What is it about?

Detecting single photons – the fundamental units of light – is challenging because the energy of a single photon is very small. The detectors we use convert the small energies of a photon and generate a signal. To do so, we pass a current through a thin wire made of a superconducting material. At a temperature close to absolute zero, the current passes through the wire without a resistance, which is called superconduction. If a photon is absorbed, it heats up the wire with its energy. The superconduction of the wire will then break down, and as a result a resistance is measured. This mechanism can detect single photons with very high efficiencies: over 90% of the photons absorbed will generate a signal. To achieve superconducting temperatures in the wire, the detector must be cooled to near absolute zero. These temperatures are typically reached by specialised refrigerator, called a cryostat. Unfortunately, the electronic connection from the coldest part of the cryostat to the rest the experiment at room temperatures conducts a considerable amount of heat. This heat conduction of the electronic connection limits the maximal number of single photon detectors which can be operated in one cryostat. Our method shows an alternative for the operation of superconducting single photon detectors in a cryostat. To deliver the supply power to the superconducting detector, we replace the electrical input by an optical fibre and photodiode. Light is guided by an optical fibre from the outside to the photodiode, which converts the light into electrical power and provides the current for the superconducting detector. This has the added benefit of being less prone to noise and interference.

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Why is it important?

We show with our method that we can provide the electrical power inside a cryostat for superconducting single photon detectors via an optical connection. In this configuration, the photodiode acts as a current source at cryogenic temperatures. The power for the photodiode is then guided through a single mode fibre. In contrast, a coaxial cable has a larger thermal conduction when providing the input power for the detector in the cryostat. In addition, the single mode fibres are poor electrical conductors such that no electrical noise can be picked up or transmitted through the connection.

Perspectives

The photodiode used in this method converts optical power to an electrical current output at the low temperature region of a cryostat. In my opinion this integration of a cryogenic current source opens a variety of technical possibilities for low temperature electronics. On the one hand, this enables a better noise performance due the electrical isolation and low heat conduction, which enhances the operation of additional circuits at cryogenic temperatures. On the other hand, superconducting detector can be independently driven while additional amplifiers, logic circuits and other cryogenic circuits can be added. In addition to the thermal and noise enhancements, a great strength of the photodiode can be exploited by pulsing the optical input. The photodiode will convert the pulsed light to a pulsed current. Given that a photodiode has a large enough bandwidth, the pulsed currents can significantly modulate the detection efficiency and turn the detector on and off. This pulsing could be used to selectively switch the detector only on when light is expected to arrive on the detector. Experiments with a strong excitation pulse and delayed signal pulses can benefit from this method because the strong excitation pulse can blind the superconducting detector. In my opinion, new capabilities in light detection can be therefore unfolded. This current pulsing of the superconducting detectors is challenging with coaxial cables and room temperature current sources because the pulses must be transmitted through the connection without additional noise pickup and interference.

Frederik Thiele
Universitat Paderborn

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This page is a summary of: Opto-electronic bias of a superconducting nanowire single photon detector using a cryogenic photodiode, APL Photonics, August 2022, American Institute of Physics, DOI: 10.1063/5.0097506.
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