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The hotspot relaxation time (τth) was systematically studied on superconducting nanowire single-photon detectors (SNSPDs) made from ultrathin NbN films with different substrates and chemical compositions. The τth values were measured to be between 9.2 ps and 41.1 ps at the bath temperature of 2.15 K. The temperature dependence of τth ∝ TC−1.5 was observed, which indicated a dominant electron-phonon scattering relaxation mechanism in disordered NbN films. In addition, the increase in τth results in a higher latching probability in NbN-based SNSPDs with the same electrical response time (τe). The experimental results confirmed that the devices suffered latching when the ratio of the electric and thermal relaxation time was less than 133. This observation is an interesting reference for the design and process optimization of SNSPDs. [ABSTRACT FROM AUTHOR]

We investigated the formation and dissolution of natural oxide layers on 4-nm ultrathin NbTiN films at room temperatures and after heat treatment. By x-ray reflection (XRR) at a low angle and adopting a tri-oxidized-layer model to fit the XRR data, the thickness of each oxidized layer and the actual NbTiN layer below was obtained. This was confirmed by scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy. The x-ray photoelectron spectroscopy of the NbTiN film indicated the presence of the oxidized layers Nb 2 O 5 , TiO 2 , and NbO on the surface. Besides, the analysis of the films gave consistent results for the oxidized layer with saturation values of approximately 1.3 ± 0.1 nm. We found that the thickness of oxidized layers remains unchanged under heat treatment of 200 °C. Our approach can help us further understand the surface properties of ultrathin NbTiN films for improving the performance of NbTiN-based devices. [ABSTRACT FROM AUTHOR]

The superconducting nanowire single-photon detector (SNSPD) is inherently sensitive to polarization due to its anisotropic nanowire structure. We studied the polarization sensitivity of SNSPDs with ultranarrow nanowires (20–40 nm wide) by simulation and fabricated SNSPDs with 30-nm-wide nanowires. The SNSPD has a maximum system detection efficiency of 20% and a polarization extinction ratio (PER) of 18. The deviation of the PER between the experiment and the simulation can be explained by a trapezoidal cross section of the nanowire. [ABSTRACT FROM PUBLISHER]

Thin \Bi2\Sr2\CaCu2\O8 + \delta (Bi-2212) single crystals with the typical thicknesses of 20–200 nm are prepared with mechanical exfoliation, which can be firmly attached to a substrate by van der Waals and/or capillary force instead of using any glue. The thin Bi-2212 single crystals are interesting for superconducting device fabrication and applications as well as the study of low-dimension superconductivity. A stack of intrinsic Josephson junctions including eight junctions is demonstrated. [ABSTRACT FROM AUTHOR]

Single photon detection is one of the key technologies for quantum key distribution in quantum communication. As a novel single photon detection technology, superconducting nanowire single photon detector (SNSPD) surpasses conventional semiconducting single photon detectors with high count rate and low dark count rate. In this article, we introduce SNSPD fabricated from NbN ultrathin superconducting film and lab-based SNSPD system. The characteristics of single photon response pulse of SNSPD are analyzed in detail. Also discussed is the relationship between waveform of single photon response and system bandwidth. Circuit model is made to analyze the performance of SNSPD. The simulation result agrees well with the experimental data. Those results are valuable for understanding the mechanism of SNSPD and building future SNSPD system for quantum communication. [ABSTRACT FROM AUTHOR]

Presents an article about how mesa-structured intrinsic Josephson junctions are frabricated in Bi&sub2;Sr&sub2;CaCu&sub2;O&sub8;+x single crystals. Observations made of typical current-voltage characteristics of intrinsic Josephson junctions; Inclusion of multiple quasiparticle branches, surface junction with critical curent lower than those of inner junctions, and subgap structures on quasiparticle branches; How the energy gap voltage of the intrinsic Josephson junctions at 30 K is about 20 mV.

