Superconducting nanowire single‐photon detectors for quantum information

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)

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.

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.

<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>

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.

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.

A large-format mid-infrared single-photon imager with very low dark count rates would enable a broad range of applications in fields like astronomy and chemistry. Superconducting nanowire single-photon detectors (SNSPDs) are a mature photon-counting technology as demonstrated by their figures of merit such as high detection efficiencies and very low dark count rates. However, scaling SNSPDs to large array sizes for mid-infrared applications requires sophisticated readout architectures in addition to superconducting materials development. In this work, an SNSPD array design that combines a thermally coupled row-column multiplexing architecture with a thermally coupled time-of-flight transmission line was developed for mid-infrared applications. The design requires only six cables and can be scaled to larger array sizes. The demonstration of a 64-pixel array shows promising results for wavelengths between 3.4 μm and 10 μm, which will enable the use of this single-photon detector technology for a broad range of new applications.

Superconducting single-photon detectors (SSPDs) based on NbN nanowires possess properties such as high counting rate, low dark count rate and small timing jitter, which are promising characteristics for a quantum key distribution system. An SSPD array has recently attracted attention, because it can provide a higher counting rate, and spatial and photon-number resolutions. We propose readout electronics using single-flux-quantum (SFQ) circuits to process a large number of output signals from an SSPD array. We designed and tested an interface circuit between an SSPD and SFQ circuit, and successfully observed SFQ pulse generation for an input pulse with a current amplitude of 17.4 ~ 36.4 muA , which is comparable with an output current amplitude of the SSPD. The interface circuit was also operational for an input pulse duration of 1.6 ns, which is shorter than the typical recovery time of the SSPD.

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.

Today Superconducting Nanowire Single-Photon Detectors (SNSPDs) are commonly used in different photon-starved applications, including testing and diagnostics of VLSI circuits. Detecting very faint signals in the near-infrared wavelength range requires not only good detection efficiency, but also very low Dark Count Rate (DCR) and jitter. For example, low noise is crucial to enable ultra-low voltage optical testing of integrated circuits. The effect of detector temperature and background thermal radiation on the noise of superconducting single-photon detectors made of NbN meanders is studied in this paper. It is shown that two different regimes can be identified in the DCR vs. bias current characteristics. At high bias, the dark count rate is dominated by the intrinsic noise of the detector, while at low bias current it is dominated by the detection of stray photons that get onto the SNSPD. Changing the detector temperature changes its switching current and only affects the high bias branch of the characteristics: a reduction of the DCR can be achieved by lowering the SNSPD base temperature. On the other hand, changing the temperature of the single-photon light source (e.g. the VLSI circuit under test) only affects the low bias regime: a lower target temperature leads to a smaller DCR.

Low-temperature optical photon detectors for quantum information applications

Single photon technology is enabling for quantum communication and processing. Combining single photon sources, routing circuitry and highly efficient detectors in a high yield, low-loss and scalable fashion represent today a major challenge. Nanophotonic devices allow for the realization of on-chip small footprint architectures for generating, manipulating and detecting single photons. Waveguide-integrated superconducting nanowire single photon detectors are today state-of-the art devices capable of detecting single photons propagating in an integrated photonic circuit with high efficiency, speed and low timing uncertainty. Their excellent metrics in the near-infrared allows to directly interface on-chip devices with existing C-Band communication technology systems. To move towards high communication speed, efficient detectors able to sustain Gcps count rate are desirable. Here we present the design process and the experimental characterization of a new generation of fast detectors, consisting of µm-long NbN superconducting nanowires embedded in a SOI two-dimensional photonic crystal cavity. Almost 70% efficiency at telecommunication wavelengths has been achieved with negligible dark counts and sub-ns recovery time, which could enable Gcps count rates.

Photonic integrated circuits (PICs) have enabled novel functionality in quantum optics, quantum information processing and quantum communication. PICs based on Silicon Nitride (Si3N4) provide low-loss passive components and are compatible with efficient superconducting nanowire single-photon detectors (SNSPDs). For realizing functional quantum photonic systems, the integration with active phase-shifters is needed which is challenging at the cryogenic temperatures needed for operating SNSPDs. Here we demonstrate a cryo-compatible phase shifter using a low-voltage opto-mechanical modulator and show joint operation with SNSPDs at 1.3 K. We achieve a half-wave voltage of 4.6 V, single-photon detection with 88% on-chip detection efficiency (OCDE) and a low timing jitter of 12.2 ps. Our approach allows for operating reconfigurable quantum photonic circuits with low dissipation in a cryogenic setting.