. The process for performing direct absolute spectrom-
etry of claim 12, wherein the EIT medium comprises an
atomic vapor.

18. The process for performing direct absolute spectrom-
etry of claim 12, wherein the atomic vapor comprises a
plurality of cesium atoms.

+ * RR Oe


US 10,641,655 B2

Sheet 2 of 11

May 5, 2020

U.S. Patent

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US 10,641,655 B2

3

resolution to GHz and which involve calibration by an
atomic cell to provide a wavelength reading. Instead, the
direct absolute spectrometer involves EIT and provides
sub-MHz resolution and direct and absolute wavelength
determination.

Direct absolute spectrometer performs direct absolute
spectrometry. In an embodiment, with reference to FIG. 1,
direct absolute spectrometer 200 includes first light source
202 that provides single photon light 204; second light

source 208 that provides pump light 210; optical combiner |

214 in optical communication with first light source 202 and
in optical communication with second light source 208.
Second light source 208 receives single photon light 204
from first light source 202; receives pump light 210 from
second light source 208; combines single photon light 204
and pump light 210; and produces dual light 212. that
comprises single photon light 204 and pump light 210.
Direct absolute spectrometer 200 also includes optical cell
206 that receives dual light 212 from optical combiner 214;
and electromagnetically induced transparent (EIT) medium
216 disposed in optical cell 206. EIT medium 216 is
optically transparent to single photon light 204 in a presence
of pump light 210 such that EIT medium 216 produces
output light 220 that includes pump light 210 and EIT light
222. Further, EIT medium 216 is optically opaque to single
photon light 204 in an absence of pump light 210 such that
EIT medium 216 does not produce output light 220 when
single photon light 204 is in an absence of pump light 210.

Direct absolute spectrometer 200 also includes filter 226 in 3

optical communication with EIT medium 216. Filter 226
receives output light 220 from optical cell 206 when output
light 220 is produced by EIT medium 216; and filters pump
light 210 from output light 220; and provides EIT light 222

free from pump light 210. EIT light 222 is a direct and 3

absolute metric of a linewidth and a wavelength of the single
photon light 204 such that direct absolute spectrometer 200
provides direct and absolute determination of the linewidth
of the single photon light 204 and direct and absolute
determination of wavelength at a maximum of intensity of
the single photon light 204.

Direct absolute spectrometer 200 can include magnetic
shield 218 disposed on optical cell 206 to shield EIT
medium 216 from a magnetic field external to optical cell
206. Moreover, detector 224 can be disposed in optical
communication with filter 226, wherein detector 224
receives EIT light 222 from filter 226 and produces EIT
signal 230, based on EIT light 222.

According to an embodiment, with reference to FIG. 2,
second light source 208 includes laser 232 that produces
laser light 234; wavelength tuner 236 in optical communi-
cation with laser 232 and that receives laser light 234 from
laser 232 and produces pump light 210. Wavelength locker
238 can be disposed in optical communication with laser
232, wherein wavelength locker 238 receives laser light 234
from laser 232; and produces laser lock 240 that is commu-
nicated to laser 232 to lock a wavelength of laser light 234.
Wavelength locker 238 can include reference cell 242 in
optical communication with laser 232, wherein wavelength
locker 238 receives laser light 234 from laser 232 and
produces reference signal 244, based on laser light 234.
Feedback unit 246 is in communication with reference cell
242, receives reference signal 244 from reference cell 242,
and produces laser lock 240, based on reference signal 244.
Second light source 208 further can include tuner controller
248 in electrical communication with wavelength tuner 236,
Tuner controller 248 produces control signal 256 that is

40

&

4

communicated to wavelength tuner 236 to control produc-
tion of pump light 210 by wavelength tuner 236.

First light source 202 produces single photon light 204. It
is contemplated that first light source 202 can include single
photon sources that can be used for quantum information
systems. Single photon light 204 can have a wavelength
from ultra-violet (UV) to infrared, and more specifically for
the wavelength of atomic transitions. A linewidth of single
photon light 204 can be from 100 kHz to 100 MHz,
specifically in a few or tens of MHz. Moreover, the intensity
of the light can be as weak as a single photon level.

