WO2023062603A1

WO2023062603A1 - Superconducting electromagnetic radiation sensor and operation method thereof

Info

Publication number: WO2023062603A1
Application number: PCT/IB2022/059882
Authority: WO
Inventors: Federico PAOLUCCI; Francesco GIAZOTTO; Nadia LIGATO; Elia STRAMBINI
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 2021-10-14
Filing date: 2022-10-14
Publication date: 2023-04-20
Also published as: IT202100026432A1; EP4417027A1

Abstract

A method for measuring electromagnetic radiation in GHz and higher frequency bands comprises arranging an active region (100), a detection region (200), and a feed region (300). In the active region (100) a superconducting inductor (110) and a sensing element (120) are arranged electrically in series. In the supply region (300) a current source (310) is arranged to form a closed electric circuit with said superconductive inductor and said sensing element. In the sensing region (200) a magnetic field sensor (210) is coupled (211) with said superconducting inductor. The sensing element (120) is a monolithic structure obtained by depositing a superconducting material so as to form a one-dimensional filament (121) and two electrodes (124, 125) made of the same superconducting material. By injecting a constant current less than or equal to the critical current of the filament into said sensing element, it is possible to detect a change of magnetic field in said superconducting inductor as a result of a change of current in said sensing element due to a transition of said filament from a superconducting condition to a resistive condition caused by the incident electromagnetic radiation.

Description

SUPERCONDUCTING ELECTROMAGNETIC RADIATION SENSOR AND OPERATION METHOD THEREOF

DESCRIPTION

Field of the invention

[0001] The present invention relates to high-sensitivity superconductive electromagnetic radiation sensors that can be used, for example, in the fields of security and defence, telescopes, quantum computers, quantum cryptography, etc.

[0002] In particular, the invention relates to a method of measuring electromagnetic radiation by means of a superconductive sensor, a superconductive sensor implementing such a method, and an electromagnetic radiation detector containing an array of such superconductive sensors.

Brief description of the prior art

[0003] Ultra-sensitive radiation sensors such as, for example, Transition Edge Sensors (TES), are known. Their sensitivity increases as the operating temperature decreases, and their fabrication requires the use of superconductors with very low critical temperature (Tc). Therefore they are difficult to synthesize with any chosen Tc. Moreover, their properties are determined by the materials used during their fabrication and they cannot be modulated during operation. TES sensors of known technique are for example described in US5090819, US5264375, US5880468.

[0004] A sensor that is an evolution of TES is known, called Josephson Escape Sensor (JES), described in "Hypersensitive Tunable Josephson Escape Sensor for Gigahertz Astronomy," Federico Paolucci, Nadia Ligato, Vittorio Buccheri, Gaia Germanese, Pauli Virtanen, and Francesco Giazotto, Phys. Rev. Applied 14, 034055 -21/09/2020. In analogy to TES, JES also exploits the change in resistance of a superconductor during the transition to the dissipative state, whereby the absorption of even very weak radiation, causing the transition from superconducting to resistive regime, provides a detectable signal. It comprises two conductors of Al joined together by a one-dimensional Josephson junction, consisting of an Al/Cu bilayer forming a fully superconducting unit. This structure provides resistance versus temperature characteristics that can be precisely controlled by a bias current, thus allowing in situ control of the operating temperature or escape temperature, and hence sensitivity. A JES sensor can operate at the critical superconductor temperature with an equivalent noise power of about 6x1 O’²⁰ W/Hz^1/2 and a frequency resolution of about 100 GHz. With a JES, it is possible to realize, by polarizing it with higher injection currents and thus lowering the escape temperature, bolometers capable of measuring an intrinsic thermal fluctuation noise equivalent power on the order of 10’²⁵ W/Hz^1/2. In addition, calorimeters can be made with a frequency resolution of about 2 GHz.

[0005] A limitation of both the currently proposed TES sensors and the JES sensor is that the side electrodes, which serve the function of Andreev mirrors, are made of a superconductor with a much higher critical temperature (Tc.e) than that of the active region (TC.RA), i.e. , TC.RA « Tc.e, to make sure that the "hot" quasi-particles remain confined to the active region, thus allowing the detector sensitivity to be maximized.

[0006] The fabrication of these TES or JES detectors, therefore, necessarily requires multiple fabrication steps to obtain the sensing element (active region) from a superconductor or a normal metal/superconductor bilayer, and to obtain side electrodes, albeit of the same or comparable thickness, of much greater width and made from a superconducting material with much higher Tc (TC.RA « Tc.e,).

[0007] Also known from US6812464 is a Superconducting Single Photon Detector (SSPD), whose active region consists of a single strand of a single material, e.g., NbN, of small transverse size, but not one-dimensional, polarized near its critical current. Despite this geometry, which allows for simpler fabrication and to achieve the sensitivity of a TES, the sensor, in addition to requiring to have both the active region and the surrounding regions at the critical temperature, fails to have high definition and ability to control in situ sensitivity.

[0008] In Paolucci F. et al: "Development of highly sensitive nanoscale transition edge sensors for gigahertz astronomy and dark matter search", Journal Of Applied Physics, vol. 128, 194502, 18 November 2020 (XP012251729), and also in Paolucci F. et al: "GHz Superconducting Single-Photon Detectors for Dark Matter Search", Instruments, vol. 5, 14, 1st April 2021 (XP055930380), two Josephson escape sensor (JES) are disclosed that implement a heat confinement (Andreev mirrors) by using lateral electrodes made of a superconductor of wider superconducting energy gap than the one-dimensional wire active region. In these references, the working temperature of the sensor (escape temperature) is tuned by simple current injection.

