WO2009101257A1 - Proximity josephson sensor - Google Patents

Proximity josephson sensor Download PDF

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Publication number
WO2009101257A1
WO2009101257A1 PCT/FI2009/050110 FI2009050110W WO2009101257A1 WO 2009101257 A1 WO2009101257 A1 WO 2009101257A1 FI 2009050110 W FI2009050110 W FI 2009050110W WO 2009101257 A1 WO2009101257 A1 WO 2009101257A1
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Prior art keywords
piece
radiation
superconductor material
sensor
coupled
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PCT/FI2009/050110
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French (fr)
Inventor
Francesco Giaziotto
Tero HEIKKILÄ
Giampiero Pepe
Panu Helistö
Arttu Luukanen
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Teknillinen Korkeakoulu
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Priority to EP09711446.6A priority Critical patent/EP2253028A4/en
Publication of WO2009101257A1 publication Critical patent/WO2009101257A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/10—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors
    • G01J5/20—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors using resistors, thermistors or semiconductors sensitive to radiation, e.g. photoconductive devices
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N60/00—Superconducting devices
    • H10N60/10—Junction-based devices
    • H10N60/12—Josephson-effect devices

Definitions

  • the invention concerns in general the field of solid-state radiation sensors.
  • the invention concerns radiation sensors that utilize superconductivity and related phenomena in detecting electromagnetic radiation in terahertz ranges.
  • Solid-state radiation sensors are in general based on allowing the observed radiation hit a sensor and measuring some electric and/or magnetic effects that the absorbed radiation caused in the sensor.
  • In order to detect continuous electromagnetic radiation in the terahertz range it is common to use an antenna-coupled microbolometer, exam pl es of wh i ch a re known e .g . from pu bl ication s US 6,242,740 and US 7,078,695.
  • the most sensitive sensors today are superconducting microcalohmeters.
  • the present disclosure addresses the above-mentioned limitations of prior art by aiming at a solid-state superconductive sensor with high sensitivity, low noise characteristics, simple structure and advantageous properties regarding mass production.
  • a sensor in which a conductive region between two superconducting contacts is dimensioned so that a critical supercurrent value of the junction has a strong dependence on the electron temperature in the conductive region.
  • an impedance transformer coupled to at least one of said first and second pieces of superconductor material for performing impedance transformation between a device impedance of the radiation sensor and space impedance of space surrounding the radiation sensor, and
  • a measurement circuit configured to convert a change in kinetic inductance of the junction into an electric measurement signal.
  • An imaging system according to the invention is characterised in that it comprises at least one radiation sensor of the kind described above.
  • a series connection in which a piece of normal conductor material is connected between two pieces of superconductor material is called an SNS junction.
  • SNS junction At the interfaces where a superconductor material meets a normal conductor material the so-called proximity effect takes place, meaning diffusion of Cooper pairs into the normal conductor material and diffusion of electronic excitations into the superconductor material. If the distance between the superconductor materials through the normal conductor material in an SNS junction is small enough, the whole junction structure constitutes a Josephson junction and can be described with various characteristics, like the so-called critical supercurrent value that is defined by material and structural parameters.
  • the critical supercurrent value can be made to depend strongly on electron temperature in the normal conductor material. If the SNS junction appears as the sensor element of a suitable radiation-receiving structure, received electromagnetic radiation causes an increase in the electron temperature of the normal conductor material. The changes in critical supercurrent are associated with changes in the kinetic inductance of the junction, which in turn can be measured with suitable readout means.
  • an SNS junction as the sensor element generates important advantages over prior art superconductor sensors, which typically utilized a superconductor wire or a bilayer of two different superconductors as the region where the absorbed radiation causes measurable effects.
  • the material volume in which the radiation-induced temperature increase takes place can be made very small, which means that an actualized temperature increase (and the corresponding change in critical supercurrent and kinetic inductance) per received radiation power is significant.
  • Fig. 1 illustrates the principal structure of a radiation sensor based on an SNS junction
  • fig. 2 illustrates curves of critical supercurrent vs. electron temperature
  • fig. 3 illustrates tinning aspects of changes in electron temperature
  • fig. 4 illustrates relative change in kinetic inductance vs. photon frequency in pulsed mode
  • fig. 5 illustrates relative change in kinetic inductance vs. received optical power in continuous mode
  • fig. 6 illustrates an exemplary measurement system
  • fig. 7 illustrates an exemplary SQUID-based readout principle
  • fig. 8 illustrates signal to noise ratio vs. photon frequency
  • fig. 9 illustrates noise equivalent power vs. phonon temperature
  • fig. 10 illustrates resolving power vs. photon frequency
  • fig. 11 illustrates an exemplary RF probing based readout principle
  • fig. 12 illustrates an exemplary imaging system
  • fig. 13 illustrates a method according to an embodiment of the invention.
  • the designation "superconductor material” means a material that is capable of achieving a superconducting (resistance-free) state
  • the designation “conductor material” means a material that is an electric conductor but not in the superconducting state.
  • the state of the material depends on the relation of the operating temperature to the material-dependent critical temperature. If the operating temperature is lower than the critical temperature, the material is in the superconducting state.
  • the critical temperature varies between different materials: for niobium and its various compounds, the critical temperature is typically of the order of 9 K, for aluminium slightly above 1 K whereas for example for copper, silver or gold the transition temperature (if it exists) has not been reached with present technology.
  • Two d ifferent materials taken together can be said to constitute a conductor-superconductor pair if the transition temperature for one (the conductor) is so much lower than for the other (the superconductor) that cooling to a certain temperature between the transition temperatures would make the decisive difference in their conductivity.
  • a material is a superconductor if it can be cooled into superconductivity with the cooling means available in the system. Temperatures below 1 kelvin require quite advanced cooling arrangements, while temperatures below 4.2 kelvins are routinely achieved with closed-cycle cryocoolers and liquid helium cooling. So-called high temperature (Type II) superconductor materials become superconductive even at significantly higher temperatures.
  • Fig. 1 illustrates a series coupling in which a piece 101 of conductor material is connected between first 102 and second 103 pieces of superconductor material.
  • S superconductor
  • N for (normal) conductor
  • the characteristic energy gap also known as the excitation energy of
  • Cooper pairs of the superconductor material is ⁇ .
  • a » hD / l 2 E Th , where % is the reduced Planck constant and E T h is the so-called Thouless energy (characteristic energy scale of electrons in a disordered conductor) of the conductor material.
  • the length / of the piece 101 is small enough for tunnelling through the SNS junction to occur, which means that the structure constitutes a Josephson junction.
