Cryogenic Amplifiers for a Superconducting Nanowire Single Photon Detector System

A superconducting nanowire single photon detector system designed for $2.3\mu \mathrm{m}$ wavelength deployed into a single photon light detection and ranging setup. This wavelength takes advantage of lower solar flux and less atmospheric absorption. © 2019 The Author(s)

We developed a superconducting nanowire single-photon detection (SNSPD) system based on Gifford-McMahon cryocooler for quantum communication applications. Environmental factors which may influence the system performance are intensively studied. Those factors include temperature fluctuations, the ambient magnetic field and the background radiation. By optimizing the bias circuit, the stability of SNSPD system to electrical noise and disturbance was effectively enhanced, thus making it more suitable for field application. A 4-channel SNSPD system with quantum efficiency higher than 4% at the dark count rate of 10 Hz for λ=1550 nm is integrated and applied into a quantum key distribution (QKD) experiment. QKD was successfully carried out over 100 km optical fiber with the final secure key rate of 1.6 kbps and the quantum bit error rate of less than 2%.

A superconducting nanowire single photon detector system designed for 2.3µm wavelength deployed into a single photon light detection and ranging setup. This wavelength takes advantage of lower solar flux and less atmospheric absorption.

Space-to-ground photon-counting optical communication links supporting high data rates over large distances require enhanced ground receiver sensitivity in order to reduce the mass and power burden on the spacecraft transmitter. Superconducting nanowire single-photon detectors (SNSPDs) have been demonstrated to offer superior performance in detection efficiency, timing resolution, and count rates over semiconductor photodetectors, and are a suitable technology for high photon efficiency links. Recently photon detectors based on superconducting nanowires have become commercially available, and we have assessed the characteristics and performance of one such commercial system as a candidate for potential utilization in ground receiver designs. The SNSPD system features independent channels which can be added modularly, and we analyze the scalability of the system to support different data rates, as well as consider coupling concepts and issues as the number of channels increases.

Superconducting nanowire single photon detector (SNSPD) plays a significant role in plenty of fields such as quantum information, deep space laser communication and lidar, while the mechanism of the photon response process still lacks a recognized theory. It is prerequisite and essential for fabricating high-performance SNSPD to understand in depth and clarify the photon response mechanism of the SNSPD. As mature theories on the SNSPD response progress, hot-spot model and vortex-based model both have their disadvantages: in the former there exists the cut-off wavelength and in the later there is the size effect, so they both need further improving. The Cut-off wavelength means that the detection efficiency of the SNSPD drops to zero with the increase of light wavelength, which is indicated by the hot-spot model but not yet observed in experiment. The size effect implies that the vortex does not exist in the weak link with the width less than 4.41<i>ξ</i>, where <i>ξ</i> is the GL coherence length. Phase slip is responsible for the intrinsic dissipation of superconductors, which promises to expound the SNSPD photon response progress and to establish a complete theory. This paper reviews and discusses the fundamental conception, the development history and the research progress of the hot-spot models, i.e. the vortex-based model and the superconductor phase slips, providing a reference for studying the SNSPD photon response mechanism.

<sec>Facing the demand for applications such as wide-area terrain mapping and space-based atmospheric measurements, there is an urgent need to develop miniaturized single-photon detection systems with low power consumption that can be adapted to airborne platforms. Superconducting nanowire single-photon detectors (SNSPDs) have been applied to quantum information, bioimaging, deep space communication and long-range lidar with the advantages of high quantum efficiency, low dark count rate and fast detection rate. However, traditional SNSPD usually operates at 2.1 K or even lower, and the required cryogenic systems are large in size and weight, which are not easy to apply to airborne platforms. Up to now, there has been no report on SNSPD applied to airborne platforms. How to apply SNSPD to airborne platforms is an urgent problem to be solved.</sec><sec>In this work, we design and make an SNSPD with an operating temperature of 4.2 K. The superconducting detector chip is a four-channel photon-number-resolving device with a photosensitive area of 60 μm × 60 μm, which is coupled to a 200-μm-diameter fiber by a beam compression system with a quantum efficiency of 50% at 1064 nm and a temperature of 4.2 K. Finally, the time characteristics of a single channel are tested in response to different photon numbers. The timing jitter of four-photon response is smallest, and the half-height width is 110 ps. This work not only supports airborne applications, but also has positive implications for promoting the development of general-purpose miniaturized SNSPD systems and their applications.</sec>

