Superconducting circuit and signal processing unit equipped therewith

Abstract

A superconducting (SC) circuit is provided that can be used as a high-speed, low-loss RF switch at cryogenic temperatures. The SC circuit comprises a combination of an SC element and a transistor switch. The transistor switch is coupled to one port of the SC element and configured to switch between two states in response to an applied control signal. The other port of the SC element is maintained at a desired potential by using an electronic component (e.g., a current or voltage source). When the SC element is in the SC state and the transistor switch is in the first state, a first electrical bias is provided across the SC element, thereby allowing the SC element to transmit radio signals. When the SC element is in the SC state and the transistor switch is in the second state, the SC element has either a second electrical bias that transitions the SC element from the SC state to the normal state, or a third electrical bias that changes the kinetic inductance of the SC element to a target value, but leaves the SC element in the SC state, allowing radio signals to pass through, or blocks radio signals.

Description

TECHNICAL FIELD

[0001] The present disclosure generally relates to the field of signal processing circuits. More particularly, the present disclosure relates to superconducting (SC) circuits that can be used as high-speed low-loss high-frequency (RF) switches at cryogenic temperatures, and to signal processing units comprising said SC circuits. BACKGROUND ART

[0002] Due to their ultra-low power consumption and high speed, superconducting (SC) circuits hold great promise as components of future quantum computing systems and associated cryogenic electronics. In this regard, it is of particular interest to develop switching devices that can be electrically tuned between a superconducting state and a normal (resistive) state at radio frequency (RF), thereby enabling and blocking RF signal propagation.

[0003] Currently, there are no high-speed low-loss RF switches that can operate at cryogenic temperatures. Existing complementary metal-oxide-semiconductor (CMOS) switches have too high power consumption for reasonable operation. Mechanical RF switches also exist, but they are very slow, and their operating power level is sufficiently high to briefly heat the cryostat, which causes long measurement delays in their operation. SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

[0004] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure.

[0005] The object of the present disclosure is to provide a technical solution that enables efficient RF switching (in terms of power loss and speed) at cryogenic temperatures. MEANS FOR SOLVING THE PROBLEM

[0006] The above objectives are achieved by the features of the independent claims in the attached claims. Further embodiments and examples are evident from the dependent claims, the modes for carrying out the invention, and the attached drawings.

[0007] According to a first embodiment, an SC circuit is provided. The SC circuit comprises an SC element, a transistor switch, and an electronic component. The SC element has a first port and a second port and also has an adjustable motion inductance. The transistor switch is coupled to the first port of the SC element and is configured to switch between a first state and a second state in response to an applied control signal. The electronic component is coupled to the second port of the SC element. When the SC element is in an SC state and the transistor switch is in the first state, the transistor switch and the electronic component are configured to provide a first electrical bias for the SC element, which causes a radio signal supplied to one of the first and second ports of the SC element to pass through the SC element. When the SC element is in the SC state and the transistor switch is in the second state, the transistor switch and the electronic components are configured to provide the SC element with either (i) a second electrical bias that transitions the SC element from the SC state to the normal state, thereby preventing a radio signal from passing through the SC element, or (ii) a third electrical bias that changes the kinetic inductance of the SC element to a target value, but depending on the target value of the kinetic inductance, either keeping the SC element in the SC state to allow a radio signal to pass through, or preventing a radio signal from passing through the SC element. By combining the SC element and the transistor switch in this way, it is possible to form a high-speed, low-loss RF switch that can operate at cryogenic temperatures for multiple purposes. For example, such an RF switch may be used as a key component of future quantum computing systems, providing a means to overcome bottlenecks associated with the burden of cabling for quantum processor control. It may also be used not only for controlling and reading out sensor arrays, but also as a substitute for high-loss cryogenic elements. Furthermore, the SC circuit configured in this way may be combined with different CMOS logic circuits so that the switching between the first and second states of the transistor switch can be performed through the logic circuit itself, which can reduce the overall complexity of the system.Furthermore, the implementation of scenario (ii) has the advantage that the SC element is not heated and no quasiparticles are formed. In this case, the SC circuit has a better response time, but its insulation is worsened.

[0008] In one exemplary embodiment of the first aspect, the SC circuit further comprises a planar transmission line comprising a dielectric substrate and an SC signal strip extending on or through the dielectric substrate. In this embodiment, the SC elements are embedded in the SC signal strip. The use of the planar transmission line may be advantageous in scaling a quantum computer into which the SC circuit of the first aspect can be integrated. Furthermore, quantum computers typically require many analog RF signals that are currently supplied by a large number of expensive coaxial cables, typically extending from room temperature to cryogenic regions. The planar transmission line can replace at least some of these coaxial cables, thereby further reducing the power loss of the SC circuit of the first aspect.