In direct absolute spectrometer 200, second light source
208 can include laser 232, beam splitter 258, and a wave-
length locking system 238, and can generate a stable pump

5 light whose wavelength is locked to a certain atomic tran-

sition wavelength. Pump light 210 can have a wavelength
from ultra-violet (UV) to infrared, and more specifically for
the wavelength of atomic transitions. The linewidth of pump
light 210 can be narrow and within tens of KHz. Moreover,
the locking accuracy of the laser can be within tens of kHz.
It is contemplated that single photon light 204 and pump
light 210 have a wavelength difference, which is usually an
amount of difference in the ground energy levels of the EIT
media 216. In an embodiment, when the EIT media is

s Cesium atom, the wavelength difference is 9.2 GHz.

Optical combiner 214 combines single photon light 204
and pump light 210 to produce dual light 212. Optical
combiner 214 can be a polarizing beam splitter or an optical
polarizing combiner that combines single photon light 204
and pump light 210 in perpendicular polarization states.
Optical combiner 214 can have anti-reflection (AR) coating
on surfaces to reduce the loss of single photon light 201 and
pump light 212. Dual light 212 can have both wavelengths
of single photon light 201 and pump light 212, and which
co-propagate in the same direction.

In direct absolute spectrometer 200, optical cell 206 can
bea container with EIT materials. The cell can be made from
quartz or glasses. The shape of the cell can be a cylinder. The
input and output surfaces can have AR coating to reduce the
loss

EIT medium 216 is disposed in optical cell 206. EIT
medium 216 can be any elements with EIT phenomena.
Exemplary EIT media include alkali elements. In an
embodiment, EIT medium 216 is an atomic vapor. Accord-
ing to an embodiment, EIT medium 216 is a vapor of cesium
atoms, and its working wavelengths are around 852 nm and
895 nm. It is contemplated that there is a difference between
a wavelength of EIT light 222 and pump light 210. In an
embodiment, the difference is 9.2 GHz, which corresponds
to an energy difference between two energy levels of a
ground state of cesium atom.

Magnetic shield 218 is disposed on optical cell 206 to
shield the external magnetic field and can include a plurality
of layers. Exemplary materials for magnetic shield 218

5 include j1-metal. A structure of optical cell 206 can include

three layers of cylindrical tubes.

In EIT medium 216, dual light 212 interacts with EIT
medium 216. Here, because of the EIT phenomena, the dual
light 212 interacts with EIT medium 216 and can have an
interference resulting in a narrow transparent window
around the wavelength range of single photon light 204
Dual light 212 becomes interacting light 228, which
includes pump light 210 and partial narrow selection of
single photon light 204 that is transmitted through the

5 transparent window (becoming EIT light 222). Interacting

light 228 exits EIT medium 216 of optical cell 206 as output
light 220 that includes pump light 210 and EIT light 222 if


US 10,641,655 B2

7
response to combining single photon light 204 and pump
light 210; subjecting EIT medium 216 to dual light 212;
producing, in EIT medium 216, interacting light 228 as dual
light 212 propagates through EIT medium 216; producing,

from EIT medium 216, output light 220 from interacting 5

light 228 when EIT medium 216 is optically transparent to
single photon light 204 in presence of pump light 210,
output light 220 including: pump light 210; and EIT light
222; and absorbing single photon light 204 when single
photon light 204 propagates in EIT medium 216 in absence
of pump light 210, such that EIT medium 216 does not
produce output light 220; filtering output light 220 to
remove pump light 210; and producing, from filtering output
light 220, EIT light 222 in absence of pump light 210 to
perform direct absolute spectrometry.

The process for performing direct absolute spectrometry
further can include producing EIT signal 230 from EIT light
222 afier filtering output light 220 to remove pump light
210.

The process for performing direct absolute spectrometry
further can include adjusting a wavelength of pump light
210; and changing a wavelength of EIT light 222 in response
to adjusting the wavelength of pump light 210.

The process for performing direct absolute spectrometry
further can include directly determining a linewidth of single
photon light 204 based on adjusting the wavelength of pump
light 210.