  • the electron temperature in piece 101 is T e and the critical supercurrent of the SNS junction is / c .
  • the critical supercurrent of the SNS junction is / c .
  • Formula (1 ) shows that (if the assumptions made above are valid) the critical supercurrent / c of the SNS junction of fig. 1 depends exponentially on the electron temperature T e in the piece 101 and is independent of the so-called phonon temperature or bath temperature, which could be described as the general temperature of the system achieved by subjecting it to cryogenic cooling.
  • the applicability to radiation detection of the SNS junction of fig. 1 comes from the fact that absorbed electromagnetic radiation can be used to increase the electron temperature of piece 101. As a result, the critical supercurrent of the junction is lowered.
  • the sensor may be configured to receive and absorb electromagnetic radiation by providing a suitable impedance transformer that performs an impedance transformation between the device impedance of the radiation sensor and space impedance of space surrounding the radiation sensor.
  • the impedance transformer is schematically illustrated as the blocks 104 and 105, each coupled to its respective piece of superconductor material.
  • a measurement circuit can be used to convert a change in the electron temperature and critical supercurrent in piece 101 into an electric measurement signal.
  • the SNS junction comprises a section made of normal conductor (and not superconductor) material
  • the density of states is not divergent at the minigap edge, contrary to bulk superconductors, and thereby the generation-recombination noise is reduced.
  • the minigap E 9 is around h ⁇ 5 GHz, and thereby radiation at low frequencies v ⁇ E 9 Ih, which typically is not part of the measured signal of interest, should not couple into the sensor.
  • the ratio A/Erh 230, so the criterion of ⁇ being significantly larger than E n is satisfied.
  • the critical supercurrent / c can be calculated numerically following the scheme outlined in TT. Heikkila, J. Sarkka, and F. K. Wilhelm, Phys. Rev. B 66, 184513 (2002), with the result shown as the dashed curve 201 in fig. 2.
  • the solid curve 202 in fig. 2 illustrates the critical supercurrent / c as a function of temperature calculated from the approximation given in formula (1 ). Agreement with the other calculation is extremely good at temperatures higher than approximately 0.2 K.
  • Kinetic inductance is known as the manifestation of the inertial mass of mobile charge carriers in alternating electric fields, expressed in terms of an equivalent series inductance. It can be measured with similar circuit topolog ies that would be used to define the value of an un known series inductance.
  • additional requirements for the measurement circuit come from the fact that the measurement should be as undestructive as possible, i.e. it should not interfere with the radiation-induced variations of electron temperature in the conductor material.
  • Fig. 3 is a schematic illustration of electron temperature (curve 301 ) in the conductor material as a function of time, when a single photon is received and absorbed by the radiation sensor.
  • the time and temperature axes are here illustrated without units, because it is more important to consider qualitatively the behaviour of T e .
  • the photon energy can be entirely dissipated inside the conductor material whereas the setup is designed such that the energy required for the measurement is dissipated away from the sensor, within the measurement circuit described later.
  • the volume of the piece of conductor material should be minimized while the distance between the superconductors can be chosen according to the desired operating temperature and measured frequency of the radiation. This can be done by adjusting the Thouless energy E Th through the length /.
  • Exemplary conductor materials include but are not limited to normal metals such as copper, silver, gold, palladium or aluminium (the latter assuming that the operating temperature is above its critical temperature). The fabrication of such SNS junctions is rather straightforward and can be done such that the matching of the SNS junction to the impedance transformer becomes almost perfect.
  • ⁇ L k /L° k as a function of v, with selectable parameter values as given above and at different phonon temperatures Tbath- It can be seen that for a relatively low value 200 mK of T bat h the value of ⁇ L k /L° k is around 14% for a 1 THz photon and as high as 163% for a 10 THz photon. At a higher phonon temperature of 1 K the variation in kinetic inductance is not as large, but ⁇ L k /L° k is still about 3% at 1 THz and about 35% at 10 THz. Kinetic inductance variations this large allow for a very large signal to noise ratio for single-photon detection.
  • the radiation sensor is used as a bolometer, i.e. in continuous mode in which the mean received optical power should be resolved.
  • a small conductor volume means that the dissipated power produces a high power density per unit volume
  • a low coupling constant means that the accumulated thermal energy of the electrons is not easily lost through phonon interaction. Achieving small conductor volume has already been considered above.
  • the criterion for low electron-phonon coupling constant can be optimized by suspending the piece of conductor material only at its ends, and only through the superconductor pieces that act as thermal filters.
  • the sensor is typically enclosed in an evacuated enclosure.
  • Fig. 6 illustrates schematically the principle of receiving electromagnetic radiation 601 with a radiation sensor 602, which comprises the SNS junction, antenna branches 604 and 605 coupled to the pieces of superconductor material to act as the impedance transformer, and a measurement circuit 603.
  • the task of the measurement circuit 603 is to convert a change in electron temperature in the piece of conductor material (N) into an electric measurement signal.
  • Additional parts of the system are a cooling system, an operating power delivery system and a control system, which are schematically illustrated as block 611.
  • the cooling system is important, because the operating principle of the radiation sensor is based on superconductivity, which is reached when the cool ing system is configured to keep the radiation sensor or array of sensors at a cooled temperature low enough to make the appropriate parts superconductive.
  • the other block 612 that is schematically illustrated in fig. 6 receives the electric measurement signal from the measurement circuit 603 and processes it for further storage and displaying.
  • a processing arrangement that takes care of the necessary operations, which may comprise for example readout, amplification, filtering, time synchronization, address generation, pulse shaping, image encoding, frame forming, storing, and the like.
  • Fig. 7 illustrates one alternative, in which the conversion into electric signal is based on measuring the magnetic field induced by that part of a constant current that does not go through the conductor that is actually the target of the measurement.
  • a constant current source 701 feeds a constant bias current l b into a parallel coupling, one branch of which comprises an inductor 702.
  • the other branch of the parallel coupling comprises the series coupling of the first piece of superconductor material, piece of conductor material and the second piece of superconductor material, which - due to the varying kinetic inductance in the conductor material - appear in fig. 7 as a variable inductor 703.
  • a magnetic field sensor 704 is configured to sense the magnetic field of the inductor 702. In fig. 7 the magnetic field sensor 704 appears in the form of a SQUID (superconducting quantum interference device) with direct current (DC) readout.
  • SQUID superconducting quantum interference device
  • the bias current l b is divided into two component currents, of which current / c is the critical supercurrent that flows through the SNS junction and current k is the current that flows through the inductor 702.
  • current / c is the critical supercurrent that flows through the SNS junction
  • current k is the current that flows through the inductor 702.
