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Applied Physics Letters

Analysis of the detection response of waveguide-integrated superconducting nanowire single-photon detectors at high count rate

Simone Ferrari, Vadim V. Kovalyuk, Andreas Vetter, Changhyoup Lee, Carsten Rockstuhl, Alexander V. Semenov, Gregory N Gol'tsman, Wolfram H. P. Pernice

Published Sep 2, 2019• 13 citations
Publisher

Abstract

Nanophotonic circuitry and superconducting nanowires have been successfully combined for detecting single photons, propagating in an integrated photonic circuit, with high efficiency and low noise and timing uncertainty. Waveguide-integrated superconducting nanowire single-photon detectors (SNSPDs) can nowadays be engineered to achieve subnanosecond recovery times and can potentially be adopted for applications requiring Gcps count rates. However, particular attention shall be paid to such an extreme count rate regime since artifacts in the detector functionality emerge. In particular, a count-rate dependent detection efficiency has been encountered that can compromise the accuracy of quantum detector tomography experiments. Here, we investigate the response of waveguide-integrated SNSPDs at high photon flux and identify the presence of parasitic currents due to the accumulation of charge in the readout electronics to cause the above-mentioned artifact in the detection efficiency. Our approach allows us to determine the maximum photon count rate at which the detector can be operated without adverse effects. Our findings are particularly important to avoid artifacts when applying SNSPDs for quantum tomography.

