Integrated and DC-powered superconducting microcomb.

Frequency combs, specialized laser sources emitting multiple equidistant frequency lines, have revolutionized science and technology with unprecedented precision and versatility. Recently, integrated frequency combs are emerging as scalable solutions for on-chip photonics. Here, we demonstrate a fully integrated superconducting microcomb that is easy to manufacture, simple to operate, and consumes ultra-low power. Our turnkey apparatus comprises a basic nonlinear superconducting device, a Josephson junction, directly coupled to a superconducting microstrip resonator. We showcase coherent comb generation through self-started mode-locking. Therefore, comb emission is initiated solely by activating a DC bias source, with power consumption as low as tens of picowatts. The resulting comb spectrum resides in the microwave domain and spans multiple octaves. The linewidths of all comb lines can be narrowed down to 1 Hz through a unique coherent injection-locking technique. Our work represents a critical step towards fully integrated microwave photonics and offers the potential for integrated quantum processors.

Enhancing LiDAR performance using threshold photon-number-resolving detection.

Single-photon light detection and ranging (LiDAR) is widely used to reconstruct 3D scenes. Nevertheless, depth and reflectivity maps obtained by single-photon detection usually suffer from noise problems. Threshold LiDAR techniques using photon-number-resolving detectors were proposed to suppress noise by filtering low photon numbers, but these techniques renounce multiple levels of information and could not be compatible when it comes to high-noise low-signal regime. In this manuscript, we propose a detection scheme which combines the noise suppression of threshold detection with the signal amplification of photon-number-resolving detectors to further enhance LiDAR performance. The enhancement attained is compared to single-photon and threshold detection schemes under a wide range of signal and noise conditions, in terms of signal-to-noise-ratio (SNR), detection rate and false alarm rate, which are key metrics for LiDAR. Extensive simulations and real-world experiments show that the proposed scheme can reconstruct better depth and reflectivity maps. These results enable the development of high-efficient and low-noise LiDAR systems.

A compact multi-pixel superconducting nanowire single-photon detector array supporting gigabit space-to-ground communications.

Classical and quantum space-to-ground communications necessitate highly sensitive receivers capable of extracting information from modulated photons to extend the communication distance from near-earth orbits to deep space explorations. To achieve gigabit data rates while mitigating strong background noise photons and beam drift in a highly attenuated free-space channel, a comprehensive design of a multi-functional detector is indispensable. In this study, we present an innovative compact multi-pixel superconducting nanowire single-photon detector array that integrates near-unity detection efficiency (91.6%), high photon counting rate (1.61 Gcps), large dynamic range for resolving different photon numbers (1-24), and four-quadrant position sensing function all within one device. Furthermore, we have constructed a communication testbed to validate the advantages offered by such an architecture. Through 8-PPM (pulse position modulation) format communication experiments, we have achieved an impressive maximum data rate of 1.5 Gbps, demonstrating sensitivities surpassing previous benchmarks at respective speeds. By incorporating photon number information into error correction codes, the receiver can tolerate maximum background noise levels equivalent to 0.8 photons/slot at a data rate of 120 Mbps-showcasing a great potential for daylight operation scenarios. Additionally, preliminary beam tracking tests were conducted through open-loop scanning techniques, which revealed clear quantitative dependence indicating sensitivity variations based on beam location. Based on the device characterizations and communication results, we anticipate that this device architecture, along with its corresponding signal processing and coding techniques, will be applicable in future space-to-ground communication tasks.

High-timing-precision detection of single X-ray photons by superconducting nanowires.

Precisely acquiring the timing information of individual X-ray photons is important in both fundamental research and practical applications. The timing precision of commonly used X-ray single-photon detectors remains in the range of one hundred picoseconds to microseconds. In this work, we report on high-timing-precision detection of single X-ray photons through the fast transition to the normal state from the superconductive state of superconducting nanowires. We successfully demonstrate a free-running X-ray single-photon detector with a timing resolution of 20.1 ps made of 100-nm-thick niobium nitride film with an active area of 50 µm by 50 µm. By using a repeated differential timing measurement on two adjacent X-ray single-photon detectors, we demonstrate a precision of 0.87 ps in the arrival-time difference of X-ray photon measurements. Therefore, our work significantly enhances the timing precision in X-ray photon counting, opening a new niche for ultrafast X-ray photonics and many associated applications.