[0009] In one exemplary embodiment of the first aspect, the signal strip is at least partially coated with a normal-conducting metal layer or an SC metal layer having a critical temperature lower than the critical temperature of the SC element. In this embodiment, the normal-conducting metal layer or SC metal layer may function as a quasiparticle trap and / or thermal equilibrium configuration. In other words, such a coating layer may be used to induce relaxation of (non-equilibrium) quasiparticles arising from the current flowing through the SC circuit and / or to enhance electron-phonon (ep) coupling or interaction, thereby lowering the electronic temperature of the SC circuit according to the first aspect.

[0010] In one exemplary embodiment of the first aspect, the signal strip has two ends, each coupled to a different contact pad. In this embodiment, each of the contact pads (in addition to or instead of the signal strip) is at least partially coated with a normal-conducting metal layer or an SC metal layer having a critical temperature lower than the critical temperature of the SC element. The contact pads can be used to couple the SC circuit to a similar SC circuit or other signal processing circuit (e.g., a logic circuit). With respect to the normal-conducting metal layer or SC metal layer covering the contact pads, each of them can also be used as a quasiparticle trap and / or thermal equilibrium configuration. In other words, they can also be used to induce relaxation of (non-equilibrium) quasiparticles resulting from the current flowing through the SC circuit and / or to enhance electron-phonon (ep) coupling or interaction, thereby lowering the electronic temperature of the SC circuit according to the first aspect.

[0011] In one exemplary embodiment of the first aspect, the planar transmission line is one of a coplanar waveguide, a microstrip, and a strip line. These types of planar transmission lines can result in low RF loss in the SC circuit according to the first aspect.

[0012] In one exemplary embodiment of the first aspect, the SC circuit further comprises at least one impedance matching element embedded in the planar transmission line. The at least one impedance matching element is configured to provide impedance matching between the planar transmission line and the SC element. By using the impedance matching element, RF losses in the SC circuit according to the first aspect can be further reduced.

[0013] In one exemplary embodiment of the first aspect, the transistor switch comprises at least one of a field-effect transistor (FET), a metal-oxide-semiconductor FET, a ferroelectric FET, a bipolar transistor, and a superconducting transistor. By combining these types of transistor switches with the SC element, the RF switching can be performed more efficiently.

[0014] In one exemplary embodiment of the first aspect, the SC element comprises at least one SC nanowire or at least one Josephson junction (JJ). By combining these types of SC elements with the transistor switch, the RF switching can be performed more efficiently.

[0015] In one exemplary embodiment of the first aspect, each of the at least one SC nanowire is shaped as a meander. The meander-shaped SC nanowire may provide a more efficient implementation of scenario (ii), namely, the kinetic inductance of the SC element is changed by a third electrical bias, but the SC element itself remains in an SC state. In this case, the SC circuit according to the first aspect can be more efficiently used as an adjustable isolator, a bandpass / low / highpass filter, or an adjustable tank circuit.

[0016] In one exemplary embodiment of the first aspect, the at least one SC nanowire comprises an array of series or parallel SC nanowires embedded in the SC signal strip. By using a series or parallel arrangement of SC nanowires and appropriately selecting their number, a desired critical current value for the entire SC element can be achieved.

[0017] In one exemplary embodiment of the first aspect, each SC nanowire in the array of parallel SC nanowires has a different cross-section. This embodiment, in which the SC nanowires have different cross-sections (meaning each SC nanowire has a different critical current), allows for partial SC-to-normal transitions that can be used to increase current sensitivity. For example, small-diameter SC nanowires included in the SC element may, when their critical current is exceeded, cause the entire SC element to return to a normal state due to Joule heating or quasi-particle poisoning. Similarly, the SC nanowires may be designed to localize hot spots, thereby enabling multi-stage responses (e.g., the application of adjustable filters).

[0018] In one exemplary embodiment of the first aspect, the at least one JJ comprises an array of series or parallel JJs embedded in the SC signal strip. By using a series or parallel arrangement of JJs and appropriately selecting their number, a desired critical current value for the entire SC element can be achieved.

[0019] In one exemplary embodiment of the first aspect, the electronic component comprises at least one of a voltage source, a current source, a diode, another transistor switch, a resistor, a capacitor, and an inductive isolator. By using these types of electronic components, the second port of the SC element can be more efficiently kept at the correct potential, thereby making it possible to obtain a first or second electrical bias simply by adjusting the control signal supplied to the transistor switch.

[0020] In one exemplary embodiment of the first aspect, the SC element has a first electrical impedance in a first direction from a first port to a second port and a second electrical impedance in a second direction from a second port to a first port. In this embodiment, the first and second electrical impedances may be selected such that, in the case of a first or third electrical bias, the SC element is configured to allow a radio signal to pass in the first direction and prevent the radio signal from returning in the second direction, or the SC element is configured to allow a radio signal to pass in the second direction and prevent the radio signal from returning in the first direction. With the SC element configured in this way, the SC circuit according to the first aspect can be additionally used as an RF isolator.

