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Free-space laser communication technologies. Conference

A wide bandwidth source of weak coherent pulses using feedback from superconducting nanowire single photon detectors

Nathaniel Wilson, Brian E. Vyhnalek

Published Jan 24, 2025• 0 citations
Publisher

Abstract

In support of NASA Glenn Research Center’s Quantum Metrology Laboratory (NQML) and Real Time Optical Receiver project (RealTOR) a wide bandwidth source of weak coherent pulses has been implemented with a mode locked c-band laser in combination with a home-built pulse picker. For component characterization in both projects there is a need for a laser source which exhibits short pulses on the order of 1ps as well as a variable repetition rate and mean photon number per pulse. The setup that has been implemented is operable over a wide bandwidth ranging from approximately 4 MHz to 1.4 GHz, and variable mean photon number per pulse. The system implements a novel feedback mechanism using a superconducting nanowire single photon detector and high-resolution time tagger to optimize its extinction ratio via photon arrival times of the weak coherent pulses. This feedback allows for frequency domain studies to be performed with automatic control. The system design as well as characterization data of its performance are presented here.

DOI
10.1117/12.3043562

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In support of NASA Glenn Research Center’s Quantum Metrology Laboratory (NQML) and Real Time Optical Receiver project (RealTOR) a wide bandwidth source of weak coherent pulses has been implemented with a mode locked c-band laser in combination with a home-built pulse picker. For component characterization in both projects there is a need for a laser source which exhibits short pulses on the order of 1ps as well as a variable repetition rate and mean photon number per pulse. The setup that has been implemented is operable over a wide bandwidth ranging from approximately 4 MHz to 1.4 GHz, and variable mean photon number per pulse. The system implements a novel feedback mechanism using a superconducting nanowire single photon detector and high-resolution time tagger to optimize its extinction ratio via photon arrival times of the weak coherent pulses. This feedback allows for frequency domain studies to be performed with automatic control. The system design as well as characterization data of its performance are presented here.
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Single photon source is indispensable for realization of photon based quantum communication technologies such as quantum key distribution (QKD). Weak coherent pulse source is a practical, efficient and robust solution to generate stream of single photons, probabilistically. In this paper, a methodology for design and development of weak coherent pulse (WCP) source with high count rate of upto 10 Mcps is proposed and implemented. The generation of optical pulses with narrow pulse width and repetition rate of up to 100 MHz is achieved by developing a miniaturized pulse laser module. The weak coherent pulses are generated by heavily attenuating the laser pulses so as to reach the mean photon no. per pulse, μ, close to 0.1. WCP source performance in terms of mean photon number, second order coherence and photon count rate is also investigated. The measured g(2)[0] of ~0.98 shows that the developed WCP source has a very good quality.
Determining the precise number of photons in a weak optical pulse is of crucial importance for imaging, computing or communication applications that use single photons or few photons to increase the performance of classical techniques. Single photons or pair of photons can for instance be used to enhance the resolution of optical imaging techniques or to transmit information with ultimate security using quantum cryptography. In order to maintain this performance gain in such applications, the exact number of photons must be measured and monitored. In this contribution we present a method to measure the number of photons in an optical pulse by using 7 superconducting nanowire single photon detectors. Since a single SNSPD is not sensitive to the number of incoming photons, the optical signal multiplexing is necessary to measure photon correlations among the channels and thus reconstruct the input photons statistics. The readout and counting of the photon arrivals is performed with a fast multi-channel Asynchronous-Correlated-Digital-Counter (ACDC) implemented in the Programmable Logic (PL) of a Xilinx ZYNQ-7020 SoC device. The test of the technique has been performed coupling a weak laser via a fiber multiplexer to an array of individually fiber-coupled detectors.
Single-photon sources and detectors are key enabling technologies for photonics in quantum information science and technology (QIST). QIST applications place high-level demands on the performance of sources and detectors; it is therefore essential that their properties can be characterized accurately. Superconducting nanowire single-photon detectors (SNSPDs) have spectral sensitivity from visible to beyond 2 μm in wavelength, picosecond timing resolution (Jitter <100 ps FWHM) and the capacity to operate ungated with low dark counts (<1 kHz). This facilitates data acquisition at high rates with an excellent signal-to-noise ratio. We report on the construction and characterization of a two-channel SNSPD system. The detectors are mounted in a closed-cycle refrigerator, which eliminates reliance on liquid cryogens. Our specification was to deliver a system with 1% efficiency in both channels at a wavelength of 1310 nm with 1 kHz dark count rate. A full width at half maximum timing jitter of less than 90 ps is achieved in both channels. The system will be used to detect individual photons generated by quantum-optical sources at telecom wavelengths. Examples include single-photon sources based on quantum dots (emitting at 1310 nm). The SNSPD system's spectral sensitivity and timing resolution make it suited to characterization of such sources, and to wider QIST applications.
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.
We have developed a cryogenic measurement system for single-photon counting, which can be used in optical experiments requiring high time resolution in the picosecond range. The system utilizes niobium nitride superconducting nanowire single-photon detectors which are integrated in a time-correlated single-photon counting (TCSPC) setup. In this work, we describe details of the mechanical design, the electrical setup, and the cryogenic optical components. The performance of the complete system in TCSPC mode is tentatively benchmarked using 140 fs long laser pulses at a repetition frequency of 75 MHz. Due to the high temporal stability of these pulses, the measured time resolution of 35 ps (FWHM) is limited by the timing jitter of the measurement system. The result was cross-checked in a Coherent Anti-stokes Raman Scattering (CARS) setup, where scattered pulses from a β-barium borate crystal have been detected with the same time resolution.