Superconducting nanowire single photon detectors (SNSPD) offer excellent performance for infrared single photon detection, combining high efficiency, low timing jitter, low dark count rates and high photon counting rates. Promising application areas for SNSPDs include quantum key distribution, space-to-ground communications and single photon remote sensing [1]. SNSPDs are typically made with ultrathin niobium nitride (NbN) films with thickness 4 nm and a superconducting transition temperature above 9 K. NbN offers high performance in the near infrared but their sensitivity drops at wavelengths beyond 2 um. There is growing interest in potential photon counting applications in the mid infrared domain (for example remote sensing of greenhouse gases in the atmosphere [2]). One way to overcome the wavelength limit in NbN SNSPDs is to use films with a lower superconducting energy gap [3]. Here we report on the study of SNSPDs fabricated with thin films of titanium nitride (TiN). We compare TiN films deposited by atomic layer deposition (ALD) and by magnetron sputtering. The TiN films range in thickness from 5 to 60 nm, with superconducting transition temperatures from ~1 K to 3.5 K. We have analyzed the films via transmission electron microscopy and variable angle spectroscopic ellipsometry. We characterize TiN SNSPDs performance from near to mid-infrared at wavelengths (1-4 um) with fast optical parametric oscillator (OPO) source. We compare the performance of TiN SNSPDs to devices based on other lower gap materials: MoSi, NbTiN, WSi. [1] Natarajan et al Superconductor Science and Technology 25 063001 (2012) [2] Abshire et al Laser Applications to Chemical, Security and Environmental Analysis, (Optical Society of America, 2008) paper LMA4 [3] Verma et al Applied Physics Letters 105 022602 (2014)

.SignificanceInterstitial fiber-based spectroscopy is gaining interest for real-time in vivo optical biopsies, endoscopic interventions, and local monitoring of therapy. Different from other photonics approaches, time-domain diffuse optical spectroscopy (TD-DOS) can probe the tissue at a few cm distance from the fiber tip and disentangle absorption from the scattering properties. Nevertheless, the signal detected at a short distance from the source is strongly dominated by the photons arriving early at the detector, thus hampering the possibility of resolving late photons, which are rich in information about depth and absorption.AimTo fully benefit from the null-distance approach, a detector with an extremely high dynamic range is required to effectively collect the late photons; the goal of our paper is to test its feasibility to perform TD-DOS measurements at null source–detector separations (NSDS).ApproachIn particular, we demonstrate the use of a superconducting nanowire single photon detector (SNSPD) to perform TD-DOS at almost NSDS by exploiting the high dynamic range and temporal resolution of the SNSPD to extract late arriving, deep-traveling photons from the burst of early photons.ResultsThis approach was demonstrated both on Monte Carlo simulations and on phantom measurements, achieving an accuracy in the retrieval of the water spectrum of better than 15%, spanning almost two decades of absorption change in the 700- to 1100-nm range. Additionally, we show that, for interstitial measurements at null source–detector distance, the scattering coefficient has a negligible effect on late photons, easing the retrieval of the absorption coefficient.ConclusionsUtilizing the SNSPD, broadband TD-DOS measurements were performed to successfully retrieve the absorption spectra of the liquid phantoms. Although the SNSPD has certain drawbacks for use in a clinical system, it is an emerging field with research progressing rapidly, and this makes the SNSPD a viable option and a good solution for future research in needle guided time-domain interstitial fiber spectroscopy.

Superconducting nanowire single photon detector (SNSPD) has been widely used in many fields such as quantum computing, quantum key distribution and laser radar, due to its high detection efficiency, low dark count rate, high counting rate, and low timing jitter. In most cases, the SNSPD works under the DC-bias mode that can detect single photons arrived at any time. In some cases such as satellite laser ranging and single-photon laser radar where the light pulses arrive regularly, the AC-bias mode enables the SNSPD to work with higher counting rates and lower background dark counts, which however requires complicated readout due to the low signal-to-noise ratio of the photon response. In this work, we report on an AC-biased SNSPD system with a self-differential readout circuit. The system includes a 2-pixel SNSPD consisting of two parallel nanowires, which are biased with 100 MHz sinusoidal current. The output signals of these two nanowires are amplified and combined for the differential readout of the photon response. The resulting response pulse possesses a signal-to-noise ratio ten times higher than that extracted before self-differential readout. In addition, the dark counts are reduced by a factor of 4, and the count rates are increased by a factor of 1.5, in comparison with those under the DC-bias mode. This work provides a specific method to read out the AC-biased SNSPD.

Single photon response of superconducting nanowire single photon detector

Superconducting nanowire single photon detector (SNSPD) has been widely used in many fields such as quantum communication due to its extremely high detection efficiency, low dark count rate, high count rate, and low timing jitter. Compared with conventional single-photon detectors with planar structure, SNSPD is typically made a periodical meandering structure consisting of parallel straight nanowires. However, owing to its unique linear structure, the detection efficiency of SNSPD is dependent on the polarization state of incident light, thus limiting SNSPD’s applications in unconventional fiber links or other incoherent light detection. In this paper, a polarization-insensitive SNSPD with high detection efficiency is proposed based on the traditional meandering nanowire structure. A thin silicon film with a high refractive index is introduced as a cladding layer of nanowires to reduce the dielectric mismatch between the nanowire and its surroundings, thereby improving the optical absorption efficiency of nanowires to the transverse-magnetic (TM) polarized incident light. The cladding layer is designed as a sinusoidal-shaped grating structure to minimize the difference in optical absorption efficiency between the transverse electric (TE) polarized incident light and the TM polarized incident light in a wide wavelength range. In addition, the twin-layer nanowire structure and the dielectric mirror are used to improve the optical absorption efficiency of the device. Our simulation results show that with the optimal parameters, the optical absorption efficiency of nanowires to both of the TE polarized incident light and TM polarized incident light has a maximum of over 90% at 1550 nm, and the corresponding polarization extinction ratio is less than 1.22. The fabricated device possesses a maximum detection efficiency of 87% at 1605 nm and a polarization extinction ratio of 1.06. The measured detection efficiency exceeds 50% with a polarization extinction ratio less than 1.2 in a wavelength range from 1505 nm to 1630 nm. This work provides a reference for high-efficiency polarization-insensitive SNSPD in the future.

