Operation¶

Cryostat¶

In order to use the detectors, the system must be at vacuum and cooled to its base temperature. The temperature of the system can be verified through the driver’s web interface. Once the system has reached the base temperature quoted on the characterization sheet, the superconducting behavior of the detectors can be confirmed (see Basic system performance check for a set of quick tests that might be performed). Details on how to operate the Retina and Atlas drivers can be found in the current section or in the standalone user manuals.

Cool-down¶

Prior to beginning a cool-down, read all Safety information and ensure all Installation steps have been followed. To cool down the system, please follow the below steps:

Turn on the driver and access the temperature tab.
Slowly open the vacuum valve at the rear of the cryostat until it is fully open.
Connect your vacuum pump, ensuring the o-ring is clean.
Turn on the vacuum pump.
Once the pressure reaches 1E-5 mbar, turn the compressor on. A pressure of 1E-5 mbar is recommended to ensure the system cools down to its specified base temperature and operates optimally for an extended period without requiring re-evacuation. However, if time is limited or the pump’s performance is constrained, cooling can be initiated at a pressure of 1E-4 mbar.
Leave the pump and compressor running for the cool-down time specified in your system’s characterization sheet or overnight, ensuring there is no power interruption during this period.
Check the temperature of the system is stable and within ± 0.1 K of the base temperature quoted on your system’s characterization sheet.
Check the base pressure of the cryostat. The pressure sensor should indicate values of the order of 5E-7 mbar. Significantly higher values may indicate an issue with vacuum integrity.
Fully close the vacuum valve, and then turn off the vacuum pump. In the closed position, the vacuum valve knob should feel finger-tight. If the valve is not closed properly, the vacuum integrity of the system may be compromised which is potentially dangerous for the health of all vacuum components. Always verify that the valve is fully closed before stopping the evacuation process and disconnecting the pump.
Stop the pump and wait until the pump fully stops before disconnecting the hose.
Seal the vacuum-valve port with a blank metal cap. The vacuum valve must be properly capped to minimize the risk of accidental opening and vacuum leaks.

The system should now be ready to use. In the event that the system does not reach its base temperature, please consult Troubleshooting.

NOTE Single Quantum systems are designed to maintain their base temperature continuously over several months of operation. Systems with an optimized configuration may remain cold for more than a year. However, unless otherwise agreed, we recommend regenerating the cryostat at least once a year or as soon as its temperature begins to rise. Regeneration consists of a complete warm-up, re-evacuation, and cooldown cycle.

Warm up¶

In order to warm up the system, simply turn off the compressor. The system should reach room temperature in 12 hours.

WARNING Do not disconnect the helium flex lines until the system has reached room temperature.

CAUTION Do not move the system until it has reached room temperature.

Operation of SNSPDs¶

The SNSPDs in your system operate in the superconducting regime, at a temperature below their critical temperature and a bias current below their switching current. Above the switching (critical) current, superconductivity can no longer be maintained. The absorption of a photon locally disrupts the superconducting state, as described in Principle of operation.

To use the SNSPDs, connect your optical fiber to the appropriate port on the front panel of the system and switch on your light source. Ensure that the photon flux from the source does not exceed the maximum count rate of the detectors.

Use the Home tab in the driver software to optimize the input polarization by identifying the setting that maximizes the count rate. Next, perform a bias sweep using the Sweep tab to determine the optimal bias current for your experiment. Set the selected bias current in the Home tab, where you can also monitor the count rate in real time.

Detailed instructions for operating the electronic drivers, including setting the bias current, performing sweeps, and using remote-control functions, are available in the dedicated sections: Atlas driver Operation and Retina driver: Product description.

Basic system performance check¶

Once the system has cooled down to its base temperature, to assess whether the system is operating normally, we recommend performing the following tests (refer to following section or the standalone manuals for the details on driver operation):

Check the temperature tab and compare the base temperature reached by the system with the value stated in its characterization sheet. The temperature should be within ± 0.1 K of the characterized value.
Verify the trigger levels set in the drivers software. They should have a values typically in the range of 100 - 200 mV.
Perform a current sweep with integration time of 1 second which includes the critical currents of all detectors as quoted on the system’s characterization sheet. After the sweep is completed, ensure that the critical currents of all detectors are within ± 0.5 µA of their characterized values.
Analyze the dark count behavior from the current sweep results. The dark count rate should increase smoothly with the bias current. There should be no sudden spikes in the dark count rate.
Perform a quick detector check with a light source (if possible). Connect a fiber carrying a controlled light signal to each detector of interest one by one. The observed count rate should correspond to the system’s detector efficiency values listed in the characterization Sheet. Ensure that the correct type of optical fiber is used for the test. If non-polarization maintaining fibers are used, optimize polarization for each channel before recording the final count rate.

