IT202100026432A1 - Method of controlling a superconducting electromagnetic radiation sensor and sensor implementing this method - Google Patents

Description

Description of the industrial invention entitled "Method of control of a superconducting sensor of electromagnetic radiation and sensor implementing this method?"

DESCRIPTION

Scope of the invention

[0001] Does the present invention concern highly sensitive superconducting electromagnetic radiation sensors? usable, for example, in the field of security and defense, telescopes, quantum computers, quantum cryptography, etc.

[0002] In particular, the invention refers to a method of measuring electromagnetic radiation by means of a superconducting sensor, to a superconducting sensor which implements this method, and to an electromagnetic radiation detector which contains an array of superconducting sensors as follows: made.

Description of the prior art

[0003] Ultrasensitive radiation sensors such as, for example, Transition Edge Sensors (TES) are known. Their sensitivity? grows as the working temperature decreases, and their realization requires the use of superconductors with very low critical temperature (TC), and therefore difficult to synthesize with a Tc chosen at will. Furthermore, their properties? are determined by the materials used during their manufacture and cannot be modulated during operation. TES of prior art are described for example in US5090819, US5264375, US5880468.

[0004] ? I know a sensor that? an evolution of the TES, called Josephson Escape Sensor (JES), described in ?Hypersensitive Tunable Josephson Escape Sensor for Gigahertz Astronomy?,

Phys. Rev. Applied 14, 034055 ? 09/21/2020. In analogy to the TES, the 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 the superconducting to the resistive regime, provides a detectable signal. It comprises two Al conductors joined together by a one-dimensional Josephson junction, consisting of an Al/Cu bilayer forming a single unit. completely superconducting. This structure provides resistance versus temperature characteristics that can be precisely controlled by a bias current, thus allowing the working temperature or leakage temperature, and therefore sensitivity, to be controlled in situ. Can a JES sensor? operate at the critical temperature of the superconductor with an equivalent noise power of approximately 6?10<-20 >W/Hz<1/2 >and a frequency resolution of approximately 100 GHz. With a JES ? It is possible to create, by polarizing it with higher injection currents and therefore lowering the escape temperature, bolometers capable of measuring an equivalent power of intrinsic thermal fluctuation noise, of the order of 10<-25 >W/Hz<1/2>. Furthermore, ? It is possible to create calorimeters with a frequency resolution of approximately 2 GHz.

[0005] A limitation of both the currently proposed TES sensors and the JES sensor? that the lateral electrodes, which act as Andreev mirrors, are made with a superconductor with a much higher critical temperature (Tc,e)? high compared to that of the active region (TC, RA), that is? TC,RA << TC,e, to ensure that the quasi-particles are ?hot? remain confined to the active region, thus allowing sensitivity to be maximized? of the detector.

[0006] The creation of these TES or JES detectors, therefore, necessarily requires more manufacturing step to obtain the sensitive element (active region) from a superconductor or from a normal metal/superconducting bilayer, and to obtain lateral electrodes, although of equal or comparable thickness, of much greater width and made with a superconducting material with TC much more? high (TC,RA << TC,e).

[0007] ? also known from US6812464 a Superconducting Single Photon Detector (SSPD) whose active region is? formed from a single wire of a single material, for example NbN, of small transverse dimensions, but not one-dimensional, polarized near its critical current. Despite this geometry, which allows for a more? simple manufacturing and achieving sensitivity? of a TES, the sensor, in addition to requiring both the active region and the surrounding regions at the critical temperature, is unable to have high definition and possibility? to check the sensitivity in situ.

Summary of the invention

[0008] Purpose of the invention? provide a method of measuring electromagnetic radiation using a superconducting sensor that allows accurate measurements in both the GHz and THz bands and higher, and which can achieve high sensitivity? throughout the GHz band.

[0009] ? Another object of the invention is to provide a method of measuring electromagnetic radiation using a superconducting sensor which allows the sensor to be used with both high sensitivity and that at low and medium sensitivity? as the injection current varies.

[0010] ? Another object of the invention is to provide a method of measuring electromagnetic radiation using a superconducting sensor which allows a quick return to the superconducting state after a detection of electromagnetic radiation.

[0011] ? another object of the invention is to provide a method of measuring electromagnetic radiation using a superconducting sensor which allows operation with both direct current injection and alternating current injection.

[0012] ? Another object of the invention is to provide a superconducting sensor that implements this method.

[0013] ? It is another object of the invention to provide such a superconducting sensor that is more simple implementation compared to similar existing sensors.

[0014] ? It is another object of the invention to provide such a superconducting sensor that is capable of being integrated into a sensor array.