• From the flux flow instability in current-voltage characteristics of NbN films, the critical vortex velocity (ʋ *) and quasiparticle inelastic scattering time (τ E) of NbN films can be obtained within the framework of the Larkin-Ovchinnikov theory. • The critical vortex velocity ʋ *, at which the instability occurs, led to the transition from the power law ʋ *∝ B −1/2 dependent to field-independent behavior, while the nonequilibrium quasiparticles are uniformly distributed. • The quasiparticle inelastic scattering time obeys T −n temperature dependence, where the exponent is 2.5 and 2.3 for the 10 and 15-nm-thick NbN films, respectively, indicating a dominant electron-phonon scattering relaxation mechanism in disordered NbN films. • At 8 K, the value of τ E decreases from 88 to 12 ps, and film thickness decreased from 15 to 10 nm. Thinner films films with shorter τ E is a feature that could be desirable in the design of rapid photon detectors. A deep understanding of the quasiparticle inelastic scattering rate in superconducting materials is beneficial for the further development of ultrasensitive and rapid superconducting detectors. From the flux flow instability in current–voltage characteristics of NbN superconducting films, we have derived the critical vortex velocity (ʋ*) and quasiparticle inelastic scattering time (τ E) of the 10- and 15-nm-thick NbN films at different magnetic fields and temperatures within the framework of the Larkin–Ovchinnikov model. The critical vortex velocity drove the transition from the power law ʋ * ∝ B −1/2 dependent to field-independent behavior where the nonequilibrium quasiparticles are uniformly distributed. The inelastic scattering time of quasiparticles obeys T −n temperature dependence, where the exponents are 2.5 and 2.3 for the 10 and 15-nm-thick NbN films, respectively. As the thickness of the film decreases from 15 to 10 nm, a reduction of τ E from 88 to 12 ps is observed at 8 K. Moreover, the small value of τ E for ultra-thin films can be associated with the low-average value of the superconducting gap, which is precisely what the detector needs. [ABSTRACT FROM AUTHOR]

In this work, the gate modulation characteristics of superconducting nanowire transistors (SNTs) were investigated under different on-chip gate voltage configurations. By fabricating NbTiN-SNTs with symmetric side gate, we studied the critical current suppression of SNTs under single, opposite, and double voltage gate. We figure out that the gate voltage and leakage current can be adjusted by the gate configuration. Moreover, it revealed an approximately twofold increase in modulation voltage for the opposite gate configuration as compared to the single gate configuration, implying a possible electric field tuned superconductivity. When simultaneously apply voltage on both gates, the onset and offset voltages kept unchanged, but the leakage current is then two times higher than that of single gate. In addition, under all these three types of gate configuration, the suppression of superconductivity coincided with the onset of leakage current, suggesting a potential charge injection mechanism. Our findings demonstrate that both the electric field and charge injection play important roles on the suppression of superconductivity in SNTs, which also contributes to the further optimization and applications of SNT. [ABSTRACT FROM AUTHOR]

The intrinsic detection performance of superconducting nanowire single photon detectors (SNSPDs) is highly dependent on the superconducting properties of underlying thin films. This report outlines the enhancement of detection performance for single telecom wavelength photons in disordered NbTiN SNSPD at 4.2 K. By increasing the nitrogen content and deposition pressure, the NbTiN films show suppression in critical temperature and an increase in sheet resistance. Notably, the resulting SNSPDs display a broader saturation plateau at 2.2 K, leading to superior detection performance at 4.2 K. With the disordered 7-nm-thick NbTiN films, we fabricated SNSPDs with system detection efficiency up to 83% for 1550 nm photons at 4.2 K. Moreover, these devices also show saturated intrinsic detection efficiency for 2000 nm photons. With the features outlined, the devices can be integrated into the idle 4.2 K stage of the dilution refrigerator for applications in optical quantum information processing or utilize for detecting laser radar signals in airborne platforms. [ABSTRACT FROM AUTHOR]

We have developed a dc-SQUID amplifier with intra-coil resistors based on second-order parallel gradiometers. The measured results show that intra-coil resistors can significantly increase the flux–voltage transfer coefficient VΦ as well as smooth out resonances and produce a smooth flux–voltage (V–Φ) characteristic curve. It has been observed through a comparison of various intra-coil damping resistors that the flux–voltage transfer coefficient VΦ as a function of the resistance of the intra-coil resistors rises as the corresponding electrical conductance increases. The measured results of the flux noise at 4.2 K demonstrate explicitly that the flux–voltage transfer coefficient can greatly increase detection sensitivity and successfully reduce the noise contribution from the electronics operating at room temperature. [ABSTRACT FROM AUTHOR]

Quantum key distribution (QKD) holds the potential to establish secure keys over long distances. The distance of point-to-point QKD secure key distribution is primarily impeded by the transmission loss inherent to the channel. In the quest to realize a large-scale quantum network, increasing the QKD distance under current technology is of great research interest. Here we adopt the 3-intensity sending-or-not-sending twin-field QKD (TF-QKD) protocol with the actively-odd-parity-pairing method. The experiment demonstrates the feasibility of secure QKD over a 1002 km fibre channel considering the finite size effect. The secure key rate is 3.11 × 10 − 12 per pulse at this distance. Furthermore, by optimizing parameters for shorter fiber distances, we conducted performance tests on key distribution for fiber lengths ranging from 202 km to 505 km. Notably, the secure key rate for the 202 km, the normal distance between major cities, reached 111.74 kbps. [ABSTRACT FROM AUTHOR]