The process for performing direct absolute spectrometry

further can include absolutely determining a peak wave- 3

length of single photon light 204 based on adjusting the
wavelength of pump light 210.

In the process, combining single photon light 204 with
pump light 210 can include polarizing with polarizing beam

splitter, which can combine single photon light 204 with 3

pump light 210 with perpendicular polarization states.

In the process, producing dual light 212 in response to
combining single photon light 204 and pump light 210 can
include aligning single photon light 204 and pump light 210
to be copropagating.

In the process, subjecting EIT medium 216 to dual light
212 causes EIT and generates a narrow transparent window
at EIT medium.

In the process, producing, in EIT medium 216, interacting
light 228 as dual light 212 propagates through EIT medium
216 transmitting part of single photon light 204 through to
make EIT light 222.

In the process, producing, from EIT medium 216, output
light 220 from interacting light 228 when EIT medium 216
is optically transparent to single photon light 204 in presence
of pump light 210. The part of light 204 passing through the
transparent window and pump light 210 includes output
light 220.

In the process, absorbing single photon light 204 when
single photon light 204 propagates in EIT medium 216 in
absence of pump light 210 occurs when single photon light
204 is in the absorption area of EIT media, and it is absorbed
without pump light 210

In the process, filtering output light 220 to remove pump
light 210 can include subjecting output light 220 to filters
that block pump light 210 and transmit EIT light 222
through optical cell 206

In the process, producing, from filtering output light 220,
EIT light 222 in absence of pump light 210 absorbing single
photon light 204 by EIT media in absence of pump light 210.

In the process, producing EIT signal 230 from detector
224 occurs for EIT light 222 after filtering output light 220

8

8
to remove pump light 210. EIT light 222 is detected by
detector 224 and the detector 224 produces corresponding
electrical EIT signal 230.

In the process, adjusting a wavelength of pump light 210
is implemented by a wavelength tuner, which can include an
acousto-optic modulator.

In the process, changing a wavelength of EIT light 222 in
response to adjusting the wavelength of pump light 210 is
implemented by EIT. Wavelengths of EIT light 222 and
pump light 210 satisfy the two photon resonance selection
rules.

In the process, directly determining a linewidth of single
photon light 204 based on linearly adjusting the wavelength
of pump light 210. A center wavelength of transparent
window of EIT changes linearly with the wavelength of
pump light 210, and the spectrum of single photon light 204
can be obtained directly by scanning the wavelength of
pump light 210.

In the process, absolutely determining a peak wavelength
of single photon light 204 is based on adjusting the wave-
length of pump light 210. The pump light is locked to an
atomic transition so the measured spectrum of single photon
light 204 is the absolute value and does include calibration.

Direct absolute spectrometer 200 and the process for
performing direct absolute spectrometry has numerous ben-
eficial uses that includes characterizing narrow bandwidth
single photon source and spectral measurements for high
spectral resolution.

Moreover, direct absolute spectrometer 200 and for per-
forming direct absolute spectrometry have numerous advan-
tageous properties. In an aspect, direct absolute spectrometer
200 provides spectrum measurement with high spectral
resolution, accuracy and sensitivity. Direct absolute spec-
trometer 200 and performing direct absolute spectrometry
unexpectedly include EIT that by which measurement of a
spectrum of single photon source is made and provides
ultra-high resolution measurement of the spectrum.

The articles and processes herein are illustrated further by
the following Example, which is non-limiting,

EXAMPLE

Spectral characterization of single photon sources with
ultra-high resolution, accuracy and sensitivity.

In future quantum communication systems, single pho-
tons, as the information carriers, possess very narrow lin-
ewidths and accurate wavelengths for an efficient interaction
with quantum memories. Spectral characterization of such
single photon sources is performed with very high spectral
resolution, wavelength accuracy, and detection sensitivity.
In this Example, we show precise characterization of spec-
tral properties of narrow-linewidth single-photon sources
using an atomic vapor cell using electromagnetically-in-
duced transparency (EIT). By using an atomic cesium vapor

‘5 cell, we have demonstrated a spectral resolution of better

than 150 kHz, an absolute wavelength accuracy of within 50
kHz, and detection sensitivity for optical signals as weak as
-117 dBm.