[0021] According to a second aspect, a signal processing unit is provided. The signal processing unit comprises at least one SC circuit according to the first aspect, and a control subunit configured to generate a control signal and supply the control signal to the transistor switch of each of the at least one SC circuit, thereby causing the transistor switch to operate in either a first state or a second state. The signal processing unit configured in this way may be used for various types of signal processing at cryogenic temperatures (for example, it may function as an RF splitter, a combiner, or a multiplexer, the latter being possible, for example, when the array of SC nanowires having different cross-sections is included in the SC element).

[0022] In one exemplary embodiment of the second aspect, when each of the at least one SC circuit comprises the planar transmission line, the signal processing unit further comprises a signal generator coupled to the planar transmission line in each of the at least one SC circuit. The signal generator is configured to generate a radio signal to be supplied to the SC element. This embodiment may allow RF signals to be generated at cryogenic temperatures, which may be beneficial in some usage scenarios, for example when it is necessary to minimize propagation loss.

[0023] Other features and advantages of the present disclosure will become apparent upon reading the following detailed description and reviewing the accompanying drawings.

[0024] The present disclosure is described below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] [Figure 1A] It is a schematic top view of a superconducting (SC) circuit according to a first exemplary embodiment. [Figure 1B] It is a schematic cross-sectional view of the superconducting (SC) circuit according to the first exemplary embodiment. [Figure 2] It is an electrical diagram of the SC circuit according to the first exemplary embodiment having a linear SC nanowire. [Figure 3] It is an electrical diagram of an SC circuit according to the first exemplary embodiment having meandering SC nanowires. [Figure 4] It is an electrical diagram of an SC circuit according to the first exemplary embodiment having SC nanowires of complex shape. [Figure 5] It is a schematic top view of an SC circuit according to the second exemplary embodiment. [Figure 6] It is a schematic top view of an SC circuit according to the third exemplary embodiment. [Figure 7] It is a schematic block diagram of a signal processing unit according to one exemplary embodiment. DETAILED DESCRIPTION OF EMBODIMENTS

[0026] Various embodiments of the present disclosure will be further described in more detail with reference to the accompanying drawings. However, the present disclosure may be embodied in many other forms and should not be construed as limited to any particular structure or function discussed in the following description. In contrast, these embodiments are provided to make the description of the present disclosure detailed and complete.

[0027] According to the detailed description of embodiments of the present invention, it will be apparent to those skilled in the art that the scope of the present disclosure encompasses any embodiment thereof disclosed herein, regardless of whether the embodiment is implemented independently or in cooperation with any other embodiment of the present disclosure. For example, the apparatus disclosed herein may be actually implemented using any number of embodiments provided herein. Furthermore, it should be understood that any embodiment of the present disclosure may be implemented using one or more of the elements set forth in the appended claims.

[0028] The term "exemplary" is used herein to mean "serving as an example". Unless otherwise stated, any embodiment described as "exemplary" herein should not be construed as being preferred or advantageous over other embodiments.

[0029] Any positioning terms such as “left,” “right,” “top,” “bottom,” “above,” “below,” “upper,” and “lower” may be used herein for convenience to describe the relationship of one element or feature to one or more other elements or features as shown in the drawings. It is clear that the positioning terms are intended to encompass different orientations of the apparatus disclosed herein, in addition to the orientation shown in the drawings. For example, if the apparatus is virtually rotated 90 degrees clockwise in the drawings, the elements or features described as “left” and “right” relative to the other elements or features will be oriented “above” and “below,” respectively, of those other elements or features. Therefore, the positioning terms used herein should not be construed as any limitation of this disclosure.

[0030] In this specification, numerical terms such as “first,” “second,” etc., may be used to describe various embodiments, elements, or features, but these embodiments, elements, or features should not be limited by these numerical terms. In this specification, these numerical terms are used solely to distinguish one embodiment, element, or feature from another. For example, the first state discussed below may be called the second state without departing from the teachings of this disclosure, and vice versa.

[0031] The exemplary embodiments disclosed herein provide technical solutions that enable the mitigation or even elimination of the drawbacks of the prior art mentioned in the “Background Art” section. In particular, the technical solutions disclosed herein relate to an SC circuit in which a combination of a superconducting (SC) element and a transistor switch is used. The SC element has two ports and an adjustable kinetic inductance. The transistor switch is coupled to a first port of the SC element and configured to switch between two states in response to a control signal supplied thereto. The second port of the SC element is maintained at a desired (fixed or stray) potential by using an electronic component (e.g., a current or voltage source). When the SC element is in the SC state and the transistor switch is in the first state, a first electrical bias is provided across the SC element, thereby allowing the SC element to transmit a radio signal. When the SC element is in the SC state and the transistor switch is in the second state, the SC element has either a second electrical bias that transitions the SC element from the SC state to the normal state to prevent the passage of radio signals, or a third electrical bias that changes the motion inductance of the SC element to a target value or a user-defined value, but depending on the target value of the motion inductance, either keeps the SC element in the SC state to allow radio signals to pass through, or prevents radio signals from passing through the SC element. The SC circuit configured in this way can be used as a high-speed, low-loss RF switch configured to operate at cryogenic temperatures for multiple purposes.

