Superconductivity - Part 22-3: Superconducting strip photon detector - Dark count rate

IEC 61788-22-3:2022 is applicable to the measurement of the dark count rate (DCR, RD) of superconductor strip photon detectors (SSPDs). It specifies terms, definitions, symbols and the measurement method of DCR that depends on the bias current (Ib) and operating temperature (T).
NOTE The data of measurement results in Annex A are based on measurements of one institute only. The standard will be updated after the data of a complete round robin test are available.

Supraconductivité - Partie 22-3: Détecteur de photons à bande supraconductrice - Taux de comptage en obscurité

L'IEC 61788-22-3:2022 s'applique au mesurage du taux de comptage en obscurité (DCR, RD) des détecteurs de photons à bande supraconductrice (SSPD). Elle spécifie les termes, les définitions, les symboles, ainsi que la méthode de mesurage du DCR qui dépend du courant de polarisation (Ib) et de la température de fonctionnement (T).
NOTE Les données des résultats de mesure figurant à l’Annexe A sont fondées sur les mesures d'un seul établissement. La norme sera mise à jour lorsque les données d'un essai interlaboratoire complet seront disponibles

General Information

Status
Published
Publication Date
18-Aug-2022
Technical Committee
Current Stage
PPUB - Publication issued
Completion Date
19-Aug-2022
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IEC 61788-22-3
Edition 1.0 2022-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Superconductivity –
Part 22-3: Superconducting strip photon detector – Dark count rate
Supraconductivité –
Partie 22-3: Détecteur de photons à bande supraconductrice – Taux de
comptage en obscurité
IEC 61788-22-3:2022-08(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 61788-22-3
Edition 1.0 2022-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Superconductivity –
Part 22-3: Superconducting strip photon detector – Dark count rate
Supraconductivité –
Partie 22-3: Détecteur de photons à bande supraconductrice – Taux de
comptage en obscurité
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.050 ISBN 978-2-8322-4070-0

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® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – IEC 61788-22-3:2022 © IEC 2022
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 8

2 Normative references ...................................................................................................... 8

3 Terms, definitions and abbreviated terms ........................................................................ 8

3.1 Terms and definitions .............................................................................................. 8

3.2 Abbreviated terms ................................................................................................. 10

4 Principle of the measurement method ............................................................................ 10

5 Apparatus ...................................................................................................................... 11

5.1 Detector packaging ............................................................................................... 11

5.2 Cryogenic system ................................................................................................. 11

5.3 Measurement system ............................................................................................ 13

6 Measurement procedure ................................................................................................ 14

6.1 Measurement of temperature ................................................................................ 14

6.2 Measurement of switching current ......................................................................... 14

6.3 Measurement of R .............................................................................................. 15

7 Standard uncertainty ..................................................................................................... 16

7.1 Type A uncertainty ................................................................................................ 16

7.2 Type B uncertainty ................................................................................................ 16

7.3 Uncertainty budget table ....................................................................................... 17

7.4 Uncertainty requirement ........................................................................................ 18

8 Test report ..................................................................................................................... 18

8.1 Identification of device under test (DUT) ............................................................... 18

8.2 Measurement conditions and results ..................................................................... 18

8.3 Miscellaneous optional report ............................................................................... 19

Annex A (informative) Results of the round robin test........................................................... 20

A.1 DUT packages ...................................................................................................... 20

A.2 Measurement conditions ....................................................................................... 20

A.3 Measurement results ............................................................................................. 21

Bibliography .......................................................................................................................... 25

Figure 1 – Example of one dark count pulse in the pulse train in inset .................................... 9

Figure 2 – Schematic curve of R as a function of normalized bias current ........................... 11

Figure 3 – Schematic diagram of a typical DCR measurement system ................................... 12

Figure 4 – Equivalent circuit of the DCR measurement .......................................................... 13

Figure 5 – Typical current-voltage (I-U) curve of an SSPD .................................................... 15

Figure A.1 – Photograph of the DUT with an SSPD and a temperature sensor ...................... 20

Figure A.2 – I-U curve and R curves ................................................................................... 22

Table 1 – Uncertainty budget table for R ............................................................................. 18

Table A.1 – Test data of DUT ................................................................................................ 22