[0009] In Gallop J et al: "Nanobridge SQUIDs as calorimetric inductive particle detectors", Superconductor Science Nano Technology, vol. 28, 084002, 13 July 2015 (XP055309233) a monolithic SQUID used as superconducting radiation detector is disclosed, working near the critical temperature of the whole system, but never transitioning to a resistive status. This structure presents nanobridges that are not one-dimensional.

Summary of the invention

[0010] An object of the invention is to provide a method of measuring electromagnetic radiation using a superconductive sensor that enables accurate measurements in both the GHz band and the THz band and above, and that can achieve high sensitivity throughout the GHz band.

[0011] It is another object of the invention to provide a method of measuring electromagnetic radiation using a superconductive sensor that allows the sensor to be used at both high sensitivity and low and medium sensitivity as the injection current changes.

[0012] It is another object of the invention to provide a method of measuring electromagnetic radiation by means of a superconductive sensor that enables a fast return to the superconductive state after a detection of electromagnetic radiation.

[0013] It is another object of the invention to provide a method of measuring electromagnetic radiation by means of a superconductive sensor that enables operation with both direct current injection and alternating current injection.

[0014] It is another object of the invention to provide a superconductive sensor implementing such a method.

[0015] It is another object of the invention to provide such a superconductive sensorthat is easier to implement than similar existing sensors. [0016] It is another object of the invention to provide such a superconductive sensor that is adapted to integration into a sensor array.

[0017] These and other objects are achieved by a method for measuring electromagnetic radiation in the GHz band, THz or higher frequencies, comprising the steps of:

- arranging an active region, a detection region and a power supply region;

- arranging in said active region a superconducting inductor and a sensing element in series with each other, wherein said sensing element is of a superconducting material having a predetermined critical temperature;

- arranging in said power supply region a source of injection electric current so as to form a closed electric circuit with said superconductive inductor and said sensing element, and injecting a predetermined constant electric current through said superconductive inductor and said sensing element;

- arranging in said sensing region a magnetic field sensor coupled with said superconducting inductor and maintaining said sensing region below a second temperature;

- detecting by means of said magnetic field sensor changes in magnetic field caused by changes in current flowing through said superconducting inductor and caused by incident electromagnetic radiation in said active region;

According to the invention said sensing element is obtained by the following steps:

- depositing a predetermined superconducting material in said active region so as to form a one-dimensional filament, said superconducting material having a predetermined critical temperature, wherein said superconducting material has a predetermined superconducting coherence length, and said one-dimensional filament has width and thickness less than said superconducting coherence length, and determining the critical current of the filament;

- depositing in said active region two electrodes made of the same superconducting material as said filament, said filament being arranged between said electrodes and forming a monolithic structure with said electrodes, said electrodes having a thickness substantially equal to said filament and having a width at least 10 times the width of said filament, preferably between 20 and 50 times the width of said filament,

- maintaining said active region at a first temperature below said critical temperature;

- injecting a constant current into said sensing element of an intensity less than or equal to the critical current of said filament, so that said filament is traversed by a constant current;

- detecting by means of said magnetic field sensor coupled with said superconducting inductor a variation of the magnetic field in said superconducting inductor following a current variation in said sensitive element due to a transition of said filament from a superconductive condition to a resistive condition due to the effect of electromagnetic radiations which affect said filament.

[0018] Applying the method according to the invention, in a manner analogous to TES and JES, due to the injection of current capable of maintaining the filament near the temperature of transition from the superconductive state to the resistive state, ad also known as escape temperature, which is lower than the critical temperature, when an absorption of incident electromagnetic radiation by the sensing element, which may be even a single photon, occurs, the energy of said radiation is sufficient to cause the transition. At the instant that the filament of the sensing element makes such a transition from the superconductive state to the resistive state, there is a consequent sharp decrease in the current circulating in the circuit, which is measured through the magnetic field sensor coupled with said superconductive inductor.

[0019] According to the invention, notwithstanding the very low thickness of the side electrodes, below the superconducting coherence length of the superconducting material, the transition does not occur in the side electrodes, which at the time of detection remain in the superconductive state despite being made of the same material as the filament, due to the fact that the side electrodes are at least 10 times larger than said filament, preferably between 20 and 50 times.

[0020] In particular, the lower the escape temperature in the filament, which depends on the injection current, compared to the critical temperature of the filament, which is the same as that of the side electrodes, the more the latter operate as energy filters, realizing a perfect thermal confinement useful for high- sensitivity measurements, e.g., suitable for obtaining measurements of single photons throughout the GHz range. In fact, the lower the frequency of the incident radiation, the lower the energy associated with it, and thus the lower the power in the case of single photons. To achieve this very high sensitivity according to the invention, it is possible to lower the first temperature so that, by injecting a current close to the critical current of the filament, it is possible to keep the filament very close to the transition to the resistive state, and to achieve this transition with minimal incident energy and without the transition affecting the side electrodes.

[0021] Advantageously, for very high sensitivities the first temperature is much less than one-third of the critical temperature of the side electrodes, i.e. , Te,w«0.3Tc,B. Specifically, for sensitive element in Nb it is between 10 mK and 1 K. In contrast, at the same first temperature, injecting a current lower than the critical filament current into the sensing element results in a higher first temperature and lower sensitivity than above. This feature allows the sensor to be used at different degrees of sensitivity and at different working temperatures, i.e., the aforementioned first temperature corresponding to the escape temperature of the filament.

[0022] In case lower sensitivities are required, the first temperature can be set Te,w~0.3-0.6Tc,B and, depending on the materials used, for example with Nb sensing element raised even above 1 K. In case of this feature, less expensive and less bulky chillers are required to reach the first temperature.

