ARL-TR-8229 ● DEC 2017

US Army Research Laboratory

Setting Single Photon Detectors for Use with 

an Entangled Photon Distribution System

by Daniel E Jones, Drew Weninger, and Michael Brodsky

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ARL-TR-8229● DEC 2017

US Army Research Laboratory

Setting Single Photon Detectors for Use with 

an Entangled Photon Distribution System

by Daniel E Jones and Michael Brodsky

Computational and Information Sciences Directorate, ARL 

Drew Weninger

Intern, US Naval Academy, Annapolis, MD

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4. TITLE AND SUBTITLE

Setting Single Photon Detectors for Use with an Entangled Photon Distribution 

System

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6. AUTHOR(S)

Daniel E Jones, Drew Weninger, and Michael Brodsky

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US Army Research Laboratory

Computational and Information Sciences Directorate (ATTN: RDRL-CIN-T)

Aberdeen Proving Ground, MD 21005

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

It is often a difficult task to find the optimal operating conditions of a single photon detector (SPD). The proper use and 

measurement of the entangled photon pairs generated by an entangled photon pair source require at least 2 SPDs operating at 

their highest possible detection efficiency and lowest possible dark count rate and with the lowest amount of counts due to 

afterpulsing. However, tuning of the detector bias and threshold values results in a tradeoff in detector performance where 

increasing the detector efficiency also increases afterpulsing and the dark count rate. In this report we discuss the work of a 

student intern from the US Naval Academy and how he learned to optimize the detector settings to detect and evaluate the 

entangled photon pairs generated by an entangled photon pair source.

15. SUBJECT TERMS

single photon detectors, entangled photon pairs, quantum networks, quantum state tomography, detection efficiency, 

entanglement distribution

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Michael Brodsky

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Contents

List of Figures iv

Acknowledgments v

1. Introduction 1

2. Generation, Detection, and Measurement of Entangled Photon 

Pairs 1

2.1 Generation of Entangled Photon Pairs 2

2.2 Detection and Measurement of Entangled Photon Pairs 3

3. Experiment Procedure 4

3.1 Physical Setup of Equipment and Identification with EPA Software 4

3.2 Setting Detector Parameters for Optimal Performance 5

3.3 Operation of Entangled Photon Distribution System with EPA 

Software 8

3.4 Measurement of Entangled Properties of Entangled Photon Pairs 10

4. Data and Results 11

4.1 Two-Photon Interference 11

4.2 Quantum State Tomography 12

5. Conclusions 12

6. References 14

List of Symbols, Abbreviations, and Acronyms 16

Distribution List 17
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List of Figures

Fig. 1 Diagram of experiment setup, which includes wavelength-division 

multiplexer (WDM) and coincidence counting (CC) electronics......... 2

Fig. 2 Diagram of how the equipment is connected: red dots represent 

electrical ports to be connected by SMA cables, and black dots 

represent fiber ports to be connected by optical fibers......................... 5

Fig. 3 Plot of coincidence counts as the polarization of the idler PA is rotated 

around the Poincare sphere. The TPI fringes show >70.7% visibility 

when the signal is horizontally (blue curve) and right circularly (red 

curve) polarized, violating a Bell inequality for proof of the 

polarization entanglement of the photon pairs generated by the EPS. 11

Fig. 4 Plot of the density matrix describing the quantum state of the photon 

pairs generated by the EPS. Various parameters are displayed in the 

table to show the entanglement of the photon pairs generated by the 

EPS...................................................................................................... 12

Fig. 5 Midshipman intern Drew Weninger operates the entangled photon 

distribution system in the ARL Quantum Network Testbed .............. 13
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v

Acknowledgments

Research was sponsored by the US Army Research Laboratory (ARL) and 

performed with a student intern from the US Naval Academy (Drew Weninger).

