Manipulation of superconducting channels by electron injection

Abstract

The present invention is particularly directed to a method of operating a superconducting channel. The method relies on a device including a potential superconducting material, a gate electrode, and an electrically insulating medium. The channel is defined by the potential superconducting material. A gate electrode is positioned adjacent to the channel, with an end face of the gate electrode facing a portion of the channel. The electrically insulating medium is positioned in such a way as to electrically insulate the gate electrode from the channel. The channel is made superconducting by cooling the device. A voltage difference is then applied between the gate electrode and the channel to inject electrons into the channel through the electrically insulating medium, thereby generating a gate current between the gate electrode and the channel. The critical current I of the channel is C Electrons are injected with sufficient average energy to modify .

Description

[Technical field]

[0001] The present invention relates generally to techniques for manipulating superconducting channels, such as metal wires, for example to switch such channels or to modulate signals transmitted within such channels. The present invention may be used in particular in quantum processing devices or to tune the resonant or cutoff frequencies of electrical circuits containing such superconducting channels. [Background technology]

[0002] Superconducting circuits are very promising as building blocks for quantum computing architectures and related cryogenically controlled electronics due to their low power consumption and high speed. In this context, it is desirable to develop switching devices (e.g., three-terminal devices) that can be electrically tuned between superconducting and resistive states at high frequencies. Ultimately, such devices would enable new functionalities for which semiconductor counterparts do not exist, such as cryogenic switches, ultrasensitive detectors, amplifiers, circulators, and multiplexers. Several electrically controlled superconducting switches have been proposed, which are based on the injection of non-equilibrium quasiparticles in Josephson junctions. However, Josephson junctions usually entail limited source-drain critical currents and the need to operate in a magnetic field-free environment. Therefore, architectures that do not rely on Josephson junctions are being intensively investigated. Such pioneering approaches are based on three- or four-terminal devices, where an electric current, a locally generated Oersted field, or heat drives the superconducting channel into the normal state.

[0003] Recent advances in quantum computing make such technology more relevant than ever for industrial applications. Quantum computing directly exploits quantum mechanical phenomena such as superposition and entanglement to perform operations on entangled qubits (i.e., the states stored in qubits). For example, superconducting circuits are relatively easy to fabricate using current technology and therefore make them promising candidates for further scaling up quantum information technology.

[0004] Devices used in switching solutions for radio-frequency (RF) applications typically suffer from one or more of the following drawbacks: they have finite resistance in the on-state leading to signal dissipation and / or attenuation, they have limited on / off rates, they are electromechanical in nature and therefore lead to long-term overheating and reduced switching speeds in cryogenic applications, or they are macroscopically large and fairly costly, or a combination thereof. Summary of the Invention

[0005] According to a first aspect, the invention is embodied as a method of operating a superconducting channel. The method relies on a device including a potential superconducting material, a gate electrode, and an electrically insulating medium. The channel is defined by the potential superconducting material. The gate electrode is positioned adjacent to the channel such that an end face of the gate electrode faces a portion of the channel. The electrically insulating medium is positioned in such a way as to electrically insulate the gate electrode from the channel. The device is cooled such that the channel can become superconducting. A voltage difference is then applied between the gate electrode and the channel to inject electrons into the channel through the electrically insulating medium, thereby generating a gate current between the gate electrode and the channel. The critical current I of the channel is determined by the voltage difference I C Electrons are injected with sufficient average energy to modify

[0006] The proposed solution has further advantages and can be used in several applications. For example, the scheme allows both the inductance and the resistance of the superconducting channel to be tuned. Interestingly, the applied voltage difference can be positive or negative. In particular, the gate can be operated with very low power consumption. The critical current I of the channel C The speed at which the switching frequency can be adjusted (e.g., less than 100 ns, down to values ​​approaching 1 ps in embodiments) is the most important advantage. This in turn allows very high off / on switching frequencies to be achieved. Thus, the superconducting device can be advantageously operated as a switch, as in the embodiments, to locally suppress the superconductivity (e.g., as a switch) and to modulate the signal passing through the channel.

[0007] The device can be integrated, for example, on existing quantum chips. It has been shown that the method works well enough at temperatures below 3.4 K when TiN is used as the superconducting material. In particular, the method can be applied for signal multiplexing purposes. In other applications, the device is configured as an electric circuit, such as an LC circuit or any circuit in which the superconducting channel is configured as a wire-like inductor. In this way, the resonant frequency of the circuit can be adjusted by changing the inductance of the wire. Similarly, the resonator can form part of the coupling element of a frequency-tunable coupler between the qubits of a quantum computer. In a further application, the cutoff frequency of a filter containing a wire-like inductor is adjusted by changing the inductance of the wire. For completeness, the underlying device is easy to manufacture, can have very small dimensions, and can be manufactured in an integrated manner.

[0008] In an embodiment, by applying a voltage difference, a critical current I Cto zero, thereby generating a local electrical resistance in the channel, at least at the level of some of the channels, which will locally eliminate the superconductivity at the level of some of the channels, which can be used advantageously for switching applications.

[0009] The applied voltage difference can then be switched off, as in the embodiment, so that the potential superconducting material self-resets to a fully superconducting state. In this way, in a controllable way, the electrical resistance of the channel can be locally increased at the level of a portion of the channel opposite the electrode and then reversed.

[0010] Preferably, the method further comprises applying a signal to the channel prior to applying the voltage difference, such that electrical resistance developed within the channel affects transmission of the signal applied to the channel.

[0011] In a preferred embodiment, the method further includes repeatedly turning on and off the voltage difference to modulate the applied signal. The applied signal can be modulated at a frequency, for example, greater than 10 MHz and less than 10 GHz, although any other (lower) frequency can be used.

[0012] The applied voltage difference is such that the magnitude of the generated gate current is equal to the critical current I C For example, the magnitude of the gate current generated upon application of the voltage difference may be, for example, in the range of 10 pA to 10 nA.

[0013] The applied voltage difference may preferably be such that the average energy of the electrons is at least 100 times the superconducting gap of the underlying superconducting material.

[0014] In an embodiment, the applied voltage difference corresponds to the minimum value required to switch the channel into the normal conducting state, for which the impedance of the gate electrode (as determined by the insulating medium, among other things) is preferably greater than 1 GΩ.

[0015] The method preferably further comprises varying the magnitude of the applied voltage difference so as to controllably expand the region of the channel where electrical resistance occurs. More generally, the method may comprise, for example, varying the applied voltage difference so as to adjust a characteristic of the channel or a characteristic implied by this characteristic of the channel. In this way, in particular, the kinetic inductance L of the channel may be adjusted. kin This can then be utilized, for example, to tune the resonant frequency or cutoff frequency of an electrical circuit of the device, the electrical circuit including an inductor formed by a channel, the channel being configured as a wire. In a variant, the device is configured as a frequency tunable coupler connected to a quantum circuit of a quantum processing device, and a voltage difference is applied to tune the resonant frequency of the frequency tunable coupler, thereby changing the coupling between the quantum circuits.

