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The superconducting nanowire single-photon detector (SNSPD or SSPD) is a type of
optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
and
near-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
single-
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
based on a current-biased
superconducting Superconductivity is a set of physical properties observed in certain materials where Electrical resistance and conductance, electrical resistance vanishes and magnetic field, magnetic flux fields are expelled from the material. Any material e ...
nanowire A nanowire is a nanostructure in the form of a wire with the diameter of the order of a nanometre (10−9 metres). More generally, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less ...
. It was first developed by scientists at
Moscow State Pedagogical University Moscow State Pedagogical University or Moscow State University of Education is an educational and scientific institution in Moscow, Russia, with eighteen faculties and seven branches operational in other Russian cities. The institution had underg ...
and at the
University of Rochester The University of Rochester (U of R, UR, or U of Rochester) is a private research university in Rochester, New York. The university grants undergraduate and graduate degrees, including doctoral and professional degrees. The University of Roc ...
in 2001. The first fully operational prototype was demonstrated in 2005 by the
National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
(Boulder), and
BBN Technologies Raytheon BBN (originally Bolt Beranek and Newman Inc.) is an American research and development company, based next to Fresh Pond in Cambridge, Massachusetts, United States. In 1966, the Franklin Institute awarded the firm the Frank P. Brown ...
as part of the
DARPA Quantum Network The DARPA Quantum Network (2002–2007) was the world's first quantum key distribution (QKD) network, operating 10 optical nodes across Boston and Cambridge, Massachusetts. It became fully operational on October 23, 2003 in BBN's laboratories, an ...
. As of 2021, a superconducting
nanowire A nanowire is a nanostructure in the form of a wire with the diameter of the order of a nanometre (10−9 metres). More generally, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less ...
single-photon detector is the fastest single-photon detector (SPD) for
photon counting Photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). A single-photon detector emits a pulse of signal for each detected photon, in contrast to a normal photodetector, which generates an analo ...
. It is a key enabling technology for
quantum optics Quantum optics is a branch of atomic, molecular, and optical physics dealing with how individual quanta of light, known as photons, interact with atoms and molecules. It includes the study of the particle-like properties of photons. Photons have b ...
and optical quantum technologies. SNSPDs are available with very high detection efficiency, very low dark count rate and very low timing jitter, compared to other types of single-photon detectors. As of 2021, commercial SNSPD devices are available in multichannel systems in a price range of 100,000 euros.