[0032] Figures 1A and 1B show schematic diagrams of an SC circuit 100 according to a first exemplary embodiment. More specifically, Figure 1A shows a top view of the SC circuit 100, and Figure 1B shows a cross-sectional view of the SC circuit 100. The SC circuit 100 comprises a coplanar waveguide (CPW) consisting of a signal strip 102 and a ground plane 104 with gaps on both sides of the signal strip 102. As shown in Figure 1B, the signal strip 102 and the ground plane 104 may be formed on a dielectric substrate, which may have a multilayer structure comprising a base layer 106 and a dielectric layer 108 provided on the base layer 106. Each of the signal strip 102 and the ground plane 104 may be made of the same or different conductive materials, such as metal or superconductor. Some examples of suitable superconductors include, but are not limited to, Al, Ti, TiN, TiNb, TiNbN, Mo, MoN, MoSi, Nb, NbN, NbSi, SiB, SiGa, V, VN, or any combination of these with other materials that exhibit SC properties at cryogenic temperatures. Note that cryogenic temperatures as used herein may refer to temperatures in the range of 120K to absolute zero. With respect to layers 106 and 108, they are SiO2, SiN, and SiN x The dielectric material may be any insulating dielectric material such as Al2O3, HfO2, HfSiON, HfSiO, ZrO2, or any combination thereof and other insulating dielectric materials. Those skilled in the art will recognize that the configuration parameters of the CPW shown in Figures 1A and 1B, such as the gap between the signal strip 102 and the ground plane 104, the dimensions of the signal strip 102, and the thickness of the dielectric substrate, should be selected to provide a desired characteristic impedance (e.g., 50 or 75 ohms). Furthermore, to provide better impedance matching, one or more impedance matching elements, such as one or more capacitors, one or more inductors, or any combination thereof, may be added to the SC circuit 100.

[0033] The SC circuit 100 further comprises an SC element 110 embedded in the signal strip 102 such that the signal strip 102 is divided into two parts (i.e., upper and lower, referring to Figure 1A) by the SC element 110. In this embodiment, the SC element 110 is implemented as a single SC nanowire, which may be made of the same or different SC material as the signal strip 102 and the ground plane 104. The SC nanowire may have a resistance of about k ohms (e.g., 10k ohms) and a tunable kinetic inductance that depends on the bias current flowing through the SC nanowire. In other embodiments, the SC element 110 may be implemented as an array of SC nanowires of equal or different sizes (e.g., having different cross-sections) embedded in the SC signal strip 102, which are arranged in series or parallel. At the same time, this disclosure is not limited to this type of SC element 110, and several embodiments are possible, in which the SC element 110 is implemented as one or more Josephson junctions (JJs) also embedded in the signal strip 102 (in the case of multiple JJs, they may be arranged in series or parallel).

[0034] The SC circuit 110 is characterized by a combination of an SC element 110 and a transistor switch 112 which can be coupled to either the input port or the output port of the SC element 110. In embodiments disclosed herein, the term “port” may refer to the end of the SC element 110 coupled to the signal strip 102. As can be seen from Figure 1B, the transistor switch 112 is implemented as a floating-gate field-effect transistor (FET) comprising a source region 114, a drain region 116, and a floating gate 118 embedded in a dielectric layer 108. Optionally, the gate 118 may also be made of SC material (e.g., the same as that used for the signal strip 102 and the ground plane 104). Conductive pins 120 are also present, extending through the dielectric layer 108 and connecting the signal strip 102 to the source region 114 of the FET, to provide RF switching (described in more detail later). Depending on the control signal supplied to the gate 118, the FET may be in either a first state or a second state. In embodiments disclosed herein, the first and second states may refer to the open and closed states of the transistor switch 112, respectively, or vice versa. The open state of the transistor switch 112 means that no current is flowing through the FET, and the closed state of the transistor switch 112 means that the FET is conducting. It should be noted that this disclosure is not limited to this type of transistor switch 112, and in some other embodiments, the transistor switch 112 may be implemented as any other FET such as a metal oxide semiconductor FET (MOSFET), a ferroelectric FET, a junction gate FET (JFET), or as a bipolar transistor, a superconducting transistor, or any combination thereof. Furthermore, one or more passive elements, such as one or more series-connected resistors and / or inductors, may be used as part of the transistor switch 112.

