High-speed detection of 1550 nm single photons with superconducting nanowire detectors Ioana Craiciu1 Boris Korzh1 Andrew D. Beyer1 Andrew Mueller12 Jason P . Allmaras1

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High-speed detection of 1550 nm single photons

with superconducting nanowire detectors

Ioana Craiciu1,*, Boris Korzh1, Andrew D. Beyer1, Andrew Mueller1,2, Jason P. Allmaras1,

Lautaro Narvaez

, Maria Spiropulu

, Bruce Bumble

, Thomas Lehner

, Emma E. Wollman

and Matthew D. Shaw1

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr, Pasadena, California 91109,

USA

2Division of Physics, Mathematics and Astronomy, California Institute of Technology, 1200 E California Blvd,

Pasadena, California 91125, USA

3Dotfast Consulting, Untere Sparchen 16, 6330 Kufstein, Austria

*Corresponding author: ioana.craiciu@jpl.nasa.gov

ABSTRACT

Superconducting nanowire single photon detectors are a key technology for quantum information and science due to their

high efﬁciency, low timing jitter, and low dark counts. In this work, we present a detector for single 1550 nm photons with

up to 78% detection efﬁciency, timing jitter below 50 ps FWHM, 158 counts/s dark count rate – as well as a world-leading

maximum count rate of 1.5 giga-counts/s at 3 dB compression. The PEACOQ detector (Performance-Enhanced Array for

Counting Optical Quanta) comprises a linear array of 32 straight superconducting niobium nitride nanowires which span

the mode of an optical ﬁber. This design supports high count rates with minimal penalties for detection efﬁciency and

timing jitter. We show how these trade-offs can be mitigated by implementing independent read-out for each nanowire

and by using a temporal walk correction technique to reduce count-rate dependent timing jitter. These detectors make

quantum communication practical on a 10 GHz clock.

1 INTRODUCTION

Superconducting nanowire single-photon detectors (SNSPDs) can detect single photons from mid-infrared [

] to

ultraviolet [

] wavelengths, with efﬁciencies up to

>98%

[

], dark count rates of

6×10−6/s

[

], timing jitter of

3 ps

FWHM [

], and count rates of up to 800 mega-counts/s (3 dB compression) [

]. They have enabled tests of local

realism [

], key demonstrations in quantum information processing [

] and quantum communication [

], and optical

communication between the earth and the moon [

]. New applications continue to emerge as their performance improves

[

]. As quantum communication protocols in particular are leveraging the low jitter of SNSPDs to increase clock rates to

10 GHz and beyond [

], it is increasingly important to measure at high count rates while maintaining low jitter and

high efﬁciency.

When an SNSPD absorbs a photon, a hotspot is created, which disrupts the superconductivity and shunts current from

the nanowire to the readout electronics [

]. The maximum rate at which an SNSPD can count photons is limited by

the timescale at which the hotspot dissipates. One strategy to increase count rate is to decrease the thermal reset time

by choice of superconducting material [

] or by using very short nanowires [

]. However, these strategies have led

to detectors with lower internal detection efﬁciency as indicated by a lack of saturation in photon count rate with bias

current, indicating a trade-off between fast thermal reset dynamics and detection efﬁciency. Another strategy to overcome

the count rate limit is to use several parallel nanowires to measure the signal of interest. In these devices, after one

nanowire detects a photon, the other wires are still superconducting and optimally biased for detection. For ﬁber-coupled

modes, efforts have focused on connecting the nanowires in parallel with a single readout line [

]. The count rate

achievable with this method was limited by electrical cross-talk between wires. High count rates have also be achieved

in free-space-coupled detector arrays [

], but these arrays used long nanowires which had signiﬁcant longitudinal

geometric timing jitter [22], leading to a jitter of greater than 50 ps full-width-at-half-maximum (FWHM).

In this work we present the PEACOQ detector, in which 32 short nanowires in a linear array are read out individually.

