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Correlated Photon-Pair Source

  • Heralded Single-Photon Source with g(2)(τ = 0) < 0.1
  • Photon-Pair Generation at 810 nm
  • Integrated 405 nm Pump Laser


Correlated Photon-Pair Source

A second-order correlation measurement [g(2)(τ)] between signal and idler photons. The peak at τ = 0 confirms the generation of photon pairs. Data is taken at 35 mW.

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Typical Applications

  • Sub-Shot-Noise Imaging
  • 2-Photon Interference
  • Heralded g(2) Measurements
  • Absorption Spectroscopy
  • Quantum Metrology


  • Spontaneous Parametric Down-Conversion (SPDC) Source (Collinear Type-II)
  • >0.45 High-Efficiency Heralding Ratio
  • >450 kHz Pair Generation Rate
  • ±2.5 nm Wavelength Stability for Emitted Photons
  • Pump Laser Power Adjustable from 10 mW to 150 mW
  • Room Temperature Operation
  • Remote Operation of 405 nm Pump Laser (Serial RS232)
SPDC Energy Conservation
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The SPDC conversion process obeys energy and momentum conservation.
SPDC Nonlinear Process
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Collinear Type-II SPDC of one 405 nm pump photon entering a PPKTP crystal and exiting as two 810 nm output photons.

Thorlabs' Correlated Photon-Pair Source uses spontaneous parametric down-conversion (SPDC) to create a pair of energy-time entangled photons at 810 nm. With an integrated 405 nm pump laser, this self-contained source results in >450 kHz photon pairs per second and a high-efficiency heralding ratio of >0.45. A zero-time delay second-order correlation function [g(2) (τ = 0)] value of <0.1 can be achieved with this source, making it a high-brightness heralded single-photon source ideal for quantum optics applications. For complete performance specifications, please see the Specs tab; note that the heralding ratio, pair rate, and g(2)(0) values are given for the pump laser operating at 35 mW and the observed statistics will vary with the pump power and detector specifications.

In this SPDC source, a nonlinear crystal [periodically poled potassium titanyl phosphate (PPKTP)] converts one 405 nm pump photon into two 810 nm photons (the signal and idler) in a single event. The resulting signal and idler photons have type-II phase matching, which means they propagate with orthogonal polarizations (extraordinary and ordinary); see the schematics to the right. As a source that emits photon-pairs in a simultaneous event, it can be used as a heralded single-photon source. This is when the detection of one photon (idler) heralds the presence of the second photon (signal). More information about single-photon verification can be found in the Single-Photon Output tab.

SPDC810 Electrical and Fiber Connections
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Photon-Pair Source shown with P1-780PM-FC-1 patch cables (not included) connected to the signal and idler outputs. 

To efficiently collect the down-converted light, the signal and idler channel outputs are FC/PC coupled. We recommend using PM780-HP FC/PC patch cables to maintain polarization, such as P1-780PM-FC-1. If polarization information does not need to be retained, 780HP FC/PC patch cables can also be used.

The photon-pair source contains an oven to maintain the temperature of the nonlinear crystal and the wavelength of the down-converted photons, resulting in a wavelength stability of ±2.5 nm. For adequate cooling, the unit requires 1" of clearance on all sides. 

This SPDC source is factory-aligned and ready to use. If misalignment occurs but signal is still detected, x- and y-axes adjustments to the internal mirrors can be made through the access holes in the side of the housing; see the manual for details. The internal adjusters accept a 5/64" or 2 mm ball driver (not included). Please contact if no signal is detected.

The unit is shipped with a 12 V power supply with an M8 connector and an RS232 cable for operating the pump laser. For more information about these connectors, please see the Pin Diagrams tab.