[0016] According to another aspect, the invention is embodied as an apparatus. The apparatus comprises a device as described above with reference to the method. That is to say, the device in particular comprises a potential superconducting material structured (e.g. structured as a wire) to define a channel. The device is adapted to be operated at cryogenic temperatures such that the channel becomes superconducting during operation. The device further comprises a gate electrode facing towards the channel such that an end face of the gate electrode faces a portion of the channel, and an electrically insulating medium arranged to electrically insulate the gate electrode from the channel. The apparatus further comprises a control unit connected to the gate electrode. The control unit is configured to apply a voltage difference between the gate electrode and the channel to inject electrons into the channel via the electrically insulating medium, thereby generating a gate current between the gate electrode and the channel. The control unit and the device in use are adapted to determine whether the applied voltage difference increases the critical current I of the channel during operation of the device (at cryogenic temperatures). C This allows for the average energy of the injected electrons to be sufficient to change

[0017] A control unit is further connected to the channel, the control unit being configured to apply a signal to the channel and to control a critical current I of the channel during operation. C and modifying the applied signal by modifying

[0018] In an embodiment, the control unit is further configured to enable a characteristic of the channel or a characteristic implied by this characteristic of the channel to be measured during operation, while the control unit enables the applied voltage difference to be controllably modified according to the measured characteristic in order to adjust said characteristic during operation.

[0019] As described herein, the channel may optionally be structured as a wire. In an embodiment, the device further comprises an electric circuit including an inductor, the inductor including or formed by a wire. In that case, the control unit may adjust the kinetic inductance L of the wire to adjust the resonant frequency or cutoff frequency of the electric circuit. kin It may be further configured to adjust

[0020] The device preferably comprises a substrate forming an electrically insulating medium, and the gate electrode and the wires are each structured on the substrate. For example, the width of the wire is in the range of 40 to 300 nm, and the minimum gap between the end face of the gate electrode and the part of the channel is in the range of 40 nm to 300 nm. This width and the minimum gap are measured parallel to the main surface of the substrate.

[0021] The present solution can be advantageously used with a variety of superconducting materials. For example, potential superconducting materials are Ti, TiN, Nb, Ti x Nb y N z , W, WSi, Al, and Mo.

[0022] In a preferred embodiment, the device is configured as a quantum processing device, further comprising quantum circuits, and a channel of the device is connected to one or more of the quantum circuits. The device may be configured, inter alia, as a switch for switching a radio frequency signal applied to the channel, as a frequency tunable coupler for modifying the coupling between two or more of the quantum circuits, or as a frequency tunable filter for selectively tuning the coupling of the quantum circuit (connected to the frequency tunable filter) with the control circuitry and the environment (e.g., to enhance the coupling from the control line to the readout resonator and reduce the coupling of the control line to the transmon type qubit).

[0023] Methods and apparatus embodying the invention will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0024] In the accompanying drawings, like reference numbers refer to identical or functionally similar elements throughout the various views, and each of the drawings, together with the following detailed description, are incorporated into and form a part of this specification: Figures 1-5 primarily serve to explain various principles of physics underlying embodiments of the present invention, while Figures 6-11 illustrate various embodiments and advantages of the present invention, all in accordance with the present disclosure. [Brief description of the drawings]

[0025] [Figure 1A] 1 shows suppression of critical current in metal nanowires.2 Scanning electron micrograph (with artificial contrast added) of a device as included in an embodiment, with a schematic diagram of the measurement setup superimposed.3 The micrograph shows TiN nanowires and a gate electrode structured on a silicon substrate. [Figure 1B] Figure 1. Suppression of the critical current in a metallic nanowire. The differential resistance dV / dISD of the nanowire is shown as a function of the source-drain current ISD, measured by sweeping ISD starting from a negative value in the positive direction. The critical current IC and the re-trapping current IR are shown. The inset shows the temperature dependence of IC (dots) and IR (squares). [Figure 1C] 1 shows suppression of the critical current in a metallic nanowire: Critical current I C in the nanowire as a function of gate voltage V G . [Figure 1D] Figure 1 shows the suppression of the critical current in a metal nanowire. Curve showing the absolute value of the gate current I flowing between the gate and the nanowire as a function of V. The linear component of I of about 1 TΩ due to leakage in the measurement setup has been subtracted from the data. [Figure 1E]Figure 1C shows the suppression of critical current in metal nanowires. Parametric plots of IC and IG obtained from the data shown in Figures 1C and 1D. [Figure 2A] 2B shows the temperature dependence of the critical current IC of the device of FIG. 2A as a function of the gate voltage VG for different temperatures T. [Figure 2B] Figure 14. Magnetic field dependence: Critical current IC of the same device as a function of the out-of-plane magnetic field B⊥. [Figure 3A] Figure 14 shows the reproducibility with other materials: Critical current IC as a function of gate voltage VG for a Ti wire structured on a Si substrate. [Figure 3B] 1 shows the reproducibility with other materials: Gate current I G as a function of gate voltage V G for a Ti wire structured on a Si substrate. [Figure 3C] Figure 14 shows the reproducibility with other materials: Critical current IC as a function of gate current VG for TiN wires on a 25 nm SiO2 film thermally grown on a Si substrate. [Figure 3D] Figure 14 shows the reproducibility with other materials: Gate current I as a function of gate current V for TiN wires on a 25 nm SiO2 film thermally grown on a Si substrate. [Figure 4A] 1A deals with spatially resolved suppression of superconductivity.FIG. 1B is a scanning electron micrograph (e.g., with artificially added contrast) of a variation of the device of FIG. 1A as used in an embodiment, with a schematic diagram of the measurement setup superimposed. [Figure 4B] This diagram deals with the spatially resolved suppression of superconductivity. It shows the differential resistance dVi / dISD (i=1, 2, 3) close to the superconducting state of perfect suppression. The recapture current IR is shown as a dotted line. [Figure 4C] Fig. 1 deals with the spatially resolved suppression of superconductivity: the critical currents ISWi (i=1, 2, 3, 4, 5) are shown as a function of the gate voltage VG1. [Figure 4D]1 is a diagram dealing with spatially resolved suppression of superconductivity. FIG. 1 is a plot of the absolute value of the gate current IG1 as a function of the gate voltage VG1. [Figure 4E] Parametric plot of ISWi as a function of IG1. The arrow indicates the IG1 value at which the inhibition coefficient is calculated. [Figure 4F] Figure 1. Suppression factor S as a function of the distance between the gate electrode and the nanowire segment. Black markers indicate values ​​obtained by sweeping IG1, while grey markers relate to values ​​obtained by sweeping IG5. Filled and empty markers refer to positive and negative gate polarity, respectively. The solid line is a fit to an exponential function. [Figure 5A] Figure 1 shows the fast switching and self-resetting mechanisms. In detail, the time-dependent switching characteristics of the device in Figure 1A are shown as a function of the DC gate voltage bias, V. A square wave with a peak-to-peak amplitude of 0.5 V and a repetition rate of 100 kHz is added to V. [Figure 5B] Figure 5 shows the fast switching and self-resetting mechanisms. Gate current IG is shown as a function of VG measured simultaneously with the data in Figure 5A. [Figure 5C] Figure 5 shows the fast switching and self-resetting mechanisms. A line drawing of the data in Figure 5A for VG = 4.5 V (corresponding to the black markers shown on the right side of Figure 5A and at the bottom of Figure 5B). [Figure 5D] Figure 5 shows the fast switching and self-resetting mechanisms. Zoomed-in view of the data from Figure 5C near the rising transition. The vertical dashed line indicates the transition from 10% to 90% amplitude, corresponding to a rise time of 90 ns. [Figure 6A] 1 is a diagram relating to an apparatus according to an embodiment;FIG. 2 is a simplified schematic diagram of a superconducting device forming part of the apparatus;FIG. [Figure 6B]6A-6C are diagrams relating to an apparatus according to an embodiment. A more realistic representation (top view) of a superconducting device is shown, overlaid with a circuit diagram corresponding to the electronic components included in the apparatus. The device shown in FIG. 6B is equivalent to the device in FIG. 1A. [Figure 6C] 6A and 6B are diagrams relating to an apparatus according to an embodiment. [Figure 6D] 6D is a diagram relating to a device according to an embodiment, showing the electron injection mechanism. FIG. 6D should be read together with FIG. 6C. [Figure 6E] 1 shows a general architecture of a quantum processing device in which superconducting devices are used as frequency tunable couplers connected to qubits on a qubit platform in accordance with an embodiment. [Figure 7] FIG. 1 is a simplified representation of another superconducting device including a coplanar waveguide resonator, in which, as in an embodiment, a gate electrode is used to tune the resonant frequency of the resonator. [Figure 8] FIG. 13 is a simplified representation of a further superconducting device, including, as in an embodiment, a Pi section filter including an inductor and a gate electrode used to tune the resonant frequency of the circuit. [Figure 9] 1 is a flow chart showing the general steps of a method for operating a superconducting channel according to an embodiment; [Figure 10] 1 is a flow chart illustrating the general steps of a method for operating a superconducting channel according to an embodiment, and shows the steps performed to modulate a signal passing through the channel. [Figure 11] 1 is a flow chart showing the general steps of a method for manipulating a superconducting channel according to an embodiment, illustrating the steps performed to tune a property of the superconducting channel (and another property of the device determined by the property of the superconducting channel); DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The accompanying drawings show simplified representations of devices or parts of devices as described in the embodiments. The technical features shown in the drawings are not necessarily drawn to scale. Similar or functionally similar elements in the figures are assigned the same reference numbers unless otherwise indicated.

