<s>
The	O
superconducting	B-Algorithm
nanowire	I-Algorithm
single-photon	I-Algorithm
detector	I-Algorithm
(	O
SNSPD	B-Algorithm
or	O
SSPD	O
)	O
is	O
a	O
type	O
of	O
optical	O
and	O
near-infrared	O
single-photon	O
detector	O
based	O
on	O
a	O
current-biased	O
superconducting	O
nanowire	B-Algorithm
.	O
</s>
<s>
As	O
of	O
2021	O
,	O
a	O
superconducting	B-Algorithm
nanowire	I-Algorithm
single-photon	I-Algorithm
detector	I-Algorithm
is	O
the	O
fastest	O
single-photon	O
detector	O
(	O
SPD	O
)	O
for	O
photon	B-Application
counting	O
.	O
</s>
<s>
SNSPDs	B-Algorithm
are	O
available	O
with	O
very	O
high	O
detection	O
efficiency	O
,	O
very	O
low	O
dark	O
count	O
rate	O
and	O
very	O
low	O
timing	O
jitter	O
,	O
compared	O
to	O
other	O
types	O
of	O
single-photon	O
detectors	O
.	O
</s>
<s>
As	O
of	O
2021	O
,	O
commercial	O
SNSPD	B-Algorithm
devices	O
are	O
available	O
in	O
multichannel	O
systems	O
in	O
a	O
price	O
range	O
of	O
100,000	O
euros	O
.	O
</s>
<s>
The	O
SNSPD	B-Algorithm
consists	O
of	O
a	O
thin	O
( ≈	O
5nm	O
)	O
and	O
narrow	O
( ≈	O
100nm	O
)	O
superconducting	O
nanowire	B-Algorithm
.	O
</s>
<s>
The	O
length	O
is	O
typically	O
hundreds	O
of	O
micrometers	O
,	O
and	O
the	O
nanowire	B-Algorithm
is	O
patterned	O
in	O
a	O
compact	O
meander	O
geometry	O
to	O
create	O
a	O
square	O
or	O
circular	O
pixel	O
with	O
high	O
detection	O
efficiency	O
.	O
</s>
<s>
The	O
nanowire	B-Algorithm
is	O
cooled	O
well	O
below	O
its	O
superconducting	O
critical	O
temperature	O
and	O
biased	O
with	O
a	O
DC	O
current	O
that	O
is	O
close	O
to	O
but	O
less	O
than	O
the	O
superconducting	O
critical	O
current	O
of	O
the	O
nanowire	B-Algorithm
.	O
</s>
<s>
A	O
photon	B-Application
incident	O
on	O
the	O
nanowire	B-Algorithm
breaks	O
Cooper	O
pairs	O
and	O
reduces	O
the	O
local	O
critical	O
current	O
below	O
that	O
of	O
the	O
bias	O
current	O
.	O
</s>
<s>
The	O
time	O
for	O
the	O
current	O
to	O
return	O
to	O
the	O
nanowire	B-Algorithm
is	O
typically	O
set	O
by	O
the	O
inductive	O
time	O
constant	O
of	O
the	O
nanowire	B-Algorithm
,	O
equal	O
to	O
the	O
kinetic	O
inductance	O
of	O
the	O
nanowire	B-Algorithm
divided	O
by	O
the	O
impedance	O
of	O
the	O
readout	O
circuit	O
.	O
</s>
<s>
Proper	O
self-resetting	O
of	O
the	O
device	O
requires	O
that	O
this	O
inductive	O
time	O
constant	O
be	O
slower	O
than	O
the	O
intrinsic	O
cooling	O
time	O
of	O
the	O
nanowire	B-Algorithm
hotspot	O
.	O
</s>
<s>
While	O
the	O
SNSPD	B-Algorithm
does	O
not	O
match	O
the	O
intrinsic	O
energy	O
or	O
photon-number	O
resolution	O
of	O
the	O
superconducting	O
transition	B-Algorithm
edge	I-Algorithm
sensor	I-Algorithm
,	O
the	O
SNSPD	B-Algorithm
is	O
significantly	O
faster	O
than	O
conventional	O
transition	B-Algorithm
edge	I-Algorithm
sensors	I-Algorithm
and	O
operates	O
at	O
higher	O
temperatures	O
.	O
</s>
<s>
A	O
degree	O
of	O
photon-number	O
resolution	O