[0015] These and other purposes are achieved by a method for measuring electromagnetic radiation in the GHz, THz or higher band comprising the steps of:

- prepare an active region, a power region and a detection region;

- placing in said active region a superconducting inductor and a sensitive element in series with each other, wherein said sensitive element is? in a superconducting material having a predetermined critical temperature;

- placing in said power supply region a source of electric injection current so as to form a closed electric circuit with said superconducting inductor and said sensitive element, and feeding with a predetermined constant electric current which passes through said superconducting inductor and said sensitive element;

- placing in said detection region a magnetic field sensor coupled with said superconducting inductor and maintaining said detection region below a second temperature; - detect by means of said magnetic field sensor variations in magnetic field caused by current variations passing through said superconducting inductor and caused by electromagnetic radiation incident in said active region;

- in which said sensitive element? obtained through 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 a width and thickness less than called the superconducting coherence length, and determine 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 greater than said filament, preferably between 20 and 50 times the width of said filament,

- maintaining said first temperature below said critical temperature;

- inject a constant current into said intensity sensitive element? less than or equal to the critical current of the filament, so that said filament is crossed by a constant current; - detect by means of said magnetic field sensor coupled with said superconducting inductor a variation in the magnetic field generated by said superconducting inductance following a variation in current in said sensitive element due to a transition of said filament from a superconducting condition to a resistive condition for effect of electromagnetic radiation affecting said filament.

[0016] By applying the method according to the invention, in a similar way to TES and JES, due to the effect of the injection current which keeps the filament close to the transition temperature from the superconducting state to the resistive state, also called escape temperature, lower at the critical temperature, when electromagnetic radiation is absorbed by the sensitive element, which can be even a single photon, the energy of this radiation is enough to cause the transition.

At the moment in which the filament of the sensitive element makes this 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 superconducting inductor.

[0017] According to the invention the transition does not occur in the lateral electrodes, which at the moment of detection remain in the superconductive state despite being made of the same material as the filament, thanks to the fact that the filament is at least 10 times larger than said filament, preferably between 20 and 50 times the width of said filament.

[0018] In particular, all the more low ? 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 the lateral electrodes, even more so? the latter operate as energy filters, creating perfect thermal confinement useful for high sensitivity measurements, for example suitable for obtaining measurements of single photons in the entire GHz range. In fact, the lower? the frequency of the incident radiation, lower? the energy associated with it, and therefore less? 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 remain very close to the transition of the filament to the resistive state, and obtain this transition with minimum incident energy and without the transition affecting the lateral electrodes.

[0019] Advantageously, for very high sensitivities? the first temperature? much lower than one third of the critical temperature of the lateral electrodes, i.e. Te,W<<0.3TC,B. In particular, for sensitive element in Nb ? between 10 mK and 1 K. Instead, at the same first temperature, by injecting a current lower than the critical current of the filament into the sensitive element, there is a higher first temperature and a higher sensitivity. lower than the above. This feature allows you to use the sensor at different degrees of sensitivity. and at different working temperatures, i.e. the aforementioned first temperature corresponding to the escape temperature of the filament.

[0020] In case sensitivity is required? minors, the first temperature can? be Te,W~0.3-0.6TC,B and, depending on the materials used, for example with a sensitive element in Nb raised even above 1 K. In this case less expensive and bulky refrigerators are needed to reach the first temperature.

[0021] In case sensitivity is required? even lower the first temperature can? be between Te,W~0.6-0.9TC,B depending on the injection current. In particular, for sensitive element in Nb could it? reach 5 K. Even then, even less expensive and bulky chillers will be needed to reach temperatures below the first temperature. At this first temperature, in case of injection current different from zero but lower than the critical current of the filament, the escape temperature in the filament will be? slightly lower than the critical temperature of the filament, and therefore it will be able to be comparable with the latter, and therefore with the critical temperature of the electrodes, which will be energy filters that are not perfectly efficient, with imperfect thermal confinement, useful for measurements with relatively lower sensitivity, for example suitable for measurements of single photons in the field of THz or greater.

[0022] At the limit, the first temperature can? be Te,W~TC,B, and with an injection current just above zero or at the zero limit, the escape temperature of the filament will be? approximately equal to the critical temperature of the active region and the lateral electrodes. So, will the sensor 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, for example suitable for measurements of single photons in the range above THz. Advantageously, for example with a sensitive element in Nb, for such sensitivities? the first temperature can? be for example between 3 K and the critical temperature of the superconductor of said sensitive element can? be around 9 K.