Principle of operation

The SNSPD consists of a thin (≈ 5 nm) and narrow (≈ 100 nm)
superconducting Superconductivity is a set of physical properties observed in certain materials where Electrical resistance and conductance, electrical resistance vanishes and magnetic field, magnetic flux fields are expelled from the material. Any material e ...
nanowire A nanowire is a nanostructure in the form of a wire with the diameter of the order of a nanometre (10−9 metres). More generally, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less ...
. The length is typically hundreds of
micrometers The micrometre (American and British English spelling differences#-re, -er, international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American and British English spelling differences# ...
, and the nanowire is patterned in a compact meander geometry to create a square or circular pixel with high detection efficiency. The nanowire is cooled well below its superconducting critical temperature and biased with a DC
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
that is close to but less than the superconducting critical current of the nanowire. A
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
incident on the nanowire breaks
Cooper pairs In condensed matter physics, a Cooper pair or BCS pair (Bardeen–Cooper–Schrieffer pair) is a pair of electrons (or other fermions) bound together at low temperatures in a certain manner first described in 1956 by American physicist Leon Cooper ...
and reduces the local critical current below that of the bias current. This results in the formation of a localized non-superconducting region, or hotspot, with finite
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...
. This resistance is typically larger than the 50 Ohm input impedance of the readout amplifier, and hence most of the bias current is shunted to the amplifier. This produces a measurable voltage pulse that is approximately equal to the bias current multiplied by 50 Ohms. With most of the bias current flowing through the amplifier, the non-superconducting region cools and returns to the superconducting state. The time for the current to return to the nanowire is typically set by the inductive time constant of the nanowire, equal to the
kinetic inductance Kinetic inductance is the manifestation of the inertial mass of mobile charge carriers in alternating electric fields as an equivalent series inductance. Kinetic inductance is observed in high carrier mobility conductors (e.g. superconductors) and ...
of the nanowire divided by the impedance of the readout circuit. Proper self-resetting of the device requires that this inductive time constant be slower than the intrinsic cooling time of the nanowire hotspot. While the SNSPD does not match the intrinsic energy or photon-number resolution of the superconducting
transition edge sensor A transition-edge sensor (TES) is a type of cryogenic energy sensor or cryogenic particle detector that exploits the strongly temperature-dependent resistance of the superconducting phase transition. History The first demonstrations of the supe ...
, the SNSPD is significantly faster than conventional transition edge sensors and operates at higher temperatures. A degree of photon-number resolution can be achieved in SNSPD arrays, through time-binning or advanced readout schemes. Most SNSPDs are made of sputtered
niobium nitride Niobium nitride is a compound of niobium and nitrogen (nitride) with the chemical formula NbN. At low temperatures (about 16 K) NbN becomes a superconductor, and is used in detectors for infrared light. Uses *Niobium nitride's main use is as a ...
(NbN), which offers a relatively high superconducting critical temperature (≈ 10  K) which enables SNSPD operation in the temperature range 1 K to 4 K (compatible with liquid helium or modern closed-cycle
cryocooler A refrigerator designed to reach cryogenic temperatures (below ) is often called a cryocooler. The term is most often used for smaller systems, typically table-top size, with input powers less than about 20 kW. Some can have input powers as l ...
s). The intrinsic thermal time constants of NbN are short, giving very fast cooling time after photon absorption (<100 picoseconds). The absorption in the superconducting nanowire can be boosted by a variety of strategies: integration with an
optical cavity An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors or other optical elements that forms a cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and provi ...
, integration with a photonic waveguide or addition of
nanoantenna An optical rectenna is a rectenna (rectifying antenna) that works with visible or infrared light. A rectenna is a circuit containing an antenna and a diode, which turns electromagnetic waves into direct current electricity. While rectennas have l ...
structures. SNSPD cavity devices in NbN, NbTiN, WSi & MoSi have demonstrated fibre-coupled device detection efficiencies greater than 98% at 1550 nm wavelength with count rates in the tens of MHz. The detection efficiencies are optimized for a specific wavelength range in each detector. They vary widely, however, due to highly localized regions of the nanowires where the effective cross-sectional area for superconducting current is reduced. SNSPD devices have also demonstrated exceptionally low
jitter In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significa ...
– the uncertainty in the photon arrival time – as low as 3 picoseconds at visible wavelengths. Timing jitter increases as photon energy drops and has been verified out to 3.5 micrometres wavelength. Timing jitter is an extremely important property for time-correlated single-photon counting (TCSPC) applications. Furthermore, SNSPDs have extremely low rates of dark counts, i.e. the occurrence of voltage pulses in the absence of a detected photon. In addition, the deadtime (time interval following a detection event during which the detector is not sensitive) is on the order of a few nanoseconds, this short deadtime translates into very high saturation count rates and enables antibunching measurements with a single detector. For the detection of longer wavelength photons, however, the detection efficiency of standard SNSPDs decreases significantly. Recent efforts to improve the detection efficiency at
near-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
and
mid-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
wavelengths include studies of narrower (20 nm and 30 nm wide) NbN nanowires as well as extensive studies of alternative superconducting materials with lower superconducting critical temperatures than NbN (
tungsten silicide Tungsten silicide (WSi2) is an inorganic compound, a silicide of tungsten. It is an electrically conductive ceramic material. Chemistry Tungsten silicide can react violently with substances such as strong acids, fluorine Fluorine is a chemical ...
, niobium silicide, molybdenum silicide and
tantalum nitride Tantalum nitride (TaN) is a chemical compound, a nitride of tantalum. There are multiple phases of compounds, stoichimetrically from Ta2N to Ta3N5, including TaN. As a thin film TaN find use as a diffusion barrier and insulating layer between cop ...
). Single photon sensitivity up to 10 micrometer wavelength has recently been demonstrated in a tungsten silicide SNSPD. Alternative thin film deposition techniques such as
atomic layer deposition Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (also ...
are of interest for extending the spectral range and scalability of SNSPDs to large areas.
High temperature superconductors High-temperature superconductors (abbreviated high-c or HTS) are defined as materials that behave as superconductors at temperatures above , the boiling point of liquid nitrogen. The adjective "high temperature" is only in respect to previ ...
have been investigated for SNSPDs with some encouraging recent reports. SNSPDs have been created from
magnesium diboride Magnesium diboride is the inorganic compound with the formula MgB2. It is a dark gray, water-insoluble solid. The compound has attracted attention because it becomes superconductor, superconducting at 39 K (−234 °C). In terms of its ...
with some single photon sensitivity in the visible and near infrared. There is considerable interest and effort in scaling up SNSPDs to large multipixel arrays and cameras. A kilopixel SNSPD array has recently been reported. A key challenge is readout, which can be addressed via multiplexing or digital readout using superconducting single flux quantum logic.