[0035] As also shown in Figure 1A, the SC circuit 100 may optionally include contact pads 122 and 124 coupled to the (opposite) ends of the signal strip 102. These contact pads may be used to couple the SC circuit 100 to a similar SC circuit or other signal processing circuit (e.g., a logic circuit). The triangular shapes of the contact pads 122 and 124 are shown for illustrative purposes only and should not be construed as an limitation of this disclosure. In some other embodiments, the contact pads 122 and 124 may have any polygonal shape (e.g., square, rectangle, rhombus, etc.) or any curved shape (e.g., circular, ellipse, etc.). In one embodiment, one of the contact pads 122 and 124 may be replaced by a signal generator configured to produce a radio signal at cryogenic temperatures.

[0036] SC circuit 100 may be obtained by using standard semiconductor manufacturing processes. These processes may include, but are not limited to, deposition techniques (e.g., atomic layer deposition (ALD), molecular beam epitaxy (MBE), chemical beam epitaxy (CBE), metal-organic chemical vapor deposition (MOCVD), physical vapor deposition (PVD), low-pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), ultra-high vacuum CVD (UHV-CVD), etc.), lithography techniques (e.g., photolithography, ultraviolet (UV) lithography, deep ultraviolet (DUV) lithography, extreme ultraviolet (EUV) lithography, electron beam lithography (EBL), etc.), sputtering techniques (e.g., radio frequency sputtering, magnetron sputtering, etc.), etching techniques (e.g., wet etching, reactive ion etching, etc.), doping techniques (e.g., diffusion doping, ion implantation, etc.), or any combination thereof.

[0037] The operating principle of the SC circuit 100 is as follows:

[0038] First, the SC circuit 100 is cooled to an extremely low temperature in which the SC element 110 is in an SC state. During operation, the transistor switch 112 may function as a DC switch, and the SC element 110 may function as an RF switch. If the transistor switch 112 is coupled to one port of the SC element 110, the other port of the SC element 110 should be maintained at a desired (fixed or floating) potential by using electronic components (not shown in Figures 1A and 1B), such as a voltage source, a current source, a diode, another transistor switch (e.g., transistor switch 112), a resistor, a capacitor, an inductive isolator, or any combination thereof. This makes it possible to give the SC element 110 a desired electrical bias simply by adjusting the control signal supplied to the gate 118 of the FET (i.e., by opening and closing the transistor switch 112). The control signal may include positive and negative pulses in an alternating order, thereby providing repeated switching between the two states of the transistor switch 112. When the SC element 110 is in the SC state and the transistor switch 112 is in the first (i.e., open) state, there is a first electrical bias across the SC element 110, which allows the SC element 110 to pass the radio signal supplied to one of its ports. Simultaneously, when the SC element 110 is in the SC state and the transistor switch 112 is in the second (i.e., closed) state, one of the following two scenarios may occur depending on the strength of the control signal: (i) there is a second electrical bias across the SC element 110, which causes the SC element 110 to transition from the SC state to the normal state and prevents the radio signal from passing through the SC element 110; or (ii) there is a third electrical bias across the SC element, which changes the kinetic inductance of the SC element 110 to a target value, but depending on the target value of the kinetic inductance, the SC element 110 may remain in the SC state and allow the radio signal to pass through, or it may block the radio signal.

[0039] In scenario (ii), if the SC circuit 100 is used, for example, as a resonant switch, it allows a specific frequency to pass through, or blocks a specific frequency and allows other frequencies to pass through, and the resonant frequency is defined by a third electrical bias.

[0040] Furthermore, the SC element may be configured to have a first electrical impedance in a first direction from the first port to the second port, and a (different) second electrical impedance in a second direction from the second port to the first port. In this sense, the SC element may be implemented, for example, identically to or similarly to an LC circuit, in which case the impedance in the direction from the series inductance to the parallel capacitor is different from the impedance in the reverse direction (i.e., from the parallel capacitor to the series inductance). In this case, the values ​​of the first and second electrical impedances may be selected such that, in the case of a first or third electrical bias, the SC element is configured to allow the radio signal to pass in the first direction and prevent the radio signal from returning in the second direction, or vice versa (i.e., the SC element allows the radio signal to pass in the second direction and prevents the radio signal from returning in the first direction).

[0041] As described above, the third electrical bias may be such that the bias current in the nanowire representing the SC element 110 does not exceed its critical current, thereby changing its kinetic inductance and providing an RF switching function. It is well known that the kinetic inductance depends on the ratio of the critical temperature to the nanowire temperature and the current flowing through the nanowire, as described below (see, for example, Shinho Cho, Temperature and Current Dependence of Inductance in a Superconducting Meander Line, J. Korean Phys. Soc. 31(2), 337 (1997)).