Superconducting nanowire single photon detectors have recently been demonstrated as viable candidates for photon-counting optical receivers operating at data rates in excess of 100 Mbit/s. In this paper, we discuss techniques for extending these data rates to rates > 1 Gbit/s. We report on a recent demonstration of a 2-element nanowire detector array operating at a source data rate of 1.25 Gbit/s. We also describe techniques for emulating larger arrays of detectors using a single detector. We use these techniques to demonstrate photon-counting receiver operation at data rates from 780-Mbit/s to 2.5 Gbit/s with sensitivities ranging from 1.1 to 7.1 incident photons per bit.

Superconducting nanowire single photon detectors (SNSPDs) promise to combine near-unity quantum efficiency with >100 megacounts per second rates, picosecond timing jitter, and sensitivity ranging from x-ray to mid-infrared wavelengths. However, this promise is not yet fulfilled, as superior performance in all metrics is yet to be combined into one device. The highest single-pixel detection efficiency and the widest bias windows for saturated quantum efficiency have been achieved in SNSPDs based on amorphous materials, while the lowest timing jitter and highest counting rates were demonstrated in devices made from polycrystalline materials. Broadly speaking, the amorphous superconductors that have been used to make SNSPDs have higher resistivities and lower critical temperature (Tc) values than typical polycrystalline materials. Here, we demonstrate a method of preparing niobium nitride (NbN) that has lower-than-typical superconducting transition temperature and higher-than-typical resistivity. As we will show, NbN deposited onto unheated SiO2 has a low Tc and high resistivity but is too rough for fabricating unconstricted nanowires, and Tc is too low to yield SNSPDs that can operate well at liquid helium temperatures. By adding a 50 W RF bias to the substrate holder during sputtering, the Tc of the unheated NbN films was increased by up to 73%, and the roughness was substantially reduced. After optimizing the deposition for nitrogen flow rates, we obtained 5 nm thick NbN films with a Tc of 7.8 K and a resistivity of 253 μΩ cm. We used this bias sputtered room temperature NbN to fabricate SNSPDs. Measurements were performed at 2.5 K using 1550 nm light. Photon count rates appeared to saturate at bias currents approaching the critical current, indicating that the device's quantum efficiency was approaching unity. We measured a single-ended timing jitter of 38 ps. The optical coupling to these devices was not optimized; however, integration with front-side optical structures to improve absorption should be straightforward. This material preparation was further used to fabricate nanocryotrons and a large-area imager device, reported elsewhere. The simplicity of the preparation and promising device performance should enable future high-performance devices.

We report on initial fabrication efforts in the integration of superconducting nanowire single-photon detectors (SNSPDs) with vertically aligned carbon nanotubes (VACNTs) with the goal of creating a wideband single-photon detector. SNSPDs provide high detection efficiencies and low dark count rates, while the VACNTs are excellent broadband optical absorbers. Combining these technologies could potentially enable the development of highly sensitive and versatile optical sensors for a variety of applications, such as spectroscopy, optical communication, and imaging in light starved environments. We developed two fabrication processes for the integration of VACNTs on SNSPDs. The first involves capping the SNSPDs with a protective layer and growing the VACNTs directly above nanowires. Thermal and electrical characterizations of the devices demonstrated a degradation of the superconducting qualities of the SNSPDs. The second process involved suspending the SNSPDs on a thin membrane via a backside etch, where VACNTs were then grown on the backside of the membranes below the nanowires. The membrane style devices showed no degradation in the superconducting properties of the nanowires. Measurements of the membrane style devices before and after the VACNT growth display similar superconducting properties and photon count rates.

Single photon detector (SPD) is one of the key components in quantum communication (QC). To compare the capabilities of SPDs, this paper discussed the current technology of different SPDs and took some experiments. The results show avalanche photodiode (APD) can’t satisfy the development of QC. The count rate of Superconducting nanowire single photon detector (SNSPD) is much higher than APD and the other SPDs, which proves that SNSPD is feasible for high speed QC. And the lower dark count rate of SNSPD makes the lower error rate and the safer communication. Though there are some technical challenges in applications, SNSPD will have broad prospects in QC.