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arXiv:1812.06075 · IEEE Transactions on Applied Superconductivity

Geometrical Jitter and Bolometric Regime in Photon Detection by Straight Superconducting Nanowire

Artem A. Kuzmin, Steffen Doerner, Mariia Sidorova, Stefan Wuensch, Konstantin Ilin, Michael Siegel, Alexey Semenov

Published Mar 28, 2019• 6 citations
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Abstract

We present a direct observation of the geometrical jitter in single photon detection by a straight superconducting nanowire. Differential measurement technique was applied to the 180-μm long nanowire similar to those commonly used in the technology of superconducting nanowire single photon detectors (SNSPD). A non-Gaussian geometrical jitter appears as a wide almost uniform probability distribution (histogram) of the delay time (latency) of the nanowire response to detected photon. White electrical noise of the readout electronics causes broadened, Gaussian-shaped edges of the histogram. Subtracting noise contribution, we found for the geometrical jitter a standard deviation of ≈8.5 ps and the full width at half maximum (FWHM) of the distribution of ≈29 ps. FWHM corresponds to the propagation speed of the electrical signal along the nanowire of ≈6.2 × 106m/s or 0.02 of the speed of light. Alternatively the propagation speed was estimated from the central frequency of the measured first-order self-resonance of the nanowire. Both values agree well with each other and with previously reported values. As the intensity of the incident photon flux increases, the wide probability distribution collapses into a much narrower Gaussian distribution with a standard deviation dominated by the noise of electronics. We associate the collapse of the histogram with the transition from the discrete, single photon detection to the uniform bolometric regime.

DOI
10.1109/tasc.2019.2908132

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We present a direct observation of the geometrical jitter in single photon detection by a straight superconducting nanowire. Differential measurement technique was applied to the 180-μm long nanowire similar to those commonly used in the technology of superconducting nanowire single photon detectors (SNSPD). A non-Gaussian geometrical jitter appears as a wide almost uniform probability distribution (histogram) of the delay time (latency) of the nanowire response to detected photon. White electrical noise of the readout electronics causes broadened, Gaussian-shaped edges of the histogram. Subtracting noise contribution, we found for the geometrical jitter a standard deviation of ≈8.5 ps and the full width at half maximum (FWHM) of the distribution of ≈29 ps. FWHM corresponds to the propagation speed of the electrical signal along the nanowire of ≈6.2 × 106m/s or 0.02 of the speed of light. Alternatively the propagation speed was estimated from the central frequency of the measured first-order self-resonance of the nanowire. Both values agree well with each other and with previously reported values. As the intensity of the incident photon flux increases, the wide probability distribution collapses into a much narrower Gaussian distribution with a standard deviation dominated by the noise of electronics. We associate the collapse of the histogram with the transition from the discrete, single photon detection to the uniform bolometric regime.
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Detection jitter quantifies variance introduced by the detector in the determination of photon arrival time. It is a crucial performance parameter for systems using superconducting nanowire single photon detectors (SNSPDs). In this work, we have demonstrated that the detection timing jitter is limited in part by the spatial variation of the photon detection events along the length of the wire. We define this jitter source as a geometric jitter since it is related to the length and area of the SNSPD. To characterize the geometric jitter, we have constructed a differential cryogenic readout with less than 7 ps of an electronic jitter that can amplify the pulses generated from the two ends of an SNSPD. By differencing the measured arrival times of the two electrical pulses, we were able to partially cancel out the difference of the propagation times and thus reduce the uncertainty of the photon arrival time. We proved that the variation of the differential propagation time was a few ps for a 3 μm × 3 μm device, while it increased up to 50 ps for a 20 μm × 20 μm device. In a 20 μm × 20 μm large SNSPD, we achieved a 20% reduction in the overall detection timing jitter for detecting the telecom-wavelength photons by using the differential cryogenic readout.
We studied the effects of the external magnetic field and photon flux on timing jitter in photon detection by straight superconducting NbN nanowires. At two wavelengths 800 and 1560 nm, statistical distribution in the appearance times of photon counts exhibits Gaussian shape at small times and an exponential tail at large times. The characteristic exponential time is larger for photons with smaller energy and increases with external magnetic field while variations in the Gaussian part of the distribution are less pronounced. Increasing photon flux drives the nanowire from the discrete quantum detection regime to the uniform bolometric regime that averages out fluctuations of the total number of nonequilibrium electrons created by the photon and drastically reduces jitter. The difference between standard deviations of Gaussian parts of distributions for these two regimes provides the measure for the strength of electron-number fluctuations; it increases with the photon energy. We show that the two-dimensional hot-spot detection model explains qualitatively the effect of magnetic field.
Low timing jitter is a unique merit of superconducting nanowire single-photon detectors (SNSPDs) for time-correlated applications. Quantitative analysis was performed for the SNSPD system. Aided by an oscilloscope with an optimal signal amplitude, we were able to measure a full width at half-maximum system timing jitter as low as 14.2 ps for a high-switching-current SNSPD using a room-temperature low-noise amplifier. When using a time-correlated single-photon counting module, the system timing jitter was 17.3 ps. The detector's intrinsic timing jitter was estimated at ∼12.0 ps.
Superconducting nanowire single-photon detectors (SNSPDs) offer high detection speeds and excellent timing resolution, enabling numerous applications such as high-speed quantum communication and deep-space laser communication. However, at high count rates, timing jitter histograms deviate from a Gaussian profile and exhibit prominent secondary peaks, degrading jitter performance metrics such as full width at 10% maximum and full width at 1% maximum. This work identifies two distinct physical origins of these secondary peaks: multiphoton responses within a single excitation pulse and pulse pileup accompanied by time walk at high repetition rates. By experimentally isolating these mechanisms and applying corresponding statistical time-offset corrections, a near-Gaussian jitter distribution is restored. These results clarify the physics of secondary peaks in SNSPDs and provide a route to achieving simultaneous high counting rates and low timing jitter.
Superconducting nanowire single-photon detectors have emerged as a promising technology for quantum metrology from the mid-infrared to ultraviolet frequencies. Despite recent experimental successes, a predictive model to describe the detection event in these detectors is needed to optimize the detection metrics. Here, we propose a probabilistic criterion for single-photon detection based on single-vortex (flux quanta) crossing the width of the nanowire. Our model makes a connection between the dark counts and photon counts near the detection threshold. The finite-difference calculations demonstrate that a change in the bias current distribution as a result of the photon absorption significantly increases the probability of single-vortex crossing even if the vortex potential barrier has not vanished completely. We estimate the instrument response function and show that the timing uncertainty of this vortex tunneling process corresponds to a fundamental limit in timing jitter of the click event. We demonstrate a trade-space between this intrinsic (quantum) timing jitter, quantum efficiency, and dark count rate in TaN, WSi, and NbN superconducting nanowires at different experimental conditions. Our detection model can also explain the experimental observation of exponential decrease in the quantum efficiency of SNSPDs at lower energies. This leads to a pulse-width dependency in the quantum efficiency, and it can be further used as an experimental test to compare across different detection models.