Table A.2 – Temperature sensitivity and bias current sensitivity above a normalized

bias current of 0,9 ................................................................................................................. 23

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IEC 61788-22-3:2022 © IEC 2022 – 3 –

Table A.3 – u and u above a normalized bias current of 0,9 .............................................. 23

A B

Table A.4 – Budget table for R at a bias point of 5,25 µA (I /I = 0,955) .......................... 23

D b sw

Table A.5 – DCR values measured at a bias point of 5,25 µA (I /I = 0,955) ...................... 24

b sw

Table A.6 – Temperature measurement ................................................................................ 24

---------------------- Page: 5 ----------------------
– 4 – IEC 61788-22-3:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SUPERCONDUCTIVITY –
Part 22-3: Superconducting strip photon detector – Dark count rate
FOREWORD

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rights. IEC shall not be held responsible for identifying any or all such patent rights.

IEC 61788-22-3 has been prepared by IEC technical committee 90: Superconductivity. It is an

International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
90/489/FDIS 90/491/RVD

Full information on the voting for its approval can be found in the report on voting indicated in

the above table.
The language used for the development of this International Standard is English.

This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in

accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available

at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are

described in greater detail at www.iec.ch/standardsdev/publications.
---------------------- Page: 6 ----------------------
IEC 61788-22-3:2022 © IEC 2022 – 5 –

A list of all parts in the IEC 61788 series, published under the general title Superconductivity,

can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under webstore.iec.ch in the data related to the

specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it

contains colours which are considered to be useful for the correct understanding of its

contents. Users should therefore print this document using a colour printer.
---------------------- Page: 7 ----------------------
– 6 – IEC 61788-22-3:2022 © IEC 2022
INTRODUCTION

IEC 61788-22 (all parts) is a series of International Standards on superconductor electronic

devices. Superconductivity enables ultra-sensitive sensing or detection of a variety of

measurands. IEC 61788-22-1 [1] lists various types of superconductor sensors and detectors.

The strip type in this document is one of them.

A typical fundamental structure of strip type detectors is a meander superconductor line, for

example, with a thickness of less than 10 nm, a width of less than 100 nm or a few 100 nm, and

a length of a few mm. The structure is in the nanoscale. ISO TS 80004-2:2015 [2] defines the

nanoscale as a length range approximately from 1 nm to 100 nm. Because nano-objects have

one or two dimensions in the nanoscale, superconductor meander lines are categorized as a

nano-object.

The term "nanowire" is frequently used for superconductor meander lines, but it is not

recommended in this document. In the ISO vocabulary, a nanowire is defined as an electrically

conducting or semi-conducting nanofibre with two external dimensions in the nanoscale, with

the third dimension being significantly larger. The two external dimensions of the nanowires are

in the nanoscale range, approximately from 1 nm to 100 nm. When the first two dimensions

differ significantly, a "nanoplate," "nanoribbon," or "nanotape" shall be used for the meander

line shape. However, in the field of electronics, these terms are not common. In addition to the

ISO definition of nano-objects, the shape of the superconductor meander lines may not fit the

shape of common wires that have a round cross-section. Although there are cases in which a

superconductor line shape falls into the category of nanowire (e.g. a superconductor line with

a thickness of 10 nm and a width of 100 nm), the theoretical treatment of single photon detection

mechanisms still requires "strip" rather than "nanowire": the width is wider than coherence

length and thus the superconductor line has a two-dimensional nature. Therefore,

IEC 61788-22-1 assigns the word "strip" or "nanostrip" to the meander line shape. According to

the nomenclature of the standard, the strip type detector is called superconductor strip photon

detector (SSPD) or superconductor nanostrip photon detector (SNSPD). The abbreviated term

SSPD is used in this document.

SSPDs are usually cooled down to a temperature well below the critical temperature and

current-biased with a bias value close to, but smaller than, its switch current. The photon

detection mechanisms can be described by Cooper-pair breaking, leading to hotspot formation

or vortex motion, followed by electrothermal feedback creating a resistive region [3], [4].