[0027] Metallic devices that can be switched between resistive and superconducting states by applying a control signal have been under active development for over 60 years, but existing superconducting switches are either bulky or require large source-drain bias currents and gate currents to operate, making them impractical for many applications and difficult to interface with classical semiconductor electronics.

[0028] For example, devices such as those mentioned in the background section, whether based on Josephson junctions or alternative designs, are typically characterized by low gate impedances, which require large gate currents for operation and draw a large amount of power from the driving circuitry. Moreover, such devices are typically operated with large source-drain bias currents, which prevent self-resetting from the normal to the superconducting state due to Joule heating. To subsequently switch the channel back to the superconducting state, additional circuitry is required to periodically drive the source-drain current below the recapture current at which the transition to the superconducting state occurs. Operation at low current biases is enabled when complete suppression of the source-drain critical current is achieved, as has been shown recently for electrothermal devices, allowing self-resetting (from the normal to the superconducting state).

[0029] The invention relies on electrically controlled devices based on superconducting channels (e.g., metallic nanowires). Such devices can be used, for example, as switches or related devices, where the normal conducting state of the channel is obtained by injection of energetic electrons from a gate contact through an insulating barrier. Typically, the operable gate current is several orders of magnitude smaller than the critical source-drain current of the nanowire, effectively resulting in a voltage-controlled device. This switching mechanism is very fast. In embodiments, the channel can self-reset from the normal state to the superconducting state and can operate in very large magnetic fields. Such devices can be advantageously used in low-power and cryogenic applications, especially in quantum computing applications.

[0030] The following description is structured as follows: First, general embodiments and schematic variants are described (section 1). The next section deals with more specific embodiments and discusses the physics underlying the invention (section 2). The method and variants thereof are collectively referred to as "the method". Reference symbol S ij all refer to steps of the methods illustrated in the flowcharts of Figures 9-11, while the reference numbers relate to physical parts or components of the apparatus. 1. General embodiments and schematic variations

[0031] With reference to Figures 6-11, an aspect of the invention relating to a method of operating a superconducting channel 102, 112, 122 will first be described. The method relies on a superconducting device 10, 11, 12 (S10). The device is designed to operate at cryogenic temperatures. In particular, the device can be embodied as a structured multilayer device 10 (as in Figures 6A-6C), a coplanar waveguide resonator (Figure 7), or an electrical circuit including an inductor (as in Figure 8).

[0032] In either case, the device includes a potential superconducting material, for example in the form of a wire, structured to define a channel 102, 112, 122. The device further includes a gate electrode 101, 111, 121. The gate electrode faces the channel 102, 112, 122, such that an end face of the gate electrode faces a side of the channel. The gate electrode typically extends perpendicular to the channel, as can be seen in Figures 6-8. In addition, an electrically insulating medium 103 is provided, which electrically insulates the gate electrode 101, 111, 121 from the channel 102, 112, 122.

[0033] In particular, the electrically insulating medium 103 may include a portion extending between the end face of the gate electrode 101, 111, 121 and the opposite portion of the channel 102, 112, 122. That is, as assumed in Figures 6B and 6C, in which case the electrically insulating medium may physically connect the gate electrode to the channel (i.e., the electrically insulating medium acts as a mechanical insulation and also as an electrical insulation). In particular, the insulating medium 103 may be provided as a substrate layer 103 supporting the superconducting material 102 and the gate electrode 101 (see Figures 6B and 6C). In a variant, the insulating medium may be, for example, an intermediate layer extending between the channel layer and the gate layer. In another variant, the electrically insulating medium may also be structured to define a portion of vacuum (or air) that contributes to insulating the gate electrode from the channel. More generally, the electrically insulating medium may include any element for electrically insulating the gate electrode from the channel.

[0034] The devices 10, 11, 12 need to be cooled (S20) in order for the channels 102, 112, 122 to become superconducting. Next, a voltage difference is applied (S30, S32) between the gate electrodes 101, 111, 121 and the channels 102, 112, 122 in order to inject electrons into the channels through the electrically insulating medium 103. This creates a net (albeit small) gate current between the gate electrodes and the channels, which accordingly pull the channels to ground. Furthermore, the critical current I CElectrons are injected with sufficient average energy to modify

[0035] The injected electrons overcome the tunnel barrier consisting of the insulating medium 103 by tunneling or by field emission (which also involves tunneling). Note that the operation of superconducting devices 10, 11, 12 differs from that of a simple Josephson junction, since Josephson junctions are typically operated with zero voltage difference so that supercurrent can flow without voltage drop. There are situations where a voltage difference can be applied to a Josephson junction. However, this voltage is typically on the order of the magnitude of the superconducting gap, whereas the voltage applied here is much larger.