[0042]

number

[0043] Figure 2 shows the electrical diagram of an SC circuit 100 having a linear SC nanowire 200. As previously mentioned, the SC nanowire 200 can function as an SC element 110. The SC nanowire 200 has two ports 202 and 204, which are coupled on both sides to a signal strip 102. Figure 2 also shows an electronic component in the form of a DC voltage source 206. This means that the first to third electrical biases shown above can be provided across the SC nanowire 200 by maintaining port 202 of the SC nanowire 200 at a fixed potential and appropriately adjusting the control signal supplied to the transistor switch 112 coupled to port 204 (i.e., to the gate 118 of the FET).

[0044] Figure 3 shows the electrical diagram of an SC circuit 100 having meander-shaped SC nanowires 300. Here again, the SC nanowires 300 can function as SC elements 110. The SC nanowires 300 have two ports 302 and 304, which are coupled on both sides to a signal strip 102. In Figure 3, the electronic components are represented by a current source 306 used to maintain port 302 at a desired potential. Similarly, the first to third electrical biases described above can be provided across the SC nanowires 300 by appropriately adjusting the control signals supplied to a transistor switch 112 coupled to port 304 (i.e., to the gate 118 of the FET).

[0045] It should be noted that this disclosure is not limited to the nanowire shapes shown in Figures 2 and 3. In some other embodiments, one or more nanowires that can be used as SC elements 110 may have any curved (e.g., helical, S-shaped, etc.) or inclined shapes (e.g., trapezoidal, sawtooth, etc.).

[0046] Figure 4 shows an electrical diagram of an SC circuit 100 having a complexly shaped SC nanowire 400. Here again, the SC nanowire 400 can function as an SC element 110. The SC nanowire 400 has a set of sections 402 having capacitive elements between them so that impedance matching is maintained. Each section of the set of sections 402 is schematically shown as a resistor in Figure 4. The set of sections 402 may contain the same or different sections, meaning that the SC nanowire 400 may have the same or different shapes in each section. For example, each section of the set of sections 402 may be implemented as a meander, as shown in Figure 3. Furthermore, this disclosure is not limited to the three sections shown in Figure 4, and in other embodiments, the SC nanowire 400 may have two or more sections. The SC nanowire 400 has two ports 404 and 406, which are coupled on both sides to a signal strip 102. In Figure 4, the electronic component is represented by FET 408 (i.e., another transistor switch) used to maintain port 404 at the desired potential. In this case, two control signals should be used to properly provide the first to third electrical biases described above across the SC nanowire 400.

[0047] It should be noted that the CPW described above is merely one example of a planar transmission line that may be used in the SC circuit 100. In other embodiments, it may be replaced by a microstrip or strip line, which are also well known in the art. In the case of a microstrip, an SC element such as SC element 110 may be embedded in an SC signal strip provided on a dielectric substrate, and no ground plane is provided on either side of the signal strip. In the case of a strip line, an SC element such as SC element 110 may be embedded in a signal strip that penetrates the dielectric substrate.

[0048] Several other embodiments are possible in which planar transmission lines are not used in the SC circuit 100. In these embodiments, the signal strip (such as the signal strip 102) may be replaced by the core of a coaxial cable, the SC element (such as the SC element 110) may be mounted on a dielectric substrate (e.g., a printed circuit board (PCB)) and coupled to the coaxial cable using cable connectors, and the transistor switch (such as the transistor switch 112) may be embedded in the dielectric substrate.

[0049] In some other embodiments, the SC circuit 100 may be provided with a quasiparticle trap or thermal equilibrium configuration configured to relax quasiparticles and / or enhance electron-phonon (ep) coupling, thereby lowering the electronic temperature of the SC circuit 100. Such a thermal equilibrium configuration or quasiparticle trap may be implemented as a metallic region having a large volume. For example, the contact pads 122 and 124 and / or the signal strip 102 may be at least partially coated with a thick (e.g., on the order of microns) normal-conducting metal layer or SC metal layer having high ep coupling, such as gold, platinum, silver, or copper. This is mainly important only if the signal strip 102 and ground plane 104 are made of superconductors (if they are metallic, there is a moderately high thermal equilibrium or quasiparticle relaxation). If an SC metal layer is used, its critical temperature must be lower than the critical temperature of the SC element 110.

[0050] Figure 5 shows a schematic top view of the SC circuit 500 according to a second exemplary embodiment. Similar to the SC circuit 100, the SC circuit 500 includes a CPW, which comprises four signal strips 502-1, 502-2, 502-3, and 502-4, and ground planes 504 provided on both sides of each of the signal strips 502-1, 502-2, 502-3, and 502-4. The signal strips 502-1, 502-2, 502-3, 502-4, and the ground planes 504 are provided on a dielectric substrate (not shown in Figure 5), as shown in Figure 1B. In other embodiments, the CPW may be replaced with any other planar transmission line (e.g., a microstrip or strip line), and planar transmission lines may not be used in the SC circuit 500.