[0023] In case even lower sensitivities are required, the first temperature can be set between T_e,w~0.6-0.9Tc,B depending on the injection current. In particular, for sensitive element in Nb it may reach 5 K. Even then, much less expensive and less bulky chillers will be needed to reach temperatures below the first temperature. At such a first temperature, in the case of non-zero injection current but less than the critical filament current, the escape temperature in the filament will be just below the critical filament temperature, and thus it may be comparable with the latter, and thus with the critical temperature of the electrodes, which will be imperfectly efficient energy filters with imperfect thermal confinement, useful for relatively lower sensitivity measurements, e.g., suitable for single photon measurements in the range of THz or greater.

[0024] At the limit, the first temperature may be T_e,w~Tc,B, and with an injection current just above zero or at the zero limit, the filament escape temperature will be about equal to the critical temperature of the active region and side electrodes. Thus, the sensor will operate at the critical temperature without thermal confinement, i.e. , the electrodes will not act as an energy filter, and the sensor will be with relatively low sensitivity, e.g., suitable for single photon measurements in the range above THz. Advantageously, e.g., with an Nb sensing element, for such sensitivities the first temperature may be, for example, between 3 K and the critical superconductor temperature of said sensing element may be around 9 K.

[0025] In this way, the following effects are obtained:

- the critical temperature of the electrodes is always equal to the critical temperature of the filament, and this allows for Andreev's reflection in the monolithic structure of the sensing element;

- the greater width of the electrodes relative to the filament allows an injection current to approach or reach the critical current in the filament, i.e., the maximum current that allows the filament to have zero electrical resistance, while remaining far from the critical current in the electrodes, enabling the electrodes to suppress thermal diffusion;

- operation in the superconducting state decreases the heat exchange of the hot quasi-particles in the active region with phonons, since electronphonon coupling is exponentially suppressed when the escape temperature is much lower than the critical temperature of the active region due to the injection current, so that current injection allows for improved sensor performance;

- the same thickness and material of the filament and electrodes allows easy fabrication, especially the possibility of making arrays, and implementing Andreev mirrors in monolithic structures using standard fabrication processes;

- due to modulation of the working temperature with the bias current of the active region, Andreev reflection in the monolithic structure means that the Andreev mirrors can extend to form the antenna or waveguide that conveys radiation to the active region;

- due to the Andreev reflection in the side electrodes this solution also allows the electrodes to extend to the antenna or waveguide that carries the electromagnetic radiation to be measured by switching the filament from superconductive to resistive;

- it is possible to limit the current flowing in the filament under the retrapping current, i.e. , the return current from the normal state to the superconducting state, after the transition to the resistive state, and this allows the fast return to the superconducting state.

[0026] In one possible embodiment, the injection current source is a DC current source and a shunt resistor is provided in said active region in parallel with the sensing element and the superconducting inductor with respect to the injection current source.

[0027] Thus, the shunt resistor makes it possible to limit the current flowing in the sensing element under the retrapping current, i.e., the return current from the normal state to the superconducting state after the transition to the normal state due to incident electromagnetic radiation. This allows the fast return to the superconductive state in order to detect more incident electromagnetic radiation. In particular, the shunt resistor will have to have value RS<RN*IR/I, RN being the resistance of said filament when it is in resistive state, IR the retrapping current and I the injection current.

[0028] In another possible embodiment, the injection current source is an alternating current source and a load resistor is provided outside the active region in series with the sensing element and the superconducting inductor.

[0029] Thus, because when absorption of electromagnetic radiation by the sensing element occurs, due to the transition of the active region from the superconducting state to the resistive state, there is a decrease in the current circulating in the circuit, and this transition occurs in times ranging from micro- to milli-second, so-called thermalization speed, it is advantageous for the frequency of the alternating injection current to be higher than this thermalization speed so that the current circulating in the sensing element is less than the IR retrapping current at various points in the oscillation period, resulting in the immediate transition of the sensor from the resistive state to the superconductive state. In such a configuration, the sensor speed is uniquely limited by the thermalization rate.

[0030] Preferably, changes in the current circulating in said superconducting inductor due to absorption of electromagnetic radiation by said sensing element can be measured by means of a SQUID amplifier inductively coupled to said superconducting inductor. Specifically, the SQUID amplifier is maintained at a second temperature, specifically said second temperature is between 1 K and 5 K. Preferably, said second temperature is about 4 K.

[0031] According to another aspect of the invention, a sensor implementing the above method is made.

[0032] The sensor has the sensing element made of any metallic superconducting material, and preferably chosen from MoGe, Pb, Nb, Ti.

[0033] According to an aspect of the invention, an electromagnetic radiation detector can be realized with a plurality of sensors made by the method according to the invention and coupled with a multiplexing scheme based on microwave resonant circuits.

[0034] In one possible embodiment, the plurality of sensors is arranged to operate in frequency division multiplexing.

[0035] In all the above solutions, to achieve maximum sensitivity, the detector elementary units can be kept at ultra-low temperature (T<1 K, first temperature), while the amplifiers, e.g., SQUID, can reside between 1 K and 5 K (second temperature), e.g., at 4K, to minimize thermal noise, while the readout electronics can be kept at room temperature (third temperature).

