PEACOQ and 2D-Correction
Higher order correction & the PEACOQ detector
The Performance Enhanced Array for Counting Optical Quanta (PEACOQ) is a new fiber-coupled SNSPD design that achieves high count rate by spreading the photon flux across a parallel array of short niobium nitride nanowires. Each wire may achieve count rates as high as 50 MCounts/s, so the 32-wire array as a whole can handle photon rates in excess of 1 GCounts/s. The PEACOQ is described in detail in Reference [1].
From early tests of the PEACOQ, it became evident that jitter increased dramatically at high single wire count rates. One of the overarching goals the the PEACOQ project was to demonstrate near-100 ps jitter at 1 GCount/s. Therefore, we investigated the possibility of applying time walk correction techniques to this detector. This began with collecting a calibration dataset like that discussed in the mode locked laser calibration section.
A 1-GHz repetition rate 1550 nm mode locked laser was used (Pritel UOC) for calibration. The 1 GHz repetition rate was chosen so that uncorrected jitter even at the highest count rates (approaching 400 ps at the FW1%M), was smaller than the laser period. Then, each time tag may be matched to the timing of the original optical pulse. A dataset with a count rate of 20 MCounts/s was used for calibration. At this rate, there is a good balance of statistics available for
The calibration process for the PEACOQ showed that high-rate pulse distortions are primarily due to amplifier effects and the overlap of RF pulses with the overshoot or ringing effects of previous RF pulses. This occurs because the PEACOQ was designed and fabricated to minimize the intrinsic reset time of the nanowire. The time it takes for bias current to re-saturate in the device is generally faster than the time for all amplifier effects to disappear following a previous RF pulse. Fig. 1 b is the delay vs.
As before with the meandered SNSPD, there is no requirement that the calibration only be used in an application that is based on the same repetition rate of 1 Ghz. As interpolation between points on the delay vs.
Second order calibration
The 2nd order time-walk correction is a new technique that builds on the methods previously introduced in this chapter. The intrinsic reset time of the PEACOQ nanowires is considerably shorter than the time it takes an RF pulse to return to a steady zero voltage. So multiple pulses can arrive in the time it takes one RF pulse to fully decay as seen by the timing electronics. Therefore, a given RF pulse can be level shifted not only by the presence of one pulse before, but even two or three before. The calibration and correction process was extended to correct a given pulse timing measurement based on two inter-pulse time measurements
Predominant features of the 2d calibration grid seem to be orthogonal and aligned to the axes. This is a result of the parametrization chosen for
In the limit of large
Proper handling of inter-pulse arrival measurements that fall outside the 2D grid is necessary for good correction performance. When both
Like the 1st-order correction, the 2nd-order method makes the assumption that the delays to be corrected are small relative to the inter-pulse times
Fig. 3 shows how the 1st and 2nd order corrections improve jitter of the one wire of the PEACOQ detector tested. Each of these response functions have full width at half maximum (FWHM), full width at one tenth maximum (FW1/10M) and full width at one hundredth maximum (FW1/100M) metrics. The FW1/100M width is relevant for quantum communication applications that design the fundamental experiment repetition rate or bin size based on this metric. We plot how these metrics scale with count rate for the single nanowire as shown in Fig. 4.
With data available over a wide range of single-wire count rates, and the correction method, we simulate the performance of the whole PEACOQ detector with time walk correction. This is based on matching count rate measurements ( Fig. 5 ) from the full array to count rates available in the single-wire dataset used for calibration and correction. The full-array rate measurements are from another cryostat and time tagger setup that supports full-readout of the PEACOQ
Fig. 6 a shows simulated full-array jitter response functions for 3 choice count rates, normalized from their peaks. The FWHM, FW1/10M, and FW1/100M width metrics are indicated with continuous, dashed, and dotted lines, respectively.
-
Craiciu, I., Korzh, B., Beyer, A. D., Mueller, A., Allmaras, J. P., Narváez, L., Spiropulu, M., et al. (2023). High-speed detection of 1550 & #x2009; & #x2009;nm single photons with superconducting nanowire detectors. Optica, 10(2), 183–190. doi:10.1364/OPTICA.478960 ↩