Superconducting Nanowire Single Photon Detector with Optical Cavity

Increasing the detection efficiency （DE） is a hot issue in the development of the superconducting nanowire single photon detector （SNSPD）. In this work, a cavity-integrated structure coupled to the SNSPD is used to enhance the light absorption of nanowire. Ultra-thin Nb films are successfully prepared by magnetron sputtering, which are used to fabricate Nb/Al SNSPD with the curve of lOOnm and the square area of 4 × 4μm2 by sputtering and the lift-off method. To characterize the optical and electrical performance of the cavity-integrated SNSPD, a reliable cryogenic research system is built up based on a He3 system. To satisfy the need of light coupling, a packaging structure with collimator is conducted. Both DE and the dark count rates increase with lb. It is also found that the DE of SNSPD with cavities can be up to 0.17% at the temperature of 0.7K under the infrared light of 1550nm, which is obviously higher than that of the SNSPD directly fabricated upon silicon without any cavity structure.

A High-Efficiency Broadband Superconducting Nanowire Single-Photon Detector with a Composite Optical Structure

Superconducting nanowire single-photon detectors （SNSPDs） with a composite optical structure composed of phase-grating and optical cavity structures are designed to enhance both the system detection efficiency and the response bandwidth. Numerical simulation by the finite-difference time-domain method shows that the photon absorption capacity of SNSPDs with a composite optical structure can be enhanced significantly by adjusting the parameters of the phase-grating and optical cavity structures at multiple frequency bands. The absorption capacity of the superconducting nanowires reaches 70%, 72%, 60.73%, 61.7%, 41.2%, and 46.5% at wavelengths of 684, 850, 732, 924, 1256, and 1426nm, respectively. The use of a composite optical structure reduces the total filling factor of superconducting nanowires to only 0.25, decreases the kinetic inductance of SNSPDs, and improves the count rates.

Pulse-gated mode of commercial superconducting nanowire single photon detectors

High detection efficiency and low intrinsic dark count rate are two advantages of superconducting nanowire single photon detectors(SNSPDs).However,the stray photons penetrated into the fiber would cause the extrinsic dark count rate,owing to the free running mode of SNSPDs.In order to improve the performance of SNSPDs in realistic scenarios,stray photons should be investigated and suppression methods should be adopted.In this study,we demonstrate the pulsegated mode,with 500 kHz gating frequency,of a commercial SNSPD system for suppressing the response of stray photons about three orders of magnitude than its free-running counterpart on the extreme test conditions.When we push the gating frequency to 8 MHz,the dark count rate still keeps under 4% of free-running mode.In experiments,the intrinsic dark count rate is also suppressed to 4.56 × 10^(-2) counts per second with system detection efficiency of 76.4372%.Furthermore,the time-correlated single-photon counting analysis also approves the validity of our mode in suppressing the responses of stray photons.

Flux-to-voltage characteristic simulation of superconducting nanowire interference device

Inspired by recent discoveries of the quasi-Josephson effect in shunted nanowire devices,we propose a superconducting nanowire interference device in this study,which is a combination of parallel ultrathin superconducting nanowires and a shunt resistor.A simple model based on the switching effect of nanowires and fluxoid quantization effect is developed to describe the behavior of the device.The current-voltage characteristic and flux-to-voltage conversion curves are simulated and discussed to verify the feasibility.Appropriate parameters of the shunt resistor and inductor are deduced for fabricating the devices.

Photon number resolvability of multi-pixel superconducting nanowire single photon detectors using a single flux quantum circuit

Superconducting nanowire single-photon detectors(SNSPDs) are typical switching devices capable of detecting single photons with almost 100% detection efficiency. However, they cannot determine the exact number of incident photons during a detection event. Multi-pixel SNSPDs employing multiple read-out channels can provide photon number resolvability(PNR), but they require increased cooling power and costly multi-channel electronic systems. In this work, a single-flux quantum(SFQ) circuit is employed, and PNR based on multi-pixel SNSPDs is successfully demonstrated. A multi-input magnetically coupled DC/SFQ converter(MMD2 Q) circuit with a mutual inductance M is used to combine and record signals from a multi-pixel SNSPD device. The designed circuit is capable of discriminating the amplitude of the combined signals in accuracy of Φ_(0)/M with Φ_(0) being a single magnetic flux quantum. By employing the MMD2 Q circuit,the discrimination of up to 40 photons can be simulated. A 4-parallel-input MMD2 Q circuit is fabricated, and a PNR of3 is successfully demonstrated for an SNSPD array with one channel reserved for the functional verification. The results confirm that an MMD2 Q circuit is an effective tool for implementing PNR with multi-pixel SNSPDs.