Brief description of the drawings

[0036] Further characteristics and/or advantages of the present invention will become clearer with the following description of an embodiment thereof, made by way of example and not of limitation, with reference to the appended drawings in which: figures 1 and 2 show schematically in plan view and side elevation view a monolithic superconductive sensing element that can be used to implement the method according to the invention; - figure 3 shows a circuit diagram in which the sensing element of figure 1 is inserted in the implementation of the method according to the invention;

- figure 4 shows possible operating ranges of the sensor, depending on the temperature of the active region, on the diagram of the temperaturedependent resistance trend in a superconductor;

- figure 5 shows a first embodiment variant of the circuit diagram in figure 3, with DC injection current supply;

- figure 6 shows a second embodiment variant of the circuit diagram in figure 3, with supply of the injection current in alternating current;

- figure 7 shows a third realizable variant of the circuit diagram in figure 3 usable as an electromagnetic radiation detector sensor implementing the method according to the invention;

- figure 8 shows an electromagnetic radiation detector with a plurality of sensors like the one in figure 7 and which are organized according to a multiplexing scheme based on microwave resonant circuits;

- figure 9 shows a fourth implementation variant of the circuit diagram in figure 3 that can be used as an electromagnetic radiation detector sensor implementing the method according to the invention;

- figure 10 shows an electromagnetic radiation detector with sensors that implement the method according to the invention and are organized according to a multiplexing scheme based on frequency division.

Description of some preferred embodiments

[0037] In a possible embodiment, described below, of the invention, a method is provided for measuring electromagnetic radiation in the bands of GHz, THz and higher.

[0038] The method includes a sensing element 120 made from a single superconductive material, shown in Figs. 1 and 2.

[0039] The sensing element 120 has constant thickness 123 throughout its extent, and has as its active portion a one-dimensional filament 121 made in continuity with side electrodes 124, 125. [0040] The side electrodes 124,125 have thickness 126 equal to the thickness 123 of the filament, but have width 127 much greater than the width 122 of filament 121.

[0041] The width 122 of filament 121 and the thickness 123 of filament 121 and side electrodes 124, 125 are less than or equal to the superconductor coherence width _w, which is a known and characteristic dimension for each superconductor, related to the spatial dimension of Cooper pairs.

[0042] Thus, by using the sensing element 120 as a superconducting circuit element, e.g., as described below, the Cooper pairs, in the transition from the electrodes 124, 125 to the one-dimensional filament 121 in the same superconducting material, face a constriction caused by the smaller side dimensions of the superconducting coherence length

and the penetration length of the magnetic field of London AL,W.

[0043] Therefore, on the one hand, the one-dimensionality of the superconducting filament 121 makes it possible to modulate the escape temperature by injecting a limited current and, consequently, modulating the transition from the superconducting to the resistive state. Indeed, as further described below, the measurement method according to the invention, and the sensor that implements it, make it possible to finely adjust the operating temperature and measurement sensitivity through the controlled injection of an electric current, enabling operation at different temperatures depending on the electromagnetic radiation to be detected and allowing a wide versatility with respect to the requirements of the specific application.

[0044] On the other hand, sensing element 120 implements Andreev's mirrors with a single superconducting material. Specifically, sensing element 120 constitutes a monolithic Josephson junction, in which filament 121 provides a strong resistance change (AR) upon its transition into the resistive state, which can be caused by an incident radiation, e.g., even a single photon, which is the object of measurement, while side electrodes 124,125 remain in the superconductive state.

[0045] Hence, by establishing a predetermined working temperature of the sensing element 120, which preferably is well below the critical temperature of its component material, due to the fact that the sensing element 120 is made of a single superconductive material and having the same thickness 123, due to the injection current flowing through the filament 121 the escape temperature in that filament may come to be coincident with the working temperature, and thus much lower than the critical temperature.

[0046] In this way, electrodes 124 and 125 will never go above the critical temperature, so that through them electrodes Andreev mirrors are obtained that can extend even further as depicted, to form the antenna or waveguide that conveys radiation to the filament. In fact, it is possible to keep below the critical temperature not only the side electrodes 124,125 but also portions of the superconductor that precisely form the antenna or waveguide that conveys the radiation to the filament, not shown in the figures.

[0047] From a technological point of view, a single evaporation process can be used to make the sensing element 120 in combination with circuit elements as further described below, reducing the cost of fabrication and ensuring the reproducibility of the entire process, as well as the ease of producing sensor arrays. In fact, as shown below, the fact that both the 124,125 electrodes and the 121 filament are made of the same superconducting material makes it possible to use all conventional metallic superconductors of which there has long been extensive know-how.

[0048] The width 127 of the electrodes 124,125 can be at least 10 times the width of the filament 121 , preferably between 20 and 50 times the width of said filament 121 , achieving the advantages described above and below.

[0049] The fabrication of the sensing element 120 can be done with any metal with superconductive properties. In fact, by the mere fact that sensing element 120 is formed from the same material that forms the one-dimensional filament 121 and electrodes 124,125, the transition from the superconductive state to the resistive state always and only occurs in filament 121 , with amplification of the current signal that is caused by the incident radiation and the abrupt change in resistance in filament 121. Thus, the effect is always that the escape temperature of filament 121 is modulated, changing the injection current, until the critical current of filament 121 is reached, for which the sensing element 120 reaches maximum sensitivity, as further described below.

[0050] Advantageously, since the critical temperature of the onedimensional superconducting filament 121 depends on its lateral dimensions and, for some materials, e.g., of MoGe, Pb, Nb, and Ti, decreases as the cross- sectional area of the wire decreases, it is preferable to choose such materials to make the sensing element 121. In fact, using these materials to make the sensing element 120 allows one to start from a lower superconductor/normal metal transition temperature of the 121 filament than that of the side electrodes 124,125. This further broadens the range of possible applications.

[0051] According to the invention, with reference to Figure 3, a measurement method using the sensing element 120 may involve the following steps.

[0052] Arrange an active region 100, a sensing region 200 and a feed region 300. In the active region 100, a superconducting inductor 110 and the sensing element 120 are arranged, for example by thermal evaporation deposition of metals on a substrate, as described above, electrically in series with each other. The active region 100 is maintained at a first temperature that is below the critical temperature of the superconducting material of the sensing element 120, in a modulable manner as further described below.