The value of using heterogeneous detector groups for the development of time-of-flight (TOF) positron emission tomography (PET) systems.

Objective. Much recent attention on positron emission tomography (PET) is the development of time-of-flight (TOF) systems with ever-improving coincidence time resolution (CTR). This is because, when all other factors remain the same, a better CTR leads to images of better statistics and effectively increases the sensitivity of the system. However, detector designs that aggressively improve the CTR often compromise the detection efficiency (DE) and offset the benefit gained. Under this circumstance, in developing a TOF PET system it may be beneficial to employ heterogeneous detector groups to balance the overall CTR and DE of the system. In this study, we examine the potential value of this system design strategy by considering two-dimensional systems that assume several representative ways of mixing two detector groups.Approach. The study is based on computer simulation and specifically considers medium time-resolution (MTR) detectors that have a 528 ps CTR and high time-resolution (HTR) detectors that have a 100 ps CTR and a DE that is 0.7 times that of the MTR detector. We examine contrast recovery, noise, and subjective quality of the resulting images under various ways of mixing the MTR and HTR detectors.Main results. With respect to the traditional configuration that adopts only the HTR detectors, symmetric heterogeneous configurations may offer comparable or better images while using considerably fewer HTRs. On the other hand, asymmetric heterogeneous configurations may allow the use of only a few HTRs for improving image quality locally.Significance. This study demonstrates the value of the proposed system-level design strategy of using heterogeneous detector groups for achieving high effective system sensitivity by factoring into the tradeoff between the CTR and DE of the detector.

Image distortion by ambiguous multiple-photon detections in a superconducting nanowire single-photon imager and the correction method.

Scaling up superconducting nanowire single-photon detectors (SNSPDs) into a large array for imaging applications is the current pursuit. Although various readout architectures have been proposed, they cannot resolve multiple-photon detections (MPDs) currently, which limits the operation of the SNSPD arrays at high photon flux. In this study, we focused on the readout ambiguity of a superconducting nanowire single-photon imager applying time-of-flight multiplexing readout. The results showed that image distortion depended on both the incident photon flux and the imaging object. By extracting multiple-photon detections on idle pixels, which were virtual because of the incorrect mapping from the ambiguous readout, a correction method was proposed. An improvement factor of 1.3~9.3 at a photon flux of µ = 5 photon/pulse was obtained, which indicated that joint development of the pixel design and restoration algorithm could compensate for the readout ambiguity and increase the dynamic range.

van der Waals Self-Epitaxial Growth of Inch-Sized Superconducting Niobium Diselenide Films.

Ultrathin superconducting films are the basis of superconductor devices. van der Waals (vdW) NbSe2 with noncentrosymmetry exhibits exotic superconductivity and shows promise in superconductor electronic devices. However, the growth of inch-scale NbSe2 films with layer regulation remains a challenge because vdW structural material growth is strongly dependent on the epitaxial guidance of the substrate. Herein, a vdW self-epitaxy strategy is developed to eliminate the substrate driving force in film growth and realize inch-sized NbSe2 film growth with thicknesses from 2.1 to 12.1 nm on arbitrary substrates. The superconducting transition temperature of 5.1 K and superconducting transition width of 0.30 K prove the top homogeneity and quality of superconductivity among all of the synthetic NbSe2 films. Coupled with a large area and substrate compatibility, this work paves the way for developing NbSe2 superconductor electronics.

Topotactic fabrication of transition metal dichalcogenide superconducting nanocircuits.

