Ultrafast superconducting single-photon detector with a reduced activearea coupled to a tapered lensedsingle-mode fiber

Maria V. SidorovaAlexander V. DivochiyYury B. VakhtominKonstantin V. Smirnov

Ultrafast superconducting single-photon detectorwith a reduced active area coupled toa tapered lensed single-mode fiber

Maria V. Sidorova,a,* Alexander V. Divochiy,b Yury B. Vakhtomin,a,b andKonstantin V. Smirnova,b,c

aMoscow State Pedagogical University, 1 Malaya Pirogovskaya Street, Moscow 119992, RussiabClosed Joint Stock Company “Superconducting Nanotechnology” (Scontel),

5/22 Rossolimo Street, Moscow 119021, RussiacNational Research University Higher School of Economics, 20 Myasnitskaya Street,

Moscow 101000, Russia

Abstract. This paper presents an ultrafast niobium nitride (NbN) superconducting single-photondetector (SSPD) with an active area of 3 × 3 μm2 that offers better timing performance metricsthan the previous SSPD with an active area of 7 × 7 μm2. The improved SSPD demonstratesa record timing jitter (<25 ps), an ultrashort recovery time (<2 ns), an extremely low dark countrate, and a high detection efficiency in a wide spectral range from visible part to near infrared.The record parameters were obtained due to the development of a new technique providingeffective optical coupling between a detector with a reduced active area and a standard single-mode telecommunication fiber. The advantages of the new approach are experimentally con-firmed by taking electro-optical measurements. © 2015 Society of Photo-Optical InstrumentationEngineers (SPIE) [DOI: 10.1117/1.JNP.9.093051]

Keywords: superconductivity; superconducting single-photon detector; photon counting;single-mode fiber; detection efficiency; response time; timing jitter.

Paper 15028P received Apr. 24, 2015; accepted for publication Aug. 10, 2015; published onlineSep. 16, 2015.

1 Introduction

Superconducting single-photon detectors (SSPDs) are very promising for a variety of applica-tions1 requiring single-photon counting such as modern quantum optics, quantum information,2

and quantum dot photonics.3,4 Early SSPDs appeared more than a decade ago,5 and since thenthey have become the subject of extensive research worldwide.6–8 Before the advent of SSPDs,single-photon detection in the visible and near-infrared (IR) ranges was accomplished mainly byavalanche photodiodes (APDs).9 However, the spectral range of APDs is significantly limited bythe band gap of the semiconductor and, in addition, they suffer from such drawbacks as after-pulsing and a relatively high dark count rate.10 Today, SSPDs outperform APDs, since the formeroffer a lower noise level, a higher detection efficiency (DE), a higher dark count rate, and a lowertiming jitter in a wide spectral range from visible to near-IR. An attractive advantage of SSPDsis the feasibility of their integration into a single-photon counting system, which makes it pos-sible to keep the SSPD’s record parameters. This is one of the main factors making SSPDs verypromising for such applications as quantum key distribution (QKD)11 and bioluminescencedetection.12

In previous experiments, the active area of the detector was varied in a wide range.13–15

However, the active area was always selected such that it fully covered the fiber core to provideeffective coupling with a single-mode (SM) optical fiber. It is also known that the multielementSSPDs with a sufficient total area can be used for effective coupling with intrafiber radiation.3,16

*Address all correspondence to: Maria V. Sidorova, E-mail: sidorova.m.89@gmail.com

1934-2608/2015/$25.00 © 2015 SPIE

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The present paper describes an improved ultrafast SSPD with an active area of 3 × 3 μm2

coupled to an SM fiber. Owing to the new method of optical coupling, record high parameters ofthe new device were achieved.

