Superconducting single photon detectors integrated with single flux quantum readoutcircuits in a cryocoolerShigehito Miki, Hirotaka Terai, Taro Yamashita, Kazumasa Makise, Mikio Fujiwara, Masahide Sasaki, and ZhenWang Citation: Applied Physics Letters 99, 111108 (2011); doi: 10.1063/1.3640503 View online: http://dx.doi.org/10.1063/1.3640503 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/99/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A new readout scheme for imaging TES based on cooling time measurement AIP Conf. Proc. 605, 325 (2002); 10.1063/1.1457656 TES detector noise limited readout using SQUID multiplexers AIP Conf. Proc. 605, 321 (2002); 10.1063/1.1457655 AC calorimeter bridge; a new multi-pixel readout method for TES calorimeter arrays AIP Conf. Proc. 605, 313 (2002); 10.1063/1.1457653 Single SQUID multiplexer for arrays of Voltage-biased Superconducting Bolometers AIP Conf. Proc. 605, 305 (2002); 10.1063/1.1457651 SQUID-based readout schemes for microcalorimeter arrays AIP Conf. Proc. 605, 295 (2002); 10.1063/1.1457649

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Superconducting single photon detectors integrated with single fluxquantum readout circuits in a cryocooler

Shigehito Miki,a) Hirotaka Terai, Taro Yamashita, Kazumasa Makise, Mikio Fujiwara,Masahide Sasaki, and Zhen WangNational Institute of Information and Communications Technology, 588-2, Iwaoka, Iwaoka-cho,Nishi-ku, Kobe, Hyogo 651-2492, Japan

(Received 22 July 2011; accepted 30 August 2011; published online 14 September 2011)

We report the operation of superconducting single photon detectors (SSPDs) with integrated

readout electronics based on a single-flux-quantum (SFQ) circuit. The SSPDs were connected to a

SFQ readout circuit without bias tee in a cryocooler system, and detection efficiencies (DEs) were

measured to verify their correct operation. The DE curves matched well with those measured by

conventional readout electronics, and the maximum bias current for normal operation was

increased. In addition, we confirmed that the SFQ circuit correctly merged the input signals

from two SSPDs. SSPDs with a SFQ readout circuit demonstrated a favorable timing jitter of 50 ps.VC 2011 American Institute of Physics. [doi:10.1063/1.3640503]

Due to their high detection efficiency (DE), excellent

timing resolution, high counting rate, and low dark count

rate (DCR),1–4 superconducting single photon detectors

(SSPD)5 have been recently employed in a wide range of

applications such as quantum key distribution,6,7 optical

communications,8 and quantum optics studies.9 In parallel

with these utilizations, efforts to further improve their per-

formance are ongoing. The primary concern of these efforts

is to resolve the trade-off between the maximum counting

rate and DE related with the size of the active area.4,10 Divid-

ing the active area into independent multi arrays is an attrac-

tive alternative to avoid this trade-off, because this

configuration can not only simultaneously achieve a high

counting rate and DE but in addition allow spatial and

pseudo-photon-number resolution.3 The realization of large

format SSPD arrays will have a great impact on various

application fields such as biomedical, fluorescent imaging,

and light detection and ranging (LIDAR).

One of the challenging tasks in realizing large format

SSPD arrays is the development of their readout electronics.

In the conventional readout technique used for single pixel

devices, the number of high-frequency coaxial cables

increases proportionally with the number of array elements.

This causes a significant increase in the heat load from room

temperature, which makes the implementation of the SSPD

arrays in a compact refrigerator difficult. To overcome this

problem, we previously proposed applying readout electron-

ics with superconducting single-flux-quantum (SFQ) logic

circuits.11 SFQ circuits can operate in a cryogenic environ-

ment with clock frequencies potentially exceeding 100

GHz.12 The power required to drive an SFQ circuit with 10

000 Josephson junctions (JJs) is typically in the order of a

few mW.13 Since the number of JJs required for the signal

processing of an SSPD array is estimated to be around 50 per

pixel,11 the SFQ circuit can support an SSPD array with

approximately 10 000 pixels in a relatively compact Gifford-

McMahon (GM) cryocooler with a cooling capacity of

0.1 W. By implementing the SFQ circuits and SSPDs on a

work surface, the number of coaxial cables introduced from

room temperature can be drastically reduced. In previous

work, we demonstrated the operation of an SSPD connected

to a SFQ readout circuit and confirmed almost identical DE

performance characteristics to those of conventional readout

electronics.14 However, during the demonstration, the SSPD

and the SFQ circuits were cooled separately by different cry-

osystems and were connected through long RF cables. In this

letter, we report on the operation of SSPDs connected to a

SFQ readout circuit, integrated in a single cryocooler system.

We investigate the DE and latching behaviors with various

readout configurations to verify their correct operations. The

timing jitter characteristics, which are of significant impor-

tance for most applications, are also verified and compared

with those measured by conventional readout electronics.