The views and conclusions contained in this document are those of the authors and 

should not be interpreted as representing the official policies, either expressed or 

implied, of ARL or the US Government. The US Government is authorized to 

reproduce and distribute reprints for government purposes notwithstanding any 

copyright notation herein. 
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INTENTIONALLY LEFT BLANK.
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1. Introduction

The development of quantum networks has recently become a significant goal in 

the scientific community due to the possibility of demonstrating network 

functionalities that cannot be performed using classical networks.1,2 Quantum 

networks consist of many nodes connected to each other with quantum channels, 

which could be existing telecom fiber channels. The nodes generate, store, and 

manipulate quantum states such as entangled photon pairs, and the channels 

transport quantum states from node to node.3,4 The primary equipment needed to 

construct the nodes of a quantum network include sources of entangled photon pairs 

and single photon detectors (SPDs). To begin building a quantum network, a

fundamental knowledge of how to operate the entangled photon pair sources and 

SPDs is required. The proper use and measurement of the entangled photon pairs 

generated by an entangled photon pair source require SPDs operating with high 

detection efficiency, low dark count rate, and few counts due to afterpulsing. 

However, tuning the detector bias and threshold values results in a tradeoff in 

detector performance where increased detector efficiency is accompanied by 

increased dark counts and afterpulsing. 

In this report we describe how to operate the NuCrypt entangled photon distribution 

system located in the US Army Research Laboratory (ARL) Quantum Network 

Testbed. First, we provide a brief introduction to the NuCrypt entangled photon 

distribution system, which consists of an Entangled Photon Source (EPS), 

Correlated Photon Detection System (CPDS), and 2 Polarization Analyzers (PAs). 

Next, we detail the process to physically set up the system and identify all 

equipment with the NuCrypt Entangled Photon Analyzer (EPA) software. We then

explain the process required to set the bias and threshold settings of the SPDs to 

optimize the detection efficiency, dark count rate, and afterpulsing. Finally, we 

explain the process to operate the EPA software to perform quantum state 

tomography and two-photon interference (TPI) measurements of the entangled 

photon pairs generated by the EPS.

2. Generation, Detection, and Measurement of Entangled 

Photon Pairs 

The entangled photon distribution system utilized in the ARL Quantum Network 

Testbed consists of 4 primary pieces of equipment procured from NuCrypt, LLC5

by the Quantum Network group at ARL’s Computational and Information Sciences 

Directorate: an EPS, a CPDS, and 2 PAs. A basic diagram of the experimental setup 

is shown in Fig. 1.
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Fig. 1 Diagram of experiment setup, which includes wavelength-division multiplexer

(WDM) and coincidence counting (CC) electronics

2.1 Generation of Entangled Photon Pairs

Various internal components of the equipment are shown in Fig. 1 to explain their 

basic operation; however, the majority of components are not depicted. The EPS 

generates photon pairs (the individual photons are denoted signal and idler) 

entangled in time-energy and polarization6,7 using a 1552.52-nm pump laser pulsed 

at a 50-MHz rate. The laser power can be adjusted using a variable attenuator (VA) 

to adjust the entanglement properties of the photons generated by the EPS. After 

the VA, a polarization controller (PC) and polarizer are used to set the polarization 

of the pump laser to 45°. The 45° polarized pump is then incident on a polarization 

beam splitter (PBS), which transmits and reflects equal powers of horizontally (H) 

and vertically (V) polarized pulses in opposite directions around the loop of a 

dispersion-shifted fiber (DSF), which has a zero-dispersion wavelength centered 

near the pump wavelength.7,8 This satisfies the necessary phase-matching 

conditions for 2 photons from the pump pulse to generate signal and idler pair 

photons with a low probability through the χ(3) nonlinear process of four-wave 

mixing (FWM).9 

Horizontally polarized signal and idler photons are created by the H-polarized 

pump pulse, and V-polarized photons are created by the V-polarized pulse traveling 

in the opposite direction around the loop. The PBS then sends the signal and idler 

photons traveling in opposite directions (H and V pairs) to the same output port. 

Polarization controllers are included within the DSF loop to adjust the birefringence 

of the fiber such that the H and V pairs arrive at the PBS at the exact same time. 