[0036] In this situation, the current flow generated upon application of a voltage difference induces a measurable leakage current between the gate electrodes 101, 111, 121 and the channels 102, 112, 122. The magnitude of the current generated depends, typically in an exponential manner, on the applied voltage difference, and thus the operation of the device differs from the simple application of an electric field to the channel.

[0037] Measurements performed by the inventors show that a relatively small number of electrons, usually injected with energies much higher than the superconducting gap, are sufficient to cause the creation of a large number of quasiparticles that affect the critical current density of the superconducting material. The critical current density is modified at least locally at the level of a part of the channel opposite the gate electrode. This then leads to the critical current I C It affects the critical current I C characterizes the maximum current that can pass through the channel 102, 112, 122 without suppressing superconductivity. In summary, injecting energetic electrons locally modifies the quasiparticle density, affecting (at least locally) the critical current density of the superconducting material, which in turn modifies the critical current of the channel.

[0038] As described in the embodiment below, the critical current I Ccan be lowered, possibly even to zero, creating a non-zero resistance in the channel. If the critical current density is locally lowered to zero, the extent of the generated normal region (i.e., non-superconducting region) affects the overall resistance of the channel. In particular, this phenomenon can be exploited to locally turn off superconductivity and, for example, to modulate electromagnetic signals passing through the channels 102, 112, 122, as described in the embodiments below. Here, increasing the quasiparticle density not only affects the critical current, but can also affect other properties, such as the kinetic inductance of the channel, due to non-trivial mechanisms that will be described in detail later. This additional phenomenon can be exploited to tune any property of the system on a macroscopic scale, as implied by the altered state of the channel. For example, this phenomenon can be exploited to tune the resonant or cutoff frequency of an electric circuit containing the channel (e.g., configured as a wire-like inductor in an LC circuit), or to tune the resonant frequency of a tunable coupler for coupling qubits, as described in the embodiments below.

[0039] It has been shown that the physical phenomenon underlying the present invention occurs in many different types of device structures and in many different superconducting materials. In that regard, it is noted that the superconducting channel is usually made of aluminum (Al), which becomes normally conductive in very small magnetic fields. In contrast, many different superconducting materials can be used to manufacture the devices involved in the present method, including materials such as titanium nitride (TiN), which allow superconductivity to be maintained in magnetic fields much larger than Al. As a result, the devices involved in the present method can be operated in very large magnetic fields.

[0040] The proposed solution has further advantages and can be used in several applications. For example, locally tuning the critical current density of the superconducting material allows both the inductance and the resistance of the superconducting channel to be tuned. Interestingly, the applied voltage difference can be positive or negative. In particular, the channel can be operated using DC voltages with very low power (for example, less than 1 nW in the off state and 0 W in the on state). The critical current I of the channel C The speed at which the switching frequency can be adjusted (e.g., less than 100 ns, down to values ​​approaching 1 ps in embodiments) is the most important advantage. This in turn allows very high off / on switching frequencies (e.g., up to 10 GHz) to be achieved. Thus, the superconducting device can be advantageously operated as a switch, as in the embodiments, to locally suppress the superconductivity (e.g., as a switch) and to modulate the signal passing through the channel.

[0041] The device can be integrated, for example, on an existing quantum chip (e.g., on the same platform as the amplifier, or even on the qubit chip). It was shown that the method works well enough at temperatures below 3.4 K when TiN is used as the superconducting material. In particular, the method can be applied for signal multiplexing purposes, for example to switch the routing of signals on or near the quantum chip. In other applications, such as those recalled above, the device is configured as an electric circuit, such as an LC circuit or any circuit in which the superconducting channel is configured as a wire-like inductor. In this way, the resonant frequency of an LC oscillator or any other oscillator containing a wire-like inductor can be adjusted by changing the inductance of the wire. Similarly, the resonator can form part of the coupling element of a frequency-tunable coupler between qubits of a quantum computer, whereby adjusting the frequency of the resonator allows for tuning the coupling of the qubits. In a further application, the cutoff frequency of an LC filter, a pi filter, or any other filter containing a wire-like inductor is adjusted by changing the inductance of the wire. For completeness, the underlying devices 10, 11, 12 are easy to manufacture, can have very small dimensions, and can be manufactured in an integrated manner. This makes them ideal components for cryogenic applications. Beyond quantum processing and frequency tuning applications, other applications can be envisaged (e.g., detectors, amplifiers, circulators, and multiplexers can be targeted).

[0042] All this will now be described in detail with reference to a specific embodiment of the invention. First, with reference to FIGS. 9-10, the critical current I CA voltage difference can be applied (S30, S32) to reduce the potential to zero, thereby generating an electrical resistance in the channel 102, 112, 122. This resistance occurs at least locally at the level of some of the channels opposite the gate electrode. In other words, this resistance locally eliminates superconductivity at the level of some of the channels 102, 112, 122, which can be used advantageously for switching applications.

[0043] As further seen in Figs. 9-10, it is noted that the voltage difference should normally be switched off at some point (S40) so that the superconducting material 102, 112, 122 self-resets to a fully superconducting state, while the channel is maintained at the appropriate temperature, and thus at the temperature after cooling in step S20. In this way, in a controllable manner, the electrical resistance of the channel 102, 112, 122 can be locally increased (e.g., turned to a normal state) at the level of a part of the channel opposite the electrode, and then reversed. Thus, this process is reversible, despite possible hysteresis. The dimensions of the area turned to a normal state can be controlled by the applied voltage, as will be explained later.

[0044] In particular, the scheme of FIG. 9 can be used to switch or even modulate a signal routed through a channel, as described herein with reference to FIGS. 6 and 10. That is, a signal can be applied to the channel 102 (S25) and routed through the channel, for example, before applying a voltage difference (S32) to inject electrons into the channel. As described above, this causes the development of electrical resistance in the channel 102, which in turn affects the transmission of the applied signal (S25) in the channel (S50). Note that the applied signal can be an AC signal or a DC signal. For example, the signal can be an RF signal, as in quantum computing applications described later. Thus, in particular, the method can be used to switch or modulate signals, possibly at very high frequencies, which can be advantageously utilized in quantum processing applications. In such applications, the applied signal is typically an RF signal, and the source impedance of the RF signal is typically 10-100 Ω (e.g., 50 ohms), which can be rapidly and easily switched according to the method.

[0045] In particular, steps S32 and S40 can be repeatedly performed to repeatedly apply (S32) and turn off (S40) a voltage difference, taking into account modulating (S50) the signal (S25) applied to the channel 102. As described herein, the term "modulating" a signal should be understood in a broad sense to refer to a process of changing one or more characteristics of the signal (e.g., typically the waveform). In particular, the strength of the signal passing through the channel can be changed (e.g., by controlling the size of the high resistance region) or the frequency at which the signal is transmitted (e.g., by repeatedly turning the channel off and on), or both. Furthermore, the signal applied in step S25 can, for example, be an already modulated signal (e.g., a waveform whose amplitude or frequency is changed). Several applications can be contemplated. For example, a medium wave (MW) signal can be switched and routed to different ports for multiplexing purposes.