SPDC Source Housing Dimensions
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Correlated Photon-Pair Source Housing Dimensions
Optical Specifications
Operating Wavelength  810 ± 2 nm
ηsi (Detector Excluded)a,b >0.45
Max Pairs/Second >450 kHz
Wavelength Stabilitya ±2.5 nm
Temperature Control No
g(2)(τ = 0)a,c <0.1
Extinction Ratioa >17 dB
Lifetime >2500 Hours of Pump Emission
Pump Laser Powerd 10 mW to 150 mW
User Servicable No
Electrical Specifications
Input Voltage 100 - 240 V
Frequency 50 - 60 Hz
Power Consumption 25 W (Max)
Interface RS232 Serial
Environmental Requirements
Room Temperature 18 °C to 25 °C
Storage Temperature -10 °C to 60 °C
Humidity Non-Condensing
Physical Dimensions
Dimensions (L x W x H) 10.13" x 6.41" x 2.24"
(257.2 mm x 162.7 mm x 56.9 mm)
Weight 2.6 kg
  • For a Pump Laser Power of 35 mW
  • ηsi is the heralding ratio and can be determined using C/sqrt(PsPi ), where C respresents the coincidence counts and Ps and Pi are the raw counts on the signal and idler channels, respectively.
  • Second-order correlation measurement at zero time delay. See the Single-Photon Output tab for more details.
  • The SPDC810 source can be used at lower pump powers, but specifications will not be met.
SPDC810 Electrical Connections
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Correlated Photon-Pair Source Electrical Connections
Correlated Photon-Pair Source Side Panel
Callout Description
1 Signal Channel Output, 2.0 mm Narrow Key FC/PC Fiber Connector 
2 Idler Channel Output, 2.0 mm Narrow Key FC/PC Fiber Connector
3 Serial RS232 Connector for Pump Laser
4 M8 Power Connector, 12 VDC Supply
Experimental Setup for Single-Photon Generation
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Figure 1: Experimental setup for testing single-photon generation. APD-h is the avalanche photodiodes for the heralded photons, while APD-1 and APD-2 are the ones for the signal photons. TCSPC is the time-correlated single-photon counter.

Verification of Single-Photon Output

One of the most important characteristics of any single-photon source is the degree to which its output consists of only single photons. It is not enough to be able to detect a signal using single photon detectors, which can be easily achieved by attenuating a classical light source. Also, the output of a true single photon source may be contaminated with additional light due to leakage or multiphoton events. Therefore, while a coincidence peak can confirm the presence of single photons, it provides little information about the present noise. Please note that throughout this discussion, singles refers to a single detection event on one channel, ideally half of the photon pair.

Thorlabs' SPDC810 Photon-Pair Source is based on spontaneous parametric down-conversion (SPDC) and, thus, generates a pair of photons at any moment in time. An experimental setup to verify its single­-photon operation is shown in Figure 1. One of the outputs is connected directly to a single-photon detector, which in this case is a single-photon avalanche photodiode (APD). This channel is often referred to as a heralding or trigger channel, as it confirms the existence of a photon in the other arm. The signal channel is split on a 50:50 beamsplitter in a Hanbury-Brown-Twiss configuration and is connected to detectors 1 and 2. All 3 detectors are then connected to a coincidence counter, which is a time-correlated single-photon counter (TCSPC). If the output of the source truly consists only of photon pairs, there will only be two-fold coincidences between the heralding detector and detector 1 or 2, which are Ch,1 and Ch,2 respectively. This is demonstrated in Figure 2. Three-fold coincidences between all three detectors Ch,1,2 should not occur, as there are only two photons present.

SPDC810 Histogram
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Figure 2: Coincidence Histogram for Ch,1 and Ch,2, which are the coincidences between APD-h and APD-1 or -2, respectively. The peak at 50 ns confirms pair emission; a heralding photon reaches APD-h and 50 ns later (an added time delay) a signal photon reaches APD-1 or APD-2. Data acquired using Thorlabs' SPDC810 Photon-Pair Source.