Since signal and idler photons are generated through FWM with a very low 
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probability, it is highly unlikely that a pair is created by both the horizontal and 

vertical pump pulses at the same time. Therefore, only one pair is output from the 

PBS at a given time, and the PBS eliminates all knowledge of which path was taken 

(i.e., the polarization is unknown), thus creating the following polarizationentangled Bell state at the output of the PBS7

:

|φφ+⟩ = 1

√2 (|HH⟩ss|HH⟩ii + |VV⟩ss|VV⟩ii). (1)

The entangled photon pair is then directed to a WDM, which “chops” the broadband 

signal and idler spectra into 4 separate channels on the 100-MHz ITU (International 

Telecommunications Union) grid symmetric about the pump wavelength of 

1552.52 nm (channel 31). The signal channels are channels 26–29, and the idler 

channels are channels 33–36. The channels are anticorrelated in 

frequency/wavelength due to energy conservation of the FWM process. All 8 of 

these signal and idler output channels are directly accessible with fiber connections 

on the front panel of the EPS.

2.2 Detection and Measurement of Entangled Photon Pairs

The EPS generates polarization-entangled photon pairs; however, the PAs and the

CPDS are needed to detect the photons and measure their entangled properties. 

Upon exiting the EPS, the signal and idler photons are each sent to a PA before 

being detected by the gated SPDs within the CPDS. The PAs contain various 

waveplates and a polarizer to adjust the polarization of any incoming light signal. 

The CPDS contains all necessary electronics to record the amount of photons 

detected by each detector (known as single counts) and the amount of occurrences 

where photons were detected by both detectors at the exact same time (known as a 

coincidence count). This setup allows measurements to be taken in the specific 

polarization bases required to perform quantum state tomography and TPI 

measurements. 

Quantum state tomography is the process by which a quantum system (i.e.,

entangled pairs generated by the EPS) is completely characterized by measuring 

many copies of the unknown quantum state.10 For an entangled photon source such 

as ours, this requires measuring the amount of single and coincidence counts at 36 

different polarization settings of the 2 PAs. After taking these 36 measurements, 

the singles and coincidence counts recorded by the CPDS are analyzed by the EPA 

software, and the quantum state of the generated entangled photon pairs is 

determined. Likewise, TPI measurements consist of adjusting the polarization of 

the signal to H using the PA and then measuring the coincidence counts when 

adjusting the polarization of the idler PA in 30° increments of the Poincare 
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sphere.11,12 When measuring an entangled pair source such as ours, this results in a 

sinusoidal variation of the coincidence counts when plotted versus the polarization 

of the idler PA. After performing the measurements with an H-polarized signal, the 

signal polarization is adjusted to a right-circular polarization (R) using the PA, and 

the idler polarization is once again rotated in 30° increments while measuring the 

coincidence counts.

3. Experiment Procedure

3.1 Physical Setup of Equipment and Identification with EPA 

Software

To set up the entangled photon distribution system, the EPS, CPDS, and PAs must 

first be connected to a laboratory computer installed with the NuCrypt EPA 

software via USB cables. Next, the EPS, CPDS, and PAs must be appropriately 

connected using optical fibers and SubMiniature version A (SMA) cables as shown 

in Fig. 2. To simplify the diagram, only ports that are actively used for the purpose 

of this report are shown. Before each fiber connection is made, the optical fiber 

patch cord that is used to connect the 2 ports must be properly cleaned using a fiber 

cleaning device such as the Optipop R optical connector cleaner.13 The fiber tip 

also must be examined and determined to be defect-free using an optical fiber 

microscope so that a sufficient connection is made between the 2 optical fiber ports. 

To detect 2 photons of an entangled pair, specific output channels of the EPS are 

used. The correlated pairs to be used are the following: 28/34, 29/33, 27/35, and 

26/36 (Fig. 2). If other combinations of 2 channels are used, the resulting 

measurements performed will show no evidence of entanglement.
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Fig. 2 Diagram of how the equipment is connected: red dots represent electrical ports to 

be connected by SMA cables, and black dots represent fiber ports to be connected by optical 

fibers

After physically connecting all of the ports, the EPA software must be opened on 

the laboratory computer. For the computer to recognize all of the equipment, the 

user must hit the “identify” button in the devices panel of the EPA user interface. 