Superconducting nanocircuits, which are usually fabricated from superconductor films, are the core of superconducting electronic devices. While emerging transition-metal dichalcogenide superconductors (TMDSCs) with exotic properties show promise for exploiting new superconducting mechanisms and applications, their environmental instability leads to a substantial challenge for the nondestructive preparation of TMDSC nanocircuits. Here, we report a universal strategy to fabricate TMDSC nanopatterns via a topotactic conversion method using prepatterned metals as precursors. Typically, robust NbSe2 meandering nanowires can be controllably manufactured on a wafer scale, by which a superconducting nanowire circuit is principally demonstrated toward potential single photon detection. Moreover, versatile superconducting nanocircuits, e.g., periodical circle/triangle hole arrays and spiral nanowires, can be prepared with selected TMD materials (NbS2, TiSe2, or MoTe2). This work provides a generic approach for fabricating nondestructive TMDSC nanocircuits with precise control, which paves the way for the application of TMDSCs in future electronics.

Polarizer-free measurement of the full Stokes vector using a fiber-coupled superconducting nanowire single photon detector with a polarization extinction ratio of ∼2.

The characterization and manipulation of polarization state at single photon level are of great importance in research fields such as quantum information processing and quantum key distribution, where photons are normally delivered using single mode optical fibers. To date, the demonstrated polarimetry measurement techniques based on a superconducting nanowire single photon detector (SNSPD) require the SNSPD to be either highly sensitive or highly insensitive to the photon's polarization state, therefore placing an unavoidable challenge on the SNSPD's design and fabrication processes. In this article, we present the development of an alternative polarimetry measurement technique, of which the stringent requirement on the SNSPD's polarization sensitivity is removed. We validate the proposed technique by a rigorous theoretical analysis and comparisons of the experimental results obtained using a fiber-coupled SNSPD with a polarization extinction ratio of ∼2 to that obtained using other well-established known methods. Based on the full Stokes data measured by the proposed technique, we also demonstrate that at the single photon level (∼ -100 dBm), the polarization state of the photon delivered to the superconducting nanowire facet plane can be controlled at will using a further developed algorithm. Note that other than the fiber-coupled SNSPD, the only component involved is a quarter-wave plate (no external polarizer is necessary), which when aligned well has a paid insertion loss less than 0.5 dB.

All-Water Etching-Free Electron Beam Lithography for On-Chip Nanomaterials.

Electron beam lithography uses an accelerated electron beam to fabricate patterning on an electron-beam-sensitive resist but requires complex dry etching or lift-off processes to transfer the pattern to the substrate or film on the substrate. In this study, etching-free electron beam lithography is developed to directly write a pattern of various materials in all-water processes, achieving the desired semiconductor nanopatterns on a silicon wafer. Introduced sugars are copolymerized with metal ions-coordinated polyethylenimine under the action of electron beams. The all-water process and thermal treatment result in nanomaterials with satisfactory electronic properties, indicating that diverse on-chip semiconductors (e.g., metal oxides, sulfides, and nitrides) can be directly printed on-chip by an aqueous solution system. As a demonstration, zinc oxide patterns can be achieved with a line width of 18 nm and a mobility of 3.94 cm2 V-1 s-1. This etching-free electron beam lithography strategy provides an efficient alternative for micro/nanofabrication and chip manufacturing.

Fast and accurate measurement of the polarization-dependent detection efficiency of superconducting nanowire single photon detectors.

Superconducting nanowire single photon detectors (SNSPDs) have been extensively investigated due to their superior characteristics, including high system detection efficiency, low dark count rate and short recovery time. The polarization sensitivity introduced by the meandering-type superconductor nanowires is an intrinsic property of SNSPD, which is normally measured by sweeping hundreds of points on the Poincaré sphere to overcome the unknown birefringent problem of the SNSPD's delivery fiber. In this paper, we propose an alternative method to characterize the optical absorptance of SNSPDs, without sweeping hundreds of points on the Poincaré sphere. It is shown theoretically that measurements on the system detection efficiencies (SDEs) subject to cases of four specific photon polarization states are sufficient to reveal the two eigen-absorptances of the SNSPD. We validate the proposed method by comparing the measured detection spectra with the spectra attained from sweeping points on the Poincaré sphere and the simulated absorption spectra.

64-Pixel Mo80Si20 superconducting nanowire single-photon imager with a saturated internal quantum efficiency at 1.5 µm.