Applications

Many of the initial application demonstrations of SNSPDs have been in the area of
quantum information Quantum information is the information of the state of a quantum system. It is the basic entity of study in quantum information theory, and can be manipulated using quantum information processing techniques. Quantum information refers to both th ...
, such as
quantum key distribution Quantum key distribution (QKD) is a secure communication method which implements a cryptographic protocol involving components of quantum mechanics. It enables two parties to produce a shared random secret key known only to them, which can then be ...
and optical
quantum computing Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Though ...
. Other current and emerging applications include imaging of infrared photoemission for defect analysis in
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
circuitry, single photon emitter characterization,
LIDAR Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
, on-chip
quantum optics Quantum optics is a branch of atomic, molecular, and optical physics dealing with how individual quanta of light, known as photons, interact with atoms and molecules. It includes the study of the particle-like properties of photons. Photons have b ...
, optical
neuromorphic computing Neuromorphic engineering, also known as neuromorphic computing, is the use of electronic circuits to mimic neuro-biological architectures present in the nervous system. A neuromorphic computer/chip is any device that uses physical artificial ne ...
, fibre optic temperature sensing, optical time domain reflectometry, readout for
ion trap An ion trap is a combination of electric and/or magnetic fields used to capture charged particles — known as ions — often in a system isolated from an external environment. Atomic and molecular ion traps have a number of applications in phys ...
qubits, quantum plasmonics, single electron detection, single α and β particle detection,
singlet oxygen Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as or ), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambie ...
luminescence detection, deep space optical communication,
dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
searches and
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
detection. A number of companies worldwide are successfully commercializing complete single-photon detection systems based on superconducting nanowires, includin
Single QuantumPhoton SpotScontelQuantum Opus
ID Quantique ID Quantique (IDQ) is a Swiss company, based in Geneva, Switzerland, and provides quantum key distribution (QKD) systems, quantum safe network encryption, single photon counters, and hardware random number generators. It was founded in 2001 a ...

PhoTec
an
Pixel Photonics
Wider adoption of SNSPD technology is closely linked to advances in
cryocooler A refrigerator designed to reach cryogenic temperatures (below ) is often called a cryocooler. The term is most often used for smaller systems, typically table-top size, with input powers less than about 20 kW. Some can have input powers as l ...
s for 4 K and below, and SNSPDs have recently been demonstrated in miniaturized systems.


References

{{Reflist, 30em Particle detectors Photodetectors Radiometry Sensors Superconducting detectors Quantum optics Superconductivity Optoelectronics Photonics Optical metrology Engineering