2 Operation and Experimental Basis

2.1 Superconducting Single-Photon Detector Configuration and Principle ofOperation

The SSPD is made of a superconducting nanowire in the form of a meander. For operating asa detector, the superconducting nanowire is biased by a current at the operating temperature. Theabsorption of a single photon by the superconducting nanowire results in the formation of aregion with a nonequilibrium concentration of quasiparticles. That can cause the current densityto exceed the critical level and lead to the formation of a resistive region across the supercon-ducting nanowire and subsequent generation of a voltage response. For effective couplingbetween the SSPD and incident optical radiation, the nanowire was fabricated in the form ofa meander with a maximum possible filling factor and a size comparable with the core diameterof the SM fiber (about 9 μm). Typically, the SSPD is made of a superconducting strip having theform of a 7 × 7 μm2 meander with a total length of 0.25 mm. Such a long superconducting striphas a significant kinetic inductance, which exceeds the geometric (or magnetic) value. Thus, thekinetic inductance becomes the main factor determining the operating speed of the SSPD.17

2.2 Superconducting Single-Photon Detector Coupled with a Tapered LensedFiber and Experimental Setup

The SSPD presented herein was made of a 4-nm-thick NbN film deposited directly on a siliconsubstrate covered by a thermal SiO2 layer. Using electron-beam lithography, a 110-nm-widemeander-shaped nanowire was patterned in the film. The period and the active area of themeander were 200 nm and 3 × 3 μm2, respectively.

The goal was to retain the high DE of the system demonstrated earlier for SSPDs with an activearea of 7 × 7 μm2. To this end, a special tapered lensed fiber (manufactured by Nanonics ImagingLtd., Israel) was applied (Fig. 1). Note that the tapered lensed fibers and smaller area detectors wereapplied earlier but separately and for other purposes. No efforts to effectively match small-areadetectors and SM fibers were made. This work tackled the problem of matching a lensed fiber witha small-area detector, which cannot be solved by the self-alignment method. It is expected thatthe reduction of the active area will improve the detector’s timing performance.

One side of a standard SM fiber was given the shape of a cone with gradually decreasingdiameters of both the cladding and core with a microlens at the very end. The tapered lensedfiber thus designed could focus a beam coming out of the fiber into a spot with a diameter of3.0� 0.3 μm for radiation wavelengths of 1.2 to 1.6 μm with losses below 0.5 dB. Such a focal

Fig. 1 Microlens at the end of the standard telecommunication single-mode (SM) fiber. Light prop-agates from right to left and is focused into a spot with a diameter of 3.0� 0.3 μm at a distance ofseveral microns.

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spot is much smaller than the SM field diameter of standard SM fibers and effectively matchesthe reduced active area of the SSPD.

The tapered lensed fiber was placed in a standard ferrule connector for physical contact fer-rule channel 126 μm in diameter so that the focal plane of the lens coincided with the flat surfaceof the ferrule. In the tapered lensed fiber and ferrule assembly, a 15- to 20-μm shift of the beam’sspot center relative to the center of the ferrule channel was observed. Because of this, the authorscould not apply the most simple and useful method of self-aligned fiber coupling to the detector.This method is based on precision fabrication of a detector chip and placing it in a zirconia splitsleeve.18 Here, it is required that the fiber core-to-ferrule concentricity be no less than the align-ment accuracy, which usually does not exceed 2 μm. Therefore, for effective optical coupling, amethod was developed based on using a FINEPLACER lambda (Finetech GmbH, Germany)submicron placement system, which was adapted to our goals and combined with the ultraviolet(UV) curing technology. This method provides precision alignment of a magnified image of thedetector’s active area with a magnified image of the optical fiber and mechanical connection ofthe detector to the fiber. The detector chip was attached to the ferrule with the tapered lensed fiberinside using an UV-cured adhesive. Unfortunately, this adhesive is thermally unstable and failsafter several operation cycles of the detector. To provide the stability and long lifetime of thedetector after alignment and use the UV-cured adhesive, the unit was fixed by a mechanicalclamp. The alignment accuracy in this method is 0.5 μm or higher, which makes it possibleto achieve effective coupling between the detector and focused radiation coming out fromthe tapered lensed fiber. Thus, the SSPD chip is first glued and wire-bonded to a coplanar wave-guide line and then is precisely aligned with the tapered lensed fiber placed in a holder. Theholder, in turn, is placed on a cold plate inside a Gifford–McMahon multichannel cryogenicrefrigeration system, which can simultaneously cool down four SSPDs to 2.2 K. The SSPDchip with the coplanar line is connected via a CuNi coaxial cable to the DC bias and RFinput of a bias-T with a current source. The RF port of the bias-T is connected to a set of ultra-wideband amplifiers (1 MHz to 7 GHz) with a total gain of 43 dB. The total bandwidth ofthe readout circuit with regard to losses in the bias-T and coaxial cable exceeds 4 GHz.