[0053] Then, a source 310 of injection electric current is arranged in the supply region 300 to form a closed electric circuit with the superconducting inductor 110 and the sensing element 120 to generate a predetermined constant electric current flowing through the superconducting inductor 110 and the sensing element 120. The supply region 300 can be maintained at a third temperature, which can be room temperature, or other temperature of practical attainment according to the specific instrument made.

[0054] A magnetic field sensor 210 is arranged in the detection region 200, which may include, for example, an inductor 211 , coupled with the superconducting inductor 110. The detection region 200 is maintained below a second temperature, above the first temperature of the active region 100, and suitable for the ideal operation of the magnetic field sensor 210. The function of the magnetic field sensor 210 is to measure changes in the current circulating in the superconducting inductor due to the absorption of electromagnetic radiation by the sensing element 120, which results in a change in the magnetic field generated by the superconducting inductor 110, and which is sensed by the inductor 211. These current variations can then be transmitted to a control unit not shown, for example by means of a transmission element 212, which may be an additional inductor or other element. [0055] The magnetic field sensor 210 may advantageously be a SQUID amplifier, as shown in examples later described, well known to be the most sensitive detector of magnetic field variation at present, and not described in more detail as known to the branch engineer.

[0056] Thus, by injecting through the source 310 a constant current into the sensing element 120, of an intensity less than or comparable to the critical current of the filament 121 , the filament 121 is also traversed by such a constant current that keeps it in the superconductive state under boundary conditions such that, in the presence of electromagnetic radiation affecting said filament 121 , there is a rapid transition of it to the resistive state, with abrupt rise in the resistance of the filament 121 , and consequently abrupt change in the current flowing through the superconductive inductor 110. This makes it possible to detect by means of the magnetic field sensor 210 coupled with the superconducting inductor 110 a change of magnetic field in the superconducting inductor 110, as a result of the said change of current in the sensing element 120 due to a transition of said filament 121 from a superconducting condition to a resistive condition.

[0057] According to the invention, also with reference to Fig. 4, it is possible to maintain the first temperature of the region 100 of Fig. 3 at a chosen value and to inject into the sensing element 120 and thus into said filament 121 an injection current which, denoting by Te.wthe escape temperature of filament 121 , and by TC.B the critical temperature of the side electrodes 124, 125 of Figs. 1 and 2, can be:

- substantially equal to the critical current of filament 121 while maintaining the first temperature T_e.w«0.3Tc.B,

- substantially less than the critical current of filament 121 and maintain the first temperature T_e.w~0.3-0.6Tc.B,

- always, however, lower than the critical temperature of filament 121 , keeping the first temperature T_e,w~0.6-1Tc,B i.e., just below or substantially equal to the critical temperature of said filament 121 .

[0058] As can be seen, the escape temperature of the filament T_e,w is not a factory datum of the sensing element 120, but is influenced by the first temperature, i.e., the temperature of the active region 100, which will depend on the choice of sensitivity to be achieved by the sensor 120, and will determine the critical value of the injection current that the filament 121 can withstand just before it abruptly switches from the superconductive state in the event of the incidence of a radiation to be measured.

[0059] Thus, in the above three respective situations, without changing the sensor, but simply choosing the first temperature for the active region 100 and modulating the injection current (I) accordingly, the following can be obtained:

- JES with perfect thermal confinement, when the escape temperature of the filament 121 is much lower than the critical temperature of the filament itself and of the side electrodes 124,125, i.e. , T_e.w«0.3Tc.B, the side electrodes 124,125 operate as energy filters, i.e., as a so-called Andreev mirror for heat. Thus, the detector, operating with l—IC, W, behaves as a JES with perfect thermal confinement and very high sensitivity.

- JES with partial thermal confinement, in the case of escape temperature albeit much lower than the critical temperature, with less distant values, such as T_e,w~0.3-0.6Tc,B.ln that case, the detector will behave as a JES with less than perfect thermal confinement since the tolerable I from filament 121 will be lower than the above case. In that case, the side electrodes 124,125 will be imperfectly efficient energy filters. Consequently, the sensitivity of the detector thus operated will be intermediate between the case above and the case below.

- JES without significant thermal confinement or TES without thermal confinement, when the escape temperature in the filament is slightly lower than the critical temperature of the filament 121 , e.g., T_e,w~0.6-0.9Tc,B and thus comparable with the critical temperature of the electrodes 125,125, the latter will behave as energy filters with imperfect thermal confinement, useful for relatively lower sensitivity measurements, e.g., suitable for single photon measurements in the THz range.

- TES configuration, at the limit, when the first temperature is close to the critical material temperature, i.e., T_e,w~Tc,B, the tolerable injection current from filament 121 is about zero (I— >0), i.e., the escape temperature of filament 121 is about equal to the critical temperature T_c of the whole sensing element 120 comprising both filament 121 and side electrodes 124,125. Thus, the sensor will operate at the critical temperature, essentially in a TES configuration, without the presence of thermal confinement, i.e. , the electrodes will not act as a filter, and therefore with low sensitivity, as both filament 121 and side electrodes 124 and 125 will transition together from the superconductive to the resistive state at the time of incidence of a radiation to be measured.