Then, graphical depictions of the EPS, CPDS, and PA should be labeled with the 

serial numbers of the equipment that is connected to the computer via USB. Verify 

that all equipment was detected by the EPA software and that all serial numbers 

listed on the EPA interface match the serial numbers of the actual equipment. If any 

piece of equipment is not properly identified, verify that the equipment is indeed 

powered on and properly connected to the computer before re-identifying the 

equipment with the EPA software. If all equipment is properly identified by the 

EPA software, set the detector parameters.

3.2 Setting Detector Parameters for Optimal Performance

The NuCrypt CPDS in the Quantum Networks Testbed contains 2 single photon 

avalanche detectors (SPADs), a specific type of SPD. The basic operation principle 

of a SPAD is that the diode is kept at a user-specified bias voltage until a gate pulse 

(in our case, supplied by the electrical clock output of the EPS) is received, which 

raises the diode voltage by a set amount over the breakdown voltage. The 

breakdown voltage of a SPAD is the voltage at which increasing the diode voltage 

causes the diode to “break down”, triggering the output of a large avalanche voltage 

pulse. By increasing the diode voltage above the breakdown voltage, any small 
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voltage change in the diode (as small as that provided by one photon incident on 

the detector) triggers an avalanche pulse. This output avalanche pulse is then 

compared with the user-determined threshold voltage, and if the output voltage 

exceeds the threshold value, the detector “clicks” (i.e., the detector registers that it 

received a photon). For more details, see Cova et al.14 and the NuCrypt CPDS 

documentation.15

Therefore, to set the SPDs of the CPDS to their optimal operating conditions, the 

bias and threshold values must be adjusted by the user to maximize detection 

efficiency and minimize the dark count rate and afterpulsing (specifications 

provided by NuCrypt quote approximately 20% detection efficiency, dark count 

rates on the order of 10–5 per gate pulse, and afterpulsing <4%). As mentioned in 

Section 2, it can be tricky to find the optimal operating conditions because

increasing the detection efficiency also increases the dark count rate and counts due 

to afterpulsing. 

A new CPDS is initially set so that the detector bias is close to the breakdown 

voltage, meaning that many dark counts occur because they are due to spontaneous 

voltage variations that exceed the breakdown voltage. To properly adjust the 

detector bias and threshold values, the user must use the following procedure:

1) Make sure that no light is incident on the detector. Next, the user must open 

TeraTerm on the computer connected to the CPDS. TeraTerm is a program 

that directly controls the CPDS in a manner similar to the command prompt 

on a computer. From the main menu of the TeraTerm window, type “5” to 

start the detector in “continuous mode”. In this mode TeraTerm constantly 

outputs the number of detections registered by the detector per the userdefined number of gate pulses. 

2) Type “<” to lower the bias voltage so that the detector never breaks down

(i.e., there are no counts [continuous mode constantly outputs zero]). Once 

you have reached this bias voltage, press the space bar to stop lowering the 

bias voltage. 

3) Set the threshold so that the count rate is approximately 10–5 per gate 

(10 counts per million gates). The threshold value is adjusted by typing “+” 

(to increase) or “–“ (to decrease). Similar to adjusting the bias voltage, 

pressing the space bar will also stop changing the threshold value. After 

reaching the desired threshold value, type “e” to exit continuous mode and 

return to the main menu. 

4) Type “6” to scan the gate phase of the detector. This command scans the 

detector over a small amount of time and outputs the number of counts at 
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each “gate phase” (time value). Ideally, the number of counts for all phases 

would be the same value of approximately 10–5; however, this is unlikely 

after the first attempt. There will likely be some phase values where there 

are very many counts and others where there are very few counts. Type “m” 

to set the detector to the gate phase at which the maximum counts were 

measured. Now repeat step 3 and rescan the gate phase until there are no 

longer any phases that result in a large number of counts. This step is crucial 

because the large counts would be mistaken for light pulses if the threshold 

is not adjusted properly to reduce them.