[0023] In this way the following effects are obtained:

- the critical temperature of the electrodes? always equal to the critical temperature of the filament, and this allows obtaining the Andreev reflection in the monolithic structure of the sensitive element;

- the greater width of the electrodes with respect 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 very far from the critical current in the electrodes, allowing the suppression of thermal diffusion at the electrodes;

- operation in the superconducting state decreases the heat exchange of the hot quasiparticles of the active region with the phonons, since? the electron-phonon coupling? exponentially suppressed when the escape temperature ? much lower than the critical temperature of the active region thanks to the injection current, the current injection allows to improve the performance of the sensor;

- the same thickness and the same material of the filament and electrodes allows for easy realizability, in particular the possibility of to create arrays, and to implement Andreev mirrors in monolithic structures using standard manufacturing processes;

- thanks to the modulation of the working temperature with the polarization current of the active region, the Andreev reflection in the monolithic structure makes s? that the Andreev mirrors can extend to form the antenna or waveguide that conveys the radiation to the active region;

- thanks to the Andreev reflection in the lateral electrodes, this solution also allows the electrodes to be extended up to the antenna or wave guide which carries the electromagnetic radiation to be measured by passing 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 rapid return to the superconducting state.

[0024] In a possible embodiment, the injection current source is ? a source of direct current and ? provision is made for the provision of a shunt resistor in said active region in parallel to the sensitive element and to the superconducting inductance with respect to the injection current source.

[0025] In this way, the shunt resistor allows limiting the current flowing in the sensitive 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 electromagnetic radiation accident. There? allows the rapid return to the superconducting state in order to detect other 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 ? in resistive conditions, IR the retrapping current and I the injection current.

[0026] In another possible embodiment, the injection current source is an alternating current source and ? provision is made for the provision of a load resistance outside the active region in series with the sensitive element and the superconducting inductance.

[0027] In this way, since? when the absorption of electromagnetic radiation by the sensitive 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 speed? of thermalization, ? Is it advantageous for the frequency of the alternating injection current to be higher than this speed? of thermalization so that the current circulating in the sensitive element is lower than the IR retrapping current at different points of the oscillation period, with the consequent immediate transition of the sensor from the resistive state to the superconducting state. In this configuration, the speed? of the sensor? only limited by speed? of thermalization.

[0028] Preferably, variations in the current circulating in said superconducting inductor due to the absorption of electromagnetic radiation by said sensitive element can be measured through a SQUID amplifier coupled inductively to said superconducting inductor. In particular, the SQUID amplifier is maintained at a second temperature, in particular said second temperature ? between 1 K and 5 K. Preferably, said second temperature is ? of approximately 4 K.

[0029] According to another aspect of the invention, a sensor is created which implements the method described above.

[0030] The sensor has the sensitive element made of any metallic superconducting material, and preferably chosen from MoGe, Pb, Nb, Ti.

[0031] According to one aspect of the invention, an electromagnetic radiation detector can be achieved with a plurality? of sensors made with the method according to the invention and coupled with a multiplexing scheme based on microwave resonant circuits.

[0032] In a possible realization the plurality? of sensors? arranged to operate in frequency division multiplexing.

[0033] In all the aforementioned solutions, to achieve maximum sensitivity, the units? detector elements can be kept at ultra-low temperature (T<1 K, first temperature), while amplifiers, for example SQUID, can reside between 1 K and 5 K (second temperature), for example at 4K, to minimize the thermal noise, while the reading electronics can be kept at room temperature (third temperature).

Brief description of the drawings

[0034] Will further characteristics and/or advantages of the present invention be more apparent? clearly with the following description of one of its embodiments, made by way of example and not by way of limitation, with reference to the attached drawings in which:

- figures 1 and 2 schematically show a plan and lateral elevation view of a monolithic superconducting sensitive element that can be used to implement the method according to the invention;

- figure 3 shows a circuit diagram in which ? inserted the sensitive element of figure 1 in the implementation of the method according to the invention; - figure 4 shows possible operating ranges of the sensor, based on the temperature of the active region, on the diagram of the resistance trend as a function of temperature in a superconductor;

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

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

- figure 7 shows a third embodiment of the circuit diagram of figure 3 which can be used as a sensor to detect electromagnetic radiation which implements 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 embodiment of the circuit diagram of figure 3 which can be used as a sensor to detect electromagnetic radiation which implements 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

[0035] In a possible embodiment of the invention, described below, a method is provided for measuring electromagnetic radiation in the GHz, THz and higher bands.

[0036] The method involves a sensitive element 120 made of a single superconducting material, shown in Figs. 1 and 2.