At the cost of increased readout complexity, this nanowire array can detect photons in a single 1550 nm optical mode at a

world-leading count rate of 1.5 giga-counts/s (Gcps) at 3 dB compression, along with a system detection efﬁciency (SDE)

of up to 78% and low timing jitter. We demonstrate a strategy for maintaining a timing jitter below 46 ps FWHM at count

arXiv:2210.11644v1 [quant-ph] 21 Oct 2022

rates up to 1 Gcps by building on a recently developed temporal walk correction technique [

]. A detector with high

efﬁciency and low timing jitter at high count rates, as well as low dark counts, is required to detect faint optical signals at

high clock rates, such as in quantum communication or deep-space optical communication where ampliﬁcation of optical

pulses is not feasible. For example, the PEACOQ can count single-photons at 250 Mcps with an SDE of 70% and, with

realistic improvements to our measurement setup, a jitter of 86 ps full-width-at-1%-maximum (FW1%M). This would

enable a quantum communication protocol based on a 10 GHz pulsed source with mean photon number per pulse at the

detector of hni=0.025.

2 PEACOQ OVERVIEW

Figure 1. a) Photograph of the PEACOQ detector. Inset is a scanning electron micrograph showing the nanowires,

microstrips (dark meandered lines), and coplanar waveguides (lighter lines connected to top and bottom of microstrips,

also visible on photograph). The active region, indicated by the arrow, is a linear array of nanowires covering an area of

13µm ×30µm

. b) Normalized efﬁciency (black circles, left axis) and dark count rate (red squares connected by solid line,

right axis) as a function of bias current in one nanowire. Solid black curve is a ﬁt to the internal detection efﬁciency

equation (see main text). c) Idetect (blue) and Iswitch (red) for all nanowires in the array. Note that the switching currents

measured in this readout conﬁguration are slightly lower than in b) due to different measurement set-ups (see

Measurement Setup section)

Figure 1a shows the PEACOQ detector. The active area of the detector consists of a linear array of 32 parallel niobium

nitride (NbN) nanowires on a 400 nm pitch. The wires are 120 nm wide,

30 µm

-long, straight sections (no meanders). A

two-part transmission line connects each side of each nanowire to bonding pads at the edges of the PEACOQ chip. The

ﬁrst part is an adiabatically tapered microstrip, comprised of a NbN conductor, a dielectric layer (SiO

), and a gold mirror

plane below. The second part is a coplanar waveguide (CPW) comprised of the NbN and contact metals (Ti/Nb/Ti/Au/Ti)

on SiO

(no gold plane), with a conductor width of

9 µm

, and a conductor-to-ground spacing of

3.625 µm

. There is a short

2/12

tapered region connecting the conductors of the microstrip and CPW. The symmetric coupling on both sides of the wire

allows for differential readout. In combination with a differential ampliﬁer, differential readout enables cancellation of

geometric jitter [

], common-mode noise rejection, and larger signal to noise, all of which can reduce overall timing

jitter.

The total inductance of the nanowire, which is dominated by the kinetic inductance of the superconducting NbN

(nanowire and waveguides), is a key design parameter that affects the maximum count rate and timing jitter. The

microstrips serve the purpose of increasing the kinetic inductance of the nanowires, to prevent latching. Short nanowires

are used in the design to minimize longitudinal geometric jitter [

], but the kinetic inductance of the

30 µm

-long nanowire

≈50

nH is too low, and would lead to latching of the device at low bias currents [

]. Each microstrip has a kinetic

inductance of

≈315

nH each, controlled by varying the width proﬁle along the taper, with

Lk=Lsq R`

w(s)ds

, where

Lsq =194 pH is the sheet inductance, w(s)is the width along the microstrip (which varies between 120 nm and 1.5 µm),

and

is the arc length of the microstrip. The microstrips are meandered to allow them to be roughly equal in length,

leading to equal inductance across the 32 nanowires in the array. The sheet inductance is estimated from the reset time of

the device

τreset =Lk

, which is measured in the Maximum Count Rate section. Device fabrication is described in the

Supplemental Materials.

Figure 1a also shows the overall shape of the detector chip. The circular area at the bottom in the picture is 2.5 mm in

diameter, designed to align with a standard FC/PC ﬁber ferrule. Self-alignment between the optical mode and the detector

active area [

] is enabled by a custom designed ceramic sleeve with a wider opening (1.9 mm) that ﬁts around both

ferrule and detector.