[0051] The SC circuit 500 further comprises three SC elements 506-1, 506-2, and 506-3, by which signal strips 502-1, 502-2, 502-3, and 502-4 are interconnected. More specifically, SC element 506-1 interconnects signal strips 502-1 and 502-2, SC element 506-2 interconnects signal strips 502-2 and 502-3, and SC element 506-3 interconnects signal strips 502-2 and 502-4. Each of the signal strips 502-1, 502-2, 502-3, and 502-4 may be made of the same material as signal strip 102, and the ground plane 504 may be made of the same material as ground plane 104. Each of the SC elements 506-1, 506-2, and 506-3 may be made of the same material as the SC element 110 and may be mounted as an SC nanowire, similar to the SC element 110. In some other embodiments, each of the SC elements 506-1, 506-2, and 506-3 may be mounted as one or more SC nanowires or JJs.

[0052] The SC circuit 500 further comprises four transistor switches 508-1, 508-2, 508-3, and 508-4, each of which is embedded in the dielectric substrate of the CPW, similar to the transistor switch 112. Furthermore, each of the transistor switches 508-1, 508-2, 508-3, and 508-4 may also be implemented as an FET, bipolar transistor, superconducting transistor, or any combination thereof. For ease of operation, there may be a set of contact pads 510, each of which is coupled to one of the transistor switches 508-1, 508-2, 508-3, and 508-4 (for example, if the transistor switches 508-1, 508-2, 508-3, and 508-4 are implemented as FETs, each of the contact pads may be coupled to the gate of the corresponding FET), and control signals may be supplied to the transistor switches 508-1, 508-2, 508-3, and 508-4 via the contact pads.

[0053] The SC circuit 500 may optionally include other contact pads 512-1, 512-2, 512-3, and 512-4 coupled to signal strips 502-1, 502-2, 502-3, and 502-4, respectively. These contact pads can be used to couple the SC circuit 500 to a similar SC circuit or other signal processing circuit (e.g., a logic circuit). Again, the triangular shapes of the contact pads 512-1, 512-2, 512-3, and 512-4 are shown for illustrative purposes only and should not be construed as an limitation of this disclosure. In some other embodiments, the contact pads 512-1, 512-2, 512-3, and 512-4 may have any polygon (e.g., square, rectangle, rhombus, etc.) or any curved shape (e.g., circle, ellipse, etc.). In some other embodiments, one of the contact pads 512-1, 512-2, 512-3, and 512-4 may be replaced with a built-in signal generator configured to generate a radio signal at cryogenic temperatures.

[0054] The operating principle of the SC circuit 500 is the same as that of the SC circuit 100. For example, assuming that a radio signal is supplied to element 506-1 via contact pad 512-1, and that transistor switches 508-1 and 508-2 are in a first (i.e., open) state and transistor switches 508-3 and 508-4 are in a second (i.e., closed) state, this leads to scenario (i) (i.e., an electrical bias across each of transistor switches 508-3 and 508-4 makes them resistive again), in which case the radio signal propagates through SC element 506-1 towards contact pad 512-2. Thus, by properly opening and closing transistor switches 508-1, 508-2, 508-3, and 508-4, the radio signal can be directed in one of four directions or its propagation can be completely blocked (provided that the second electrical bias described above is provided across each of transistor switches 508-1, 508-2, 508-3, and 508-4). In other words, with the appropriate selection of components / geometric shapes, the SC circuit 500 can function as a splitter.

[0055] Figure 6 shows a schematic top view of an SC circuit 600 according to a third exemplary embodiment. The SC circuit 600 comprises two parts 602 and 604, part 604 obtained by cutting everything to the left of line AA* in Figure 5 from the SC circuit 500, and part 602 obtained by mirroring everything to the left of line AA* in Figure 5 to the right with respect to line AA* itself (except for transistor switch 508-2). The operating principle of the SC circuit 600 is the same as that of the SC circuits 100 and 500.

[0056] Figure 7 shows a schematic block diagram of a signal processing unit 700 according to one exemplary embodiment. The signal processing unit 700 comprises an SC circuit 500 and a control subunit 702 configured to generate control signals and supply them to transistor switches 508-1, 508-2, 508-3, and 508-4 via a set of contact pads 510, thereby causing each of the transistor switches 508-1, 508-2, 508-3, and 508-4 to operate in either a first or second state. The control subunit 702 may be implemented as a CPU, general-purpose processor, memristor, neuromorphic computing chip, artificial intelligence (AI) chip (also known as AI hardware), single-purpose processor, microcontroller, microprocessor, application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), digital signal processor (DSP), composite programmable logic device, etc. It should also be noted that the control subunit 702 may be implemented as any combination of one or more of the above. As an example, the control subunit 702 may be a combination of four microcontrollers, each configured to supply a control signal to one of the transistor switches 508-1, 508-2, 508-3, and 508-4 via a corresponding contact pad of a set of contact pads 510. In other embodiments, the signal processing unit 700 may have more than one SC circuit 500, or may have SC circuits 100 and / or 600 in addition to or instead of SC circuit 500.

