Superconducting nanowire single-photon detector
Encyclopedia
The superconducting nanowire single-photon detector (SNSPD) is a type of near-infrared and optical single-photon
detector based on a current-biased superconducting
nanowire. It was first developed by scientists at Moscow State Pedagogical University
in 2001.
nanowire. The length is typically hundreds of microns, 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
that is close to but less than the superconducting critical current of the wire. A photon
incident on the nanowire breaks Cooper pairs 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
. 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
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 offer the intrinsic energy or photon-number resolution of the superconducting transition edge sensor
, the SNSPD is significantly faster than conventional transition edge sensors. Most SNSPDs are made of niobium nitride
(NbN), which offers a relatively high superconducting critical temperature (≈ 10 K
) and a very fast cooling time (<100 picoseconds). NbN devices have demonstrated optical detection efficiencies as high as 67% with count rates in the hundreds of MHz. NbN devices have also demonstrated jitter – the uncertainty in the photon arrival time – of less than 50 picoseconds, as well as very low rates of dark counts, i.e. the occurrence of voltage pulses in the absence of a detected photon. For the detection of longer wavelength photons, however, the detection efficiency decreases significantly. Recent efforts to improve the detection efficiency at infrared wavelengths include studies of narrower NbN nanowires as well as studies of materials with lower superconducting critical temperatures than NbN.
, such as quantum key distribution and quantum computing. Another demonstrated application is imaging of infrared photoemission for defect analysis in CMOS
circuitry. Other applications that are being explored include LIDAR
and ultra-long distance classical communication.
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
detector based on a current-biased superconducting
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
nanowire. It was first developed by scientists at Moscow State Pedagogical University
Moscow State Pedagogical University
The Moscow State Pedagogical University, previously known as the Moscow University for Women, the Second Moscow State University, the Moscow State Pedagogical Institute and the Moscow State V. I...
in 2001.
Principle of operation
The SNSPD consists of a thin (≈ 5 nm) and narrow (≈ 100 nm) superconductingSuperconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
nanowire. The length is typically hundreds of microns, 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
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
that is close to but less than the superconducting critical current of the wire. A photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
incident on the nanowire breaks Cooper pairs 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
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
. This resistance is typically larger than the 50 ohm input impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
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
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 Kinetic inductance is the manifestation of the inertial mass of mobile charge...
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 offer the intrinsic energy or photon-number resolution of the superconducting transition edge sensor
Transition edge sensor
A transition edge sensor or TES is a type of cryogenic particle detector that exploits the strongly temperature-dependent resistance of the superconducting phase transition.-Principle of operation:...
, the SNSPD is significantly faster than conventional transition edge sensors. Most SNSPDs are made of niobium nitride
Niobium nitride
Niobium nitride is a compound of niobium and nitrogen with the chemical formula NbN. At low temperatures , niobium nitride becomes a superconductor, and is used in detectors for infrared light.-Uses:...
(NbN), which offers a relatively high superconducting critical temperature (≈ 10 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
) and a very fast cooling time (<100 picoseconds). NbN devices have demonstrated optical detection efficiencies as high as 67% with count rates in the hundreds of MHz. NbN devices have also demonstrated jitter – the uncertainty in the photon arrival time – of less than 50 picoseconds, as well as very low rates of dark counts, i.e. the occurrence of voltage pulses in the absence of a detected photon. For the detection of longer wavelength photons, however, the detection efficiency decreases significantly. Recent efforts to improve the detection efficiency at infrared wavelengths include studies of narrower NbN nanowires as well as studies of materials with lower superconducting critical temperatures than NbN.
Applications
Many of the initial application demonstrations of SNSPDs have been in the area of quantum informationQuantum information
In quantum mechanics, quantum information is physical information that is held in the "state" of a quantum system. The most popular unit of quantum information is the qubit, a two-level quantum system...
, such as quantum key distribution and quantum computing. Another demonstrated application is imaging of infrared photoemission for defect analysis in CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
circuitry. Other applications that are being explored include LIDAR
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
and ultra-long distance classical communication.