  • the signal to noise ratio S/N can be readily expressed as
  • ⁇ n is the flux sensitivity of the dc SQUID
  • ⁇ is its bandwidth.
  • Fig. 8 illustrates the S/N ratio as a function of the frequency v of the received photon. In calculating the curves of fig. 8 the following parameter values were used:
  • NEP TFN NEPTFN
  • N EPSQUID solid line
  • the optical NEP is of the same order of magnitude, because the resistance of the device can be easily matched to suitable antenna types, like common broadband self-similar lithographic antennas.
  • Fig. 10 illustrates an exemplary calculation of the resolving power (2 ⁇ hv/AE ) of a rad iation sensor accord ing to an embod iment of the invention , where ⁇ E « 2V2 In 2NEP TFN (v) ⁇ e _ ph is the energy resolution of full width at half maximum, for a number of different phonon temperatures Tbath- Resolving power values between about 1 .2 and about 2.3 can be achieved in the 5 - 70 THz frequency range for phonon temperatures at or above 400 mK, which shows that the proximity Josephson sensor is suitable for far- and mid-infrared singe-photon detection.
  • Fig. 11 illustrates an alternative readout principle, based on radio frequency probing of the resonance frequency of a resonant circuit, a part of which is the
  • An oscillating input signal in a suitable radio frequency range is coupled to the input port 1101 at one end of an input side transmission line 1102.
  • the resonant circuit which here comprises the SNS junction (illustrated here as a variable inductance) 1103, a superconducting parallel inductance (or simply a superconducting wire) 1104 and coupling capacitances 1105 and 1106 that complete the loop.
  • SNS junction illustrated here as a variable inductance
  • superconducting parallel inductance or simply a superconducting wire
  • capacitances 1105 and 1106 that complete the loop.
  • One point of the resonant circuit is coupled to an output side transmission line 1107, one end of which constitutes the output port 1108.
  • the varying kinetic inductance of the SNS junction changes the resonance frequency of the resonant circuit, which can be measured by analysing the signal appearing at the output port 1108, when the probing signal (the oscillating signal coupled to the input port 1101 ) is known.
  • the probing signal the oscillating signal coupled to the input port 1101
  • the level (and/or phase) of an oscillating signal transmitted through the resonant circuit one could analyse (i.e. measure the level and/or phase) an oscillating signal reflected by said resonant circuit.
  • the input and output ports would naturally be on the same side of the resonant circuit and not on opposite sides like in fig. 11.
  • Fig. 12 illustrates schematically an imaging system according to an embodiment of the invention.
  • Electromagnetic radiation 601 is directed from an object under study (not shown) through a optical system 1201 to an array of radiation sensors 1202.
  • the radiation sensors in the array may all be identical, or the array may contain e.g. a selection of differently dimensioned radiation sensors to better shape the desired frequency response.
  • a proximity Josephson sensor an easy way to optimize operating characteristics is to select the dimensions (especially length) of the piece of conductor material that connects the pieces of superconductive material together.
  • a multiplexing arrangement 1203 that facilitates feeding the necessary bias currents and voltages, probing signals and other inputs to the individual sensors and reading the outputs of the measurement circuits in the individual sensors.
  • Multiplexing techniques for feeding, controlling and reading out arrays of SQUID-based, RF probing based, or other appropriate measurement circuits are widely known.
  • the RF probing based technique of fig. 1 1 is easily multiplexed by making the transmission lines 1102 and 1107 long enough, coupling a number of resonant circuits of the kind shown in fig. 11 in parallel to the transmission lines, and dimensioning each resonant circuit to exhibit a different enough resonance frequency.
  • the imaging system of fig. 12 comprises a cooling system 1204.
  • Cryogenic cooling is well known and numerous suitable technical solutions are readily available. How low the desired temperature is depends very much on the superconductor materials that have been selected. It is advantageous to utilize superconductor materials with a transition temperature at or higher than 4.2 K, because e.g. closed-circuit cryocooling can be easily adopted to maintain cooled temperatures in the range between 1 and 4.2 Kelvin.
  • the imaging system of fig. 12 comprises an image production system that is configured to read the multiplexed output signals of individual radiation sensors and to convert these output signals into an electronic representation of an image.
  • an analog interface 1205 is responsible for interfacing tasks in analog domain, like signal amplification, bias and operating power generation and delivery, and passing through possible multiplexed address and feedback signals.
  • a digital interface 1206 implements interfacing tasks in digital domain, like time reference generation, address generation for the multiplexing scheme, and feedback signal processing.
  • a control computer 1207 has a control bus connection to and from the interface blocks 1205 and 1206, and a serial data bus for conveying data to and from the digital interface block 1206. The final electronic representation of an image is formed and displayed in the control computer 1207 with the appropriate image processing software.
  • Fig. 13 illustrates schematically a method for detecting electromagnetic radiation according to an embodiment of the invention.
  • the method comprises receiving electromagnetic radiation through an impedance transformer, directing an electric current induced by said received electromagnetic radiation through a series coupling of a first piece of superconductor material, a piece of conductor material, and a second piece of superconductor material, and converting a change in electron temperature in said piece of conductor material into an electric measurement signal through the change in the critical supercurrent of the junction and thereby its kinetic inductance.
  • the converting may comprise measuring changes in the magnetic field of an inductor coupled in parallel with said series coupling and fed with an electric current from a common current source in parallel with said series coupling.
  • the converting may comprise measuring reflection or transmission of an oscillating signal by a resonant circuit, a part of which is said series coupling.
  • the method may comprise cooling the series coupling to a temperature below 4.2 Kelvin, depending on the desired accuracy, detected radiation frequency and used materials.
  • the method may comprise directing electromagnetic radiation from an object under study to an array of radiation sensors that implement said receiving and directing, reading output signals of individual radiation sensors in said array of radiation sensors, and converting said output signals into an electronic representation of an image.

Abstract

A radiation sensor comprises first (102) and second (103) pieces of superconductor material, and a piece (101) of conductor material connected between them. An antenna branch (104) is coupled to said first piece (102) of superconductor material, and an optionalsecond antenna branch (105) is coupled to said second piece (103) of superconductor material. Ameasurement circuit (603) is configured to convert a change in electron temperature in said piece (101) of conductor material intoan electric measurement signal throughthe change in the kinetic inductance of the device.

Description

Proximity Josephson sensor
TECHNICAL FIELD
The invention concerns in general the field of solid-state radiation sensors. In particular the invention concerns radiation sensors that utilize superconductivity and related phenomena in detecting electromagnetic radiation in terahertz ranges.