Fig. 1(a) shows a microphotograph of the SSPD device.

The SSPDs used in this experiment consist of an NbN mean-

dering nanowire covering an area of 15� 15 lm with an opti-

cal cavity structure on the nanowire area. The 4-nm-thick

NbN films were fabricated to be a 100-nm-wide nanowire

with 200-nm pitch. The optical cavity structure consists of an

Ag mirror and a k/4 SiO cavity. The nanowire superconduct-

ing critical temperature, Tc, and critical current, Ic, were

10.2–10.5 K and 29 lA, respectively. The SSPDs with optical

cavities were then mounted into compact fiber-coupled pack-

ages, which can achieve high optical coupling efficiencies.

The fabrication of the SSPD and its implementation in fiber-

coupled packages are described in detail elsewhere.1,15 Fig.

1(b) shows a microphotograph of the SFQ readout circuit used

in this experiment. The circuit was fabricated using an SRL

2.5 kA/cm2 Nb standard process.16 A front-end circuit, called

a magnetically coupled DC/SFQ (MC-DC/SFQ) converter,

consists of an input transformer connected to a 50 X load re-

sistor, and a DC/SFQ converter, where SFQ pulses are gener-

ated at the rising edge of each input pulse. The detailed circuit

parameters of the MC-DC/SFQ converter are described in

Ref. 11. Four MC-DC/SFQ converters were placed as input

ports for the SSPDs’ output signals, and the generated

SFQ pulses at the converters were transmitted to a

a)Author to whom correspondence should be addressed. Electronic mail:

s-miki@nict.go.jp.

0003-6951/2011/99(11)/111108/3/$30.00 VC 2011 American Institute of Physics99, 111108-1

APPLIED PHYSICS LETTERS 99, 111108 (2011)

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signal-processing unit, which can merge four inputs into one

output without any clock pulses. The merged signals were

then converted into rectangle voltage pulses by a voltage

driver. The voltage driver was designed so that the amplitude

and duration of the output voltage pulses were approximately

1.8 mV and 1.6 ns, respectively. This conversion of the pulse

shape is necessary to detect the SFQ pulses because the SFQ

pulse itself is a tiny voltage pulse with a maximum duration

of 4 ps. The fiber-coupled SSPD packages and SFQ readout

circuit were implemented in a pulse-tube cryocooler system

with a cooling capacity of 0.7 W and an operation temperature

of 2.8 K, as shown in Fig.1(c). The sample stage where the

SSPD and SFQ were placed was surrounded by a l-metal

cylinder to shield the magnetic field. Furthermore, the SFQ

circuit was inserted into a l-metal container to avoid any

vortex traps.

To verify the SSPD operation connected to the SFQ

readout circuit, the system DE and DCR of the SSPD device

as a function of the bias current to the SSPD (ISSPD) were

measured for three types of readout electronics configura-

tions. The schematic views of the three types of readout elec-

tronics configurations are shown in Fig. 2(a). The ISSPD was

estimated by dividing the applied voltage by the total resist-

ance placed between the voltage source and the SSPD. Dur-

ing measurements, a 1550 nm wavelength continuous laser

diode was used as an input photon source and was heavily

attenuated so that the output power at the input port was

equal to 106 photons per second. Fig. 2(b) shows the ISSPD

dependence of the system DE at a 1550 nm wavelength for

the three types of electronic readout configurations. Arrows

indicate the switching current points at which the device was

locked to a resistive state and did not produce any output

pulses. The DE and DCR curves measured by conventional

readout electronics are termed as “original curves” and are

denoted in Fig. 2(b) by blue squares. We then connected the

SSPD and SFQ circuit through a conventional bias tee (Pico-

second Pulse Labs. PSPL-8854A), denoted by the green line

in Fig. 2(a), and placed them into a low temperature sample

stage. The DEs and DCR measured by these readout electron-

ics are plotted by green triangles in Fig. 2(b). Even though the

DE and DCR curves match well with the original curve, the

SSPD went into a resistive state at the ISSPD of 26 lA, which

is lower than the ISSPD of 27.5 lA at which the SSPD went

into a resistive state when using the conventional readout elec-

tronics. This switching point reduction was observed only by

cooling the bias tee at 2.8 K regardless of the connection of

the SFQ readout circuit. To find out the reason, we checked

the capacitance value in the bias tee by using a precise LCR

meter, and the capacitance of 170 nF at room temperature was

reduced to 4.0 nF at 2.8 K. According to the simulation pro-

gram with integrated circuit emphasis (SPICE) using an

equivalent circuit of SSPD,4 reduction of the capacitance

results in an increase in overshooting of the ISSPD recovery.

The maximum current flow back to the SSPD was calculated

to be 27.5 lA, compared to the initial ISSPD of 26.0 lA for the

capacitance of 4.0 nF, while considerably lower overshooting

was calculated for the capacitance of 170 nF. This excess cur-

rent value almost corresponds to the switching point in con-

ventional readout electronics. In addition to this switching

problem, a large number of bias tee components will occupy a

lot of space in the cryocooler, making the implementation of

large-scale SSPD arrays difficult.