Although an exact detection model has not been established yet, it is true that photon absorption

leads to Cooper pair breaking that creates quasiparticles because the photon energy in a

telecommunication wavelength band (~ 1 eV) is typically 2 to 3 orders of magnitude higher than

the binding energy of a Cooper pair (~ meV). The photon absorption may create a normal-

conducting local-hotspot in the nanostrip. With an electrothermal feedback process, the normal

conducting domain expands across the width of the nanostrip and along the current flow

direction, leading to a voltage drop in the superconductor nanostrip. Other possible models are

vortex-antivortex depairing, in which two vortices move toward the opposite strip edges, and

single vortex crossing. Such vortex motion also creates a voltage drop, which can be followed

by resistive domain creation with the same electrothermal feedback mechanism. Because of

the resistive domain in the strip, the bias current is diverted to a readout circuit. The normal

conducting region will be cooled down rapidly and finally disappear. The above process

produces a voltage pulse which corresponds to an event of single photon absorption.

Typical application areas of SSPDs include quantum information, laser communication, light

detection and ranging, fluorescence spectroscopy and quantum computing. The SSPDs

outperform such single photon detectors as photomultipliers and avalanche photodiodes in

performance measures listed in the next paragraph. Due to the increasing needs for ultra-

sensitive photon detection in a range of visible to mid-infrared wavelengths, the SSPD market

___________
Figures in square brackets refer to the Bibliography.
---------------------- Page: 8 ----------------------
IEC 61788-22-3:2022 © IEC 2022 – 7 –

is growing quickly. The standardization of SSPDs is beneficial to not only the industrial

application, but also detector development.

For photon detection, there are fundamental parameters, such as detection efficiency, timing

jitter, dead time and dark count rate. The dark count rate affects the measurement of other

parameters. For this reason, priority is given to the dark count rate. This document

(IEC 61788-22-3) defines a measurement method of dark count rate (DCR).
---------------------- Page: 9 ----------------------
– 8 – IEC 61788-22-3:2022 © IEC 2022
SUPERCONDUCTIVITY –
Part 22-3: Superconducting strip photon detector – Dark count rate
1 Scope

This part of IEC 61788 is applicable to the measurement of the dark count rate (DCR, R ) of

superconductor strip photon detectors (SSPDs). It specifies terms, definitions, symbols and the

measurement method of DCR that depends on the bias current (I ) and operating temperature

(T).

NOTE The data of measurement results in Annex A are based on measurements of one institute only. The standard

will be updated after the data of a complete round robin test are available.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
dark count
count recorded without any incident photon

Note 1 to entry: An example of one dark count is shown in Figure 1. The inset of Figure 1 shows a pulse train of

many dark counts, which have the same pulse shape.
---------------------- Page: 10 ----------------------
IEC 61788-22-3:2022 © IEC 2022 – 9 –
Figure 1 – Example of one dark count pulse in the pulse train in inset
3.1.2
dark count rate
DCR
number of dark counts per unit of time
Note 1 to entry: R is equal to the sum of R and R as defined below.
D Db Di
3.1.3
background dark count rate
DCR originating from blackbody radiation of optical components and stray photons
3.1.4
intrinsic dark count rate

DCR originating from spontaneous occurrence of resistance inside a superconductor strip

3.1.5
bias current

direct current flowing through a superconductor strip that forms an SSPD to hold operating

condition
3.1.6
switch current
maximum bias current for photon counting operation

Note 1 to entry: The I value can be determined as the highest supercurrent on a static current-voltage (I-U) curve.

Since a strip goes to normal conducting state locally by electrothermal feedback mechanism, the I value is usually

lower than the critical current, at which the whole strip becomes the normal conducting state.

---------------------- Page: 11 ----------------------
– 10 – IEC 61788-22-3:2022 © IEC 2022
3.1.7
normalized bias current
I /I
b sw
bias current divided by switch current
3.1.8
retrapping current

current at which an SSPD resumes a superconducting state from a normal conducting state

when the bias current is reduced from a high value above I
3.2 Abbreviated terms
R dark count rate
R background dark count rate
R intrinsic dark count rate
I bias current
I switch current
T temperature
t time interval
I retrapping current
V output pulse amplitude
N number of measurements at a specific I and T
u type A standard uncertainty of R
A D
u type B standard uncertainty of R
B D
4 Principle of the measurement method