A superconducting nanowire single-photon imager (SNSPI) uses a time-multiplexing method to reduce the readout complexity. However, due to the serial connection, the nanowire should be uniform so that a common bias can set all segments of the nanowire to their maximum detection efficiency, which becomes more challenging as the scalability (i.e., the length of the nanowire) increases. Here, we have developed a 64-pixel SNSPI based on amorphous Mo80Si20 film, which yielded a uniform nanowire and slow transmission line. Adjacent detectors were separated by delay lines, giving an imaging field of 270 µm × 240 µm. Benefiting from the high kinetic inductance of Mo80Si20 films, the delay line gave a phase velocity as low as 4.6 µm/ps. The positions of all pixels can be read out with a negligible electrical cross talk of 0.02% by using cryogenic amplifiers. The timing jitter was 100.8 ps. Saturated internal quantum efficiency was observed at a wavelength of 1550 nm. These results demonstrate that amorphous film is a promising material for achieving SNSPIs with large scalability and high efficiency.

Probabilistic Energy-to-Amplitude Mapping in a Tapered Superconducting Nanowire Single-Photon Detector.

A spectrum-resolved photon detector is crucial for cutting-edge quantum optics, astronomical observation, and spectroscopic sensing. However, such an ability is rarely obtained because a direct linear conversion from weak single-photon energy to a readable electrical signal above the noise level without causing an avalanche is challenging. Here, we overcame these difficulties by building a probabilistic energy-to-amplitude mapping in a tapered superconducting nanowire single-photon detector and combining a computational reconstruction to obtain equivalent spectral resolving capacity. Distinguished dependence of pulse amplitude distributions on varied input spectra has been observed experimentally. As the energy-to-amplitude mapping is probabilistic, statistical measurements are required. By collecting around a few hundred photons, we have demonstrated wavelength perception over a wide spectral range from 600 to 1700 nm with a resolution of 100 nm. These findings represent a new approach to designing spectrum-sensitive SNSPDs for low-light spectroscopic applications.

Interfacial Engineered Vanadium Oxide Nanoheterostructures Synchronizing High-Energy and Long-Term Potassium-Ion Storage.

Potassium ion hybrid capacitors (KICs) have drawn tremendous attention for large-scale energy storage applications because of their high energy and power densities and the abundance of potassium sources. However, achieving KICs with high capacity and long lifespan remains challenging because the large size of potassium ions causes sluggish kinetics and fast structural pulverization of electrodes. Here, we report a composite anode of VO2-V2O5 nanoheterostructures captured by a 3D N-doped carbon network (VO2-V2O5/NC) that exhibits a reversible capacity of 252 mAh g-1 at 1 A g-1 over 1600 cycles and a rate performance with 108 mAh g-1 at 10 A g-1. Quantitative kinetics analyses demonstrate that such great rate capability and cyclability are enabled by the capacitive-dominated potassium storage mechanism in the interfacial engineered VO2-V2O5 nanoheterostructures. The further fabricated full KIC cell consisting of a VO2-V2O5/NC anode and an active carbon cathode delivers a high operating voltage window of 4.0 V and energy and power densities up to 154 Wh kg-1 and 10 000 W kg-1, respectively, surpassing most state-of-the-art KICs.

Single-Detector Spectrometer Using a Superconducting Nanowire.

Designing a spectrometer without the need for wavelength multiplexing optics can effectively reduce the complexity and physical footprint. On the basis of the computational spectroscopic strategy and combining a broadband-responsive dynamic detector, we successfully demonstrate an optics-free single-detector spectrometer that maps the tunable quantum efficiency of a superconducting nanowire into a matrix to build a solvable mathematical equation. Such a spectrometer can realize a broadband spectral responsivity ranging from 660 to 1900 nm. The spectral resolution at the telecom is sub-10 nm, exceeding the energy resolving capacity of existing infrared single-photon detectors. Meanwhile, benefiting from the optics-free setup, precise time-of-flight measurements can be simultaneously achieved. We have demonstrated a spectral LiDAR with eight spectral channels. This spectrometer scheme paves the way for applying superconducting nanowire detectors in multifunctional spectroscopy and represents a conceptual advancement for on-chip spectroscopy and spectral imaging.

Electronic Modulation of Non-van der Waals 2D Electrocatalysts for Efficient Energy Conversion.