3 Experimental Results

3.1 Detection Efficiency of the Superconducting Single-PhotonDetector System

The experiments conducted measured the DE of the SSPD system, which is defined as a ratio ofthe output count rate of the system to an input photon flux measured on the input of the SM fiber.Since the SSPD is a polarization-sensitive device, optical measurements were made for thepolarization direction at which the absorption of light is maximal. The number of 250- to300-mV voltage pulses at the output of the detector was measured by an Agilent 53131A

Fig. 2 Current dependences of the dark count rate (filled circles) and detection efficiency (DE) at1550 and 1310 nm (empty circles and empty triangles, respectively). An inset plots the DE againstthe dark count rate at 1550 and 1310 nm.

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(United States) pulse counter. The power of the incident optical beam at the input of the SM fiberwas determined with an Ophir IRG300 (United States) power meter with a sensitivity as high asseveral picowatts. DE measurements were carried out at an input photon flux of 108 photons∕sat two standard telecommunication wavelengths (power equal to 12.8 pW for 1550 nm and15.2 pW for 1310 nm). Such a photon flux corresponds to 106 cps on SSPD output for eachpercentage of the DE. The SSPD dark count rate was measured when the optical input wasblocked and background photons with wavelengths longer than 1.6 μm were filtered out.19

To check the stability of coupling between the tapered lensed fiber and detector, the dependenceof the DE on the bias current was measured before and after more than 10 thermal cycles.No changes in the DE were observed, which confirms a high thermal-cycling stability of ourcoupling method. The dependence of the DE on the bias current at wavelengths of 1310 and1550 nm after many thermal cycles is presented in Fig. 2. The inset to this figure shows thedependence of the DE on the dark count. At a 10-Hz dark count rate, the DE is roughlyequal to 28% and 23% at 1310 and 1550 nm, respectively. With regard to the SSPD opticalabsorptivity, the presented experimental results indicate the achievement of effective opticalcoupling between the SSPD and the tapered lensed fiber.

3.2 Superconducting Single-Photon Detector Output Pulse Duration

The kinetic inductance of the new ultrafast SSPD with an active area of 3 × 3 μm2 is smaller thanthat of the conventional device. This inductance is the main feature of a simple phenomenologi-cal model17 and determines the SSPD recovery time. Therefore, the decreased kinetic inductanceof the SSPD with a smaller active area makes it possible to improve the timing performance andrecovery time. However, it was shown20 that when the kinetic inductance of the SSPD dropsconsiderably, the so-called latchup effect arises, wherein the device is locked in a resistivestate and cannot detect photons. On the other hand, it has been reported that in detectorswith similar dimensions and kinetic inductances, the latchup effect was not observed at aload resistance of 50 Ω; it starts limiting the maximal bias current of the SSPD at higherload resistances.13,16,21 The shape of the pulsed voltage response from the detector shown inFig. 3(b) was determined using a Tektronix DPO70404C (United States) oscilloscope with abandwidth of 4 GHz. The 10% to 90% rise time of the voltage pulse does not exceed 200 ps,its fall time is no longer than 2 ns, and the FWHM of the voltage pulse is 1 ns. Thus, the fall timeof the pulse decreased approximately 10 times in comparison with the fall time in the conven-tional detector [Fig. 3(a)], which signifies a change in the kinetic inductance of the supercon-ducting strip while the pulse rise time changed twofold. This discrepancy cannot be explained byan insufficient bandwidth of the readout circuit and will be investigated in detail in the future.

3.3 Detector’s Dead Time

To measure the dead time of the detector, a method was employed that is similar to that describedby Kerman et al.17 The respective experimental setup is shown in Fig. 4(a). It consists of an

Fig. 3 Amplified output voltage pulses: (a) superconducting single-photon detector (SSPD) withan active area of 7 × 7 μm2 and (b) ultrafast SSPD with a 3 × 3 μm2 active area.