[0037] The sensitive element 120 has a constant thickness 123 for its entire extension, and has as its active portion a one-dimensional filament 121 made in continuity. with lateral electrodes 124, 125.

[0038] The lateral electrodes 124,125 have a thickness 126 equal to the thickness 123 of the filament, but have a width 127 much greater than the width 122 of the filament 121.

[0039] The width 122 of the filament 121 and the thickness 123 of the filament 121 and of the side electrodes 124,125 are less than or equal to the coherence width of the superconductor ?w, which ? a known and characteristic dimension for each superconductor, linked to the spatial dimension of the Cooper pairs.

[0040] In this way, using the sensitive element 120 as a superconducting circuit element, for example as described further forward, the Cooper pairs, in passing from the electrodes 124, 125 to the one-dimensional filament 121 in the same superconducting material, face a constraint caused by the smaller lateral dimensions of the superconducting coherence length ?w and the penetration length of the London magnetic field ?L, w.

[0041] Therefore, on the one hand, the one-dimensionality? of the superconducting filament 121 makes it possible to modulate the escape temperature, injecting a limited current and, consequently, modulating the transition from the superconducting to the resistive state. In fact, how more? described above, the measurement method according to the invention, and the sensor that implements it, allow fine adjustment of the working temperature and sensitivity. measurement through the controlled injection of an electric current, allowing operation at different temperatures depending on the electromagnetic radiation to be detected and allowing for wide versatility. with respect to the requirements of the specific application.

[0042] On the other hand, the sensing element 120 implements Andreev mirrors with a single superconducting material. In particular, the sensitive element 120 constitutes a monolithic Josephson junction, in which the filament 121 provides a strong variation in resistance (?R) at the moment of its transition into the resistive state, which can be caused by an incident radiation, for example even a single photon, which is the object of the measurement, while the lateral electrodes 124,125 remain in the superconductive state.

[0043] Therefore, by establishing a predetermined working temperature of the sensitive element 120, which preferably is well below the critical temperature of the material that composes it, thanks to the fact of making the sensitive element 120 in a single superconducting material and having the same thickness 123, due to the effect of the injection current that passes through the filament 121 the escape temperature in this filament could? arrive at coinciding with the working temperature, and therefore much lower than the critical temperature.

[0044] In this way, the electrodes 124 and 125 will never go above the critical temperature, so through these electrodes Andreev mirrors are obtained which can extend to form the antenna or the wave guide that conveys the radiation to the filament. Indeed, ? It is possible to maintain below the critical temperature not only the lateral electrodes 124,125, but also portions of superconductor which form the antenna or wave guide which conveys the radiation to the filament, not shown in the figures.

[0045] From a technological point of view, to create the sensitive element 120? It is possible to use a single evaporation process for the creation of the entire sensor, in combination with circuit elements such as pi? described above, reducing production costs and ensuring reproducibility? of the entire process, as well as? facilitates the production of sensor arrays. In fact, as shown more? further, the fact that both the electrodes 124,125 and the filament 121 are made of the same superconducting material allows the use of all conventional metallic superconductors for which there has been extensive know-how for some time.

[0046] The width 127 of the electrodes 124,125 can? be at least 10 times larger than the filament 121, preferably between 20 and 50 times the width of said filament 121, obtaining the above and more advantages? described below.

[0047] The realization of the sensitive element 120 can? be made with any metal with properties? superconducting. In fact, for the sole fact that the sensitive element 120 is formed by the same material that forms the one-dimensional filament 121 and the electrodes 124,125, the transition from the superconductive state to the resistive state always and only occurs in the filament 121, with amplification of the current signal which is caused by the incident radiation and the sudden change in resistance in the filament 121. In this way, the effect is always obtained that the escape temperature of the filament 121 is modulated, by modifying the injection current, until the critical current of the filament 121 is reached , for which the sensitive element 120 reaches maximum sensitivity, as more? described below.

Advantageously, since? the critical temperature of the one-dimensional superconducting filament 121 depends on its lateral dimensions and, for some materials, for example MoGe, Pb, Nb, and Ti, decreases as the cross-section of the wire decreases, ? It is preferable to choose these materials to make the sensitive element 121. In fact, the use of these materials to make the sensitive element 120 allows starting from a superconductor/normal metal transition temperature of the filament 121 that is lower than that of the lateral electrodes 124,125 . There? further expands the range of possible applications.

According to the invention, with reference to Figure 3, a measurement method that uses the sensitive element 120 can foresee the following phases.

