Polarization dependent performance of superconducting nanowire single-photon detectors (SNSPDs) remains one of the obstacles to free space applications of SNSPDs. Here, we study the wavelength dependence of polarization anisotropy in two most widely used SNSPD geometries: meander and fractal nanowires based on niobium nitride thin films. In particular, we observe that polarization sensitivity becomes more pronounced at longer wavelengths, accompanied by an oscillatory behavior of photon count rate (PCR) on polarization angle. Furthermore, we report a wavelength-dependent shift in the polarization angle corresponding to maximum and minimum PCR and also find that experimental data consistently exhibit higher polarization anisotropy than predicted by simulations—likely due to fabrication-induced substrate effects. We deduce that the polarization anisotropy is a direct result of significant difference between the refractive indices of substrate and nanowire to that of the air, which surrounds them. We also outline potential directions for future work in improving our understanding of polarization anisotropy in SNSPDs. Our results provide new insight into the geometric factors governing polarization sensitivity in SNSPDs and underscore the potential of fractal nanowires in enabling broadband, polarization-independent single photon detection.

The advancement of superconducting nanowire single-photon detectors (SNSPDs or SSPDs) is intrinsically linked to breakthroughs in superconducting materials engineering. These detectors are prized for their excellent sensitivity and performance across a wide wavelength range, from ultra-violet to mid-infrared (mid-IR). Plasma-enhanced atomic layer deposition (ALD) is an advanced deposition technique that enables exceptional film uniformity across large wafers, making it ideal for a variety of applications. In this study, ALD NbTiN films are fabricated into SNSPDs that exhibit single-photon response across near- to mid-IR wavelengths. A 5 nm thick NbTiN film was patterned into devices consisting of similar to 100 nm wide meandered nanowires, and the nanowire cross-section and composition were analysed by transmission electron microscopy. The devices were tested in a closed-cycle cryostat with a base temperature of 0.9 K. Photon detection rate and timing jitter were measured across the wavelength range 1.5-4.4 & micro;m under low dark count conditions, confirming single-photon sensitivity across the measured wavelength bands. This work introduces the ALD-grown NbTiN films as a new platform for SNSPD detectors, achieving mid-IR single-photon sensitivity. The intact film quality after fabrication demonstrates the potential of ALD processes for scalable infrared detector technologies.

Single-photon time-of-flight light detection and ranging (LiDAR) is a versatile technique for the measurement of absolute distances and for depth profiling. It has a wide variety of applications (e.g., land surveying, autonomous car navigation, underwater imaging) with the potential to achieve high-resolution three-dimensional images over long ranges when the key components of the measurement system are of a suitably high specification. In this work, a novel, high-efficiency, and low timing jitter superconducting nanowire single-photon detector, in conjunction with a custom single-pixel scanning transceiver system, and the time-correlated single-photon counting technique, enable the acquisition of millimeter-scale resolution depth images of scenes at standoff distances of hundreds of meters. A 1550 nm wavelength fiber laser was coupled to the monostatic transceiver to provide the illumination. The system was eye-safe with the maximum average optical output power being <= 3.5 mW for measurements of a scene at a standoff distance of 1 km. The overall system instrumental response was approximately 13 ps full width half maximum. This enabled 1 mm depth features on a reference board and a human head to be clearly resolved when measured by the system in broad daylight at standoff distances of 45 and 325 musing per-pixel acquisition times of between 0.25 and 1 ms. These high-resolution results demonstrate the enormous potential of such a system to acquire detailed depth and intensity images of scenes from long distances in daylight or darkness conditions. This could lead to step change improvements in applications such as facial and human activity recognition and the imaging of scenes through clutter and atmospheric obscurants. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Superconducting Transition Edge Sensors (TESs) are a promising technology for fundamental physics applications due to their low dark count rates, excellent energy resolution, and high detection efficiency. On the DESY campus, we have been developing a program to characterize cryogenic quantum sensors for fundamental physics applications, particularly focused on TESs. We currently have two fully equipped dilution refrigerators that enable simultaneous TES characterization and fundamental physics searches. In this paper, we summarize the current status of our TES characterization, including recent calibration efforts and efficiency measurements, as well as simulations to better understand TES behavior and its backgrounds. Additionally, we summarize some physics applications that we are already exploring or planning to explore. We will give preliminary projections on a direct dark matter search with our TES, where exploiting low-threshold electron scattering in superconducting materials allows us to search for sub-MeV-scale dark matter. We are also working toward performing a measurement of the even-number photon distribution (beyond one pair) of a quantum-squeezed light source. Finally, if it proves to meet the requirements, our TES detector may be used as a second, independent detection system to search for an axion signal at the ALPS II experiment.

