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IEEE Transactions on Quantum Engineering

Superconducting Nanostrip Photon-Number-Resolving Detector as an Unbiased Random Number Generator

Pasquale Ercolano, Mikkel Ejrnaes, Ciro Bruscino, Syed Muhammad Junaid Bukhari, Daniela Salvoni, Chengjun Zhang, Jia Huang, Hao Li, Lixing You, L. Parlato, Giovanni Piero Pepe

Published Jan 1, 2024• 6 citations
Publisher

Abstract

Detectors capable of resolving the number of photons are essential in many applications, ranging from classic photonics to quantum optics and quantum communication. In particular, photon-number-resolving detectors based on arrays of superconducting nanostrips can offer a high detection efficiency, a low dark count rate and a recovery time of a few nanoseconds. In this work, we use a detector of this kind for the unbiased generation of random numbers by following two different methods based on the detection of photons. In the former, we exploit the property that the light is equally distributed on each strip of the entire detector, whereas in the latter we exploit the fact that, for a high average number of photons, the parity of the Poissonian distribution of the number of photons emitted by the laser tends to be zero. In addition, since these two methods are independent, it is possible to use them at the same time.

DOI
10.1109/tqe.2024.3432070

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Detectors capable of resolving the number of photons are essential in many applications, ranging from classic photonics to quantum optics and quantum communication. In particular, photon-number-resolving detectors based on arrays of superconducting nanostrips can offer a high detection efficiency, a low dark count rate and a recovery time of a few nanoseconds. In this work, we use a detector of this kind for the unbiased generation of random numbers by following two different methods based on the detection of photons. In the former, we exploit the property that the light is equally distributed on each strip of the entire detector, whereas in the latter we exploit the fact that, for a high average number of photons, the parity of the Poissonian distribution of the number of photons emitted by the laser tends to be zero. In addition, since these two methods are independent, it is possible to use them at the same time.
Single photon detectors based on superconducting nanostrips demonstrated high performance at 1550 nm, combining high efficiency and low noise. Arranged in arrays, these devices can implement photon-number resolution, expanding the range of applicability of superconducting detectors. A detector of this kind was used in our recent work to implement the annihilation operator and the characterization of quantum states in terms of their photon number distribution. The reconstructed distributions were consistent with the theoretical predictions, exhibiting fidelities higher than 99.9%. In this work, this procedure was repeated with each pixel singularly to define a detector fidelity matrix, evaluating the fidelity among all the possible pixel combinations. In this way, we introduce an additional tool that quantifies not only the quality of the reconstructed photon number distribution when compared to the theoretical expectation, but also the mutual fidelity when different pixels are chosen as independent, and then the fabrication uniformity.
Photon-number-resolving detectors (PNRDs) are the ultimate optical sensors. Superconducting-nanostrip photon detectors (SNSPDs), traditionally known as ON-OFF detectors, have recently been found to have photon number resolving capability without multiplexing. This discovery positions them to become true PNRDs. However, their practical use is limited by the need to precisely detect tiny signal differences with low signal-to-noise ratios within sub-nanosecond time frames. We overcome this challenge using optical sampling with a dual-output Mach-Zehnder modulator (DO-MZM) and an ultra-short pulsed laser. By adjusting the DO-MZM's bias voltage to nearly balance the outputs, this method enables sensitive detection of picosecond-order signal differences, achieving a temporal resolution of 1.9 ps and facilitating real-time photon number resolution. We applied this method to produce various non-classical quantum states, enhancing their non-classicality through photon number resolution. This advancement marks a significant shift from principle verification to practical application for SNSPD-type PNRDs in diverse quantum optics fields.
Efficient and precise photon-number-resolving detectors are essential for optical quantum information science. Despite this, very few detectors have been able to distinguish photon numbers with both high fidelity and a large dynamic range, all while maintaining high speed and high timing precision. Superconducting nanostrip-based detectors excel at counting single photons efficiently and rapidly, but face challenges in balancing dynamic range and fidelity. Here, we have pioneered the demonstration of 10 true photon-number resolution using a superconducting microstrip detector, with readout fidelity reaching an impressive 98% and 90% for 4-photon and 6-photon events, respectively. Furthermore, our proposed dual-channel timing setup drastically reduces the amount of data acquisition by 3 orders of magnitude, allowing for real-time photon-number readout. We then demonstrate the utility of our scheme by implementing a quantum random-number generator based on sampling the parity of a coherent state, which guarantees inherent unbiasedness, robustness against experimental imperfections and environmental noise, as well as invulnerability to eavesdropping. Our solution boasts high fidelity, a large dynamic range, and real-time characterization for photon-number resolution and simplicity with respect to device structure, fabrication, and readout, which may provide a promising avenue towards optical quantum information science.
Photon number resolving detectors find space in many fields, such as quantum optics, boson sampling and fluorescence spectroscopy. In particular, the reconstruction of the input photon distribution is essential in quantum communications to detect photon-number-splitting attacks. In this work, we discuss the operation configurations of a photon number resolving detector based on superconducting nanostrips at a wavelength of 1550 nm from a temporal point of view. We set a time binning and acquired the number of recorded pulses per bin by means of a time-to-digital converter. We studied the predictions of two theoretical models and compared them to the experimental data in order to analyse their operation regimes depending on the binwidth and to employ them for the reconstruction of the input photon distribution. We applied this method to a continuous-wave laser source, showing that the former can be used for the characterization of non-pulsed light sources, even with a photon emission rate so low that the dark count rate of a superconducting nanostrip is not negligible.
Photon-number-resolving superconducting photon detectors promise to simplify detection schemes where only photon statistics matters in the few-photon regimes. Employed in one- and two-photon subtraction protocols, they allowed reconstruction of the photon number distribution for coherent and thermal states obtained from a pulsed laser at telecom wavelengths. The experimental setup is based on photon detection events recorded using a photon-number-resolving detector based on an array of eight superconducting NbN nanostrips working at 2.2 K. Photon subtraction was implemented using a low-reflectivity beam splitter and two superconducting nanostrip single photon detectors with an efficiency higher than 90% and a dark count rate of 1 cps. Our results confirm that our scheme accurately reproduces the theoretically expected effects of the annihilation operator on the states, achieving a fidelity higher than 99%.