5) Return to the main menu by typing “e” and put the detector in continuous 

mode once again by typing “5”. Now raise the bias by typing “>” until the 

count rate reaches the recommended dark count rate of approximately 10–4

per gate (100 counts per million gates). Type “e” to return to the main menu.

6) Type “6” to scan the gate phase again and confirm that there is very little 

variation in the count rate. There should not be any phase values with a large 

number of counts that could be mistaken as a light pulse.

7) Connect a pulsed photon source to the detector and scan the gate phase by 

pressing “6”. When a light source is connected to the detector, a peak in the 

counts at a certain gate phase signifies that the pulse arrives at the detector 

at the same time as the time represented by that value of the gate phase. 

Obviously, we want to set the detector phase such that it “sees” the 

incoming pulse, so type “m” to select the gate phase that maximizes the 

count rate. The pulsed photon source that is used for the purposes of this 

report is one of the signal or idler channels of the EPS. See Section 3.3 to 

determine how to operate the EPS and generate the entangled photon pairs. 

8) To verify that the detectors are indeed optimized, the user must directly 

measure the efficiency with a calibrated measurement device. 

a. To do this, assemble a system of fiber couplers and attenuators such 

that an incoming signal is almost entirely transmitted through one 

output but experiences >50 dB of attenuation through another 

output. This is done by cascading a 99:1 fiber coupler with >30 dB 

of additional attenuation at the 1% output of the fiber coupler. This 

is required so that the power of the high-transmission channel can 

be measured with a power meter without sending too strong of a 

signal to the SPD (which will be placed at the output of the 

attenuated channel).
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b. Measure the relative transmission of the attenuated channel and the 

high-transmission channel using a high-power light source and a 

sensitive power meter (able to measure signal powers of 

approximately –80 dBm). 

c. Connect the “residual pump” port of the EPS to a variable attenuator 

and connect the variable attenuator to the input of the attenuated 

fiber coupler system built in step 8a. Connect the high-transmission 

output to the same power meter used in step 8b and connect the 

attenuated output to the SPD being tested. 

d. Adjust the variable attenuator so that the power measured by the 

power meter corresponds to an average of one photon incident on 

the SPD. The power value that corresponds to an average of one 

photon incident on the SPD can be easily calculated from the 

equation for the energy of one photon, the number of photons 

incident on the power meter, and the relative transmission of the 

high-transmission path and the attenuated path of the system built in 

step 8a (measured in step 8b). The energy of one photon is given by 

= hcc

λλλλ , where h is Planck’s constant, c is the speed of light, λ is the 

wavelength, and n is the refractive index of silica fiber. The number 

of photons incident on the power meter is given by NNPPPP = PP

ffrrrrrr EE

, 

where P is the reading on the power meter and frep is the frequency 

(repetition rate) of the residual pump of the EPS (50 MHz).

e. Measure the number of single counts detected by the SPD and divide 

this by the number of detection gates to determine the efficiency of 

the SPD. Also measure the dark count rate and the afterpulsing using 

the EPA software.

f. Verify that that the detector is operating with approximately 20% 

efficiency, <10–4 dark counts/gate, and <4% afterpulsing. If the 

detector does not perform to these specifications, make slight 

changes to the detector bias and threshold values to improve the 

performance.

3.3 Operation of Entangled Photon Distribution System with 

EPA Software

Now that the detector bias and threshold values have been set, return to the EPA 

interface to operate all components of the entangled photon distribution system. 
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1) Properly adjust the pump polarization to 45° by clicking the “eps init” (short 

for initialize) button on the EPA user interface. When the EPS is first 

powered on, the laser polarization is set to some default value not equal to 

45°. To properly generate the entangled state of Eq. 1, the pump must be 

45°, as explained in Section 2.1. By clicking the “eps init” button, the EPS 

automatically adjusts the internal PC (Fig. 1) and monitors the power of the 

EPS after the PBS. The PC is adjusted until the power is maximized, 

signifying that the polarization has been properly adjusted. The message 

panel on the EPA interface will display a notification when the adjustments 

are complete. The power reading on the front panel of the EPS will also 

reach its maximum value after initializing, at which point the power of the 

EPS pump laser will be set to a value of 3600 in the EPA software. The 

practical range of powers that can be set using the EPA interface ranges 

from 3000 to 3600, corresponding to a peak power of approximately 

5–150 μW (measured at the raw output port of the EPS).