Large-area superconducting nanowires fabricated based on laser exposure on photoresist

To realize large-scale micro/nanofabrication process in superconducting devices,the nano laser direct writing(NLDW)as a potential tool was implemented.In this paper,thermal effect induced laser-matter(superconducting film)interaction based on laser direct writing on Nb films and laser exposure on photoresist were investigated by simulation and experiment.To avoid nanoscale thermal effect on superconducting films,large-scale superconducting nanoarrays with the area up to 100μm×100μm based on laser exposure on photoresist were fabricated.Compared with laser direct writing on superconducting films,which lead to the degradation of superconducting performance such as critical current,transition temperature,the superconducting nanoarrays based on laser exposure on photoresist maintain excellent superconducting performance without degradation.Besides that,by further optimizing the process parameters and the thickness of photoresist,the nanowires with the width down to nanometers are plausible.Compared with the traditional electron beam and ion beam process,to some extent,the nanowires fabrication process based on NLDW provides a more efficient and cost-effective path for the fabrication of large-area superconducting devices/circuits.

Wideband cryogenic amplifier for a superconducting nanowire single-photon detector

We present a low-power inductorless wideband differential cryogenic amplifier using a 0.13-μm Si Ge Bi CMOS process for a superconducting nanowire single-photon detector(SNSPD).With a shunt-shunt feedback and capacitive coupling structure,theoretical analysis and simulations were undertaken,highlighting the relationship of the amplifier gain with the tunable design parameters of the circuit.In this way,the design and optimization flexibility can be increased,and a required gain can be achieved even without an accurate cryogenic device model.To realize a flat terminal impedance over the frequency of interest,an RC shunt compensation structure was employed,improving the amplifier’s closed-loop stability and suppressing the amplifier overshoot.The S-parameters and transient performance were measured at room temperature(300 K)and cryogenic temperature(4.2 K).With good input and output matching,the measurement results showed that the amplifier achieved a 21-d B gain with a 3-d B bandwidth of 1.13 GHz at 300 K.At 4.2 K,the gain of the amplifier can be tuned from 15 to 24 d B,achieving a 3-d B bandwidth spanning from 120 k Hz to 1.3 GHz and consuming only 3.1 m W.Excluding the chip pads,the amplifier chip core area was only about 0.073 mm^(2).

Geometric origin of intrinsic dark counts in superconducting nanowire single-photon detectors

The dark count is one of the key physical issues for superconducting nanowire single-photon detectors(SNSPDs)that limits various applications for optical quantum information and classical optics.When the bias current approaches the switching current of SNSPDs,the dark count is actually dominated by the intrinsic dark counts(iDCs).However,the origin of iDCs and its relation to constrictions remains unclear for practical SNSPDs.We herein systematically characterize the iDCs of the SNSPDs with and without artificial geometric constrictions by applying the differential readout method.For these devices with constrictions,we have observed distinct Gaussian distributions in the temporal distribution of iDCs,in which the time difference between the distributions is consistent with the geometric distance between constrictions,and the rates of iDCs produced by each constriction are in good agreement with constrictions'widths.With respect to practical SNSPDs,surprisingly,we also observe several Gaussian distributions in the temporal domain and it shows no significant dependence on the devices’sizes,demonstrating that the iDCs of SNSPDs are mainly dominated by a few specific constrictions.

Quest towards ultimate performance in superconducting nanowire single photon detectors

In 2007,superconducting nanowire single photon detectors(SSPD or SNSPD)[1]made an outstanding impact in the field of quantum information technology by demonstrating quantum key distribution(QKD)over a 200-km optical fiber with a 42-dB optical loss using a practical SNSPD system[2].This successful demonstration was realized thanks to its

Tunable coplanar waveguide resonator with nanowires

A tunable superconducting half-wavelength coplanar waveguide resonator （CPWR） with Nb parallel nanowires - 300 nm in width embedded in the center conductor was designed, fabricated, and measured. The frequency shift and the amplitude attenuation of the resonance peak under irradiation of 404-nm pulse laser were observed with different light powers at 4.2 K. The RF power supplied to such a CPWR can serve as current bias, which will affect the light response of the resonator.

32×32 NbN SNSPD array based on thermally coupled row-column multiplexing architecture

We report a superconducting nanowire single‐photon detector(SNSPD)array aiming for a near‐infrared 1550‐nm wavelength that consists of 32×32 nanowire pixels and an area of 0.96 mm×0.96 mm.Unlike most reported large‐scale SNSPD arrays with amorphous films,NbN superconducting nanowires are employed in our array,which allows the detector operation at 2.3 K provided by a compact two‐stage Gifford–McMahon cryocooler.Thermally coupled row–column multiplexing is employed in our arrays to avoid current redistribution and loss of electrical signal occurring in the electrically coupled row–column architecture.The fabricated detector array shows a pixel yield of 94%and maximal intrinsic efficiencies of 77%and 96%at 1550 nm and 405 nm,respectively.The timing jitter and the thermal coupling probability are also investigated.