Prepare an active region 100, a detection region 200 and a power supply region 300. In the active region 100, for example by deposition by thermal evaporation of metals on a substrate, a superconducting inductor 110 and the sensitive element 120 are arranged as described above , electrically in series with each other. The active region 100 is maintained at a first temperature which is below the critical temperature of the superconducting material of the sensitive element 120, in a modulable way as more described below.

Then, a source 310 of injection electric current is arranged in the power supply region 300, so as to form a closed electric circuit with the superconducting inductor 110 and with the sensitive element 120, to generate a predetermined constant electric current that passes through the ?superconducting inductor 110 and the sensing element 120. The power region 300 can? be maintained at a third temperature, which can? be the room temperature, or another temperature that can be practically reached based on the specific instrument made.

A magnetic field sensor 210 is arranged in the detection region 200, which can include for example an inductor 211, coupled with the superconducting inductor 110. The detection region 200 is kept below a second temperature, higher than the first temperature of the active region 100, and suitable for the ideal operation of the magnetic field sensor 210. The magnetic field sensor 210 has the function of measuring the variations in the current circulating in the superconducting inductor due to the absorption of electromagnetic radiation by the sensitive element 120, which determine a variation in the magnetic field generated by the inductor superconducting 110, and which is sensed by the inductor 211. Such current variations can then be transmitted to a unit control not shown, for example by means of a transmission element 212, which can? be an additional inductor or other element.

The magnetic field sensor 210 can? advantageously be a SQUID amplifier, as indicated in more examples? described above, well known for currently being the most? sensitive detector of magnetic field variations, and not described in greater detail as it is known to those skilled in the art.

Therefore, by injecting through the source 310 a constant current into the sensitive element 120, of intensity? lower or comparable to the critical current of the filament 121, the filament 121 is also ? crossed by this constant current which keeps it in the superconducting state in limit conditions such that, in the presence of electromagnetic radiation affecting said filament 121, there is a rapid transition to the resistive state, with a sudden increase in the resistance of the filament 121, and consequently sudden change in the current passing through the superconducting inductor 110. This allows us to detect, by means of the magnetic field sensor 210 coupled with the superconducting inductor 110, a change in the magnetic field in the superconducting inductance 110, following the aforementioned change in current in the element sensitive 120 due to a transition of said filament 121 from a superconducting condition to a resistive condition.

According to the invention, also with reference to Fig. 4, it is? It is possible to maintain the first temperature of the region 100 of Fig. 3 at a preselected value and inject into the sensitive element 120 and therefore into said filament 121 an injection current which, indicating with Te,W the escape temperature of the filament 121, and with TC,B is the critical temperature of the lateral electrodes 124,125 of Fig. 1 and 2, can? to be:

- substantially equal to the critical current of the filament 121 maintaining the first temperature Te,W<<0.3TC,B,

- substantially lower than the critical current of the filament 121 and maintain the first temperature Te,W~0.3-0.6TC,B,

- always, however, lower than the critical temperature of the filament 121, maintaining the first temperature Te,W~0.6-1TC,B i.e. slightly below or substantially equal to the critical temperature of said filament 121. How? It is possible to note that the escape temperature of the Te,W filament is not ? a factory data of the sensitive element 120, but ? influenced by the first temperature, i.e. by the temperature of the active region 100, which will depend? from the choice of sensitivity? what do you want to achieve with the 120 sensor, and will determine? the critical value of the injection current that the filament 121 can? endure shortly before abruptly switching from the superconducting state in the event of the incidence of radiation to be measured.

In this way, in the aforementioned three respective situations, without modifying the sensor, but simply by 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 ? much lower than the critical temperature of the filament itself and of the lateral electrodes 124,125, i.e. Te,W<<0.3TC,B, the lateral electrodes 124,125 act as energy filters, that is? from so-called Andreev mirror for heat. Therefore, the detector, operating with I~IC,W, behaves like a JES with perfect thermal confinement and very high sensitivity.

- JES with partial thermal confinement, in case of escape temperature although much lower than the critical temperature, with less distant values, such as Te,W~0.3-0.6TC,B. In this case the detector will behave? like a JES with thermal confinement that is not perfect since the I tolerable by the filament 121 will be? lower than the above case. In this case the lateral electrodes 124,125 will be energy filters that are not perfectly efficient. Consequently, the sensitivity? of the detector like this? will he be operated on? intermediate between the case above and the one reported 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. Te,W~0.6-0.9TC,B and therefore comparable with the critical temperature of the electrodes 125,125, the latter will behave as energy filters with imperfect thermal confinement, useful for measurements with relatively lower sensitivity, for example suitable for measurements of single photons in the THz field.