can	O
be	O
achieved	O
in	O
SNSPD	B-Algorithm
arrays	O
,	O
through	O
time-binning	O
or	O
advanced	O
readout	O
schemes	O
.	O
</s>
<s>
Most	O
SNSPDs	B-Algorithm
are	O
made	O
of	O
sputtered	O
niobium	O
nitride	O
(	O
NbN	O
)	O
,	O
which	O
offers	O
a	O
relatively	O
high	O
superconducting	O
critical	O
temperature	O
( ≈	O
10K	O
)	O
which	O
enables	O
SNSPD	B-Algorithm
operation	O
in	O
the	O
temperature	O
range	O
1	O
K	B-Operating_System
to	O
4	O
K	B-Operating_System
(	O
compatible	O
with	O
liquid	O
helium	O
or	O
modern	O
closed-cycle	O
cryocoolers	O
)	O
.	O
</s>
<s>
The	O
intrinsic	O
thermal	O
time	O
constants	O
of	O
NbN	O
are	O
short	O
,	O
giving	O
very	O
fast	O
cooling	O
time	O
after	O
photon	B-Application
absorption	O
(	O
<	O
100	O
picoseconds	O
)	O
.	O
</s>
<s>
The	O
absorption	O
in	O
the	O
superconducting	O
nanowire	B-Algorithm
can	O
be	O
boosted	O
by	O
a	O
variety	O
of	O
strategies	O
:	O
integration	O
with	O
an	O
optical	O
cavity	O
,	O
integration	O
with	O
a	O
photonic	O
waveguide	O
or	O
addition	O
of	O
nanoantenna	O
structures	O
.	O
</s>
<s>
SNSPD	B-Algorithm
cavity	O
devices	O
in	O
NbN	O
,	O
NbTiN	O
,	O
WSi	O
&	O
MoSi	O
have	O
demonstrated	O
fibre-coupled	O
device	O
detection	O
efficiencies	O
greater	O
than	O
98%	O
at	O
1550nm	O
wavelength	O
with	O
count	O
rates	O
in	O
the	O
tens	O
of	O
MHz	O
.	O
</s>
<s>
They	O
vary	O
widely	O
,	O
however	O
,	O
due	O
to	O
highly	O
localized	O
regions	O
of	O
the	O
nanowires	B-Architecture
where	O
the	O
effective	O
cross-sectional	O
area	O
for	O
superconducting	O
current	O
is	O
reduced	O
.	O
</s>
<s>
SNSPD	B-Algorithm
devices	O
have	O
also	O
demonstrated	O
exceptionally	O
low	O
jitter	O
–	O
the	O
uncertainty	O
in	O
the	O
photon	B-Application
arrival	O
time	O
–	O
as	O
low	O
as	O
3	O
picoseconds	O
at	O
visible	O
wavelengths	O
.	O
</s>
<s>
Timing	O
jitter	O
increases	O
as	O
photon	B-Application
energy	O
drops	O
and	O
has	O
been	O
verified	O
out	O
to	O
3.5	O
micrometres	O
wavelength	O
.	O
</s>
<s>
Timing	O
jitter	O
is	O
an	O
extremely	O
important	O
property	O
for	O
time-correlated	O
single-photon	O
counting	O
(	O
TCSPC	O
)	O
applications	O
.	O
</s>
<s>
Furthermore	O
,	O
SNSPDs	B-Algorithm
have	O
extremely	O
low	O
rates	O
of	O
dark	O
counts	O
,	O
i.e.	O
</s>
<s>
the	O
occurrence	O
of	O
voltage	O
pulses	O
in	O
the	O
absence	O
of	O
a	O
detected	O
photon	B-Application
.	O
</s>
<s>
For	O
the	O
detection	O
of	O
longer	O
wavelength	O
photons	B-Application
,	O
however	O
,	O
the	O
detection	O
efficiency	O
of	O
standard	O
SNSPDs	B-Algorithm
decreases	O
significantly	O
.	O
</s>
<s>
Recent	O
efforts	O
to	O
improve	O
the	O
detection	O
efficiency	O
at	O
near-infrared	O
and	O
mid-infrared	O
wavelengths	O
include	O
studies	O
of	O
narrower	O
(	O
20nm	O
and	O
30nm	O
wide	O
)	O
NbN	O
nanowires	B-Architecture
as	O
well	O
as	O
extensive	O
studies	O
of	O
alternative	O
superconducting	O