Optical cavity with backside optical coupling is one of the prevalent optical structures for superconducting nanowire single photon detector. A single layer anti-reflection coating (ARC) on the backside of the substrate is often adopted to enhance the transmittance to the substrate. We here apply a multilayer ARC to further increase the transmittance from 94.5% to 99.5%. An NbTiN SNSPD made on such a substrate with cavity structure presents a system detection efficiency of 90.1% at a dark count rate (DCR) of 100 Hz, which is the best value reported for backside optical coupled SNSPD at 1550 nm. It shows a timing jitter of ∼40.7 ps and the recovery time constant of ∼22.9 ns. [ABSTRACT FROM AUTHOR]

Maximum count rate is a crucial parameter of superconducting nanowire single-photon detector (SNSPD) for quantum communication and laser communication. However, when increasing the incident photon flux, the SNSPD device with small active area is apt to latch due to the AC coupling of room temperature amplifier in the readout circuit, which limits SNSPD to reach a high count rate. We proposed a simple way by inserting an electrical attenuator in series with small-active area SNSPD to avoid the latching effect, thus improving maximum count rate effectively. The count rate with the system detection efficiency reduced by half increased by 6 times for SNSPD with an active area of Φ15 μm. [ABSTRACT FROM AUTHOR]

The Journal of Low Temperature Physics has published a special issue containing original contributions to the 20th International Conference on Low Temperature Detectors (LTD20). The conference, hosted by the Korean Society of Superconductivity & Cryogenics and the Institute for Basic Science, took place in Daejeon, South Korea, from July 23 to July 28, 2023. The conference focused on the latest developments in cryogenic detectors and their applications, covering topics such as sensor physics, quantum phenomena, and various scientific fields. The special issue includes around 150 papers based on conference contributions, which have undergone a thorough review process. [Extracted from the article]

On-chip single photon detection is crucial for implementing on-chip quantum communication, quantum simulation, and calculation. Superconducting nanowire single-photon detectors (SNSPDs) have become one of the essential techniques to achieve high-efficiency, on-chip, single-photon detection at scale due to their high detection efficiency, low dark count rate, and low jitter. Silicon carbide (SiC) has emerged as a promising integrated photonics platform due to its nonlinear optical processing capabilities, compatibility with CMOS technology, and outstanding quantum properties as a device for single photon sources. However, achieving high-efficiency superconducting nanowire single-photon detection on SiC substrates has yet to be demonstrated. In this study, we deposited polycrystalline NbN thin films onto 4H-SiC substrates. We also ensured that the deposited NbN thin film had a flat surface with a roughness less than 1 nm on the C-side 4H-SiC substrate through optimized chemical mechanical polishing. The NbN-SNSPD achieved a saturated quantum efficiency covering the color center emission bandwidth wavelength range (from 861 to 1550 nm) of the 4H-SiC material. These results offer a promising solution for high-efficiency single-photon detection on fully integrated quantum optical chips on 4H-SiC substrates. [ABSTRACT FROM AUTHOR]

We report a broadband superconducting nanowire single-photon detector for the near-infrared waveband. The optical absorptance in this band is enhanced by utilizing a reflective aluminum mirror and a SiO2 layer as an optical cavity. A broadband system detection efficiency (SDE) over 50% from 950 to 1600 nm with a low dark count rate of 10 Hz is realized by coupling a HI 1060 FLEX single-mode fiber. The SDEs reaches 78.3%, 70.9%, and 57.7% at the wavelengths of 1064, 1310, and 1550 nm, respectively. A system timing resolution (jitter) of 83 ps at 1550 nm is obtained. The detector may be useful for broadband applications such as earth observation, environment monitoring, and biomedicine. [ABSTRACT FROM PUBLISHER]