BACKGROUND OF THE INVENTION
Solid-state radiation sensors are in general based on allowing the observed radiation hit a sensor and measuring some electric and/or magnetic effects that the absorbed radiation caused in the sensor. In order to detect continuous electromagnetic radiation in the terahertz range (usually understood to cover frequencies of about 0.3-3 THz) it is common to use an antenna-coupled microbolometer, exam pl es of wh i ch a re known e .g . from pu bl ication s US 6,242,740 and US 7,078,695. For measuring pulsed excitations, the most sensitive sensors today are superconducting microcalohmeters. Other prior art publications that touch upon the subject of superconductive radiation sensors include US 6,812,464; US 6,528,814; US 6,455,849; US 6,239,431 ; US 5,532,485; US 4,869,598; and US 4,851 ,680. In many commercially interesting applications, like security imaging, it is desirable to operate also in the long wave infrared range (3-30 THz) and/or mid-range infrared, up to and beyond 100 THz.
Known drawbacks of radiation sensors utilizing superconductivity involve usually factors like limited sensitivity, insufficient signal to noise ratio, strict requirements for cryogenic cooling, complicatedness in structure, and large variations between the performance characteristics of individual sensor units. There exists a need for structurally simple and operationally reliable sensor structures that could be mass produced in the form of detector chips or matrices and that would have high sensitivity, good signal to noise ratio and advantageous noise equivalent power (NEP) characteristics.
SUMMARY OF THE INVENTION
The present disclosure addresses the above-mentioned limitations of prior art by aiming at a solid-state superconductive sensor with high sensitivity, low noise characteristics, simple structure and advantageous properties regarding mass production.
The objectives of the invention are achieved by a sensor, in which a conductive region between two superconducting contacts is dimensioned so that a critical supercurrent value of the junction has a strong dependence on the electron temperature in the conductive region.
A radiation sensor according to the invention is characterised in that it comprises:
- a first piece of superconductor material,
- a second piece of superconductor material, - a piece of conductor material connected between said first and second pieces of superconductor material to form a junction,
- an impedance transformer coupled to at least one of said first and second pieces of superconductor material for performing impedance transformation between a device impedance of the radiation sensor and space impedance of space surrounding the radiation sensor, and
- a measurement circuit configured to convert a change in kinetic inductance of the junction into an electric measurement signal.
An imaging system according to the invention is characterised in that it comprises at least one radiation sensor of the kind described above.
A method for detecting electromagnetic radiation according to the invention is characterised in that it comprises:
- receiving electromagnetic radiation through an impedance transformer,
- directing an electric current induced by said received electromagnetic radiation through a series coupling of a first piece of superconductor material, a piece of conductor material, and a second piece of superconductor material, which together constitute a junction, and
- converting a change in electron temperature in said piece of conductor material into an electric measurement signal through a change in the critical supercurrent and kinetic inductance of the junction.
A series connection in which a piece of normal conductor material is connected between two pieces of superconductor material is called an SNS junction. At the interfaces where a superconductor material meets a normal conductor material the so-called proximity effect takes place, meaning diffusion of Cooper pairs into the normal conductor material and diffusion of electronic excitations into the superconductor material. If the distance between the superconductor materials through the normal conductor material in an SNS junction is small enough, the whole junction structure constitutes a Josephson junction and can be described with various characteristics, like the so-called critical supercurrent value that is defined by material and structural parameters.
It has been found out that, especially if the distance between the superconductor materials through the normal conductor material in an SNS junction is not too short, the critical supercurrent value can be made to depend strongly on electron temperature in the normal conductor material. If the SNS junction appears as the sensor element of a suitable radiation-receiving structure, received electromagnetic radiation causes an increase in the electron temperature of the normal conductor material. The changes in critical supercurrent are associated with changes in the kinetic inductance of the junction, which in turn can be measured with suitable readout means.
Using an SNS junction as the sensor element generates important advantages over prior art superconductor sensors, which typically utilized a superconductor wire or a bilayer of two different superconductors as the region where the absorbed radiation causes measurable effects. For example, the material volume in which the radiation-induced temperature increase takes place can be made very small, which means that an actualized temperature increase (and the corresponding change in critical supercurrent and kinetic inductance) per received radiation power is significant.
The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS Fig. 1 illustrates the principal structure of a radiation sensor based on an SNS junction, fig. 2 illustrates curves of critical supercurrent vs. electron temperature, fig. 3 illustrates tinning aspects of changes in electron temperature, fig. 4 illustrates relative change in kinetic inductance vs. photon frequency in pulsed mode, fig. 5 illustrates relative change in kinetic inductance vs. received optical power in continuous mode, fig. 6 illustrates an exemplary measurement system, fig. 7 illustrates an exemplary SQUID-based readout principle, fig. 8 illustrates signal to noise ratio vs. photon frequency, fig. 9 illustrates noise equivalent power vs. phonon temperature, fig. 10 illustrates resolving power vs. photon frequency, fig. 11 illustrates an exemplary RF probing based readout principle, fig. 12 illustrates an exemplary imaging system, and fig. 13 illustrates a method according to an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION AND ITS EMBODIMENTS
In this document the designation "superconductor material" means a material that is capable of achieving a superconducting (resistance-free) state, and the designation "conductor material" means a material that is an electric conductor but not in the superconducting state. The state of the material depends on the relation of the operating temperature to the material-dependent critical temperature. If the operating temperature is lower than the critical temperature, the material is in the superconducting state. The critical temperature varies between different materials: for niobium and its various compounds, the critical temperature is typically of the order of 9 K, for aluminium slightly above 1 K whereas for example for copper, silver or gold the transition temperature (if it exists) has not been reached with present technology. Two d ifferent materials taken together can be said to constitute a conductor-superconductor pair if the transition temperature for one (the conductor) is so much lower than for the other (the superconductor) that cooling to a certain temperature between the transition temperatures would make the decisive difference in their conductivity.
As a part of a system, a material is a superconductor if it can be cooled into superconductivity with the cooling means available in the system. Temperatures below 1 kelvin require quite advanced cooling arrangements, while temperatures below 4.2 kelvins are routinely achieved with closed-cycle cryocoolers and liquid helium cooling. So-called high temperature (Type II) superconductor materials become superconductive even at significantly higher temperatures.