DOI
10.1063/1.5113652

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Nanophotonic circuitry and superconducting nanowires have been successfully combined for detecting single photons, propagating in an integrated photonic circuit, with high efficiency and low noise and timing uncertainty. Waveguide-integrated superconducting nanowire single-photon detectors (SNSPDs) can nowadays be engineered to achieve subnanosecond recovery times and can potentially be adopted for applications requiring Gcps count rates. However, particular attention shall be paid to such an extreme count rate regime since artifacts in the detector functionality emerge. In particular, a count-rate dependent detection efficiency has been encountered that can compromise the accuracy of quantum detector tomography experiments. Here, we investigate the response of waveguide-integrated SNSPDs at high photon flux and identify the presence of parasitic currents due to the accumulation of charge in the readout electronics to cause the above-mentioned artifact in the detection efficiency. Our approach allows us to determine the maximum photon count rate at which the detector can be operated without adverse effects. Our findings are particularly important to avoid artifacts when applying SNSPDs for quantum tomography.
Fast and efficient detection of single photons with high timing accuracy is a crucial requirement in most quantum optics experiments and enables novel sensing and imaging solutions. Superconducting nanowire single-photon detectors (SNSPD) achieve technology-leading performance in terms of detection efficiency, dark count rate, timing jitter, and detector dead times. However, conventional SNSPDs with high system detection efficiency typically rely on resonant enhancement of the absorption efficiency, thus only achieving attractive detector benchmarks over narrow spectral windows. Waveguide-integrated SNSPDs allow for leveraging the wideband material absorption in superconducting nanowires by absorbing light in a traveling-wave geometry but have been limited to low system detection efficiencies due to interface losses when coupling to optical fibers. Here, we show how high system detection efficiencies of 22%–73% are realized over a broad wavelength range from 532 nm to 1640 nm in a single waveguide-integrated SNSPD device. We accomplish efficient coupling between optical fibers and waveguide-integrated nanowire detectors by employing a 3D interface, produced in direct laser writing, that relies on total internal reflection for achieving a broad transmission bandwidth. We further find low timing jitter of 25.7 ps and detector decay times of 9.8 ns, allowing for single-photon counting with high repetition rates up to 100 MHz. Our work paves the way for an efficient single-photon detector solution that combines the spectral requirements of an extremely wide range of quantum optics experiments in a single device. The coupling approach and SNSPD-integration with nanophotonic circuits are further well-suited for realizing large-scale detector arrays.
Superconducting nanowire single-photon detectors (SNSPDs) provide high efficiency for detecting individual photons while keeping dark counts and timing jitter minimal. Besides superior detection performance over a broad optical bandwidth, compatibility with an integrated optical platform is a crucial requirement for applications in emerging quantum photonic technologies. Here we present SNSPDs embedded in nanophotonic integrated circuits which achieve internal quantum efficiencies close to unity at 1550 nm wavelength. This allows for the SNSPDs to be operated at bias currents far below the critical current where unwanted dark count events reach milli-Hz levels while on-chip detection efficiencies above 70% are maintained. The measured dark count rates correspond to noise-equivalent powers in the 10(-19) W/Hz(-1/2) range and the timing jitter is as low as 35 ps. Our detectors are fully scalable and interface directly with waveguide-based optical platforms.
Nanophotonic integrated circuits are emerging as a promising platform for quantum photonics. A key building block are waveguide integrated detectors with superior performance. Detectors based on superconductor nanowires (SNSPDs) attached to optical waveguides have been shown to provide high efficiency and good timing performance, as well as broad bandwidth. To move towards applications in high bandwidth quantum communication and processing, ultrafast single-photon detectors with high efficiency are needed. The speed of meander type SNSPDs is limited because the required high absorption efficiency necessitates long nanowires deposited on top of the waveguide. This enhances the kinetic inductance and makes the detectors slow. We overcome this problem by aligning the nanowire perpendicular to the waveguide to realize devices with a length below 1 µm. By integrating the nanowire into a photonic crystal cavity, we recover high absorption efficiency, thus enhancing the detection efficiency by more than an order of magnitude. Our cavity enhanced superconducting nanowire detectors are fully embedded in silicon nanophotonic circuits and efficiently detect single photons at telecom wavelengths. The detectors possess sub-nanosecond decay (~ 120 ps) and recovery times (~ 510 ps), and thus show potential for GHz count rates at low timing jitter (~ 32 ps).
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.
Superconducting Nanowire Single Photon Detectors (SNSPD) have set leading performance benchmarks in terms of detection efficiency, low noise operation, speed, and timing accuracy. Besides scaling up the numbers of individually addressable nanowire detector channels, integration with nanophotonic functionalities as well as optical and electrical multiplexing will be key to advance single photon counting applications, such as quantum key distribution, optical quantum information processing and simulations of sampling problems. Here we show how integration of SNSPDs with nanophotonic circuit components gives rise to novel use cases. We combine highperformance waveguide-integrated SNSPDs with nano-electromechanical phase-shifters, high speed electro-optic modulators, sub-wavelength grating structures, directional coupler networks, on-chip delay lines and highly efficient and broadband 3D-interconnects to optical fibers, achieving crucial active and passive functionalities for integrated quantum photonics. We showcase the capabilities and practical relevance of photonic integrated SNSPD solutions for the example of high-rate quantum key distribution receivers.
With recent advances in quantum technologies for applications such as communication, cryptography, computing, metrology and sensing, the performance and scalability of single-photon detection as a vital key component is becoming increasingly important. At the same time, ongoing efforts in the development of high-performance photonic integrated circuits (PIC) benefit the miniaturization and scalability of these quantum technologies. Waveguide-integrated superconducting nanowire single-photon detectors (WI-SNSPDs) allow to combine excellent performance metrics, such as high detection efficiency, low dark-count rates and low timing jitter below 20 ps with the scalability and functionality that PIC platforms such as Si3N4 provide. We have previously demonstrated broadband efficient single-photon detection with a single device over a range from visible to mid-infrared wavelengths and ultra-fast detector recovery times allowing for up to GHz count rates. Here, we present the utilization of WI-SNSPDs for discrete-variable quantum cryptography receivers with the complete photonic circuitry embedded together with the single-photon detectors on a single silicon chip, where the secret-key rates greatly benefit from the short recovery times of the detectors especially for metropolitan distances. We further realize a fully packaged 64 channel WI-SNSPD matrix for use in a wavelengthdivision multiplexed QKD setup.