The development of superconductor integrated circuits (SCIC) places increasing demands on electronic design automation (EDA) tools. Circuit simulation is a crucial step in the design process of superconducting quantum interference devices (SQUID) and single flux quantum (SFQ) circuits. Over the years, there have been many SC circuit simulators, like JSPICE, JSIM, WRspice, JoSIM, PSCAN2, JSICsim, PrimeSim HSPICE, Spectre, and more. The previous studies have compared the differences in results among some simulators for the same circuit cases. However, designers of SC circuits still face challenges when choosing simulators and setting simulation parameters. The performance of these simulators lacks comprehensive and quantitative evaluations to date. To evaluate the performance of the simulators, we focused on three aspects: the differences among the IV results, accuracy, and speed. For characterizing the accuracy of the simulators, we proposed a method that uses the relative error between the numerical and analytical solutions at the L - C resonance point on the IV curve of a dc SQUID. In this article, we have selected five representative simulators JoSIM, JSIM, WRspice, PSCAN2, and JSICsim for our study. By using multiple cases of the bare and coupled dc SQUID, multiple IV curves, and the analytical solution as a reference, we comprehensively compared the performance of these simulators. Additionally, we quantitatively examined the impact of two key simulation parameters, namely, the maximum allowed simulation timestep (max timestep) and relative tolerance (RelTol), on the performance of these simulators. Our results show that the normalized voltage differences in the IV curves of different simulators are relatively small (within 0.06) in regions far from the L - C resonance point, while they increase significantly near the L - C resonance point (maximum is 0.4). PSCAN2 exhibits a significant relative error of approximately 16% when the max timestep is 0.6ps and RelTol is 1 × 1 0 − 1 , which is close to its default RelTol value. Our work provides some insights and references for the designers of SC circuits on how to choose simulators and set simulation parameters. • Comprehensively studied the performance of the five simulators (JoSIM, JSIM, WRspice, PSCAN2, and JSICsim) from three aspects: the differences among the IV results, accuracy, and speed. • Proposed a reliable method to characterize the accuracy of simulators by using the relative error between the numerical and analytical solutions at the L - C resonance point of the dc SQUID's IV curve. • Evaluated and compared the performance of simulators, and quantitatively examined the impact of simulation parameters (max timestep and RelTol) on the simulators' performance. [ABSTRACT FROM AUTHOR]

• Measurements of infrared lidar signals with SNSPDs. • Test of SNPSDs performance to retrieve optical properties of atmospheric aerosol. • Comparison of aerosol backscatter at 1064 and 532 nm acquired with SNSPDs and PMT. Infrared lidar systems are powerful tools in many environmental applications where high accuracy level, real time observations and eye safe operations are essential. The main challenge for infrared lidar systems rests on the lack of high-performance detectors operating at such wavelengths. A promising solution is represented by Superconducting Nanostrip Single Photon Detectors (SNSPDs), thanks to their optimal efficiency and noise at infrared wavelengths with a system detection efficiency close to one, a dark count rate of less than 1 count/s as well as few nanoseconds dead time and picosecond time resolution. Here, we report on the use of SNSPDs for atmospheric lidar observations at 1064 nm and compare the results with those obtained simultaneously at 532 nm using a photomultiplier (PMT) as detector. Our experimental findings show the potentiality of SNSPDs for future lidar applications in the infrared domain. [ABSTRACT FROM AUTHOR]

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%. [ABSTRACT FROM AUTHOR]

We report on the development and characterization of a passive heterodyne hot electron bolometer (HEB) imager operating at 850 GHz. HEB detectors provide unprecedented sensitivity and spectral resolution at terahertz frequencies covering the frequency range of 0.5-5 THz. Terahertz imagers based on HEB technology have sufficient sensitivity to operate in a passive imaging mode, thus eliminating the need for active illumination. We demonstrated a fully automated passive imaging system based on our HEB technology. The front-end heterodyne detector includes a quasi-optically coupled HEB device in close proximity to a monolithic microwave integrated circuit low-noise IF amplifier. We integrated a compact commercial harmonic multiplier local oscillator (LO) source to provide the LO biasing for the HEB detector. Our high spectral resolution terahertz imager has a noise equivalent temperature difference value of 0.4 K and a spatial resolution of approximately 4 mm. [ABSTRACT FROM AUTHOR]

We investigate the current–voltage characteristics and the interface properties of epitaxial NbN/TaN/NbN Josephson junctions in this study. The crystal structure and interface properties of the NbN/TaN/NbN junctions are determined using x-ray diffraction and cross-sectional scanning transmission electron microscopy, and the epitaxial growth of the NbN/TaN/NbN trilayer exhibited a smooth and clear interface. The characteristic voltage can be easily tuned over more than one or two orders of magnitude by precisely and independently controlling the barrier thickness and resistivity. For the barrier resistivity of 79 mΩ cm, the junctions show excellent Josephson properties with a characteristic voltage of 2.04 ± 0.08 mV at 4.2 K, corresponding to a large normal metal coherence length of 2.51 ± 0.03 nm and a fast diffusion rate of 24.9 ± 0.4 mm2/s. A high junction quality was also confirmed by the large characteristic voltage of 0.88 ± 0.03 mV at 10.0 K, making the intrinsically shunted NbN junctions promising for use in higher speed and temperature-superconducting devices and circuits. [ABSTRACT FROM AUTHOR]