Fig. 1 illustrates a series coupling in which a piece 101 of conductor material is connected between first 102 and second 103 pieces of superconductor material. Using the letters S for superconductor and N for (normal) conductor, we may designate the series coupling in fig. 1 an SNS junction. We assume that the piece
101 has a length / and is made of a material the diffusion coefficient of which is D, and that the characteristic energy gap (also known as the excitation energy of
Cooper pairs) of the superconductor material is Δ. Of the mutual relations of Δ, D, and / we assume that A » hD / l2 = ETh , where % is the reduced Planck constant and ETh is the so-called Thouless energy (characteristic energy scale of electrons in a disordered conductor) of the conductor material. We further assume that the length / of the piece 101 is small enough for tunnelling through the SNS junction to occur, which means that the structure constitutes a Josephson junction.
The electron temperature in piece 101 is Te and the critical supercurrent of the SNS junction is /c. For ETh « kBTe « Δ , where kB is the Boltzmann constant, the Josephson current Ij of the SNS junction is ld = I0 sin(ø), where φ is the phase difference between the pieces 102 and 103 of superconductor material. According to the references A. D. Zaikin and G. F. Zharkov, Sov. J. Low Temp. Phys. 7, 184 (1981 ); and F. K. Wilhelm, A. D. Zaikin, and G. Schόn, J. Low Temp. Phys. 106, 305 (1997), under the assumptions made above a formula for the critical supercurrent /c can be written as
Figure imgf000006_0001
Parameters in formula (1 ) that have not yet been explained above are the elementary charge e and the normal-state resistance RN of the SNS junction, defined in turn as RN = pl/A, where p = (vpe2D)"1 is the resistivity of the conductor material of piece 101 , A is its cross section, and vp is the density of states at the Fermi level in the conductor material. Formula (1 ) shows that (if the assumptions made above are valid) the critical supercurrent /c of the SNS junction of fig. 1 depends exponentially on the electron temperature Te in the piece 101 and is independent of the so-called phonon temperature or bath temperature, which could be described as the general temperature of the system achieved by subjecting it to cryogenic cooling. The applicability to radiation detection of the SNS junction of fig. 1 comes from the fact that absorbed electromagnetic radiation can be used to increase the electron temperature of piece 101. As a result, the critical supercurrent of the junction is lowered. The sensor may be configured to receive and absorb electromagnetic radiation by providing a suitable impedance transformer that performs an impedance transformation between the device impedance of the radiation sensor and space impedance of space surrounding the radiation sensor. In fig. 1 the impedance transformer is schematically illustrated as the blocks 104 and 105, each coupled to its respective piece of superconductor material. A measurement circuit can be used to convert a change in the electron temperature and critical supercurrent in piece 101 into an electric measurement signal. Before going into practical implementations we make a theoretical analysis and simulation of the response of a radiation sensor according to an embodiment of the invention to received photons.
Even if the SNS junction comprises a section made of normal conductor (and not superconductor) material, a supercurrent through the junction occurs due to the proximity effect, giving rise in the local density of states of the conductor material to an energy minigap of size E9 = c{φ)ETh with c(0) ~ 3.1 (see F. Zhou, P. Charlat, B. Spivak, and B. Pannetier, J. Low Temp. Phys. 1 10, 841 (1998)). Due to the minigap, both heat capacity and electron-phonon coupling are reduced inside the conductor material , which actually improves the resolution of the sensor. Moreover, the density of states is not divergent at the minigap edge, contrary to bulk superconductors, and thereby the generation-recombination noise is reduced. Below we will introduce some exemplary parameter values, with which the minigap E9 is around h ■ 5 GHz, and thereby radiation at low frequencies v < E9 Ih, which typically is not part of the measured signal of interest, should not couple into the sensor.
We assume that the piece 101 of conductor material is a rectangular slab having a thickness of 10 nanometres, a width of 100 nanometres and a length / of 1 micrometre, giving a volume Ω = 10~21 m3. We further assume that the conductor material is silver and the superconductor material is niobium, giving vF = 1.0 x 1047 J"1m"3, D = 0.01 m2s"1, RN = 38Ω, En = Q-Q μeV, and Δ = 1.52 meV. The ratio A/Erh = 230, so the criterion of Δ being significantly larger than En is satisfied. If we fix φ = π/2 and AIEn =230, the critical supercurrent /c can be calculated numerically following the scheme outlined in TT. Heikkila, J. Sarkka, and F. K. Wilhelm, Phys. Rev. B 66, 184513 (2002), with the result shown as the dashed curve 201 in fig. 2. This calculated critical supercurrent saturates at about 1.7 μA at Te = 50 mK and is suppressed by a factor of about 20 when temperature rises to 1 K. This calculated behaviour is in good conformity with the above-explained exponential dependence on Te.
The solid curve 202 in fig. 2 illustrates the critical supercurrent /c as a function of temperature calculated from the approximation given in formula (1 ). Agreement with the other calculation is extremely good at temperatures higher than approximately 0.2 K.
Measuring the critical supercurrent of a Josephson junction can be accomplished for example by measuring the kinetic inductance Lk, because there is a relatively simple relation Lk = h/(2elc ). Kinetic inductance is known as the manifestation of the inertial mass of mobile charge carriers in alternating electric fields, expressed in terms of an equivalent series inductance. It can be measured with similar circuit topolog ies that would be used to define the value of an un known series inductance. In an SNS junction additional requirements for the measurement circuit come from the fact that the measurement should be as undestructive as possible, i.e. it should not interfere with the radiation-induced variations of electron temperature in the conductor material. Some possible measurement circuits are described in detail later.
Fig. 3 is a schematic illustration of electron temperature (curve 301 ) in the conductor material as a function of time, when a single photon is received and absorbed by the radiation sensor. The time and temperature axes are here illustrated without units, because it is more important to consider qualitatively the behaviour of Te. With a proper design of the impedance transformer, the photon energy can be entirely dissipated inside the conductor material whereas the setup is designed such that the energy required for the measurement is dissipated away from the sensor, within the measurement circuit described later. The electron temperature Te in the conductor material is elevated from the initial level Tbath, uniformly along its length, over a time scale set by the diffusion time τo = I2ID. With parameter values as given above, τo ~ 10~10 s. After the photon has been fully absorbed, Te relaxes towards Tbath over a time scale set by the electron-phonon interaction time τe_ph = i/(α7" ϋ 3 af/J ), where α « 0.34∑/(/c|vF ) and Σ is the electron- phonon coupling constant. An exemplary value for silver is Σ = 5 x 108 WiTf3K"5, with which we get τe-Ph ~ 1 x 10"4 ... 1 x 10~7 s in the temperature range between 0.1 K and 1.0 K. This shows that the relaxation time τe-Ph of the electron temperature is much longer than the diffusion time τo.