Superconducting nanowire single-photon detectors are widely used for detecting individual photons across various wavelengths from ultraviolet to near-infrared range. Recently, there has been increasing interest in enhancing their sensitivity to single photons in the mid-infrared spectrum, driven by applications in quantum communication, spectroscopy and astrophysics. Here, we present our efforts to expand the spectral detection capabilities of U-shaped NbTiN-based superconducting nanowire single-photon detectors, fabricated in a 2-wire configuration on a silicon-on-insulator substrate, into the mid-infrared range. We demonstrate saturated internal detection efficiency extending up to a wavelength of 3.5 μm for a 5 nm thick and 50 nm wide NbTiN nanowire with a dark count rate less than 10 counts per second at 0.9 K and a rapid recovery time of 4.3 ns. The detectors are engineered for integration on waveguides in a silicon-on-insulator platform for compact, multi-channel device applications.

Detector timing jitter is a key parameter in advanced photon counting applications. Superconducting nanowire single-photon detectors offer the fastest timing jitter in the visible to telecom wavelength range and have demonstrated single-photon sensitivity in the mid-infrared spectral region. Here, we report on timing jitter in a NbTiN nanowire device from 1.56 to 3.5 μm wavelength, achieving a FWHM jitter from 13.2 to 30.3 ps. This study has implications for emerging time-correlated single-photon counting applications in the mid-infrared spectral region.

The ability to detect individual light quanta – single photons – is prized across many fields of physics from astronomy to quantum optics. Superconducting photon detectors offer exceptional performance in terms of sensitivity, spectral range and timing resolution. In this review, we introduce the underlying physics of photon absorption in superconducting devices. We then present detailed case studies of contemporary superconducting detector technologies for photon counting at visible and infrared wavelengths. We conclude with a perspective on future developments in this exciting area.

Superconducting Nanowire Single photon Detectors (SNSPDs) offer unparalleled performance for IR photon counting, combining close to unity quantum efficiency, low intrinsic noise and ultrafast timing jitter. The ability of SNSPDs to count photons in mid-IR band up to 7 um wavelength opens up new possibilities in quantum optics, laser ranging, free space Quantum Key Distribution (QKD) and astronomy. Here we report on development of mid-IR SNSPDs including device design, fabrication, optimisation of superconducting materials and characterisation. We present a characterisation setup covering 1.5 - 4.2 um spectral region based on tuneable optical parametric oscillator with picosecond long pulses. We then demonstrate the viability of mid infrared SNSPDs for a variety of applications and report the results from single photon light detection and ranging (LIDAR) experiment with 2.3 um photons. This work paves the way for future app in free space QKD, deep space communication and astronomy.

We report on the extended infrared single-photon response of niobium nitride superconducting nanowires deposited by atomic layer deposition. The superconducting nanowire single-photon detectors are based on 4.65 nm thick NbN, patterned into 100 nm meanders, and characterized at 2.5 K. We verify single-photon sensitivity from 1310 to 2006 nm with saturated response at shorter wavelengths.

Quantum-enhanced optical technologies operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. Currently, sources of entangled photons are available at visible, near-infrared and telecom wavelengths but are strongly underdeveloped at longer wavelengths. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.

We demonstrate two-photon interference and polarization entanglement at 2090 nm, constituting a crucial leap towards free-space mid-infrared quantum communication systems in a spectral region with high atmospheric transparency and reduced solar background.

We demonstrate two-photon interference and polarization entanglement at 2090 nm, constituting a crucial leap towards free-space mid-infrared quantum communication systems in a spectral region with high atmospheric transparency and reduced solar background.

In this work we design, fabricate and characterize superconducting nanowire single photon detectors (SNSPDs) optimized for mid infrared operation. The mid infrared is of interest for free space applications due to lower solar background than at shorter wavelengths as well as low atmospheric absorption. We show a proof-of-principle LIDAR imaging experiment at 2.3µm showing the viability of using SNSPDs for a variety of applications in the mid infrared.

We demonstrate millimetre-scale depth imaging up to 325 meters by deployment of a novel superconducting nanowire single-photon detector with 13 ps FWHM instrument response function at 1550 nm wavelength.

Quantum-enhanced optical systems operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. However, to date, sources of entangled photons have been realized mainly in the near-infrared 700- to 1550-nm spectral window. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.

In this work, we show a proof-of-principle benchtop single-photon light detection and ranging (LIDAR) depth imager at 2.3µm, utilizing superconducting nanowire single-photon detectors (SNSPDs). We fabricate and fiber-couple SNSPDs to exhibit enhanced photon counting performance in the mid-infrared. We present characterization results using an optical parametric oscillator source and deploy these detectors in a scanning LIDAR setup at 2.3µm wavelength. This demonstrates the viability of these detectors for future free-space photon counting applications in the mid-infrared where atmospheric absorption and background solar flux are low.

Quantum-enhanced optical systems operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. However, to date, sources of entangled photons have been realized mainly in the near-infrared 700- to 1550-nm spectral window. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.

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)