- TES configuration, at the limit, when the first temperature ? close to the critical temperature of the material, i.e. Te,W~TC,B, the injection current that can be tolerated by the filament 121? approximately zero (I?0), i.e. the escape temperature of the filament 121 ? approximately equal to the critical temperature Tc of the entire sensitive element 120 which includes both the filament 121 and the lateral electrodes 124,125. So, will the sensor operate? at the critical temperature, substantially in 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 the filament 121 and the lateral electrodes 124 and 125 will pass together from the superconductive state to the resistive state at the moment of incidence of a radiation to be measured.

The possibility? to control in situ the escape temperature of the filament 121 in the totally monolithic structure of the sensitive element 120 without, therefore, jumps in the intrinsic critical temperature of the material between electrodes 124, 125 and filament 121, ensures the following advantages compared to existing technologies:

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

- ? It is possible to use only a superconducting material through standard manufacturing processes, allowing to lower production costs, create arrays easily as well as guaranteeing reproducibility. of the system;

- ? Is it possible to reduce the starting Tc of the sensitive element 120 with the appropriate choice of superconducting material, depending on the maximum sensitivity? that must reach the sensor;

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

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

- 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 all things being equal? of working temperature, its efficiency is much greater than existing sensors;

- the small dimensions of the active region 100 which are much smaller than 1 m<3> and the Andreev mirrors obtainable in the lateral electrodes 124,125 ensure a high efficiency of the sensor;

- the invention can? immediately use the TES reading circuits or a simple direct or alternating current bias;

- once the sensor has been created, ? It is possible to implement it in detectors with greater or lesser 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 by choosing an appropriate polarization current.

As shown in Figure 5, in one possible embodiment, the injection current source 310 can comprising a direct current source 311, for example a DC power supply, disposed in the power supply region 300 at room temperature (third temperature). Then ? provision is made for the provision of a shunt resistor 130 in said active region 100, inserted in parallel to the sensitive element 120 and to the superconducting inductance 110 with respect to the injection current source 310. Shunt resistor 130 can? have a value RS<RN<.>IR/I, RN being the resistance of said filament 121 when ? in resistive conditions, IR the retrapping current and I the injection current. The shunt resistor 130, made in the active region 100, has the role of limiting the current flowing in the filament 120 under the IR retrapping current, i.e. the return current from the normal state to the superconducting state, after the transition to the resistive state with resistance RN. There? it allows the rapid return to the superconducting state, in times ranging from milliseconds to microseconds. Thus, a SQUID 220 sensor or other magnetic field sensor can detect photons in sequence without the overheating of the filament 120 disturbing the measurement.

Alternatively, as shown in Figure 6, in another possible embodiment, the injection current source 310 comprises an alternating current source 312 arranged in the power supply region 300. In this case, ? provision is made for the provision of a load resistor 313 outside the active region 100 in series with the sensitive element 120 and with the superconducting inductance 110.

Then, ? Is it possible to use an AC 311 generator of voltage V, placed at room temperature, connected in series to the load resistor 313, with resistance RL? RN, also kept at room temperature, to the superconducting inductor 110 with inductance L, kept at the working temperature, for example <1 K, and to the sensitive element 120. This causes? that with the absorption of a photon, the filament 121 makes the transition to the resistive state with a consequent decrease in the current circulating in the I=V/(RL+RN) circuit. By imposing the frequency of the bias signal lower than the speed? of thermalization of the filament 121, i.e. from micro- to millisecond, ? It is possible to avoid the shunt resistance foreseen for the direct current configuration of Fig. 5. In fact, the circulating current will be? lower than IR at different points of the oscillation period resulting in the sensor transition from normal to superconducting state. Here too, the SQUID 220 sensor or other magnetic field sensor can? detect photons sequentially.

With reference to figure 7, an electromagnetic radiation detector can? be configured to provide a single pixel of a multiplexed array based on microwave resonant circuits, as shown in Figure 8.

In this case, the sensor or pixel 10? configured as one of the embodiments described above, for example that of figure 5, and can? be coupled to an RLC 400 circuit, also maintained at the second temperature of the 200 region.

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

As described above, the absorption of radiation by the sensor or pixel 10 causes a variation in the value of the resistance of the filament 121 (Fig. 3, Fig. 5) of the sensitive element 120 and therefore of the current flowing through the inductor 110. The change in the current flowing through the inductor 110 causes the variation of the inductance linked to the Josephson junction 220 and therefore of the resonant frequency of the RLC 400 circuit. The RLC 400 circuit is powered through a power circuit 500, made up of a signal generator operating at radio frequency 501, a load impedance 502 and an amplifier 503. The resulting measurement is always given in the region 300 located at the third temperature, by a circuit 600 formed by a signal amplifier 601 which provides an output signal 602. The amplifier 600 can? be achieved through a high electron mobility transistor (HEMT) 601. The variation of the output signal 602 is therefore linked to the absorption of radiation by the sensor or pixel 10.