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adjustable optical delay stage and a PLS 780/5.5 1550-nm laser as a source of optical excitation.The laser pulse was several tens of picoseconds duration. The laser beam was split by a50%∕50% fiber beam splitter, and the resulting beams followed two optical paths. The firstbeam passed through the other 50%∕50% fiber beam splitter toward the ultrafast SSPD(arm 1). The second beam passed through the optical delay stage and then recombined withthe initial beam in the second beam splitter (arm 2). Both optical paths had fiber attenuatorsand polarization controllers for the powers and polarizations of the beams in the opticalpaths to be the same. The detector was biased by a current corresponding to a dark countrate of 10 Hz. The adjustable delay line was used to generate pairs of laser pulses passing tothe detector through the SM fiber.

Unlike Ref. 17, where the probability of detecting both pulses was measured as a function ofthe pulse separation, this study measured the probability of detecting only the second pulse asa function of the delay time. These measurements were made by the histogram method usinga Tektronix DPO70404C high-frequency oscilloscope. Two histograms were constructed forwhen the beams passed through the arms to the detector. Since the FWHM for each histogramis determined by the experimental setup and is kept constant during the experiment, the totalnumber of voltage pulses for each histogram (or the detection probability) is proportional to theheight of the histogram.

Figure 4(b) shows the normalized SSPD output voltage pulses that were obtained by takinghistograms at delay time range. It is seen that the height of the histogram changes when the timedelay becomes shorter than 4 ns. The detector’s dead time was determined as the delay time atwhich the histogram height changes by 3 dB: it was found to be about 2 ns. For delay times lessthan 1.5 ns, the histograms have a constant height of 0.2. This is explained by a response due tothe second photon in a pair in the absence of a response caused by the first photon. When the DEof the detector is 23%, the probability of such events is roughly equal to 0.18, which is in goodagreement with the measured value (0.2). Filled circles in Fig. 4(b) were obtained from the timedependence of the recovery current and the current dependence of the DE.

The measurements were taken at 1550 nm. Since the shorter wavelength DE of the detectortends to saturation, the dead time at a wavelength of about 1000 nm can be estimated as no longerthan 1.5 ns.

3.4 Timing Jitter

The timing jitter of our ultrafast SSPD system was measured (Fig. 5). As a source of photons,a 1560-nm pulsed laser (PErL, Avesta Ltd., Russia) was used. The SSPD operated at T ¼ 2 K

Fig. 4 (a) Experimental setup for dead-time measurements with the adjustable optical delay stageand (b) the normalized SSPD detection probability versus the delay time (histogram method). Thehistograms refer to the second optical pulse from the pair. Filled circles are obtained fromthe experimental data for the recovery current and DE on time.

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and was biased by a current corresponding to a 10-Hz dark count rate. The results shown inFig. 5 were obtained by histogram measurements using the DPO70404C Tektronix oscilloscope.For this ultrafast detection system, the jitter measured at the FWHM was below 25 ps, which isabout half the typical jitter value (<50 ps) for the conventional 7 × 7 μm2 SSPD. Similar mea-surements were taken using widely used Becker & Hickl (TCSPC B&H, Germany) photoncounting electronics, and the same jitter value was obtained.

4 Conclusions

An ultrafast single-photon detection system was developed based on an SSPD with a reducedactive area of 3 × 3 μm2. Using a tapered lensed SM fiber for optical input and effective SSPD-fiber optical coupling, this study managed to considerably improve the timing performance,while also saving a high system DE and a low dark count rate achieved earlier in the conventionalSSPD with an active area of 7 × 7 μm2 (Table 1).

Acknowledgments

This work was jointly supported by the Ministry of Education and Science of the RussianFederation (contract no. 3.2655.2014/K) and the program at the President of the RussianFederation in support of leading scientific schools (Grant No. 1918.20.2014.2).

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Fig. 5 Timing jitter histogram for the ultrafast SSPD system (the vertical axis plots normalizedcounts).

Table 1 Parameters of two types of SSPDs at 1550 nm.

ParametersSSPD with an activearea of 7 × 7 μm2

Ultrafast SSPD with anactive area of 3 × 3 μm2

DE (%) ∼25

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FWHM (ns) <5 <1

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Biographies for the authors are not available.

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