[0060] The ability to control the escape temperature of filament 121 in situ in the totally monolithic structure of sensing element 120 without, therefore, inherent critical material temperature jumps between electrodes 124,125 and filament 121 ensures the following advantages over existing technologies:

- the working temperature, i.e., the minimum escape temperature achievable with the injection current, and the sensitivity of the sensor can be changed during operation depending on the specific application and the characteristics of the experimental set-up;

- it is possible to use only a superconductive material through standard fabrication processes, allowing lower production costs, easy fabrication of arrays as well as ensuring reproducibility of the system;

- it is possible to decrease the starting Tc of the sensing element 120 with the appropriate choice of superconductive material, depending on the maximum sensitivity to be achieved by the sensor;

- modulation of the working temperature, i.e., the escape temperature, is possible due to the small lateral dimensions of the filament 121 and the injection current, allowing for the first time Andreev's mirrors to be made in a monolithic structure, i.e., made from a single material;

- the invention avoids the use of different superconductive materials at specific critical temperatures, creating a universal platform for ultrasensitive photon detection;

- the sensor works at the escape temperature, which, depending on the injection current and the first working temperature chosen, can be much lower than the critical temperature of the superconductor, so that at the same working temperature, its efficiency is much higher than existing sensors; - the small size of the active region 100 which is much less than 1 pm3 and the Andreev mirrors obtainable in the side electrodes 124,125 ensure high efficiency of the sensor;

- the invention can immediately use the readout circuits of TESs or a simple DC or AC bias;

- once the sensor is made, it can be implemented in detectors with higher or lower sensitivity by appropriately sizing systems that maintain the first temperature in the active region 100 and the second temperature in the detection region 200, and choosing an appropriate bias current.

[0061] As shown in Figure 5, in one possible implementation, the injection current source 310 may include a DC current source 311 , such as a DC power supply, arranged in the supply region 300 at room temperature (third temperature). In such a case, a shunt resistor 130 is arranged in said active region 100, inserted in parallel with the sensing element 120 and the superconducting inductor 110 with respect to the injection current source 310. The shunt resistor 130 can have a value RS<RN*IR/I, RN being the resistance of said filament 121 when it is in resistive condition, IR the retrapping current and I the injection current. The shunt resistor 130, realized in the active region 100, has the role of limiting the current flowing in filament 120 under the retrapping current IR, i.e., the return current from the normal state to the superconducting state, after the transition to the resistive state with resistance RN. This allows the fast return to the superconductive state, in times ranging from milliseconds to microseconds. Thus, a SQUID 220 sensor or other magnetic field sensor can detect photons sequentially without overheating of the filament 120 disturbing the measurement.

[0062] Alternatively, as shown in Figure 6, in another possible embodiment, the injection current source 310 includes an alternating current source 312 arranged in the supply region 300. In this case, a load resistor 313 is arranged outside the active region 100 in series with the sensing element 120 and the superconducting inductor 110.

[0063] In such a case, it is possible to use an AC generator 311 of voltage V, placed at room temperature, connected in series with load resistor 313, with resistance RL < RN, also kept at room temperature, with superconducting inductor 110 with inductance L, kept at working temperature, e.g. , <1 K, and with sensing element 120. This causes that with the absorption of a photon, filament 121 makes the transition to the resistive state resulting in a decrease in the current circulating in the circuit l=V/( RL + RN). By imposing the frequency of the bias signal lower than the thermalization rate of filament 121 , i.e., micro- to millisecond, the shunt resistor provided for the DC configuration in Fig. 5 can be avoided. In fact, the circulating current will be less than IR at different points of the oscillation period resulting in the transition of the sensor from the normal to the superconducting state. Again, the SQUID 220 or other magnetic field sensor can detect photons in sequence.

[0064] Referring to Figure 7, an electromagnetic radiation detector can be configured to provide a single pixel of an array in multiplexing based on microwave resonant circuits, as shown in Figure 8.

[0065] In such a case, the sensor or pixel 10 is configured as one of the embodiments described above, for example, that in figure 5, and can be coupled to an RLC 400 circuit, which is also maintained at the second temperature of region 200.

[0066] The RLC 400 circuit can be formed as an inductance 410 coupled with the SQUID 210-220, a capacitor 420, and two transmission lines operating at radio frequency 430 and 431 , formed, for example, by coaxial cables 432 and 433. The coupling between the RLC 400 circuit and the sensor or pixel 10 is achieved through a magnetic field sensor 210, which can be implemented through a radio-frequency SQUID formed by a superconducting loop interrupted by a Josephson junction 220, and two coupling inductances 211 and 212 with the sensing element circuit 120 and the RLC 400 circuit, respectively. The resonant frequency of the RLC 440 circuit depends on the capacitance 420, the inductance 410 and the inductance 212 associated with the Josephson junction 200.

[0067] As described above, absorption of radiation by the sensor or pixel 10 causes a change in the value of the resistance of the filament 121 (Fig. 3, Fig. 5) of the sensing element 120 and thus the current flowing through the inductor 110. The change in the current flowing through inductor 110 causes a change in the inductance bound to Josephson junction 220 and thus in the resonant frequency of the RLC 400 circuit. The RLC 400 circuit is fed through a power supply circuit 500, consisting of a signal generator operating at radio frequency 501 , a load impedance 502, and an amplifier 503. The resulting measurement is given always in region 300 placed at the third temperature, by a 600 circuit formed by a signal amplifier 601 providing an output signal 602. The 600 amplifier can be realized through a high electron mobility transistor (HEMT) 601. The variation of the output signal 602 is then related to the absorption of radiation by the sensor or pixel 10.

[0068] Referring to Figure 8, an electromagnetic radiation detector may comprise a plurality of sensors 10, each representing a pixel of a sensor array, connected the according to a multiplexing scheme based on microwave resonant circuits such as that depicted in Figure 7. Each pixel 10 is coupled to a magnetic field sensor 210 as shown, for example, in Fig. 5. Each pixel 10 and each related sensor 210 are in turn coupled to a resonant circuit 400, and that inductively couples to the one magnetic field sensor 210, such as a SQUID, as described above with reference to Fig. 7. For simplicity, only three pixels 10 have been represented, which can obviously be in numbers proportionate to the resolution of the detector.