Quantum memory implements quantum repeaters and
quantum computer networks and are based on ions or atomic
ensembles. To integrate with these quantum memories,
photonic qubits must have compatible wavelengths and
linewidths. Except for a few approaches that work in the
off-resonant range to achieve broad bandwidths, such as
Raman, most quantum memory approaches work near
atomic resonant transition lines and have a bandwidth in the
order of MHz. While most entangled photon sources are


U.S. Patent May 5, 2020 Sheet 1 of 11 US 10,641,655 B2

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US 10,641,655 B2

Page 2
(56) References Cited
U.S. PATENT DOCUMENTS
2006/0256427 Al* 11/2006 Kuekes GO2F 1/03
359/321
2008/0089696 Al* 4/2008 Furuta HOA4L 9/0855
398/175
2011/0075239 Al* 3/2011 Moiseev GO2F 1/3515
359/241

OTHER PUBLICATIONS

Zhao, J., et al., “High sensitivity spectroscopy of cesium Rydberg
atoms using electromagnetically induced transparency”, Optics
Express, 2009, p. 15821, vol. 17 No. 18.

* cited by examiner


U.S. Patent

May 5, 2020 Sheet 11 of 11

US 10,641,655 B2

Counts (1/s)

Shuttle

Detuning from the Cs transition peak (MHz)

FIG. 11

905 100 100.5 101

4015 102


US 10,641,655 B2

5

single photon light 204 is ina presence of pump light 210 in
EIT medium 216. As used herein, single photon light 204 is
in a presence of pump light 210 when single photon light
204 spatially and temporally overlaps with pump light 210.

Otherwise. single photon light 204 is in an absence of pump
light 210. Accordingly, it should be understood that output
light 220 includes EIT light 222 and pump light 210 when
single photon light 204 is in a presence of pump light 210 in
EIT medium 216. Otherwise, single photon light 204 is in an
absence of pump light 210 in EIT medium 216, wherein
single photon light 204 is absorbed by EIT medium 216, and
EIT medium 216 only transmits pump light 210 instead of
output light 220,

Filter 226 receives output light 220 from optical cell 206
and filters output light 220 by removing pump light 210 and
leaving EIT light 222. Filter 226 can include a polarizer,
etalon, atomic filters, or a combination thereof, and the like.
Inan embodiment, filter 226 includes a high extinction ratio
Glen polarizer and two etalons. Moreover, the spectrum of
residual light of 210 passed the filter 226 is independent to
single photon signal 204 and can be partially removed by
post-processing.

Filter 226 transmits EIT light 222 to detector 224. Detec-
tor 224 can any optical detector that can detect a single
photon level signal. Exemplary detectors 224 include an
avalanche photodiode (APD), photomultiplier (PMT),
superconducting single photon detector, and the like. A
wavelength range that detector 224 detects can cover the
wavelength of single photon signal 204.

With regard to second light source 208, laser 232 can be 3

a laser diode, whose wavelength can be tuned by adjusting
driving current and temperature. In direct absolute spec-
trometer 200, laser light 234 is used as pump light, wherein
a wavelength of laser light 234 is locked to a certain

transition wavelength of EIT medium 216. Beam splitter 258 3

samples laser light 234, typically 1-5% thereof, and sends
this portion to wavelength locking system 238. Wavelength
locking system 238 has a reference cell 242 and feedback
unit 246 for the wavelength locking of the laser 232.
Reference cell 242 can be a cell containing reference gas for
the laser wavelength locking. Reference signal 244 can be a
signal that indicates the wavelength difference between the
light 234 and the absorption wavelength of reference cell
242. Feedback unit 246 can be a closed-loop system includ-
ing circuit to lock the laser wavelength. Laser lock signal
240 can be generated form feedback unit 246 to control laser
232. Tuner controller 248 can provide wavelength tuning to
the laser light 234. Program unit 260 can be a program to
tune the wavelength of pump light 210 and collect data from
EIT signal 230 to generate the spectrum of single photon
light 204. Program signal 258 can be a signal from program
unit 260 to control generator. Control generator 250 gener-
ates control signal 252 according to program signal 258.
Control signal 252 is a radio frequency signal generated
from control generator 250. Amplifier 254 amplifies the
control signal 252 and generates an amplified control signal
256. Amplified control signal 256 is used to drive wave-
length tuner 236 to generate pump light 210.