[0046] Quantitatively, the properties of the applied signal can be modulated (S50) at frequencies greater than 10 MHz and less than 10 GHz, thereby opening the door to multiple applications at cryogenic temperatures. Again, such frequencies present on / off switching frequencies, which should be distinguished from normal frequency modulation processes. Measurements performed by the inventors confirmed that single actuation times of less than 100 ns can be achieved. However, taking into account the limitations of the measurement setup used by the inventors led to the conclusion that the duration of a single actuation can be of the order of picoseconds. Two effects are involved: relaxation and diffusion of quasiparticles away from the injection point. The effects of relaxation and diffusion of quasiparticles have already been shown to be very fast processes (e.g., on the order of 20-50 ps or less) for comparable device structures (note that the diffusion process is probably much faster than the relaxation process), and the quasiparticle generation process is thermally induced by applying a heat pulse. However, in this case the generated quasiparticles are not in thermal equilibrium, and therefore the above estimates (e.g., 20-50 ps) can be considered as an upper limit for the actual duration of the process, which also depends on the superconducting material used.

[0047] In this case, modulation frequencies of up to 10 GHz can still be contemplated in principle, although other embodiments may require lower frequencies (e.g., around 1 KHz, 1 MHz, or 10 MHz or even lower).

[0048] Another distinguishing feature of the method is that the intensity of the gate current generated when applying the voltage difference (S30, S32) is greater than the critical current I C Much lower than (e.g., I CThe gate current therefore does not jeopardize the state of the channel, making the process sufficiently controllable. In practice, however, the gate current can be several orders of magnitude smaller than the critical source-drain current. This is explained in more detail in section 2.1. Depending on the superconductor material chosen, complete suppression of superconductivity can usually be achieved using gate currents in the range of a few tens of pA to a few nA. That is, the strength of the gate currents generated in steps S30, S32 is usually in the range of 10 pA to 10 nA. This means that the critical current I C This is in stark contrast to previously demonstrated devices, where gate currents on the order of 1000 Ω (eg, several μA) are required.

[0049] To trigger the creation of quasiparticles, relatively few electrons are injected, usually with an energy much higher than the superconducting gap. A voltage difference can be applied (S30, S32) (see diagram in FIG. 6B) such that this average energy is at least 100 times the superconducting gap of the superconducting material. The superconducting gap is defined as the energy difference between the ground state of the superconductor channel 102 and the energy of the lowest quasiparticle. In practice, the average energy of the injected electrons should, in principle, usually be 50-60,000 times the gap, although even larger ratios can be observed. A preferred ratio is in the range of 5000-20000, especially for materials based on Ti. This ratio can be, for example, about 10000. That is, for a gap of 2Δ=500 μeV, as assumed in FIG. 6D, a voltage difference of about 5 eV can be applied.

[0050] Note that the preferred ratio depends on the desired mode of operation. For switching applications, a very large ratio will give sufficient results (e.g., with a small voltage difference). For such applications, a superconductor with a small gap, such as titanium, would be preferred. However, if significant suppression of signal transmission in the off-state of the channel is desired, a superconductor with a high resistivity (in the normal state), such as TiN, would be preferred. Superconductor materials with a high resistivity in the normal state are also preferred for frequency tuning applications, since they have a high kinetic inductance.

[0051] The impedance of the gate electrode, determined mainly by the insulating medium 103, can typically be larger than 1 GΩ. Such a value corresponds to the minimum voltage difference required to locally switch the channel to the normal state. However, the gate impedance is likely to be significantly larger in practice. For instance, in the example described in section 2.1, superconductivity can be turned off at about 5 V / 1 nA = 5 GΩ. The gate impedance is likely to be larger than 1 TΩ when the critical current density starts to be affected by the applied voltage difference. On the other hand, the gate impedance can be less than 1 GΩ when the applied voltage becomes a little larger and the normal region in the channel starts to expand. This is a consequence of the exponential behavior of the switch, where the tunnel junction is driven by a resistance V G / I G is changed exponentially.

[0052] In that regard, as reflected in the flow chart of FIG. 11, the magnitude of the applied voltage difference can optionally be varied (S34) to controllably expand the region of the channel 102 where electrical resistance occurs. If desired, the voltage difference can be varied to increase the source-drain current I SD can be adjusted to keep the source-drain current I greater than the recapture current of the superconducting material. SDBy expanding the normal region through the wire, a large off-resistance can be reached, which results in a rapid heating effect. In this way, as soon as a local normal region is created, the entire wire changes to the normal state due to the rapid heating.

[0053] More generally, the magnitude of the voltage difference can be altered (S34) to adjust any local property of the channel, or any property (e.g. of the device or connected components) that is determined by this local property. To that end, in some cases, a property of interest can be monitored (S33). For example, as explained previously, altering the critical current density (e.g. a local property) affects the overall critical current of the channel (a non-local property). As described below, this can result in the kinetic inductance L of the channel. kin (another nonlocal property), which can be exploited to tune other properties, such as the resonant frequency or cutoff frequency, of an electrical circuit containing the superconducting channel.

[0054] The applied voltage difference is proportional to the kinetic inductance L of the channels 102, 112, and 122 for the following reasons: kin Electrons are injected into the channel, creating a finite quasiparticle density that drops off to zero away from the electron injection point. In the region of higher quasiparticle density, the critical current density and kinetic inductance L kin Therefore, both I C The decrease in L kin This leads to an increase in the quasiparticle density when injecting electrons, which is an incidental consequence of the increase in the quasiparticle density when injecting electrons.

[0055] As described herein, the kinetic inductance L kincan be altered (S34) to tune the frequency of an electrical circuit 120 including a superconducting channel 122, the superconducting channel 122 being configured as a wire forming an inductor 122 in the circuit. That is, a critical current I can be increased by injecting electrons as described above. C is changed, which is the kinetic inductance L of the channel 122. kin . This phenomenon can therefore be used to adjust the frequency of an electric circuit. For example, this frequency can be the resonant frequency of an LC oscillator or any other oscillator that includes a wire-like inductor 122. The resonant frequency is modulated by changing the inductance of the wire. In another application, the cutoff frequency of an LC filter (for example a pi filter as assumed in FIG. 8) or any other filter that includes a wire-like inductor is adjusted by changing the inductance of the wire thanks to the gate electrode 121. FIG. 8 shows the cutoff frequency of two capacitors C 1 , C 2 1 illustrates a pie section filter 120 including an inductor L (e.g., a wire 122) and an inductor L, and device 12 also includes an electrode 121 separated from wire 122 by an insulating medium (not shown).

[0056] Other applications may aim at tuning the resonant frequency of a coplanar waveguide (CPW) resonator 110 or a frequency tunable coupler, for example as assumed in FIG. 7. In the example of FIG. 7, the device 11 includes a CPW resonator 110, which comprises a conductive track 112 and a pair of return conductors 114 on either side of the track 112. The return conductors are separated from the conductive tracks by respective gaps. A gate electrode 111 faces towards the track 112. Again the device is controlled by a voltage via the gate electrode 111. The small electron current generated needs to flow out, therefore the central conductor 112 of the CPW resonator is connected to ground, which can be achieved for example using a very thin high kinetic inductance wire.