Optimization of SNSPD performance¶

The current at which the SNSPD is operated is called the bias current. It is through careful selection of this bias current that the operation of the SNSPDs can be tuned to your purposes. The main dependencies of system parameters on the chosen bias current are as follows:

The higher the bias current, the higher the output pulse amplitude.
The higher the bias current, the lower the timing jitter.
The higher the bias current, the higher the DCR. Check the system characterization sheet to see the DCR dependence on bias current for each detector.
The higher the bias current, the higher the efficiency. Check the system characterization sheet to see the SDE dependence on bias current for each detector.
The higher the bias current, the more prone the detector is to latching. See Section 5.7 for troubleshooting tips.

NOTE Latching is a state in which an SNSPD fails to return to its superconducting regime (normal operation). This typically occurs due to a combination of high input optical power and high bias current or as a result of noise pickup.

The higher the input photon flux, the lower the critical current and therefore the bias current must be reduced.
The higher the input photon flux, the higher the jitter.

These relationships are described in more detail during system user training. The SDE of the detectors is also dependent on the polarization of the incident light. A misaligned polarization state can cause a drop in efficiency of >50%. It is therefore important to include a polarization controller in line with the SNSPDs, and as close to the system as is viable.

It is important to ensure that the fibers used to connect to the system are of the same type as is used for detector channel inside the cryostat. The type of fiber used in each channel can be found on the system’s characterization sheet. Using the incorrect fiber type will lower the coupling efficiency and may damage the internal fibers.

It is not possible to damage the system or detectors by coupling an input optical power of 100 µW or less to the ports. However, the detectors will be unable to function at such high irradiance, and it is recommended to limit the optical power to a range in which constant latching behavior is not seen. If you are experiencing latching at low input light intensities, please refer to Troubleshooting. As the switching current of the detectors is dependent on the input photon flux, it is important to find a non-latching bias current by running a bias current sweep every time the input intensity is significantly changed.

Fiber and port cleaning¶

Each time a fiber is connected to the system, the fiber and the port must be inspected and cleaned. Gloves should be worn when handling optical fibers. 2Latching is a state in which an SNSPD fails to return to its superconducting regime (normal operation). This typically occurs due to a combination of high input optical power and high bias current or as a result of noise pickup.

WARNING Ensure that any laser sources are turned offbefore removing or inspecting fibers.

CAUTION If the connections are not adequately cleaned, scratches can occur on the fiber end faces which can limit the coupling efficiency and therefore detection efficiency. The following tools are recommended:

(Cletop) dry fiber cleaning sticks.
(Cletop-S) optical fiber cleaner.
(Cletop-S) optical cleaning cartridge.
(Sticklers) connector cleaning sticks.
(Sticklers) clean wipes.
(Thorlabs) lens cleaning tissues.
(EasyGet WifiWireless Fiber Endface Microscope) fiber and port inspection microscope. To clean the ports:
Inspect the fiber port with the fiber port inspection probe. If there is any visible dirt, continue with the cleaning process.
Wet the connector cleaning sticks with the fiber cleaning liquid .
Insert the stick into the fiber port and rotate 10 times anticlockwise.
Insert a dry stick into the fiber port and rotate 10 times anticlockwise.
Clean the optical fiber with lens cleaning tissues, clean wipes or optical fiber cleaner.

Characterization of SNSPDs¶

Single Quantum has characterized each of your detectors at a specific bias current, wavelength and photon flux. The experimental procedure for these characterization measurements is defined in the appendix of the characterization sheet.

Disassembly, storage and transport¶

Before beginning to disassemble your system, please ensure that you have all necessary packing materials. If you are lacking any packing materials, please contact our Logistics department. If you would like any additional guidance, please contact the Service department. Contact details for all departments can be found in Section 1.6.

Disassembly¶

Disconnect any optical connections and firmly screw on all fiber port caps.
Turn on driver and access temperature tab.
Close the vacuum valve, and turn offthe pump if it is connected and running.
Turn off the compressor.
Do not proceed until driver reads room temperature (~300 K).
Disconnect the driver (power, ethernet, DC cable, SMA cables).
Disconnect the pump, if connected. Apply plastic caps to vacuum port and tube.
Disconnect the power cable from the cold head.
Using two wrenches as explained in Section 3.2, remove first the supply and then the return helium lines.
Apply caps to the cryostat helium ports and plugs to the helium lines.
Using two wrenches, disconnect first the supply and then the return line from the compressor.
Cap all fiber, helium and vacuum ports.

WARNING Do not disconnect Helium lines from the system unless it is confirmed to be at room temperature.

Storage¶

The system components must be stored within their environmental tolerances. The cryostat and compressor must be stored upright. The detectors must be stored under vacuum, so do not open the valve at the rear of the cryostat before storage.

Transportation¶

If it is necessary to transport the cryostat, use the original packing material (boxes and protective foam). Ensure that a shock and tilt watch are applied to both the compressor and cryostat packaging. If a watch has been activated, it must be replaced before using the boxes to ship the system on future occasions. If a watch is activated during transportation, please contact the Single Quantum service team using the details in Contact. Once the system reaches its final destination, it can be unpacked and installed following the steps in Installation. Single Quantum’s service team can remotely help you to re-install the system. Please contact the team using contact information, with at least 2 weeks’ notice of the need for remote support.