Figure 1b shows the normalized photon count rate (PCR) versus bias current for nanowire 16. This measurement is

done at low count rates – here 82 kcps at saturation. The saturation of the count rate for currents above

10.5 µA

indicates

unity internal detection efﬁciency (IDE), which means that any photon absorbed in the nanowire will create an output

pulse that can be detected [

]. The PCR curve was ﬁt to an idealized model of the IDE as a function of current in the

nanowire,

η=1

2erfc−Inanowire−Idetect

σ

, where

Idetect

is the inﬂection point and

is a ﬁt parameter describing the slope of

the inﬂection. At low count rates the current in the nanowire is assumed to be equal to the bias current (

Inanowire =Ibias

)

meaning the nanowire is not recovering from a previous detection when it absorbs a photon. Figure 1b also shows the dark

count rate (DCR) of the same nanowire. In this measurement, dark counts are mostly due to ambient photons coupling to

the detector, as indicated by the plateau of 0.4 counts/s at bias currents below

Iswitch

. No ﬁltering or special enclosure was

used to minimize the dark count rate. Shortly before

Iswitch

, the DCR shows the typical exponential increase. After

Iswitch

the nanowire enters a regime of relaxation oscillations, then the count rate starts decreasing, indicating latching. We deﬁne

Ilatch

as the nanowire current at which DCR starts to decrease. Figure 1c shows

Idetect

and

Iswitch

for all nanowires.

Idetect

values were similar, indicating that the nanowires are fairly uniform across the array, though the slightly higher variation

in switching current indicates varying degree of constriction in the wires. All wires had large plateaus in IDE vs. current.

Since the IDE exceeds

90%

for

Ibias >Idetect +σ

, a measure of the plateau is

Iswitch −(Idetect +σ)

, which ranged from

0.7 µA

3.9 µA

for the wires in the array, with an average of

2.8µA

. In order to take advantage of this long detection

plateau, the nanowires were biased at 95% of their individual switching currents for all measurements.

3 MEASUREMENT SETUP

The PEACOQ array was measured in two different cryostats at a temperature of

0.9

K. One cryostat (Setup A), was

used to read out all 32 channels simultaneously. Setup A was initially developed as a testbed for the Deep Space Optical

Communications project [

]. The second cryostat (Setup B) was used to perform low-jitter measurements of a single

detector channel. Further details of this setup can be found in

. In each case, the device was wire-bonded to a custom

PCB board, designed for differential read-out of each of the 32 nanowires, and packaged to accommodate self-aligned

ﬁber coupling [

]. One side of each nanowire was terminated with a

50 Ω

resistor at

0.9

K for a single-ended readout of

each wire. Future work will focus on a fully differential readout.

In Setup A, microcoaxial cables connected the signal from each nanowire to a cryogenic readout board at 40 K. The

cryogenic readout board contained a resistive bias-tee and a two-stage cryogenic ampliﬁer (DC-coupled HEMT and

SiGe LNA) for each channel. At room temperature, the pulses were further ampliﬁed by a third stage of ampliﬁers to an

amplitude of

≈500

mV, then converted to time stamps using a custom 128-channel time-to-digital converter (TDC) from

Dotfast Consulting. The optical source comprised either a continuous or pulsed laser at 1550 nm, a variable attenuator,

polarization controller, and a switch that sent light either to the PEACOQ or to a power meter. An SMF-28 optical ﬁber

was used to couple light to the detector. The face of the optical ﬁber ferrule had an anti-reﬂective coating optimized for

1550 nm. The pulsed laser had a pulse width of

0.5

ps and a repetition rate of 20 MHz. Using a phase-locked loop, the

sync signal of the pulsed laser was converted to a 10 MHz signal to synchronize the TDC. The timing jitter of the readout

3/12

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High-speeddetectionof1550nmsinglephotonswithsuperconductingnanowiredetectorsIoanaCraiciu1,*,BorisKorzh1,AndrewD.Beyer1,AndrewMueller1,2,JasonP.Allmaras1,LautaroNarvaez2,MariaSpiropulu2,BruceBumble1,ThomasLehner3,EmmaE.Wollman1,andMatthewD.Shaw11JetPropulsionLaboratory,CaliforniaInstituteofTechnology...

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High-speed detection of 1550 nm single photons with superconducting nanowire detectors Ioana Craiciu1 Boris Korzh1 Andrew D. Beyer1 Andrew Mueller12 Jason P . Allmaras1

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