2) Set the number of detection gates (measurement time) to 1 million gates 

using the “gates” text box on the EPA interface and click the “other CPDS 

ctl” button to open a new window of the EPA interface. From this window, 

the bias, threshold, gate phase, and delay of one detector relative to the other 

can all be adjusted. Click the “gate phase” button for channel 1 to scan the 

gate phase of detector 1. This ensures that the detector is measuring the 

incoming signal or idler photons at the same time that they are arriving. 

Repeat this step for channel 2 to set the gate phase of detector 2 as well.

3) Delay channel 1 so that the photons arrive at both detectors at the same time. 

For technical reasons, channel 2 of the CPDS is delayed relative to channel 

1; therefore, channel 1 must be delayed with the EPA software for 

coincidences to be measured between photons incident on detectors 1 and 

2. In the “synchronization” tab of the “Other CPDS Control” panel, check 

the boxes to maximize correlation c12 and to delay channel 1. Now set the 

number of gates to 10 million and click the “search” button. This adjusts the 

delay of channel 1 over 20 different values and measures the coincidences 

of detectors 1 and 2 for each delay in order to set the delay such that the 

maximum possible number of coincidences are measured between detectors 

1 and 2.

4) Measure the dark count rate of detectors 1 and 2 by clicking the “measure” 

button in the “Dark Counts” panel of the primary EPA interface. The rates 

for both detectors should not exceed 10–4 counts per gate. If so, the detector 

bias and/or threshold settings must be readjusted as described in 
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Section 3.2. The number of gates should be ≥10 million to obtain an 

accurate measurement. The NuCrypt entangled photon distribution system 

is now fully ready to operate, and measurements of the single count rates, 

coincidence count rate, and afterpulsing of each detector can be performed 

by clicking the “measure” button on the EPA interface (the results are 

output on the message panel of the EPA software). 

5) If a TPI measurement is to be performed, the polarization of the PAs must 

first be adjusted to account for the birefringence of the fibers connecting the 

EPS and PAs. This step is not needed to perform tomography. Connect an 

external polarized light source to the “External Alignment Source” port of 

the EPS and click the “align H & D singles” button in the “Alignment” panel 

of the EPA interface. By doing this the PAs are automatically adjusted to 

various polarization settings and the single count rates of both detectors are 

measured for each polarization setting. The EPA software then analyzes the 

data and calculates the polarization transformation applied by the fibers 

connecting the EPS and the PAs and adjusts both PAs to invert this 

transform (i.e., it cancels out the transformation applied by the fibers).

3.4 Measurement of Entangled Properties of Entangled Photon 

Pairs

After completing the steps explained in the previous section, TPI measurements 

and quantum state tomography can be performed to examine the entangled behavior 

of the photon pairs generated by the EPS. A TPI measurement can be performed by 

clicking the “start” button in the “Two-Photon Interference” panel of the EPA 

interface. The PAs and CPDS automatically perform all required polarization 

adjustments and measurements (as described in Section 2) to make a sinusoidal plot 

of the coincidence counts as a function of the signal and idler polarization. The EPA 

software also automatically calculates and outputs the visibility of the TPI. The 

visibility is given by the following equation:

VV = CCmmmmmm−CCmmmmmm

CCmmmmmm+CCmmmmmm

, (2)

where Cmax denotes the maximum coincidence counts measured for any setting of 

the idler polarization (i.e., the peak of the sinusoidal TPI curve), and Cmin denotes 

the minimum coincidence counts measured for any setting of the idler polarization 

(i.e., the minimum of the TPI curve). 