[0069] This configuration maximizes the dynamic range of each pixel or sensor 10 and places no limit on the number of pixels 10. Each resonant 400 circuit is characterized by a different resonant frequency given by the value of the resistance R, capacitance C, inductance L, and Josephson inductance value of the magnetic field sensor 210.

[0070] Specifically, the absorption of radiation shifts the resonant frequency of the RLC 400 circuit related to the individual sensor or pixel 10 because the absorption of radiation causes a change in the total inductance of the related resonant circuit 400. The frequency of the resonant circuits 400 is chosen so as not to limit the operating speed of the individual sensor 700. Each resonant circuit 400 is powered simultaneously with the others and the total signal is sent to a unique amplifier located at the third temperature of region 300, e.g., room temperature.

[0071] With reference to Figure 9, in addition to what is shown in Figures 3, 4, 5 and 6, in another possible embodiment, a possible circuit suitable for use as a single pixel for a sensor array in frequency division multiplexing is shown.

[0072] The sensing element 120, in a manner similar to what described above, acts with zero resistance in the absence of photon detection, while on detection it has its own resistance Rn. In this form of embodiment there are provided, in series with The sensing element 120, an inductance 140 of value Ln, and a capacitance 150, of value Cn.

[0073] In this way, an RCL 160 circuit is made that operates at its own frequency fn=1/[2ir(LnCn)1/2] of resistance given by sensor 120, which varies in dependence on whether or not photons are received, capacitance Cn 140, and inductance Ln 150, so that it is distinct from the frequencies of other adjacent elements (as described below with reference to Fig. 10), to avoid interference between the various sensors.

[0074] The bandwidth of the sensor signal 10 will be greater than the sensor's thermalization time, so as to suppress noise outside the band of interest. The separation between the resonant frequency of the various circuits will be chosen to be greater than the band of the individual sensor. Similarly to the previous cases, with reference also to Figure 9, the RCL 160 circuit is fed by a 311 generator and an Ri 313 load impedance, placed at room temperature. In addition, a shunt resistor 130 of value Rsh is provided, which has the role of limiting the current flowing in the single RCL circuit under the IR retrapping current. Similarly to what has been described above for the other embodiments, an amplifier 220 placed at the second cryogenic temperature of region 200, such as a SQUID amplifier, and coupled to the inductance L 110 receives the signal and amplifies it into 250 to allow detection in 260. The advantage of keeping the 250 amplifier at cryogenic temperatures is the decrease in thermal noise and thus increased sensitivity of the sensor

[0075] Referring to Figure 10, an electromagnetic radiation detector may comprise a plurality of sensors 10, each representing a pixel of a sensor array, connected the according to a frequency division scheme.

[0076] Similarly to the previous cases, each RCL circuit (denoted by 160 in Fig. 9), formed by the variable resistors, and inductances and capacitances 120, 140, 150; 120', 140', 150'; 120", 140", 150"; can be supplied by a single generator 311 and a load impedance Ri 313, placed, for example, at room temperature in region 300.

[0077] Generator 311 generates a carrier signal in common with all pixels 10. The shunt impedance Rsh 130 has the role of limiting the current flowing in the single pixel 10 under the IR retrapping current. The different of oscillation frequency of individual pixel 10 shifts the relative signal to a different frequency for each channel.

[0078] Again, in a manner similar to that described above for the other embodiments, an amplifier 220 placed at the second cryogenic temperature of region 200, such as a SQUID amplifier, and coupled to the inductance L 110 receives the signal and amplifies it in 250 to allow detection in 260. An unshown demodulation circuit separates the signals from the various pixels and assigns them to the different channels. Again, the advantage of keeping the 250 amplifier at cryogenic temperatures is decreased thermal noise and thus increased sensor sensitivity.

[0079] The above description of some specific embodiments is capable of showing the invention from the conceptual point of view so that others, using the known technique, will be able to modify and/or adapt in various applications such specific embodiment without further research and without departing from the inventive concept, and, therefore, it is understood that such adaptations and modifications will be considered as equivalents of the specific embodiment. The means and materials for realizing the various functions described may be of various kinds without departing from the scope of the invention. It is understood that the expressions or terminology used are for descriptive objects only and therefore not limiting.

[0080] For example, the first and second temperatures of regions 100 and 200 may coincide with each other, particularly in lower sensitivity sensors or for simplicity of construction.

Claims

- 22 -

CLAIMS A method for measuring electromagnetic radiation in the GHz, THz and higher bands, comprising the steps of:

- providing an active region (100), a detection region (200) and a power supply region (300);

- arranging in said active region (100) a superconducting inductor (110) and a sensitive element (120) electrically in series with each other, in which said sensitive element (120) is made of a superconducting material having a predetermined critical temperature;

- arranging in said supply region (300) an injection electric current source (310) so as to form a closed electric circuit with said superconducting inductor (110) and said sensitive element (120), and supplying a predetermined constant electric current which passes through said superconducting inductor (110) and said sensitive element (120);

- arranging in said detection region (200) a magnetic field sensor (210) coupled (211 ) with said superconducting inductor (110) and keeping said detection region (200) below a second temperature, detecting by means of said sensor of magnetic field (210) magnetic field variations caused by current variations that pass through said superconducting inductor (110) and caused by an electromagnetic radiation incident in said active region (100) which activates said sensitive element (120); wherein said sensitive element (120) is obtained by means of the following steps:

- depositing a predetermined superconducting material in said active region (100) so as to form a one-dimensional filament (121 ), said superconducting material having a predetermined critical temperature, wherein said superconducting material has a predetermined length of superconducting coherence, and said one-dimensional filament has width (122) and thickness (123) less than said superconducting coherence length, and determining the critical current of the filament;