DCR is divided into two components: background DCR (R ) that originates from blackbody

radiation of optical components and stray photons at any I value and intrinsic DCR (R ) that

b Di

originates from spontaneous occurrence of resistance inside superconductor strips and is

dominant in a high I region near I .
b sw

Figure 2 shows a schematic curve of the bias current dependence of R , which is called the R

D D

curve. In the measurement setup with an SSPD coupled to an optical fibre for signal input, the

R component is dominant in a low I region, while the R component is dominant in a high I

Db b Di b

region. The R component that has a relatively weak dependence on I and equals the product

Db b

of the detection efficiency and the sum of blackbody photons and stray photons. On the other

hand, the R component is related to the events of spontaneous voltage-drop occurrence

probably due to vortex dynamics related to inherent properties of superconductor strips.

Since R strongly depends on user’s environment, R curves shall be measured in a high bias

Db D

current region of I /I (> 0,8 in Figure 2), in which R is dominant with a negligible contribution

b sw Di
of R .

The R curves shall be measured by counting output pulses for a certain period at different I

D b

points while the temperature of the SSPD is held constant at an operating temperature

recommended by a manufacturer. There is an approximately linear relation between lg(R ) and

normalized bias current in I /I > 0,8, as shown in Figure 2.
b sw
---------------------- Page: 12 ----------------------
IEC 61788-22-3:2022 © IEC 2022 – 11 –
R is dominant in the low bias region.
R is dominant in the high bias region.
Figure 2 – Schematic curve of R as a function of normalized bias current
5 Apparatus
5.1 Detector packaging

Before characterizing an SSPD, it is necessary to make a detector package. For applications,

the most important purpose of packaging is to effectively couple the light to the SSPD active

area. A high coupling efficiency ensures a high detection efficiency. However, for the

measurement of R of the SSPD, optical coupling is optional.

When optical coupling is optionally installed, fibre optical coupling is one of the most common

methods. The optical fibre shall be fixed in the block with effective and stable light coupling to

the detector. The temperature of the fibre end shall be the same as the block to minimize R .

The fibre core shall be axially aligned to the SSPD active area surface to ensure good optical

coupling.

For the measurement of R , the SSPD shall be fixed to the packaging block using conductive

silver paste or low-temperature conducting epoxy to ensure good thermal contact. The SSPD

shall be surrounded by the block material so that no blackbody radiation causes a temperature

rise of the SSPD. The block should be made of oxygen-free copper and equipped with a radio

frequency (RF) connector.
5.2 Cryogenic system

The most commonly used cryogenic system for SSPD operation is a cryostat based on a closed-

cycle mechanical cryocooler, e.g., Gifford-McMahon (GM) cryocooler or a pulse-tube cryocooler,

which provides a base temperature of less than 4 K. The packaging block is mounted on a cold

head plate with good thermal contact to obtain the identical temperature as that of the plate. It

is noted that a geomagnetic field causes no observable change in DCR, so that a magnetic

shield is unnecessary.

The temperature of the packaging block shall be measured by a calibrated temperature sensor

during the R measurement. The procedure of the temperature measurement is provided in 6.1.

---------------------- Page: 13 ----------------------
– 12 – IEC 61788-22-3:2022 © IEC 2022

The fibre and coaxial cables should be installed inside the cryostat to provide the optical and

electronic connection between the detector package and the measurement circuit at room

temperature.

As shown in Figure 3, one end of the fibre (blue line) is fixed on the detector package. The

other end of the fibre is connected to a fibre connector (blue square) on the cryostat chamber

, the fibre should be removed, then
surface at room temperature. For the measurement of R
the detector is fully shielded from blackbody radiation and stray photons.
Figure 3 – Schematic diagram of a typical DCR measurement system
---------------------- Page: 14 ----------------------
IEC 61788-22-3:2022 © IEC 2022 – 13 –
Figure 4 – Equivalent circuit of the DCR measurement
5.3 Measurement system

The schematic diagram of a typical measurement system for the DCR measurement and the

equivalent circuit are shown in Figure 3 and Figure 4, respectively. The SSPD in the detector

package is connected to the bias tee through a coaxial cable. The voltage source in se

...

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