To perform tomography, check the “stack” and “max likelihood” boxes in the 

“Tomography” panel of the EPA interface and click the “start” button. The PAs 
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will automatically adjust themselves to all of the 36 polarization settings required 

to perform tomography (as described in Section 2), and the CPDS will record the 

single and coincidence counts of both detectors. The software will also perform a 

maximum likelihood estimation to determine the density matrix of the quantum 

state generated by the EPS. The raw detector measurements are automatically saved 

to a text file named tomoXXX (XXX corresponds to a number assigned to that data 

set beginning at 000 and increasing by one for each subsequent data set), and the 

calculated density matrix and its corresponding entanglement measures are saved 

to a text file named tomoXXX_Analysis. A figure is also generated that includes a 

plot of the elements of the density matrix and a table of the fidelity, tangle, linear 

entropy, concurrence, and purity of the entangled state described by the calculated 

density matrix. Example results of TPI and tomography measurements are shown 

in Section 4.

4. Data and Results

4.1 Two-Photon Interference

TPI measurements were performed to analyze the entangled nature of the photon 

pairs generated by the EPS. The power of the pump laser was adjusted over the 

entire accessible range of powers, and the optimum power (resulting in the greatest 

TPI visibility) was found to be an EPA interface value of 3100 (~10 μW measured 

at the raw output port of the EPS). With this pump power, a TPI visibility of >85% 

was measured, easily exceeding the 70.7% visibility required to violate a Bell 

inequality for proof of polarization entanglement as seen in Fig. 3.

Fig. 3 Plot of coincidence counts as the polarization of the idler PA is rotated around the 

Poincare sphere. The TPI fringes show >70.7% visibility when the signal is horizontally (blue 

curve) and right circularly (red curve) polarized, violating a Bell inequality for proof of the 

polarization entanglement of the photon pairs generated by the EPS.

0 50 100 150 200

chan 2 basis (deg)

250 300 350

0

200

150

100

50

250

350

300

coinc

Hvis 85.7% Rvis 85.0%
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4.2 Quantum State Tomography

Quantum state tomography was also performed on the pairs generated by the EPS 

to determine the density matrix of the system. Once again, measurements were 

made over the entire accessible range of pump powers, and proof of polarization 

entanglement was clearly shown with raw concurrence and fidelity (compared with

the Bell state given by Eq. 1) values of 0.871 and 0.934. Furthermore, the 

accidental-subtracted concurrence and fidelity values of 0.975 and 0.985 also show 

the strong polarization entanglement of the pairs generated by the EPS. The density 

matrix of the entangled photon pairs generated by the EPS and the relevant 

entanglement measures are shown in Fig. 4.

Fig. 4 Plot of the density matrix describing the quantum state of the photon pairs generated 

by the EPS. Various parameters are displayed in the table to show the entanglement of the 

photon pairs generated by the EPS.

5. Conclusions

To efficiently operate an entangled photon distribution system, the user must 

optimize the performance of 2 SPDs by adjusting their bias and threshold values in 

order to reliably detect the photons generated by an entangled photon pair source. 

However, it can be difficult to optimize the 3 performance specifications of an SPD 

(high efficiency, low dark count rate, and low afterpulsing) because tuning the 

detector bias and threshold values results in a tradeoff in detector performance 

where increased detector efficiency also results in increased dark counts and 

afterpulsing. In this report we have detailed the process to optimize the SPDs used 

in the ARL Quantum Network Testbed in order to achieve 20% detection 

efficiency, <10–4 dark counts per detection gate, and <4% afterpulsing. The steps 

required to generate, detect, and evaluate the entangled properties of the entangled 

photon pairs generated by the NuCrypt EPS were then explained, and the results of 

measurements performed by a student intern from the US Naval Academy (Fig. 5) 

were reported.
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13

Fig. 5 Midshipman intern Drew Weninger operates the entangled photon distribution 

system in the ARL Quantum Network Testbed
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14

6. References

1. Kimble H. The quantum internet. Nature. 2008;453:1023–1030.

2. Van Meter R. Quantum networking. Hoboken (NJ): John Wiley & Sons; 2014.

3. Duan LM, Lukin M, Cirac J, Zoller P. Long-distance quantum communication 

with atomic ensembles and linear optics. Nature. 2001;414:413–418.

