Features

• 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)

Click to Enlarge
The SPDC conversion process obeys energy and momentum conservation.

Click to Enlarge
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.

Click to Enlarge
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 Applications@thorlabs.com 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.

Click to Enlarge
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 C_h,1 and C_h,2 respectively. This is demonstrated in Figure 2. Three-fold coincidences between all three detectors C_h,1,2 should not occur, as there are only two photons present.

Click to Enlarge
Figure 2: Coincidence Histogram for C_h,1 and C_h,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 C_h,1 and C_h,2, which are similarly distributed between the two detectors. However, three-fold coincidences C_h,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.

The measurement described above is often referred to as a heralded second-order intensity correlation g_h⁽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 g_h⁽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)

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)

For Connection to DS12 12 VDC Power Supply

Pin	Description
1	Not Connected
2	Not Connected
3	+12 V
4	Ground