[0057] While exemplary embodiments of this disclosure are described herein, it should be noted that various changes and modifications can be made to these embodiments without departing from the scope of legal protection defined by the appended claims. In the appended claims, the word “comprising” does not exclude other elements or actions, and the indefinite article “a” or “an” does not exclude the plural. The mere fact that certain means are described in different dependent claims does not imply that combinations of these means cannot be used advantageously.

Claims

1. A superconducting (SC) circuit, An SC element having a first port and a second port, and having an adjustable motion inductance, A transistor switch coupled to the first port of the SC element, configured to switch between a first state and a second state in response to an applied control signal, The SC element comprises an electronic component coupled to the second port of the SC element, When the SC element is in the SC state and the transistor switch is in the first state, the transistor switch and the electronic component are configured to provide a first electrical bias for the SC element, which causes the radio signal supplied to one of the first and second ports of the SC element to pass through the SC element. When the SC element is in the SC state and the transistor switch is in the second state, the transistor switch and the electronic component, A second electrical bias that causes the SC element to transition from the SC state to the normal state, thereby preventing the wireless signal from passing through the SC element, or An SC circuit is configured to change the motion inductance of the SC element to a target value, and to provide a third electrical bias to the SC element that, depending on the target value of the motion inductance, either keeps the SC element in the SC state and allows the wireless signal to pass through, or prevents the wireless signal from passing through the SC element.

2. The SC circuit according to claim 1, further comprising a planar transmission line comprising a dielectric substrate and a signal strip extending on or through the dielectric substrate, wherein the SC element is embedded in the signal strip.

3. The SC circuit according to claim 2, wherein the signal strip is at least partially coated with a normal conducting metal layer or an SC metal layer having a critical temperature lower than the critical temperature of the SC element.

4. The SC circuit according to claim 2 or 3, wherein the signal strip has two ends, each coupled to a different contact pad, and each of the contact pads is at least partially coated with a normal conducting metal layer or an SC metal layer having a critical temperature lower than the critical temperature of the SC element.

5. The SC circuit according to any one of claims 2 to 4, wherein the planar transmission line is one of a coplanar waveguide, a microstrip, and a strip line.

6. The SC circuit according to any one of claims 2 to 5, further comprising at least one impedance matching element embedded in the planar transmission line, wherein the at least one impedance matching element is configured to provide impedance matching between the planar transmission line and the SC element.

7. The SC circuit according to any one of claims 1 to 6, wherein the transistor switch comprises at least one of a field-effect transistor (FET), a metal oxide semiconductor FET, a ferroelectric FET, a bipolar transistor, and a superconducting transistor.

8. The SC circuit according to any one of claims 1 to 7, wherein the SC element comprises at least one SC nanowire or at least one Josephson junction (JJ).

9. The SC circuit according to claim 8, wherein each of the at least one SC nanowires is formed as a meander.

10. The SC circuit according to claim 8 or 9, wherein the at least one SC nanowire comprises an array of series or parallel SC nanowires embedded in the SC signal strip.

11. The SC circuit according to claim 10, wherein each SC nanowire in the array of parallel SC nanowires has a different cross-section.

12. The SC circuit according to claim 8, wherein the at least one JJ comprises an array of series or parallel JJs embedded in the SC signal strip.

13. The SC circuit according to any one of claims 1 to 12, wherein the electronic component comprises at least one of a voltage source, a current source, a diode, another transistor switch, a resistor, a capacitor, and an inductive isolator.

14. The SC element has a first electrical impedance in a first direction from the first port to the second port, and a second electrical impedance in a second direction from the second port to the first port, and when the first electrical impedance and the second electrical impedance are the first electrical bias or the third electrical bias, The SC element is configured to allow the wireless signal to pass in the first direction and prevent the wireless signal from returning in the second direction, or The SC circuit according to any one of claims 1 to 13, wherein the first and second electrical impedances are selected such that the SC element is configured to allow the radio signal to pass in the second direction and prevent the radio signal from returning in the first direction.

15. A superconducting (SC) circuit according to any one of claims 1 to 14, A signal processing unit comprising a control subunit configured to generate the control signals and supply them to each of the transistor switches of the at least one SC circuit, thereby causing the transistor switches to operate in either the first state or the second state.

16. The unit according to claim 15, wherein, when each of the at least one SC circuits comprises the planar transmission line, the unit further comprises a signal generator coupled to the planar transmission line of each of the at least one SC circuits, the signal generator being configured to generate the wireless signal.

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