Let us assume that the radiation sensor is used as a calorimeter, i.e. in pulsed mode in which the energy of individual received photons should be resolved. If a photon of frequency v arrives at time t = 0, the electron temperature in the cond uctor materia l ca n be d eterm i ned by solvi ng the heat eq uation Ce(dTe/dt) = Popt , in which Ce = (π2vF/c|7~ e )/ 3 is the electron heat capacity, and
Popt = (2πhv / Ω.)δ(t) is the optical input power per volume per incident photon. In writing the heat equation we neglected the spatial dependence of Te in the conductor material, as well as the interaction with the lattice phonons, the latter occurring on a time scale τe_ph » τD . From the solution of the heat equation we get Te(y) = ^]T*ath + 12% v/(πΩvF/c| ) , which shows that a small volume of the conductor material is advantageous in achieving large variations in Te. Intuitively this is easy to understand, because a small volume of the conductor material means that an amount of energy deposited therein will result in a high energy density. Exactly how small the volume of the conductor material should be depends, among others, on the accuracy at which the measurement circuit is able to detect phenomena that result from changes in the electron temperature. Above we have shown that for example a volume of conductor material in the order of 10~21 m3 can be achieved easily, and volumes even below that are also within easy reach of known technology.
Above we pointed out that to maximize the sensitivity of the sensor, the volume of the piece of conductor material should be minimized while the distance between the superconductors can be chosen according to the desired operating temperature and measured frequency of the radiation. This can be done by adjusting the Thouless energy ETh through the length /. Exemplary conductor materials include but are not limited to normal metals such as copper, silver, gold, palladium or aluminium (the latter assuming that the operating temperature is above its critical temperature). The fabrication of such SNS junctions is rather straightforward and can be done such that the matching of the SNS junction to the impedance transformer becomes almost perfect. However, if sensitivity is preferred over the ease of manufacture, for example carbon nanotubes or semiconductor wires can be used to replace the abovementioned normal metals as the N parts of the junction. In this case the details of the analysis shown above may be slightly modified, but the main idea remains the same. Above it was pointed out that variation in electron temperature in the piece of conductor material in an SNS junction has an effect on the kinetic inductance. To quantify the phenomenon we may define a relative variation of kinetic inductance as the quantity δLk/L°k = [Lk(v)- Lk(θ)]/Lk(θ), where the dependence on v means that the variation in kinetic inductance is the result of absorbing a photon of frequency v. Fig. 4 illustrates δLk/L°k as a function of v, with selectable parameter values as given above and at different phonon temperatures Tbath- It can be seen that for a relatively low value 200 mK of Tbath the value of δLk/L°k is around 14% for a 1 THz photon and as high as 163% for a 10 THz photon. At a higher phonon temperature of 1 K the variation in kinetic inductance is not as large, but δLk/L°k is still about 3% at 1 THz and about 35% at 10 THz. Kinetic inductance variations this large allow for a very large signal to noise ratio for single-photon detection.
Let us then assume that the radiation sensor is used as a bolometer, i.e. in continuous mode in which the mean received optical power should be resolved. At low temperatures close to or under 1 K the main contribution to the power of a system of electrons in a conductor is related to electron-phonon heat flux which can be modelled approximately as Qe _ph = ∑Ω(Te 5 - T*ath ). Requiring this heat flux to equal the dissipated optical power Popt under steady-state conditions gives Popt + Qe ph = 0 , wh i c h i n t u rn g i ve s Te (Popt ) = *](Popt /∑Ω) + Tb b ath . Th i s expression shows, among others, that both the conductor volume Ω and the electron-phonon coupling constant Σ should be small in order to maximize the increase in the electron temperature Te upon continuous irradiation. Again, the criteria are intuitively understandable, because a small conductor volume means that the dissipated power produces a high power density per unit volume, and because a low coupling constant means that the accumulated thermal energy of the electrons is not easily lost through phonon interaction. Achieving small conductor volume has already been considered above. The criterion for low electron-phonon coupling constant can be optimized by suspending the piece of conductor material only at its ends, and only through the superconductor pieces that act as thermal filters. The sensor is typically enclosed in an evacuated enclosure.
The relative variation of kinetic inductance is - in the case of continuous operation - defined as the quantity δLk/L°k = [Lk(Popt )- Lk(θ)]/Lk(θ), where the dependence is now on Popt and means that the variation in kinetic inductance is the result of continuously absorbing energy at a certain rate. Fig. 5 illustrates this δLk/L°k as a function of Popt, with selectable parameter values again as given above and at different phonon temperatures Tbath- At the lowest illustrated temperature Tbath = 0.2 K the value of δLk/L°k is around 130% for Popt = 10 fW and around 2000% for
POpt = 1 pW. When the phonon temperature is 1 K the corresponding values of δLk /L-I are around 1 % for Popt = 10 fW and around 100% for Popt = 1 pW.
Fig. 6 illustrates schematically the principle of receiving electromagnetic radiation 601 with a radiation sensor 602, which comprises the SNS junction, antenna branches 604 and 605 coupled to the pieces of superconductor material to act as the impedance transformer, and a measurement circuit 603. The task of the measurement circuit 603 is to convert a change in electron temperature in the piece of conductor material (N) into an electric measurement signal. Additional parts of the system are a cooling system, an operating power delivery system and a control system, which are schematically illustrated as block 611. The cooling system is important, because the operating principle of the radiation sensor is based on superconductivity, which is reached when the cool ing system is configured to keep the radiation sensor or array of sensors at a cooled temperature low enough to make the appropriate parts superconductive.
The other block 612 that is schematically illustrated in fig. 6 receives the electric measurement signal from the measurement circuit 603 and processes it for further storage and displaying. For a person skilled in the art it is known how to devise a processing arrangement that takes care of the necessary operations, which may comprise for example readout, amplification, filtering, time synchronization, address generation, pulse shaping, image encoding, frame forming, storing, and the like.
Measurement circuits for converting a change in the electron temperature, critical supercurrent, and kinetic inductance in a piece of conductor material into an electric measurement signal, without causing further change in the electron temperature through the measurement, are known as such. Fig. 7 illustrates one alternative, in which the conversion into electric signal is based on measuring the magnetic field induced by that part of a constant current that does not go through the conductor that is actually the target of the measurement. In the schematically drawn circuit diagram of fig. 7 a constant current source 701 feeds a constant bias current lb into a parallel coupling, one branch of which comprises an inductor 702. The other branch of the parallel coupling comprises the series coupling of the first piece of superconductor material, piece of conductor material and the second piece of superconductor material, which - due to the varying kinetic inductance in the conductor material - appear in fig. 7 as a variable inductor 703. A magnetic field sensor 704 is configured to sense the magnetic field of the inductor 702. In fig. 7 the magnetic field sensor 704 appears in the form of a SQUID (superconducting quantum interference device) with direct current (DC) readout.
