SNSPD background

Introduction to SNSPDs

Superconducting Nanowire Single Photon Detectors (SNSPDs) are high performance single-photon detectors that operate at cryogenic temperatures. They stand out as the most capable single photon detection technology for near-infrared and increasingly mid-infrared wavelengths. They may be customized to exhibit extraordinary performance along a number of metrics including count rate ¹², detection efficiency ¹³¹⁴, dark count rate ¹⁵¹⁰, and timing resolution ¹⁶. Some designs achieve excellent performance across all such metrics simultaneously, which makes them uniquely qualified for certain quantum communication and optics applications ¹⁷. This section elaborates on the fundamental principles underlying SNSPD operation, and reviews some recent SNSPD advancements and their relevance to certain applications.

Operating principles

An SNSPD consists of a thin superconducting nanowire patterned on a substrate biased by a DC current. The nanowire is typically made of a superconducting material such as niobium nitride (NbN), niobium titanium nitride (NbTiN), or tungsten silicide (WSi). The nanowire is connected to a readout circuit which is used to detect the electrical signals generated by the SNSPD upon absorption of a single photon.

When a photon hits the wire, it breaks a superconducting electron pair, or Cooper pair. This disruption creates quasiparticles and initiates a down-conversion cascade. This localized disruption of superconductivity in turn leads to the formation of region of reduced superconductivity called a hot-spot. The hot-spot has a higher resistance than the superconducting state, which due the initial flow of current, induces a voltage pulse from this region. This pulse is shunted out of the nanowire and into a readout circuit, which amplifies the pulse, and may include electronics for digitizing the pulse arrival time. Following the formation of the hotspot, the nanowire cools down through electron-phonon interaction with the substrate and returns back to the superconducting state, making it ready for the next photon detection.

The readout scheme typically adopted for the detection of these electrical signals involves separating the RF and DC components of the RF line from the SNSPD using a bias-T circuit. The RF component is afterwards amplified using one or more cryogenic or room temperature amplifiers. This is then followed by a free-running or TCSPC based time tagger which digitizes pulse arrival time.

Recent advancements

Recent SNSPDs advancements revolve around improving the detection efficiency, count rate, timing jitter, sensitive wavelength range, and scalability to large arrays. Ultra-fast measurements of short superconducting wires have demonstrated timing jitters as low as 3 ps and illuminated the relationship between timing jitter and intrinsic detection latency effects ¹⁶. System detection efficiencies from 98% to 99.5% have been demonstrated as well, and highlight the necessity to enhance the quality of the optical cavity surrounding the detector by using ultra-high reflectivity mirrors or distributed Bragg reflectors ¹⁴¹⁸. More recently, single photon sensitivity has been demonstrated out to 29 \(\upmu \mathrm{m}\) in thin silicon-rich WSi nanowires, paving the way for unique new SNSPD applications in astronomy and dark matter detection ¹⁹. Finally, with the development of a novel thermally coupled readout architecture, a 400,000 pixel SNSPD array has been demonstrated — more than doubling the number of pixels from any previous superconducting detector array ²⁰.

Going forward, there is a plethora of opportunities to improve the practicality of fabricating and using SNSPDs. There is a growing interest in wide superconducting strips with performance that rivals much narrower nanowire designs ²¹ thereby accommodating scalable photolithography processes. Finally, ongoing efforts are being made to increase the critical current and critical temperatures in advanced SNSPD superconducting materials.

Applications

Superconducting nanowire single-photon detectors (SNSPDs) have been employed in various applications, highlighting their versatility and potential in the field of photonics. These applications span different areas, ranging from astrophysics to biomedical imaging. This section provides a brief overview of some recent applications of SNSPDs.

Dark Matter Searches: SNSPDs have been utilized in experiments aimed at detecting dark matter particles. For example, one study has investigated using SNSPDs for direct detection of sub-GeV dark matter ²², while others have proposed using SNSPDs as photosensors in cryogenic scintillator-based experiments ²³. Another notable approach involves the search for dark photon dark matter using multilayer dielectric haloscopes ²⁴²⁵. SNSPDs are employed to achieve efficient photon detection. By improving the limits on the kinetic mixing parameter for dark photon dark matter, these experiments provide valuable insights into the nature of dark matter in the eV mass range.

Cerebral Blood Flow Measurement: SNSPDs were used for diffuse correlation spectroscopy (DCS) measurements for the observation of cerebral blood flow ²⁶. The high sensitivity of SNSPDs improved the signal-to-noise ratio and enabled the detection of low-intensity signals associated with pulsatile flow. This advancement in DCS technology has the potential to improve the understanding and diagnosis of cerebral blood flow-related conditions.

Deep Space Optical Communication: SNSPDs played a crucial role in the development of ground laser receiver (GLR) systems for deep space optical communication ²⁷. The GLR, equipped with an array of SNSPDs, received optical downlink signals from spaceborne transceivers. These detectors facilitated the processing of discrete downlink data rates over large distances, making significant contributions to the success of the Deep Space Optical Communication (DSOC) project.

Photon Counting LIDAR: SNSPDs also found application in LIDAR systems operating at 2.3 \(\upmu \mathrm{m}\) wavelength ²⁸. These detectors demonstrated enhanced photon counting performance in the mid-infrared region, making them suitable for free-space photon counting applications. This development opens up opportunities for LIDAR systems to operate in environments with low atmospheric absorption and background solar flux. In the more accessible wavelength range near 1.5 \(\upmu \mathrm{m}\), LIDAR with SNSPDs has also been demonstrated with km range ²⁹ and mm scale resolution ³⁰.

In addition to these specific applications, SNSPDs have been extensively used in quantum key distribution and quantum communication experiments. Notably, they have been employed in demonstrating high-quality quantum teleportation ¹¹ and will continue to be integral components in the development of advanced quantum networks ⁹.

This brief overview highlights the broad range of applications that have used SNSPDs, showcasing their potential to advance various fields of science and technology. With ongoing advancements in SNSPD technology and further optimization, these detectors will continue to play a vital role in enabling groundbreaking research and innovation.

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