- depositing in said active region (100) two electrodes (124,125) made of a superconducting material that consists of the same superconducting material of said filament, said filament (121 ) being arranged between said electrodes (124,125) so as to form a monolithic structure of said superconducting material with said electrodes (124,125), said electrodes (124, 125) having a thickness (126) substantially equal to said filament (121 ) and having a width (127) at least 10 times greater than said filament (121 ),

- maintaining said first temperature below said critical temperature;

- injecting a constant current into said sensitive element (120) of an intensity lower than or equal to the critical current of the filament (121 ), so that said filament (121 ) is crossed by a constant current;

- detecting by means of said magnetic field sensor (210) coupled with said superconducting inductor (110) a variation of the magnetic field in said superconducting inductor (110) following a current variation in said sensitive element (120) due to a transition of said filament (121 ) from a superconductive condition to a resistive condition due to the effect of electromagnetic radiations which affect said filament (121 ).

2 . The method according to claim 1 , wherein said injecting step is selected among:

- injecting into said sensitive element (120) and then into said filament (121 ) a current substantially equal to the critical current of said filament (121 ) and maintaining the first temperature T_e w«0.3TcB,

- injecting into said sensitive element (120) and therefore into said filament (121 ) a current substantially lower than the critical current of said filament (121 ) and maintaining the first temperature T_e,w~0.3- 0.6TC,B,

- injecting into said sensitive element (120) and then into said filament (121 ) a current comparable to the critical current and maintaining the first temperature at temperatures comparable with the critical temperature of said filament (121 ) T_e,w~0.6-1Tc,B.

3 . The method according to claim 1 , wherein said injection current source (310) comprises a direct current source (311 ) arranged in said supply region (300), and provision is made for a shunt resistor (130) in said active region (100) in parallel to the sensitive element (120) and to the superconducting inductor (110) with respect to the injection current source (310), wherein said shunt resistance (130) has value RS <RN IR/I, RN being the resistance of said filament (121 ) when it is in resistive conditions, IR the retrapping current and I the injection current. The method according to claim 1 , wherein said injection current source (310) comprises an alternating current voltage source (312) arranged in said power supply region (300), and a load resistor (313) is provided outside the active region (100) in series with said sensitive element (120) and with said superconducting inductor (110). The method according to claim 4, wherein said source of alternating current voltage (312) supplies said injection current at a frequency higher than the thermalization rate of said sensitive element (120) at the moment of the incidence of an electromagnetic radiation, in such a way that that the current circulating in the sensitive element is lower than the retrapping current IR at different points of the oscillation period, with the consequent immediate transition of the sensor from the resistive state to the superconducting state. The method according to claim 1 , wherein variations in the current circulating in said superconducting inductor due to the absorption of electromagnetic radiation by said sensing element are measured through a SQUID amplifier (220) arranged in said sensing region and inductively coupled (211 ) to said superconducting inductor. The method according to claim 1 , wherein said electrodes (124,125) have a width (127) set between 20 and 50 times the width of said filament (121 ). An electromagnetic radiation sensor in the GHz, THz and higher bands, comprising:

- an active region (100), a detection region (200) and a power supply region (300);

- a superconductive inductor (110) and a sensitive element (120) electrically arranged in series with each other in said active region (100), wherein said sensitive element (120) is made of a superconducting material having a predetermined critical temperature;

- a source (310) of injection electric current arranged in said supply region (300) and connected arranged to form a closed electric circuit with said superconductive inductor (110) and said sensitive element (120), and to supply a predetermined current constant electric current passing through said superconducting inductor (110) and said sensitive element (120);

- a magnetic field sensor (210) arranged in said detection region (200) coupled (211 ) with said superconducting inductor (110) and configured to - 25 - detect magnetic field variations caused by current variations passing through said superconducting inductor (110) and caused by an electromagnetic radiation incident in said active region (100) which activates said sensitive element (120); wherein said sensitive element (120) comprises:

- a predetermined superconducting material arranged in said active region (100) so as to form a one-dimensional filament (121 ), said superconducting material having a predetermined critical temperature, wherein said superconducting material has a predetermined coherence length, and said one-dimensional filament has width (122) and thickness (123) less than said coherence length, and determining the critical current of the filament;

- two electrodes (124,125) arranged in said active region (100) made of a superconducting material that consists of the same superconducting material of said filament, so that said filament (121 ) is arranged between said electrodes (124,125) and forms a monolithic structure of said superconducting material with said electrodes (124,125), said electrodes (124, 125) having a thickness (126) substantially equal to said filament (121 ) and having a width (127) at least 10 times greater than said filament (121 ),

- means for maintaining said first temperature of said active region (100) below said critical temperature;

- wherein said source (310) is configured to inject a constant current into said sensitive element (120) of an intensity lower than or equal to the critical current of the filament (121 ), so that said filament (121 ) is traversed by a constant current;

- said magnetic field sensor (210) configured to detect a magnetic field variation in said superconducting inductor (110) following a current variation in said sensitive element (120) due to a transition of said filament (121 ) from a superconductive condition to a resistive condition caused by electromagnetic radiations hitting said filament (121 ).

9. The electromagnetic radiation sensor as per claim 8, wherein said electrodes (124,125) have a width (127) set between 20 and 50 times the width of said filament (121 ).

10 . An electromagnetic radiation detector comprising with a plurality of sensors as per claim 8. - 26 - The electromagnetic radiation detector as per claim 9, wherein said plurality of sensors is connected according to a scheme based on microwave resonant circuits. The electromagnetic radiation detector as per claim 10, wherein said plurality of sensors is connected according to a frequency division multiplexing scheme.