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

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

In particular, the absorption of the radiation shifts the resonance frequency of the RLC 400 circuit relating to the single sensor or pixel 10 since? the absorption of a radiation causes a change in the total inductance of the relevant resonant circuit 400. The frequency of the resonant circuits 400 is ? chosen so as not to limit the speed? of operation of the single sensor 700. Each resonant circuit 400 ? powered at the same time as the others and the total signal? sent to a single amplifier placed at the third temperature of the 300 region, for example room temperature.

With reference to figure 9, in addition to what is shown in figures 3, 4, 5 and 6, in another possible embodiment, it is shown a possible circuit suitable for use as a single pixel for a frequency division multiplexed sensor array.

The sensitive element 120, in a similar way to as described above, acts with zero resistance in the absence of photon detection, while upon detection it has its own resistance Rn. in this embodiment there are provided, in series with the sensitive element 120, an inductance 140 of value Ln, and a capacitance 150, worth Cn.

In this way, an RCL 160 circuit is created which operates at its own frequency fn=1/[2?(LnCn)<1/2>] of resistance given by the sensor 120 which varies depending 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 later with reference to Fig. 10), to avoid interference between the various sensors.

The sensor signal band 10 will be? longer than the thermalization time of the sensor itself, 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 single sensor. Similarly to the previous cases, with reference also to figure 9, the RCL 160 circuit? powered by a 311 generator and a Rl 313 load impedance, placed at room temperature. Furthermore, ? a shunt resistor 130 of value Rsh is provided which has the role of limiting the current flowing in the single circuit RCL under the retrapping current IR. In a similar way to what is described above for the other embodiments, an amplifier 220 placed at the second cryogenic temperature of the region 200, such as for example a SQUID amplifier, and coupled to the inductance L 110 receives the signal and amplifies it in 250 to allow its detection in 260. The advantage of keeping the 250 amplifier at cryogenic temperatures is the decrease in thermal noise and therefore greater sensitivity? of the sensor

With reference to figure 10, an electromagnetic radiation detector can? understand a plurality? of sensors 10, each representing a pixel of a sensor array, connected according to a frequency division scheme.

Similarly to the previous cases, each RCL circuit (indicated with 160 in Fig. 9), formed by variable resistors, inductances and capacitance 120, 140, 150; 120?, 140?, 150?; 120?, 140?, 150?; can? be powered by a single generator 311 and a load impedance Rl 313, placed for example at room temperature in the region 300.

The generator 311 generates a carrier signal common to all the 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 oscillation frequency of the single pixel 10 shifts the relative signal to a different frequency for each channel.

Here too, similarly to what is described above for the other embodiments, an amplifier 220 placed at the second cryogenic temperature of the region 200, such as for example a SQUID amplifier, and coupled to the inductance L 110 receives the signal and amplifies it in 250 to allow detection in 260. A demodulation circuit not shown separates the signals coming from the various pixels and assigns them to the different channels. Here too, the advantage of keeping the 250 amplifier at cryogenic temperatures is? the decrease in thermal noise and therefore greater sensitivity? of the sensor.

The above description of some specific embodiments ? capable of showing the invention from a conceptual point of view so that others, using the known art, will be able to modify and/or adapt this specific embodiment in various applications without further research and without departing from the inventive concept, and, therefore, intends that such adaptations and modifications will be considered equivalent to the specific embodiment. The means and materials for carrying out the various functions described may be of various nature without departing from the scope of the invention. It is understood that the expressions or terminology used are purely descriptive and therefore not restrictive.

For example, the first and second temperatures of the 100 and 200 regions can coincide, particularly in multiple sensors. low sensitivity? or for simplicity? constructive.

Claims (10)