In the previous four decades, the two-coil mutual inductance (MI) technique has been widely employed in characterizing magnetic penetration depth, , of superconducting films. However, the conventional methods used to obtain are not applicable to small-sized films with common shapes, which limits the application of the MI technique in superconductivity research. Here, we first employed the fast wavelet collocation (FWC) method to a two-coil system and then proposed the possibility of directly obtaining the absolute of polygonal superconducting films with arbitrary sizes. To verify its accuracy and advantages, we extracted the values of square NbN films with different sizes using the FWC and conventional flux leakage subtraction (FLS) methods. Notably, the FLS method fails for a film, which is attributed to the significant current peak at the film edge. In contrast, the absolute extracted using the FWC method was independent of the film size. Finally, we established the applicability of the FWC method to large coil spacings, which may pave the way for integrating high-accuracy measurements with the ionic liquid gating technique. [ABSTRACT FROM AUTHOR]

We report herein the development of a superconducting nanowire single-photon detector (SNSPD) that uses a hybrid cryocooler compatible with space applications. The hybrid cryocooler incorporates a two-stage high-frequency pulse-tube cryocooler and a Joule–Thomson loop, which produces a minimum temperature of 2.2 K. The SNSPD with the structure of sandwiched nanowires integrated in the cryocooler shows a maximum detection efficiency of 93% at 1550 nm, which makes it promising for deep-space communications and quantum information applications. [ABSTRACT FROM AUTHOR]

Superconducting nanowire single-photon detectors (SNSPDs) made of ultrathin δ-NbN films have been widely applied in both visible and infrared wavelengths. For mid-infrared (MIR) wavelengths, SNSPDs made of tungsten silicide films with a lower critical temperature were reported up to 9.9 μm wavelength. In this study, we demonstrate the potential of NbN-SNSPDs for use in MIR applications. SNSPDs made of γ-Nb4N3 films (critical temperature of 5.1 K for 6.5 nm thick films) instead of δ-NbN films were fabricated. The dependence of the normalized detection efficiency on the bias current indicated a clear trend toward a saturated plateau for wavelengths up to 2145 nm. The calculated cut-off wavelengths indicated the possibility of using γ-Nb4N3 SNSPDs for longer MIR wavelengths. [ABSTRACT FROM AUTHOR]

Quantum computers promise to perform certain tasks that are believed to be intractable to classical computers. Boson sampling is such a task and is considered a strong candidate to demonstrate the quantum computational advantage. We performed Gaussian boson sampling by sending 50 indistinguishable single-mode squeezed states into a 100-mode ultralow-loss interferometer with full connectivity and random matrix—the whole optical setup is phase-locked—and sampling the output using 100 high-efficiency single-photon detectors. The obtained samples were validated against plausible hypotheses exploiting thermal states, distinguishable photons, and uniform distribution. The photonic quantum computer, Jiuzhang, generates up to 76 output photon clicks, which yields an output state-space dimension of 1030 and a sampling rate that is faster than using the state-of-the-art simulation strategy and supercomputers by a factor of ~1014. [ABSTRACT FROM AUTHOR]

We study superconducting properties of NbN thin film samples with different thicknesses and an ultra-thin NbTiN meander nanowire sample. For the ultra-thin samples, we found that the temperature dependence of upper critical field (Hc2) in parallel to surface orientation shows bending curvature close to critical temperature Tc, suggesting a two-dimensional (2D) nature of the samples. The 2D behavior is further supported by the angular dependence measurements of Hc2 for the thinnest samples. The temperature dependence of parallel upper critical field for the thick films could be described by a model based on the anisotropic Ginzburg–Landau theory. Interestingly, the results measured in the field perpendicular to the film surface orientation show a similar bending curvature but in a much narrow temperature region close to Tc for the ultra-thin samples. We suggest that this feature could be due to suppression of pair-breaking caused by local in-homogeneity. We further propose the temperature dependence of perpendicular Hc2 as a measure of uniformity of superconducting ultra-thin films. For the thick samples, we find that Hc2 shows maxima for both parallel and perpendicular orientations. The Hc2 peak for the perpendicular orientation is believed to be due to the columnar structure formed during the growth of the thick films. The presence of columnar structure is confirmed by transmission electron microscopy (TEM). In addition, we have measured the angular dependence of magneto-resistance, and the results are consistent with the Hc2 data. [ABSTRACT FROM AUTHOR]