materials	O
with	O
lower	O
superconducting	O
critical	O
temperatures	O
than	O
NbN	O
(	O
tungsten	O
silicide	O
,	O
niobium	O
silicide	O
,	O
molybdenum	O
silicide	O
and	O
tantalum	O
nitride	O
)	O
.	O
</s>
<s>
Single	O
photon	B-Application
sensitivity	O
up	O
to	O
10	O
micrometer	O
wavelength	O
has	O
recently	O
been	O
demonstrated	O
in	O
a	O
tungsten	O
silicide	O
SNSPD	B-Algorithm
.	O
</s>
<s>
Alternative	O
thin	O
film	O
deposition	O
techniques	O
such	O
as	O
atomic	O
layer	O
deposition	O
are	O
of	O
interest	O
for	O
extending	O
the	O
spectral	O
range	O
and	O
scalability	O
of	O
SNSPDs	B-Algorithm
to	O
large	O
areas	O
.	O
</s>
<s>
High	O
temperature	O
superconductors	O
have	O
been	O
investigated	O
for	O
SNSPDs	B-Algorithm
with	O
some	O
encouraging	O
recent	O
reports	O
.	O
</s>
<s>
SNSPDs	B-Algorithm
have	O
been	O
created	O
from	O
magnesium	O
diboride	O
with	O
some	O
single	O
photon	B-Application
sensitivity	O
in	O
the	O
visible	O
and	O
near	O
infrared	O
.	O
</s>
<s>
There	O
is	O
considerable	O
interest	O
and	O
effort	O
in	O
scaling	O
up	O
SNSPDs	B-Algorithm
to	O
large	O
multipixel	O
arrays	O
and	O
cameras	O
.	O
</s>
<s>
A	O
kilopixel	O
SNSPD	B-Algorithm
array	O
has	O
recently	O
been	O
reported	O
.	O
</s>
<s>
Many	O
of	O
the	O
initial	O
application	O
demonstrations	O
of	O
SNSPDs	B-Algorithm
have	O
been	O
in	O
the	O
area	O
of	O
quantum	O
information	O
,	O
such	O
as	O
quantum	O
key	O
distribution	O
and	O
optical	O
quantum	B-Architecture
computing	I-Architecture
.	O
</s>
<s>
Other	O
current	O
and	O
emerging	O
applications	O
include	O
imaging	O
of	O
infrared	O
photoemission	O
for	O
defect	O
analysis	O
in	O
CMOS	B-Device
circuitry	O
,	O
single	O
photon	B-Application
emitter	O
characterization	O
,	O
LIDAR	O
,	O
on-chip	O
quantum	O
optics	O
,	O
optical	O
neuromorphic	O
computing	O
,	O
fibre	B-Algorithm
optic	I-Algorithm
temperature	I-Algorithm
sensing	I-Algorithm
,	O
optical	B-Protocol
time	I-Protocol
domain	I-Protocol
reflectometry	I-Protocol
,	O
readout	O
for	O
ion	O
trap	O
qubits	O
,	O
quantum	O
plasmonics	O
,	O
single	O
electron	O
detection	O
,	O
single	O
α	O
and	O
β	O
particle	O
detection	O
,	O
singlet	O
oxygen	O
luminescence	O
detection	O
,	O
deep	O
space	O
optical	O
communication	O
,	O
dark	B-Application
matter	I-Application
searches	O
and	O
exoplanet	O
detection	O
.	O
</s>
<s>
A	O
number	O
of	O
companies	O
worldwide	O
are	O
successfully	O
commercializing	O
complete	O
single-photon	O
detection	O
systems	O
based	O
on	O
superconducting	O
nanowires	B-Architecture
,	O
including	O
,	O
,	O
,	O
,	O
ID	O
Quantique	O
,	O
and	O
.	O
</s>
<s>
Wider	O
adoption	O
of	O
SNSPD	B-Algorithm
technology	O
is	O
closely	O
linked	O
to	O
advances	O
in	O
cryocoolers	O
for	O
4	O
K	B-Operating_System
and	O
below	O
,	O
and	O
SNSPDs	B-Algorithm
have	O
recently	O
been	O
demonstrated	O
in	O
miniaturized	O
systems	O
.	O
</s>