The bias current lb is divided into two component currents, of which current /c is the critical supercurrent that flows through the SNS junction and current k is the current that flows through the inductor 702. When electromagnetic radiation is received, resulting in an increase of the kinetic inductance of the SNS junction, the component currents change. The variation in component current k produces a magnetic field that is detected with the magnetic field sensor 704. The magnetic flux generated by the incident radiation is Φ = Mk, where M is the mutual inductance between the SQUID and the inductor 702.
Certain mathematical expressions can be derived using the Josephson inductance model corresponding to the linearized current-phase relation. This allows for analytical expressions, but slightly underestimates the detector response, which must be compensated for in more accurate considerations. In the linearized regime, i.e. assuming that Lk « Φo, where Φo is the flux quantum, we get ', - lbΦj(Φ0 + Lle), and dlL/dlc « LlbΦ0/(Φ0 + LlJ .
In the pulsed (calohmetric) detection mode, the signal to noise ratio S/N can be readily expressed as
S _ (dΦ/dTe )δTe M(dlL/dlJdlJdTβ)δTβ
(2) N δΦnyfω δΦn4ω
where δΦn is the flux sensitivity of the dc SQUID, and ω is its bandwidth. Fig. 8 illustrates the S/N ratio as a function of the frequency v of the received photon. In calculating the curves of fig. 8 the following parameter values were used:
L = I OO nH; M = I O nH; ω = 1 MHz; δΦn = 10"7 Φ0/VRz ; and lb = 0.8/c(v). The proximity Josephson sensor is capable of giving very high S/N ratios in the 100 GHZ - 100 THz frequency range. In these exemplary calculations the S/N ratio is maximized around 40 THz where it obtains values in the order of 1.2 x 103 at Tbath = 0.2 K.
In continuous mode (bolometric) operation an important figure of merit is the noise equivalent power NEP, which is due to several uncorrelated noise sources. In the present case a major contribution is due to thermal fluctuation noise -limited NEP, known as N E PTFN and g iven by NEPTFN = -^5Zc6XQ(T6 6 + T® ath ) , wh ile the contribution due to Johnson noise is absent, thanks to the operation of the SNS junction in the dissipationless regime. The contribution of the SQUID readout to NEP (known as NEPSQUID) can be determined by setting S/N = 1 , ω = 1 Hz, and solving formula (2) for Popt. Fig. 9 illustrates NEPTFN (dashed line) and N EPSQUID (solid line) as a function of the phonon temperature Tbath- NEPSQUID is significantly smaller than NEPTFN, and the latter can be as low as in the order of 7 x 10~20 W/VJHz at 0.2 K. Further reduction of NEPTFN is possible by making the conductor volume Ω smaller as well as exploiting materials with low coupling coefficient ∑.
To be exact, the discussion so far concerns the electrical NEP. The optical NEP is of the same order of magnitude, because the resistance of the device can be easily matched to suitable antenna types, like common broadband self-similar lithographic antennas.
Fig. 10 illustrates an exemplary calculation of the resolving power (2πhv/AE ) of a rad iation sensor accord ing to an embod iment of the invention , where ΔE « 2V2 In 2NEPTFN(v)^τe _ph is the energy resolution of full width at half maximum, for a number of different phonon temperatures Tbath- Resolving power values between about 1 .2 and about 2.3 can be achieved in the 5 - 70 THz frequency range for phonon temperatures at or above 400 mK, which shows that the proximity Josephson sensor is suitable for far- and mid-infrared singe-photon detection.
Fig. 11 illustrates an alternative readout principle, based on radio frequency probing of the resonance frequency of a resonant circuit, a part of which is the
SNS junction. An oscillating input signal in a suitable radio frequency range is coupled to the input port 1101 at one end of an input side transmission line 1102.
From one point along the transmission line 1102 there is a coupling to the resonant circuit, which here comprises the SNS junction (illustrated here as a variable inductance) 1103, a superconducting parallel inductance (or simply a superconducting wire) 1104 and coupling capacitances 1105 and 1106 that complete the loop. A variety of different topologies could be presented for the resonant circuit, as is known in the art. One point of the resonant circuit is coupled to an output side transmission line 1107, one end of which constitutes the output port 1108. The varying kinetic inductance of the SNS junction changes the resonance frequency of the resonant circuit, which can be measured by analysing the signal appearing at the output port 1108, when the probing signal (the oscillating signal coupled to the input port 1101 ) is known. Instead of the level (and/or phase) of an oscillating signal transmitted through the resonant circuit one could analyse (i.e. measure the level and/or phase) an oscillating signal reflected by said resonant circuit. In that case the input and output ports would naturally be on the same side of the resonant circuit and not on opposite sides like in fig. 11.
Fig. 12 illustrates schematically an imaging system according to an embodiment of the invention. Electromagnetic radiation 601 is directed from an object under study (not shown) through a optical system 1201 to an array of radiation sensors 1202. The radiation sensors in the array may all be identical, or the array may contain e.g. a selection of differently dimensioned radiation sensors to better shape the desired frequency response. In a proximity Josephson sensor an easy way to optimize operating characteristics is to select the dimensions (especially length) of the piece of conductor material that connects the pieces of superconductive material together.
In order to utilize the large number of radiation sensors in the array simultaneously there is a multiplexing arrangement 1203 that facilitates feeding the necessary bias currents and voltages, probing signals and other inputs to the individual sensors and reading the outputs of the measurement circuits in the individual sensors. Multiplexing techniques for feeding, controlling and reading out arrays of SQUID-based, RF probing based, or other appropriate measurement circuits are widely known. For example, the RF probing based technique of fig. 1 1 is easily multiplexed by making the transmission lines 1102 and 1107 long enough, coupling a number of resonant circuits of the kind shown in fig. 11 in parallel to the transmission lines, and dimensioning each resonant circuit to exhibit a different enough resonance frequency. Multiplexing in the case of SQUID-based measurement circuits, used in a readout scheme for transition edge sensors, is known e.g. from the publication M. Kiviranta, H. Seppa, J. van der Kuur, P. de Korte: "SQU ID-based Readout Schemes for Microcalorimeter Arrays", 9th International Workshop on Low Temperature Detectors, Madison, Wisconsin, U.S.A., 23-27 JuIy, 2001.