We study superconducting properties of NbN thin film samples with different thicknesses and an ultra-thin NbTiN meander nanowire sample. For the ultra-thin samples, we found that the temperature dependence of upper critical field (Hc2) in parallel to surface orientation shows bending curvature close to critical temperature Tc, suggesting a two-dimensional (2D) nature of the samples. The 2D behavior is further supported by the angular dependence measurements of Hc2 for the thinnest samples. The temperature dependence of parallel upper critical field for the thick films could be described by a model based on the anisotropic Ginzburg–Landau theory. Interestingly, the results measured in the field perpendicular to the film surface orientation show a similar bending curvature but in a much narrow temperature region close to Tc for the ultra-thin samples. We suggest that this feature could be due to suppression of pair-breaking caused by local in-homogeneity. We further propose the temperature dependence of perpendicular Hc2 as a measure of uniformity of superconducting ultra-thin films. For the thick samples, we find that Hc2 shows maxima for both parallel and perpendicular orientations. The Hc2 peak for the perpendicular orientation is believed to be due to the columnar structure formed during the growth of the thick films. The presence of columnar structure is confirmed by transmission electron microscopy (TEM). In addition, we have measured the angular dependence of magneto-resistance, and the results are consistent with the Hc2 data. [ABSTRACT FROM AUTHOR]

We report a broadband superconducting nanowire single-photon detector for the near-infrared waveband. The optical absorptance in this band is enhanced by utilizing a reflective aluminum mirror and a SiO2 layer as an optical cavity. A broadband system detection efficiency (SDE) over 50% from 950 to 1600 nm with a low dark count rate of 10 Hz is realized by coupling a HI 1060 FLEX single-mode fiber. The SDEs reaches 78.3%, 70.9%, and 57.7% at the wavelengths of 1064, 1310, and 1550 nm, respectively. A system timing resolution (jitter) of 83 ps at 1550 nm is obtained. The detector may be useful for broadband applications such as earth observation, environment monitoring, and biomedicine. [ABSTRACT FROM AUTHOR]

Superconducting nanowire single-photon detectors (SNSPDs) with both an ultra-large-active area and a high count rate (CR) are challenging from the design and fabrication. Here, we develop a NbN SNSPD with a circular active area of 300 μm-in-diameter and use multiple approaches to boost the maximal count rate (MCR). This large-active-area SNSPD is divided into nine pixels (parts). Each pixel consists of serially connected two superconducting nanowire avalanche photodetectors (SC-2SNAP), which yielded a four-fold reduction in the kinetic inductance of a single pixel. To further increase MCR, an optimized series resistance of about 380 Ω is added to each pixel, reducing the full recovery time of each pixel from a few microseconds to approximately 90 ns. All nine pixels show uniform superconducting critical temperatures of ∼7.4 K and switching currents in the range of 15.3–16.7 μA. When the detector coupled to a single-mode fiber and illuminated with 1064-nm photons, the single-pixel exhibits a well-saturated system detection efficiency (SDE) of 67% at a dark CR of 30 Hz, and its CR exceeds 10 MHz with an SDE of 50%. When coupled to a 200-μm multi-mode fiber, the total SDE of nine pixels is approximately 42% and the total MCR exceeds 43 MHz. [ABSTRACT FROM AUTHOR]

Gaussian Boson sampling (GBS) provides a highly efficient approach to make use of squeezed states from parametric down-conversion to solve a classically hard-to-solve sampling problem. The GBS protocol not only significantly enhances the photon generation probability, compared to standard Boson sampling with single photon Fock states, but also links to potential applications such as dense subgraph problems and molecular vibronic spectra. Here, we report the first experimental demonstration of GBS using squeezed-state sources with simultaneously high photon indistinguishability and collection efficiency. We implement and validate 3-, 4- and 5-photon GBS with high sampling rates of 832, 163 and 23 kHz, respectively, which is more than 4.4, 12.0, and 29.5 times faster than the previous experiments. Further, we observe a quantum speed-up on a NP-hard optimization problem when comparing with simulated thermal sampler and uniform sampler. [ABSTRACT FROM AUTHOR]