The exploration of efficient electrocatalysts for energy conversion is important for green energy development. Owing to their high surface areas and unusual electronic structure, 2D electrocatalysts have attracted increasing interest. Among them, non-van der Waals (non-vdW) 2D materials with numerous chemical bonds in all three dimensions and novel chemical and electronic properties beyond those of vdW 2D materials have been studied increasingly over the past decades. Herein, the progress of non-vdW 2D electrocatalysts is critically reviewed, with a special emphasis on electronic structure modulation. Strategies for heteroatom doping, vacancy engineering, pore creation, alloying, and heterostructure engineering are analyzed for tuning electronic structures and achieving intrinsically enhanced electrocatalytic performances. Lastly, a roadmap for the future development of non-vdW 2D electrocatalysts is provided from material, mechanism, and performance viewpoints.

Noise-tolerant single-photon imaging with a superconducting nanowire camera.

The quality of an image is limited to the signal-to-noise ratio of the output from sensors. As the background noise increases much more than the signal, which can be caused by either a huge attenuation of light pulses after a long-haul transmission or a blinding attack with a strong flood illumination, an imaging system stops working properly. Here we built a superconducting single-photon infrared camera of negligible dark counts and 60 ps timing resolution. Combining with an adaptive 3D slicing algorithm that gives each pixel an optimal temporal window to distinguish clustered signal photons from a uniformly distributed background, we successfully reconstructed 3D single-photon images at both a low signal level (∼1 average photon per pixel) and extremely high noise background (background-to-signal ratio = 200 within a period of 50 ns before denoising). Among all detection events, we were able to remove 99.45% of the noise photons while keeping the signal photon loss at 0.74%. This Letter is a direct outcome of quantum-inspired imaging that asks for a co-development of sensors and computational methods. We envision that the proposed methods can increase the working distance of a long-haul imaging system or defend it from blinding attacks.

Fabrication of superconducting niobium nitride nanowire with high aspect ratio for X-ray photon detection.

The niobium nitride (NbN) nanowires fabricated with the high-quality ultra-thin NbN film with a thickness of 3 nm-6 nm were widely used for single photon detectors. These nanowires had a low aspect ratio, less than 1:20. However, increasing the thickness and the aspect ratio of highly-uniformed NbN nanowires without reducing the superconductivity is crucial for the device in detecting high-energy photons. In this paper, a high-quality superconducting nanowire with aspect ratio of 1:1 was fabricated with optimized process, which produced a superconducting critical current of 550 µA and a hysteresis of 36 µA at 2.2 K. With the optimization of the electron beam lithography process of AR-P6200.13 and the adjustion of the chamber pressure, the discharge power, as well as the auxiliary gas in the process of reactive ion etching (RIE), the meandered NbN nanowire structure with the minimum width of 80 nm, the duty cycle of 1:1 and the depth of 100 nm were finally obtained on the silicon nitride substrate. Simultaneously, the sidewall of nanowire was vertical and smooth, and the corresponding depth-width ratio was more than 1:1. The fabricated NbN nanowire will be applied to the detection of soft X-ray photon emitted from pulsars with a sub-10 ps time resolution.

Depth-resolved NIR-II fluorescence mesoscope.

NIR-II fluorescence imaging is a promising method for visualizing biological structures in deep tissue, owing to the advantages of significantly suppressed optical scattering and diminished autofluorescence in biological tissues. However, few NIR-II fluorescence imaging approaches can simultaneously achieve a large field of view, high resolution and superior penetration depth, while exhibiting optical sectioning capability. In this paper, we present a novel NIR-II fluorescence mesoscopy system based on the f-θ scanning scheme and confocal detection to overcome these limitations. When used with NIR-II fluorescent dyes, our setup performs NIR-II fluorescence imaging on samples as large as 7.5×7.5 mm2 with a lateral resolution of 6.3 µm. In addition, our system provides a depth-resolved imaging ability and zooming function. We successfully demonstrate in vivo cerebrovascular imaging of a mouse with local ischemia. Thus, our system provides new opportunities to explore the mechanism of cerebrovascular disease.