At least the array of radiation sensors 1202, and typically also the multiplexing arrangement 1203, needs to be cooled enough and kept at a cooled temperature to achieve superconductivity in the appropriate parts. For this purpose the imaging system of fig. 12 comprises a cooling system 1204. Cryogenic cooling is well known and numerous suitable technical solutions are readily available. How low the desired temperature is depends very much on the superconductor materials that have been selected. It is advantageous to utilize superconductor materials with a transition temperature at or higher than 4.2 K, because e.g. closed-circuit cryocooling can be easily adopted to maintain cooled temperatures in the range between 1 and 4.2 Kelvin.
The imaging system of fig. 12 comprises an image production system that is configured to read the multiplexed output signals of individual radiation sensors and to convert these output signals into an electronic representation of an image. Again, technical solutions are known as such. In the exemplary imaging system of fig. 12 an analog interface 1205 is responsible for interfacing tasks in analog domain, like signal amplification, bias and operating power generation and delivery, and passing through possible multiplexed address and feedback signals. A digital interface 1206 implements interfacing tasks in digital domain, like time reference generation, address generation for the multiplexing scheme, and feedback signal processing. A control computer 1207 has a control bus connection to and from the interface blocks 1205 and 1206, and a serial data bus for conveying data to and from the digital interface block 1206. The final electronic representation of an image is formed and displayed in the control computer 1207 with the appropriate image processing software.
Fig. 13 illustrates schematically a method for detecting electromagnetic radiation according to an embodiment of the invention. The method comprises receiving electromagnetic radiation through an impedance transformer, directing an electric current induced by said received electromagnetic radiation through a series coupling of a first piece of superconductor material, a piece of conductor material, and a second piece of superconductor material, and converting a change in electron temperature in said piece of conductor material into an electric measurement signal through the change in the critical supercurrent of the junction and thereby its kinetic inductance.
The converting may comprise measuring changes in the magnetic field of an inductor coupled in parallel with said series coupling and fed with an electric current from a common current source in parallel with said series coupling. Alternatively the converting may comprise measuring reflection or transmission of an oscillating signal by a resonant circuit, a part of which is said series coupling. In order to achieve superconductivity in appropriate materials, the method may comprise cooling the series coupling to a temperature below 4.2 Kelvin, depending on the desired accuracy, detected radiation frequency and used materials. In order to implement imaging of an object under study the method may comprise directing electromagnetic radiation from an object under study to an array of radiation sensors that implement said receiving and directing, reading output signals of individual radiation sensors in said array of radiation sensors, and converting said output signals into an electronic representation of an image.

Claims

What is claimed is:
1. A radiation sensor, comprising:
- a first piece (102) of superconductor material,
- a second piece (103) of superconductor material, - a piece (101 ) of conductor material connected between said first (102) and second (103) pieces of superconductor material to form a junction,
- an impedance transformer (104, 105) coupled to at least one of said first (102) and second (103) pieces of superconductor material for performing impedance transformation between a device impedance of the radiation sensor and space impedance of space surrounding the radiation sensor, and
- a measurement circuit (603) configured to convert a change in kinetic inductance of the junction into an electric measurement signal.
2. A radiation sensor according to claim 1 , characterized in that the volume of said piece (101 ) of conductor material is not larger than 10"21 m3.
3. A radiation sensor according to claim 1 or 2, characterized in that the measurement circuit (603) comprises:
- an inductor (702) coupled in parallel with the series coupling (703) of said first piece of superconductor material, said piece of conductor material and said second piece of superconductor material, - a current source (701 ) coupled to feed electric current to the parallel coupling of said inductor (702) and said series coupling (703), and
- a magnetic field sensor (704) configured to sense the magnetic field of said inductor (702).
4. A radiation sensor according to claim 3, characterized in that the magnetic field sensor (704) comprises a SQUID.
5. A radiation sensor according to claim 1 or 2, characterized in that the measurement circuit (603) comprises:
- a signal source coupled to feed an oscillating signal to a resonant circuit, a part of which is the series coupling (1103) of said first piece of superconductor material, said piece of conductor material and said second piece of superconductor material, and
- a signal sensor coupled to measure at least one of: level of oscillating signal transmitted through said resonant circuit, level of oscillating signal reflected back from said resonant circuit, phase of oscillating signal transmitted through said resonant circuit, phase of oscillating signal reflected back from said resonant circuit.
6. A radiation sensor according to claim 1 , characterized in that said impedance transformer comprises a first antenna branch (604) coupled to said first piece (102) of superconductor material and a second antenna branch (605) coupled to said second piece (103) of superconductor material.
7. A radiation sensor according to claim 1 , characterized in that said impedance transformer comprises a resonant cavity within the structure of the radiation sensor.
8. An imaging system, comprising an array of radiation sensors (1202) according to claim 1.
9. An imaging system according to claim 8, characterized in that the imaging system comprises:
- an optical system (1201 ) configured to direct electromagnetic radiation from an object under study to said array of radiation sensors (1202),
- a cooling system (1204) configured to keep said array of radiation sensors (1202) at a cooled temperature, and
- an image production system (1205, 1206, 1207) configured to read output signals of individual radiation sensors in said array of radiation sensors (1202) and to convert said output signals into an electronic representation of an image.
10. An imaging system according to claim 9, characterized in that the cooling system (1204) is configured to keep said array of radiation sensors (1202) at a temperature below 4.2 Kelvin.
11. A method for detecting electromagnetic radiation, comprising: - receiving electromagnetic radiation through an impedance transformer (104, 105),
- directing an electric current induced by said received electromagnetic radiation through a series coupling of a first piece (102) of superconductor material, a piece (101 ) of conductor material, and a second piece (103) of superconductor material, which together constitute a junction, and
- converting a change in electron temperature in said piece of conductor material into an electric measurement signal through a change in the critical supercurrent and kinetic inductance of the junction.
12. A method according to claim 11 , characterized in that said converting comprises measuring changes in the magnetic field of an inductor (702) coupled in parallel with said series coupling (703) and fed with an electric current from a common current source (701 ) in parallel with said series coupling (703).
13. A method according to claim 11 , characterized in that said converting comprises measuring reflection or transmission of an oscillating signal by a resonant circuit that comprises said series coupling (1103).
14. A method according to any of claims 11 -13, characterized in that the method comprises cooling said series coupling (703) to a temperature below 4.2 Kelvin.
15. A method according to any of claims 11 -14, characterized in that the method comprises:
- directing electromagnetic radiation (601 ) from an object under study to an array of radiation sensors (1202) that implement said receiving and directing, and
- reading output signals of individual radiation sensors in said array of radiation sensors and converting said output signals into an electronic representation of an image.
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