An energy-time entanglement-based dispersive optics quantum key distribution (DO-QKD) is demonstrated experimentally over optical fibers of 20 km. In the experiment, the telecom band energy-time entangled photon pairs are generated through spontaneous four-wave mixing in a silicon waveguide. The arrival time of photons is registered for key generation and security test. High-dimensional encoding in the arrival time of photons is used to increase the information per coincidence of photon pairs. The bin sifting process is optimized by a three-level structure, which significantly reduces the raw quantum bit error rate (QBER) due to timing jitters of detectors and electronics. A raw key generation rate of 151 kbps with a QBER of 4.95% is achieved, in a time bin encoding format with 4 bits per coincidence. This experiment shows that the entanglement-based DO-QKD can be implemented in an efficient and convenient way, which has great potential for quantum secure communication networks in the future. [ABSTRACT FROM AUTHOR]

Superconducting nanowires single-photon detectors (SNSPDs or SSPDs) have emerged as an attractive single-photon detection technology with high performance. Two types of SNSPDs have been developed so far. One is the standard-fiber-coupled SNSPD with the light normally incident on the meandered nanowires, the other is waveguide-coupled SNSPD with the nanowires fabricated on the surface of the waveguide which guides photons while the fiber is coupled to the waveguide. Here we propose a new type of SNSPD integrated with microfiber. The photons are guided by a microfiber and evanescently absorbed by the nanowire of SNSPD when the nanowire is parallel and very close to the microfiber. The numerical simulation results show that the fiber to detector coupling efficiency can be close to unity with optimized device structure/parameters. With a minimal total NbN nanowire length of about 300 μ m, the absorption of the nanowire can reach 90%. [ABSTRACT FROM AUTHOR]

Reduction of the size of the superconducting quantum interference device (SQUID) ring structure improves its sensitivity to magnetic moments. Nanobridge junctions are advantageous for SQUID miniaturization. However, many planar SQUIDs with nanobridge junctions have shallow flux modulation depths because of their nonideal Josephson current-phase relationships. Here, we fabricated a three-dimensional niobium nitride (NbN) miniaturized SQUID in which the nanobridge junction structures are much thinner than the superconducting ring. We grew a thick superconducting NbN film on a silicon wafer and embedded an insulating slit in the middle of the film. By setting two thin nanobridge junctions across this insulating slit, we were able to manufacture NbN miniaturized SQUIDs that, in principle, can become nano-SQUIDs. The flux modulation depths of these devices reached 36% at 4.2 K. In addition, the fabricated devices show reversible current–voltage characteristics in the absence of any shunting resistance. [ABSTRACT FROM PUBLISHER]

In high-speed photon-counting communication where the slot duration is close to the scale of detector reset time, the detection efficiency recovery characteristics of superconducting nanowire single photon detector (SNSPD) become the key factors affecting counting and communication performance. We introduced the nonlinear detection efficiency recovery model derived from the kinetic inductance model combined with the nonlinear efficiency-current relationship into the pulse position modulation (PPM) optical communication system based on the SNSPD array, and studied the impact of the recovery characteristics on the counting and communication performance of the system. On this basis, we introduce the efficiency recovery model into the correction of the channel logarithmic likelihood ratio (LLR) to estimate the SNR of each slot more accurately. Relevant results show that compared with the existing Poisson channel model, fixed deadtime model and detection efficiency exponential recovery model, this modification combined with FEC can provide the best decoding performance, especially in scenarios with high PPM order, small array pixel number and strong noise level. Besides, the nonlinear detection efficiency recovery model also reflects the best consistency with the experimental measurements of the SNSPD array in terms of response count rates near saturation. [ABSTRACT FROM AUTHOR]

We report on the response of Ba122:Co meander micron line structures to pulsed 635 nm laser radiation. A 75 nm thick Ba122:Co film was patterned into 2 µm wide and 2.4 mm long meander micron line by a standard photolithographic process and dry etching in an argon ion-milling techniques. The Ba122:Co meander micron line show T c onset = 25.9 K and a critical current density of up to 1.7 × 106 A/cm2 at T = 15 K. The I-V properties of the sample show hysteretic current-voltage characteristics over a wide range of temperatures (5–15 K). The photo response of Ba122:Co meander micron line was studied at temperatures of 5 K and 15 K. At 15 K with I b = 0.9 I c = 2.40 mA, the pulse had a fast component with a rise time of 1.3 ns and a slower component with a fall time of 15.5 ns, making Ba122:Co a strong competitor in the development of high-temperature superconducting photon detectors. [ABSTRACT FROM AUTHOR]