[0057] In other applications, the superconducting device can be used as a frequency-tunable coupler for coupling quantum circuits (e.g., superconducting qubits). For example, FIG. 6E shows a classical architecture of a quantum processing device. In that case, the quantum circuits 55 are assumed to be transmon-type superconducting qubits, although in some cases spin-based topological qubits can also benefit from the invention. Apparatus 1 further includes multiple RF-controlled components, such as couplers and frequency-tunable coupling elements. Apparatus 1 is described in detail below with reference to another embodiment of the invention.

[0058] Now, with reference to figures 6 to 8, another aspect of the invention relating to an apparatus 1 will be described. In particular, this apparatus can be a quantum processing device, or any device that includes parts operated at cryogenic temperatures. This aspect of the apparatus has already been described with reference to the method and will only be described briefly below.

[0059] The apparatus essentially comprises a device 10, 11, 12 containing a potential superconducting material. As previously explained, the potential superconducting material is structured to define a channel 102, 112, 122, preferably structured as a wire. The device is adapted to be operated at cryogenic temperatures, such that the channel 102, 112, 122 is superconducting. The device further comprises a gate electrode 101, 111, 121 facing towards the channel 102, 112, 122, whereby an end face of the gate electrode faces a portion of the channel. In addition, an electrically insulating medium 103 is arranged to electrically insulate the gate electrode 101, 111, 121 from the channel 102, 112, 122, as previously explained.

[0060] As can be further seen in Figures 6B and 6E, the apparatus 1 further comprises a control unit 20. The control unit 20 is connected to the device 10, in particular to the gate electrode 101. The control unit 20 consists, for example, of various interconnected elements 21-24, as assumed in Figure 6B. The control unit 20 is generally configured to apply a voltage difference between the gate electrode 101 and the channel 102 in order to inject electrons into the channel via the electrically insulating medium 103, thereby generating a gate current between the gate electrode 101 and the channel 102. The control unit, together with the device 10, controls the critical current I of the channel during operation of the device, due to the principles previously explained. C This allows for the injection of electrons with sufficient average energy to modify

[0061] As can be further seen in FIG. 6B, the control unit 20 can be further connected to the channel 102. Furthermore, the unit 20 can be configured to apply a signal to the channel 102 and to control the critical current I of the channel 102 during operation. C, and modifying the applied signal, thereby enabling applications such as those previously described with reference to FIG. 11. In particular, the control unit 20 can be further configured to enable a characteristic of the channel 102 (or for example any characteristic implied by this characteristic of the channel) to be measured. The unit 20 can further be designed to enable the applied voltage difference to be controllably modified according to the measured characteristic, in order to adjust the characteristic during operation. For that purpose, the control unit can include additional equipment for monitoring the characteristic to be adjusted, as further described in section 2.1.

[0062] In the example of Figure 6B, unit 20 includes a source measurement unit 23 that can both force a desired voltage and measure the current going into the gate. Additionally, unit 20 can include a voltmeter connected to channel 102. A signal generator 22 is connected to channel 102, which itself is connected to ground 104. A logic unit 21 is used to control the various components, which can potentially communicate with an external computer (not shown) and be operated from the cloud if desired.

[0063] As can be further seen in Figures 6B and 6C, the device 10 comprises a substrate 103 which serves as an electrically insulating medium 103. The gate electrode 101 and the wires 102 are each deposited and structured on the substrate 103. The gate electrode 101 and the channel 102 are preferably each made of the same material to facilitate lithography steps (e.g., a single lithography step is required to fabricate the device 10, as described in section 2.2). The superconducting material 102 and the gate material 101 may each be, for example, Ti, TiN, Nb, TiN ... x Nb y N z, W, WSi, Al, or Mo, or a combination thereof. The substrate 103 may be, for example, Si, SiO 2 , Sapphire, Si 3 N 4 , or HfO 2 , or combinations thereof. It will be apparent to one skilled in the art that other materials are contemplated. In addition, other structural designs can be used for device 10. In particular, an insulating medium can be provided as an intermediate layer or as a space separating gate 101 from channel 102.

[0064] The width of the wire 102 is typically in the range of 40-300 nm, while the minimum gap between the end face of the gate electrode 101 and the part of the channel 102 opposite is preferably in the range of 40 nm-300 nm. The width of the wire 102 and the minimum gap are measured parallel to the major surface of the substrate 103 along the direction x of Figure 6C (e.g., in this example, transversely in-plane to the longitudinal extension direction y of the wire). It is noted, however, that the width of the channel and the minimum gap can in some cases be less than 40 nm (e.g., equal to 10 nm).

[0065] FIG. 6D is a corresponding diagram showing the density of states in the channel 102, and illustrating the electron injection and quasiparticle generation processes. In this example, electrons with an average energy of 5 eV are injected. The electrons cross the insulating medium by tunneling. As explained before, the high energy of the injected electrons (e.g., much larger than the superconducting gap, in this example 5 μeV) causes the formation of quasiparticles in the channel 102.

[0066] 6E shows a general architecture of a quantum processing apparatus 1 according to an embodiment. Such an apparatus 1 generally includes one or more devices 10 and quantum circuits 55, assumed herein to be superconducting qubits of the transmon type. The superconducting channel of each device 10 is coupled to one or more of the quantum circuits 55. This coupling is preferably realized by capacitance, although inductive coupling can also be contemplated.

[0067] In detail, the device 1 shown in FIG. 6E comprises several cooling stages 61-65. The device 1 comprises in particular a first stage 61 comprising or connected to the control unit 20. The stage 61 may comprise one or more signal generators (not shown) for generating RF signals to drive the components 10, 55 as required. Such signal generators may form part of the control unit 20 and are at least connected to the control unit 20. Signals are generated and transmitted along a downward path 41 to the qubit platform, processed at the level of the qubit platform as required to perform the quantum computation, and an output signal is transmitted back to the control unit 20 along an upward path 42. The device further comprises a second stage 65 adapted to be cooled to a much lower temperature than the first stage. The superconducting qubit 55 and the device 10 are arranged in the second stage 65, which is likely to comprise various other components 51, 52, such as attenuators, circulators, isolators, etc., as known per se. The device 1 further comprises intermediate stages 62-64 between the first stage 61 and the second stage 65. Thus, the first stage, the intermediate stages 62-64 and the second stage 65 form a series (i.e. a chain) of successive stages, each intermediate stage being adapted to be cooled to a lower temperature than any previous stage in the series. Each stage 61-65 may comprise various electronic components (e.g. an attenuator for thermally balancing the RF signal from the signal generator), communication means (e.g. optical and / or electrical), cooling means, etc. Such electronic components are known per se and are not shown in FIG. 6E for the sake of brevity. The device may comprise additional stages, including a room temperature stage (not shown). The (indicative) temperatures shown in FIG. 6E are indicative of possible temperatures as achieved by the various platforms during operation. Of course, other platform temperatures may be used. For example, the upper platform 61 may be a room temperature platform.