1. A method for measuring electromagnetic radiation in the GHz, THz and higher bands, including the steps of: - prepare an active region (100), a detection region (200) and a power region (300); - arrange in said active region (100) a superconducting inductor (110) and a sensitive element (120) electrically in series with each other, wherein said sensitive element (120) is in a superconducting material having a predetermined critical temperature; - arrange in said power supply region (300) a source (310) of electric injection current so as to form a closed electric circuit with said superconducting inductor (110) and said sensitive element (120), and supply a predetermined constant electric current which passes through said superconducting inductor (110) and said sensitive element (120); - arrange in said detection region (200) a magnetic field sensor (210) coupled (211) with said superconducting inductor (110) and maintain said detection region (200) below a second temperature, detect using said sensor of magnetic field (210) variations in magnetic field 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); - in which said sensitive element (120) is? obtained through 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 superconducting coherence length, and said one-dimensional filament has width (122) and thickness (123) less than said superconducting coherence length, and determine the critical current of the filament; - deposit in said active region (100) two electrodes (124,125) made of the same superconducting material as said filament, said filament (121) being arranged between said electrodes (124,125) so as to form a monolithic structure with said electrodes (124,125), said electrodes (124,125) having thickness (126) substantially equal to said filament (121) and having width (127) at least 10 times greater than said filament (121), preferably between 20 and 50 times the width of said filament (121), - maintaining said first temperature below said critical temperature; - inject a constant current into said sensitive element (120) of intensity? lower than or equal to the critical current of the filament (121), so that said filament (121) is crossed by a constant current; - detect by means of said magnetic field sensor (210) coupled with said superconducting inductor (110) a change in magnetic field in said superconducting inductance (110) following a change in current in said sensitive element (120) due to a transition of said filament (121) from a superconducting condition to a resistive condition due to the effect of electromagnetic radiation affecting said filament (121). 2. The method according to claim 1, wherein said injecting step is choice between: - inject into said sensitive element (120) and therefore into said filament (121) a current substantially equal to the critical current of said filament (121) and maintain the first temperature Te,W<<0.3TC,B, - inject into said sensitive element (120) and therefore into said filament (121) a current substantially lower than the critical current of said filament (121) and maintain the first temperature Te,W~0.3-0.6TC,B, - inject into said sensitive element (120) and then into said filament (121) a current comparable to the critical current and maintain the first temperature at temperatures comparable to the critical temperature of said filament (121) Te,W~0.6-1TC,B . 3. The method according to claim 1, wherein said injection current source (310) comprises a direct current source (311) disposed in said power supply region (300), and is? provision is made for the provision of a shunt resistor (130) in said active region (100) in parallel to the sensitive element (120) and to the superconducting inductance (110) with respect to the source (310) of injection current, in which said shunt resistor shunt (130) has the value RS<RN IR/I, RN being the resistance of said filament (121) when ? in resistive conditions, IR the retrapping current and I the injection current. 4. The method according to claim 1, wherein said injection current source (310) comprises an alternating current voltage source (312) disposed in said power supply region (300), and is? provision is made for the provision of a load resistor (313) outside the active region (100) in series with said sensitive element (120) and with said superconducting inductance (110). 5. The method according to claim 4, wherein said alternating current voltage source (312) delivers said injection current at a frequency greater than the speed of thermalization of said sensitive element (120) at the moment of the incidence of electromagnetic radiation so that the current circulating in the sensitive element is lower than the IR retrapping current at different points of the oscillation period, with the consequent immediate transition of the sensor from the resistive state to the superconducting state. 6. 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 sensitive element are measured through a SQUID amplifier (220) arranged in said detection region and coupled inductively (211) to said superconducting inductor. 7. An electromagnetic radiation sensor in the GHz, THz and higher bands, comprising: - an active region (100), a detection region (200) and a power region (300); - a superconducting 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 in a superconducting material having a predetermined critical temperature; - a source (310) of electric injection current arranged in said power supply region (300) and connected so as to form a closed electric circuit with said superconducting inductor (110) and said sensitive element (120), and supply a predetermined current constant electrical 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) configured to 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 in the same superconducting material as said filament, so that said filament (121) is arranged between said electrodes (124,125) and forms a monolithic structure with said electrodes (124,125) , said electrodes (124,125) having thickness (126) substantially equal to said filament (121) and having width (127) at least 10 times greater than said filament (121), preferably between 20 and 50 times the width of said filament (121) , - means for maintaining said first temperature of said active region (100) below said critical temperature; - in which said source (310) ? configured to inject a constant current into said sensitive element (120) of intensity? lower than or equal to the critical current of the filament (121), so that said filament (121) is crossed by a constant current; - said magnetic field sensor (210) being configured to detect a magnetic field variation in said superconducting inductance (110) following a current variation in said sensitive element (120) due to a transition of said filament (121) from a superconducting condition to a resistive condition due to the effect of electromagnetic radiation affecting said filament (121). 8. An electromagnetic radiation detector comprising with a plurality? of sensors as per claim 7. 9. An electromagnetic radiation detector as per claim 8, wherein said plurality? of sensors? connected according to a scheme based on microwave resonant circuits. 10. An electromagnetic radiation detector as per claim 8, wherein said plurality? of sensors? connected according to a frequency division multiplexing scheme.