We next connected the SFQ readout circuit directly to

the SSPD through 10 cm-long coaxial cables without any

FIG. 1. (Color online) Photographs of (a) SSPD device, (b) SFQ readout cir-

cuit, and (c) implementation of two fiber-coupled SSPDs and a SFQ readout

circuit on a sample stage in a pulse tube cryocooler system.

FIG. 2. (Color online) (a) Schematic view of three readout electronics con-

figurations for SSPDs. The blue dashed line represents the conventional

readout electronics that do not use a SFQ readout circuit. The green dashed

line represents the readout electronics using a SFQ readout circuit connected

through a bias tee component. The red line represents the readout electronics

using a SFQ readout circuit without a bias tee component. (b) DE (filled

points) and DCR (unfilled points) versus the SSPD bias current for three

readout electronics configurations. Arrows indicate the current points at

which the device was locked to a resistive state and did not produce any out-

put pulses.

111108-2 Miki et al. Appl. Phys. Lett. 99, 111108 (2011)

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bias tee components. These connections are drawn as red

lines in Fig. 2(a). The bias current to the SSPD was supplied

through a 5 kX resistance made by molybdenum films on a

SFQ circuit chip to avoid invert current flow to the bias

source side. Since the resistance of the SSPD side including

the cable resistance was much lower than the load resistance

of 50 X in the SFQ readout circuit, even if the capacitance

was not placed between the SSPD and the SFQ readout cir-

cuit, almost all of the bias current would flow to the SSPD

side under the superconducting state. The DEs measured by

this configuration are plotted by red circles in Fig. 2(b). In

addition to the fact that the DE and DCR curves match well

with the original curves, the latching behavior rather favor-

ably improved when compared to conventional readout elec-

tronics because no overshooting associated with the insertion

of capacitance occurred. This result indicates that the SFQ

readout circuit can be directly connected to the SSPD with-

out the bias tees, which will conserve space in the cryo-

coolers. Furthermore, in the future, the SFQ readout circuit

can be integrated with the SSPD on the same chip, making

the implementation of large-scale SSPD arrays much easier.

The SFQ readout circuit shown in Fig. 1(b) has four input

ports and can merge these four inputs into one output. We

checked this merging function by connecting two SSPDs with

the SFQ readout circuit. When the number of output pulses

from the two SSPDs was set at around 10 000 and 5000,

respectively, the number of output pulses counted from the

SFQ readout circuit was around 15 000, indicating correct

operation of the merging function of the SFQ readout circuit.

The timing jitter of the SSPD with a SFQ readout circuit

against photon incident was measured by using a time-

correlated single photon counting module with 1 ps resolution.

A 1550 nm wavelength pulsed laser with 100 fs pulse width

was used as the photon source, and a synchronized trigger

pulse was used as input to the timing module. The output

pulse from the SFQ readout circuit through a low-noise ampli-

fier was also used as input to the timing module, and the time

correlation between the pulsed laser and the SSPD was

recorded. Fig. 3 shows the profile of the timing jitter, with and

without the SFQ readout circuit, at the ISSPD of 27 lA. The

timing jitter of the SSPD using the SFQ readout circuit was 50

ps full width at half maximum (FWHM), which is 10 ps wider

than that without using the SFQ circuit. It is noted that the

timing jitter was very sensitive to the ISSPD and the DC bias to

the MC-DC/SFQ converter. For example, the timing jitter at

the ISSPD of 24 lA was degraded to 57 ps, in spite of the fact

that the ISSPD was adequately higher than the minimum cur-

rent sensitivity of the SFQ readout circuit (�18.2 lA).14 This

indicates that the switching timing of the front-end JJ in the

MC-DC/SFQ converter significantly affects the timing jitter

and higher current sensitivity is required to achieve shorter

timing jitter. Further, optimization of the MC-DC/SFQ con-

verter circuit parameters will be necessary to improve the tim-

ing jitter, especially at low ISSPD values.

In conclusion, we integrated SSPDs and a SFQ readout

circuit in the same cryocooler system and verified their oper-

ation. Based on DE measurement results using various read-

out electronics configurations, we found the effective

connection between the SSPD and the SFQ that can achieve

correct and stable operation. Since this integrated readout

technology does not require any bias tee components, it may

even be possible to integrate the SSPD and SFQ readout cir-

cuit on the same chip. In addition, we confirmed the timing

jitter of 50 ps FWHM at an ISSPD of 27 lA, which is compa-

rable to that obtained by conventional readout electronics.

The results obtained by this work are important steps for-

ward in realizing practical large-scale SSPD array systems.

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FIG. 3. (Color online) SSPD timing jitter, with and without SFQ readout

circuit.

111108-3 Miki et al. Appl. Phys. Lett. 99, 111108 (2011)

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