[0068] In the example of FIG. 6E, each device 10 is configured as a frequency-tunable coupler. Each coupler 10 is connected to two or more qubits 55 in order to change the coupling between such qubits 55 during operation. Note that the qubits 55 are computational qubits that, together with the connected couplers 10, form a structure 54 of coupled qubits. Each structure 54 includes N computational qubits (e.g., fixed frequency transmons), where N≧2, and at least one coupling element 10 couples a pair of N qubits. The apparatus 1 shown in FIG. 6E comprises superconducting channels (not shown) leading to the qubits 55 and the coupling elements 10, as well as electrical connections (not shown) for operably coupling the control unit 20 to each structure 54 of coupled qubits, as required. The couplers 10 are operated as follows. As previously explained, a voltage difference is forced from the unit 20 and applied to the gate 101 of the coupler 10 in order to adjust the resonant frequency of the frequency-tunable coupler 10 and thereby change the coupling between the quantum circuits 55.

[0069] FIG. 6E shows a very schematic representation. Of course, multiple arrangements of qubits 55 and couplers 10 can be contemplated. The qubits 55 can possibly be connected to one or more tunable couplers 10 to allow the transitions between the states of the qubits to be driven parametrically, for example by modulating the energy of the tunable couplers. In addition to the parametric driving of the qubits, quantum gates can also be used, where a coupler or a tunable bus is brought into resonance with the two qubits to exchange excitations between the qubits. Another option is to use a fixed bus, the frequency of which is changed to turn off the interaction completely. More generally, the interaction of multiple qubits can be enabled or disabled by tuning the frequency of the couplers 10.

[0070] Unit 20 may, for example, comprise or be connected to a compiler and sequencer (not shown) that are typical components of a quantum processing device. The compiler determines and schedules high-level operations to execute quantum algorithms on device 1 as required. In response, the compiler is then able to determine batches of operations to be performed on qubits 55. The sequencer then breaks down such operations into single-cycle operations that may include operations such as single-qubit and two-qubit operations.

[0071] In a variant, the device 10 can in fact be configured as a switch 10 for switching RF signals applied to the channels 102, for example for multiplexing purposes, as previously explained. As long as superconductivity can be maintained within each channel 102, the device 10 can possibly be arranged in the upper stages 61-64 of the apparatus 1.

[0072] In another variation, multiple devices 10 configured as frequency-tunable filters are used, whose cutoff frequencies can be adjusted similarly to Purcell filters used to increase the coherence of qubits (e.g., while keeping pulses short) for readout purposes. That is, each frequency-tunable filter is connected to each quantum circuit (e.g., qubit or readout resonator) to block frequencies of signal resonance with the qubits while allowing control signals to pass for readout purposes.

[0073] It is noted that the apparatus according to the embodiment may also include devices 11, 12 as shown in Figs. 7, 8, with a control unit (not shown) being connected to such devices 11, 12. In particular, the apparatus may comprise an electric circuit 120 as seen in Fig. 8. This circuit has been previously described. This circuit essentially includes an inductor 122 formed by a channel and structured as a wire 122. The control unit then controls the kinetic inductance L of the wire 122 in order to adjust the cut-off frequency of the electric circuit 120. kin is used to adjust the

[0074] The above embodiments have been briefly described with reference to the accompanying drawings and may accommodate many variations. Multiple combinations of the above features may be contemplated. In the following sections, examples are provided. 2. Specific embodiments - technical implementation details 2.1 High-speed superconducting switches and their operation

[0075] In this section we describe high-speed superconducting switches and their operation. These switches have ultra-high gate impedance (much greater than 1 GΩ), are operated by the injection of energetic electrons through an insulating barrier, and can be self-reset.

[0076] A typical device is shown in Figure 1A. This device is essentially similar to the device described with reference to Figure 6 in Section 1. This device comprises a metal nanowire with a side gate, all deposited on an insulating substrate. The gate voltage V G By applying G flows, and this current is V G It depends exponentially on I GWith the onset of , superconductivity in the nanowire is weakened and eventually suppressed. Measurements show that a relatively small number of electrons injected with energies much higher than the superconducting gap are sufficient to trigger the creation of a large number of quasiparticles and to extinguish superconductivity. This mechanism is extremely efficient, and for gate currents below 1 pA, the source-drain critical current I of the nanowire is reduced by tens of μA. C The present superconducting switches allow the suppression of superconductivity. Contrary to previously presented devices, where gate currents comparable to the nanowire's critical current (e.g., a few μA) were required to operate the device, complete suppression of superconductivity is realized using gate currents in the range of tens of pA to a few nA (depending on the superconductor material in use). That is, such devices rely on low-energy electrons injected through a metallic medium. In addition to these remarkable properties, the devices presented herein operate similarly over the entire range of temperatures and magnetic fields where the superconductivity of the nanowires in the devices persists, both at positive and negative gate voltages, making these devices applicable to a wider range of environments than Josephson junctions. Finally, the present superconducting switches are extremely compact, fabricated in a single lithographic step, and can be easily integrated into existing architectures, either directly at the quantum processor level as a signal router, or as a cryogenic interface between classical voltage-driven digital electronics and superconducting current-driven circuits. In the following, the operation of the basic device is described in terms of the critical current, as well as the dependence of the critical current on gate voltage, temperature, and magnetic field. Characterization is performed using different substrates and superconductors. In this section, we further describe how the injected quasiparticles affect the superconductivity along the length of the channel, and demonstrate fast electrical switching and self-resetting back to the superconducting state. Experimental observations are presented, after which the physical causes of the phenomena, and their implications for device design, are discussed.

[0077] Figure 1A shows the superconducting device together with a schematic diagram of the measurement setup. The device consists of a 2 μm long, 80 nm wide TiN wire and a TiN side gate. The wire and the side gate are separated by an 80 nm wide gap. The gate and the nanowire were processed using electron beam lithography and dry etching of a TiN film deposited on an intrinsic Si substrate. The source-drain current I SD The measurements were performed with a low frequency lock-in technique by passing a gate voltage V through the source and recording the resulting voltage V. G is applied and the source-measure unit measures the current I G The second side gate, visible in Figure 1A, was not operated and remained grounded. Further details regarding materials, sample fabrication, and measurement techniques are reported in Section 2.2. G At I = 0, the nanowire SD The critical current I measured while starting from zero and sweeping upward or downward C In contrast, I SD When sweeping from the normal state towards zero, the recapture current I R = 1 μA and superconductivity was again established.