In an embodiment, with reference to FIG. 3 and FIG. 4,
first light source 202 produces single photon light 204
having a spectrum shown in FIG. 3. Here, direct absolute
spectrometer 200 directly determines an absolute wave-
length Lmax at maximum intensity Imax of single photon
light 204. Further, direct absolute spectrometer 200 directly
determines an absolute linewidth FWHM of single photon
light 204 shown as being from first wavelength L1 to second
wavelength L2. The determination of the absolute wave-

S

2

40

&

6

length Lmax and absolute linewidth FWHM is made directly
by direct absolute spectrometer 200. As used herein,
“directly” and variants thereof refers to the spectrum being
obtained by direct measurement instead of by calculation
from measurement results, such as lifetime. As used herein,
absolute and variants thereof (¢.g., absolutely and the like)
refers to a wavelength reading in direct absolute spectrom-
eter 200 that is directly measured in the disclosed processes
and that does not need calibration. Direct and absolute
determination of wavelength Lmax and linewidth FWHM is
a technical improvement over conventional spectrometry
that indirectly determines such parameters. Indirect deter-
mination can include bandwidth estimated by lifetime.

In an embodiment, a process for making direct absolute

5 spectrometer 200 includes disposing optical combiner 214 in

optical communication with first light source 202 and sec-
ond light source 208 such that single photon light 204
temporally and spatially overlaps pump light 210 in EIT
medium 216 in optical cell 206; disposing EIT medium 216
in optical cell 206; disposing optical cell 206 in optical
communication with optical combiner 214; and disposing
filter 226 in optical communication with optical cell 206.
Making direct absolute spectrometer 200 also can include
disposing detector 224 in optical communication with filter

S 226; disposing magnetic shield 218 on optical cell 206

Making direct absolute spectrometer 200 can include, with
respect to second light source 208, wavelength tuner 236 in
optical communication with laser 232; disposing reference
cell 242 in optical communication with laser 232; disposing
feedback unit 246 in optical communication with reference
cell 242 and in communication with laser 232 to form
wavelength locker 238; and disposing control generator 250
in communication with wavelength tuner 236. The process
also can include interposing amplifier 254 between and in
communication with wavelength tuner 236 and control
generator 250; disposing program unit 260 in communica-
tion with control generator 250.

In an embodiment, with reference to panel A of FIG. 4, a
process for performing direct absolute spectrometry
includes receiving dual light 212 by EIT medium 216. Here,
single photon light 204 has a wavelength range from first
wavelength L.A1 to second wavelength LA2 and maximum
intensity Imax at wavelength Lmax. Single photon light 204
is in presence of pump light 210 in EIT medium 216. Pump

5 light 210 has a wavelength range from first wavelength LPL

to second wavelength LP2 and maximum intensity at wave-
length LP. The spatial and temporal overlap of single photon
light 204 and pump light 210 in EIT medium 216 electro-
magnetically induces transparency in EIT medium 216 with
respect to transmission of a portion of single photon light
204 as EIT light 222 that occurs from wavelength L4 to
wavelength L5 with a maximum of intensity of EIT light 222
at wavelength L3. Accordingly, output light 220 is output
from EIT medium 216 of optical cell 206. It is further

5 contemplated that, a wavelength of pump light 210 can be

adjusted, e.g., blue shifted to a shorter wavelength Lblue. As
a result, EIT light 222, will be shifted toward shorter
wavelength Lshort. It is further contemplated that, a wave-
length of pump light 210 can be adjusted, e.g., red shifted to
a longer wavelength Lblue. As a result, EIT light 222, will
be shifted toward longer wavelength Llong. In this manner,
linewidth FWHM and wavelength Lmax of single photon
light 204 can be determined by adjusting the wavelength of
pump light 210.

In an embodiment, a process for performing direct abso-
lute spectrometry includes combining single photon light
204 with pump light 210; producing dual light 212 in