In this article, an optical interconnection system from low temperature (4 K) to room temperature was built based on a conventional Distributed Feedback Laser. The curves of P–I–V and the spectra of Distributed Feedback Laser were characterized from 296K to 4K. The laser spectrum had a significant blue shift from 296 K to 4 K, the bandwidth of 16.18 GHz and 12 Gbps eye diagram was achieved. • The study of laser characteristics extends to 4 K instead of the common 77 K. • It has been proved that the conventional directly modulated laser can normally work in a low-temperature environment. • Commercial DFB has the potential to be used in the technology of optical interconnection between low and room temperatures. [ABSTRACT FROM AUTHOR]

Polarization is one of the key parameters of light. Most optical detectors are intensity detectors that are insensitive to the polarization of light. A superconducting nanowire single photon detector (SNSPD) is naturally sensitive to polarization due to its nanowire structure. Previous studies focused on producing a polarization-insensitive SNSPD. In this study, by adjusting the width and pitch of the nanowire, we systematically investigate the preparation of an SNSPD with high polarization sensitivity. Subsequently, an SNSPD with a system detection efficiency of 12% and a polarization extinction ratio of 22 was successfully prepared. [ABSTRACT FROM AUTHOR]

Abstract: Control of the in-plane crystallographic orientation of YBa2Cu3O7−x (YBCO) films on (100) MgO substrates is of significant application value due to the selective enhancement of superconducting properties. In the present work, the preparation, crystallographic and superconducting properties of YBCO films deposited on MgO substrates are reported. Crystallographic in-plane orientation was realized by means of tailoring the pulsed laser deposition conditions and the use of interfacial buffering structures. Superconductiong properties were measured for films having different in-plane orienations. The results indicate that the 0° in-plane oriented films showed the highest current density of 1.62MA/cm2 that was attributed to the elimination of high-angle grain boudaries. Additionally, the growth mechanism of YBCO films was discussed in terms of crystallographic and thermodynamic theory. [Copyright &y& Elsevier]

• We have developed a high-frequency analysis method to remove the intrinsic resonances of dc-SQUID amplifiers from the operating region of interest, so that the distortions in the IVΦ characteristics are eliminated and ensure stable operations for the dc-SQUID amplifier without excess noise. • Based on the proposed method, we designed and fabricated a dc-SQUID amplifier. The measured results verified the reliability of the proposed method and demonstrated smooth IVΦ characteristics in the operating region of interest and a white noise level of 1.7 μΦ 0 /√Hz, corresponding to a current noise level of 15 pA/√Hz. • Based on the simulated and measured results, we concluded that these resonance frequencies of the dc-SQUID amplifier mainly originates from the lossless embedded section without the JJ and the whole geometric size including the JJ. • Based on the simulated and measured results, we deduced that these intrinsic resonance frequencies from dc-SQUID amplifier may produce crossovers at the corresponding voltages in the I-V characteristics. Further measurements and simulations will be carried out in future work. We performed high-frequency simulation to optimize the geometric size of dc-SQUID amplifiers and to remove their intrinsic resonance characteristics from the operating range. The aim is to eliminate distortions in the IVΦ characteristics that ensures low noise and stable operation over a large range of bias current. With this method, we designed and fabricated a dc-SQUID amplifier with two series-connected octagonal loops to achieve a robust operating performance. The measured results not only show smooth IVΦ characteristics without any distortion, but also demonstrate that the swing voltage is much larger than the corresponding voltage realized without this method at the same bias current. In addition, the white flux noise level as low as 1.7 μΦ 0 /√Hz was achieved at 0.1 K corresponding to a low current noise level of 15 pA/√Hz. [ABSTRACT FROM AUTHOR]

In this paper, a generation scheme for telecom band hyper-entanglement is proposed and demonstrated based on the vector spontaneous four wave mixing (SFWM) processes in optical fibers. Two kinds of two-photon states are generated, one is hyper-entangled in the degree of freedoms (DOFs) of energy-time and polarization, the other is hyper-entangled in DOFs of energy-time and discrete-frequency. Experiments of Franson-type interference, two-photon interference under non-orthogonal polarization bases and spatial quantum beating are realized to demonstrate the entanglement in energy-time, polarization and frequency, respectively. This scheme provides a simple way to realize telecom band hyper-entanglement, which has potential for large geographic-scale applications of quantum communication and quantum information over optical fibers. [ABSTRACT FROM AUTHOR]