Example data obtained using the SPDC810 photon-pair source is presented in Table 1. As expected, the singles are split according to the beamsplitter reflectance. In this case, the additional loss was due to fiber­-to-fiber coupling. The same is true for coincidences Ch,1 and Ch,2, which are similarly distributed between the two detectors. However, three-fold coincidences Ch,1,2 are very close to 0. The results confirm the true single­-photon output of the source. In addition, the experiment also confirms the particle nature of light, i.e., a photon cannot be split.

Table 1
Average Coincidences per Second Singles per Second
Ch,1 15229 Sh 130796
Ch,2 17435 S1 45376
Ch,1,2 8 S2 55128

The measurement described above is often referred to as a heralded second-order intensity correlation gh(2)(τ), where τ is the time difference between the arrival times t1 and t2. At τ = 0, which is our point of interest, it can be quantified using the following formula:

For an ideal photon pair source, the conditional probability of detecting photons at both detectors 1 and 2 at the same time (τ=0), given that a photon is detected at the heralding detector, is 0. Based on the data shown in Table 1 and using equation 1, we obtain gh(2)(0) = 0.004, which is very close to ideal performance. In addition, the second order intensity correlation has a more fundamental importance. It is used to prove the non-classical nature of light, as the value of g(2)(0) depends on the type of light being investigated:

Depending on the experimental configuration, photon pair sources can exhibit both bunched and antibunched light statistics. Thermal light is a typical example of bunched light, where the probability of photons being detected across the outputs of a beamsplitter increases for τ ≈ 0, peaking at τ = 0. In contrast, an ideal single photon source exhibits antibunching, as discussed earlier. However, if a g(2)(τ) measurement is performed only on one of the channels, with the other one ignored, then such a source will produce thermal statistics.

SPDC810 Correlated Photon-Pair Source Electrical Connections

RS232 Female Connector (On Housing)

RS-232 Connector
The RS232 connector provides connection to the pump laser.
Pin Description Pin
1 Data Carrier Detect (DCD) 6 Data Set Ready (DSR)
2 Receive Data (RXD) 7 Request to Send (RTS)
3 Transmit Data (TXD) 8 Clear to Send (CTS)
4 Data Terminal Ready (DTR) 9 Ring Indicator (RI)
5 Signal Ground (GND) - -

M8 Male Connector (On Housing)

M8 Connector
For Connection to DS12 12 VDC Power Supply
Pin Description
1 Not Connected
2 Not Connected
3 +12 V
4 Ground

Posted Comments:
Christopher Ebbers  (posted 2021-01-05 12:22:55.733)
1. Is there a manual available online for the SPDC810? 2. Are there any plans to create on of your famous teaching kits which would include the SPDC810, 2 silicon Avalanche photodiodes, beamsplitter, polarizers, & waveplates & manual with 5 or 6 experiments? Thanks in advance
YLohia  (posted 2021-01-06 10:27:56.0)
Thank you for contacting Thorlabs. The manual can be accessed by clicking the red document icon next to the part number or by following this link ( We will contact you directly to discuss your request about the educational kit.
CHRISTIAN D'HEM  (posted 2020-12-07 13:00:46.047)
Could you provide a SPDC810 with a Maxpairs/second between 10-100 Mhz (preferably 100) Best regards
YLohia  (posted 2020-12-08 02:48:34.0)
Hello Christian, thank you for contacting Thorlabs. We have reached out to you directly to discuss the feasibility of offering this.
user  (posted 2020-09-05 19:00:59.663)
Is it possible to request a minor tuning of the center wavelength of the source. Specifically, could it be possible to have it at a slightly lower wavelength of 795 or 800nm?
YLohia  (posted 2020-09-08 11:11:32.0)
Thank you for contacting Thorlabs. Our engineers will reach out to you directly to discuss the possibility of offering this.

Correlated Photon-Pair Source

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SPDC810 Support Documentation
SPDC810810 nm Correlated Photon-Pair Source with Integrated Pump Laser
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