4. Munro W, Harrison K, Stephens A, Devitt S, Nemoto K. From quantum 

multiplexing to high-performance quantum networking. Nat Photonics. 

2010;4:792–796.

5. NuCrypt, LLC. Product overview: quantum optical instrumentation. Evanston 

(IL): NuCrypt, LLC [accessed 2017 July]. http://www.nucrypt .net/quantum

-optical-instrumentation.html.

6. Kwiat PG, Mattle K, Weinfurter H, Zeilinger A, Sergienko AV, Shih Y. New 

high-intensity source of polarization-entangled photon pairs. Phys Rev Lett. 

1995;75:4337–4341.

7. Fiorentino M, Voss PL, Sharping JE, Kumar P. All-fiber photon-pair source 

for quantum communications. IEEE Photon Technol Lett. 2002;14:983–985.

8. Wang SX, Kanter GS. Robust multiwavelength all-fiber source of 

polarization-entangled photons with built-in analyzer alignment signal. IEEE 

J Sel Top Quantum Electron. 2009;15:1733–1740.

9. Inoue K. Four-wave mixing in an optical fiber in the zero-dispersion 

wavelength region. J Lightwave Technol. 1992;10:1553–1561.

10. Altepeter JB, Jeffrey ER, Kwiat PG. Photonic state tomography. In: Berman 

PR, Lin CC, editors. Advances in atomic, molecular, and optical physics. 

Amsterdam (Netherlands): Elsevier; 2005. p. 105–159.

11. Pittman TB, Strekalov DV, Migdall A, Rubin MH, Sergienko AV, Shih YH.

Can two-photon interference be considered the interference of two photons? 

Phys Rev Lett. 1996;77:1917–1920.

12. Collett E. The Poincare sphere: field guide to polarization. Bellingham (WA): 

The International Society for Optics and Photonics (SPIE); 2005. p. 10.

13. USConec Ltd. The OPTIPOP R refillable cassette cleaning tools.

Hickory (SC): USConec Ltd.; 2015 [accessed 2017 July].

http://www.usconec.com/products/cleaning_tools/optipop_r_refillable

_cassette_cleaning_tools.htm.
Approved for public release; distribution is unlimited.

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14. Cova S, Ghioni M, Lacaita A, Samori C, Zappa F. Avalanche photodiodes and 

quenching circuits for single-photon detection. Appl Opt. 1996;35:1956–1976.

15. NuCrypt, LLC. Correlated photon detection system. Evanston (IL): NuCrypt, 

LLC; 2017 [accessed 2017 July]. http://nucrypt.net/CPDS-1000.html.
Approved for public release; distribution is unlimited.

16

List of Symbols, Abbreviations, and Acronyms

ARL US Army Research Laboratory

CC coincidence counting electronics

CPDS Correlated Photon Detection System

DSF dispersion-shifted fiber

EPS Entangled Photon Source

EPA Entangled Photon Analyzer

FWM four-wave mixing

H horizontally

ITU International Telecommunications Union

PA Polarization Analyzer

PBS polarization beam splitter

PC polarization controller

R right-circular polarization

SMA SubMiniature version A

SPAD single photon avalanche detector

SPD single photon detector

TPI two-photon interference

USB Universal Serial Bus

V vertically

VA variable attenuator

WDM wavelength-division multiplexer
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17

1 DEFENSE TECHNICAL

(PDF) INFORMATION CTR

DTIC OCA

2 DIR ARL

(PDF) IMAL HRA 

RECORDS MGMT

RDRL DCL

TECH LIB

1 GOVT PRINTG OFC

(PDF) A MALHOTRA

1 DIR ARL

(PDF) RDRL CIN T

M BRODSKY
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