[0078] Figure 1B shows the I SD The differential resistance dV / dI of the nanowire measured while sweeping SD The inset shows the I C and I R The temperature dependence of I is shown in Fig. 1. C and I R The large difference between I and I is likely due to self-heating of the nanowires when they are in the normal conducting state, along with the difficulty of extracting heat through the substrate or leads. Figures 1C and 1D show the I C and I G V G are shown as functions of V G = ±2.5V, I GJust before I reaches the detection level (approximately 100 fA in the measurement setup used), C begins to decrease. V G = ±5.5V, and IG ≈ ±1nA, then I C vanishes and the nanowire reaches a normal-state resistance of 1.6 kΩ. C and I G The parametric plot of I G For small values ​​of I C Extremely fast suppression of (I G (approximately 50% of the suppression occurs within a noise level of 1 nA), followed by an I G The graph shows a slower decay that lasts until

[0079] Figures 2A and 2B show the effect of various temperatures T and out-of-plane magnetic fields B ⊥ In the case of I C The gate voltage dependences of are shown in Fig. 1D. G and V G properties and, as a result, the same V for complete suppression of superconductivity in the wire up to a critical temperature of 3.7 K or a critical magnetic field of 3.5 T. G On the other hand, for higher temperatures, I C The onset of inhibition is higher at V G (see grey arrows). ⊥ A more relevant dependence on is observed.

[0080] The inventors have demonstrated I above detection levels in over 20 TiN-based devices with a variety of gate geometries, nanowire widths (e.g., 40, 80, and 200 nm), nanowire lengths (e.g., 650 nm, 1 μm, and 2 μm), and gate-to-wire separations (e.g., 80 and 160 nm). G Simultaneously with the start of or I G Slightly earlier than the start of I CA suppression of V was observed. Similar phenomena were further observed using devices containing different substrate and superconductor materials. For example, Figures 3A and 3B show the V for a Ti nanowire similar to that of Figure 1A but with a width of 200 nm. G Regarding I C and I G In this case, a low I of about 40 pA is shown. G The normal state is reached at a critical temperature of 220 mK and a normal state resistance of 60 Ω. Figures 3C and 3D show a TiN device similar to that of Figure 1A, but with 25 nm of SiO thermally grown on a Si substrate. 2 The V in Figure 1 shows measurements performed on TiN devices deposited on a TiN layer. G operating range for very different V G Despite the operating range of I C The suppression of I G It still coincides with the onset of SiO 2 The device containing the positive V G I when the value is negative, not when the value is positive C with faster suppression of I C and V G The sharp edge of the gate electrode and the SiO 2 Given the large electric field reached on the substrate, it is expected that in the case of negative gate bias, the extraction of electrons from the gate is easier. In this case, the detection of the asymmetry of the small gate current is probably hindered by spurious current leakage in the measurement setup for high gate bias. Overall, these results show that the switching mechanism is a general phenomenon, rather than being related to a specific superconducting material. On the other hand, the obtained data also suggest that weaker superconductors require smaller gate currents for the switching mechanism to take place.

[0081] The measurements described above were performed on relatively short nanowires, and a sharp transition from zero resistance to normal state resistance was observed. In what follows we explain how superconductivity decreases from the electron injection point along the length of the nanowire. The device shown in Figure 4A consists of six TiN segments, 1 μm long and 80 nm wide. The current I SD is injected along the nanowire and a voltage V is applied across the first five segments. i were recorded simultaneously (e.g., the rightmost side contact was left floating). Figure 4B shows the I SD R Figure 4A shows a close-up of the differential resistance of the first three segments for the case of V. In addition to the varying extension of the zero-resistance state, the segments away from the injection point show an intermediate region where the resistance is greater than zero but below the normal-state value. This intermediate region indicates that normal and superconducting regions coexist within the same wire segment. The critical current, including the markers shown in Figure 4A, increases with V. G1 4C as a function of the corresponding gate current I G1 is shown in FIG. C and I G1 A parametric plot of the recapture current I is shown in Figure 4E. R The focus is on the region below I = 1.4 μA. The nonlocal measurements highlight two distinct regions. SD >I R In the case of I, all segments were switched simultaneously. After a local hot spot was created, the Joule heating mechanism warmed the surrounding TiN material in a rapid manner, and the entire channel was transformed into a normal conducting state. SD R Continuous switching occurs when the segment is closer to the biased gate, and less gate current is required to reach the normal conducting state.

[0082] Figure 4F shows the relationship between the distance |Δx| measured between the point of current injection and the center of each segment, S = (I R -I C ) / I​​C As indicated in the legend (e.g., filled markers), I C1 disappears |I G |I corresponding to the value G1 Data were extracted from the V (see, for example, the vertical arrow in Figure 4E). G5 A similar analysis, performed by sweeping , is plotted in Figure 4 with grey markers. In both cases of gate and voltage polarity, the results are highly consistent and show no dependence on the relative positions between the injection point, the measurement point, and the ground contact. In this case, the limited data range and the relatively large point-to-point scatter do not allow the functional form of S(Δx) to be determined. By fitting the results to an exponentially decaying function exp(-Δx / λ), shown as a solid line in Figure 4F, we obtain a characteristic decay length λ of about 1.2 μm.

[0083] A square wave with a peak-to-peak amplitude of 500 mV and a repetition rate of 100 kHz was applied as the DC gate bias V G The switching speed of a device similar to that of Fig. 1A was tested by adding a low-temperature bias T. An AC source-drain current of 100 nA amplitude and 100 MHz frequency was simultaneously passed through the nanowire and the resulting AC voltage drop ΔV was monitored as a function of time using a lock-in amplifier. S When a fast signal is applied and measured through the nanowire, ΔV averages to zero, reflecting the change in the circuit impedance between the resistive nanowire and the superconducting nanowire, while a positive ΔV reflects the normal conducting state. Figure 5A shows the time evolution of ΔV with respect to V. G The time-averaged gate current is shown in Figure 5B as a function of V G Switching within a 500 mV interval centered at I = 4.5 V was achieved. This gate current value is small I SD Within the limits of I C This is consistent with the results shown in Figure 1 for reaching complete inhibition of V. G= 4.5 V, highlighting the fast and reproducible switching between the two impedance states. A close-up view of the rising point is shown in Fig. 5D, where the dashed line indicates the transition from 10% to 90% of the step height, which occurs within 90 ns. Similar results are obtained regarding the decay time. Such a transient period is equal to three times the time constant of the lock-in amplifier used (e.g., 30 ns) and can therefore be considered as the best switching time measurable with the measurement setup used and, moreover, as explained in section 1, as an upper limit for the response time of the device. To evaluate the switching speed of the device more precisely, a sample specifically designed for microwave measurements and correlation techniques is investigated.

[0084] Here, a possible cause of the observed phenomenon is explained: the electrons emitted from the gate have a capacitance of 1 eV, which is much larger than the superconducting gap (e.g., 500 μeV for TiN). G The electrons reach the nanowire in a highly non-equilibrium state with energies of the order of 1000 Å. A large amount of quasiparticles are generated in the nanowire when they relax to the gap boundaries by inelastic scattering with other electrons and phonons. A sufficiently high concentration of quasiparticles results in the nanowire changing to a normal conducting state and switching the channel to a normal conducting state by erasing the superconducting gap and suppressing the de-pairing critical current. Such behavior is reminiscent of superconducting nanowire single photon detectors (SNSPDs) in which the impact of a visible or infrared photon promotes a single electron to high energy, which then triggers the generation of a large amount of quasiparticles as the single electron relaxes. In this case, no photon is required, and the high energy electron is provided directly by the gate current. The inventors have shown that the suppression of the critical current is highest at the injection point and decreases with distance, with a characteristic length scale of the order of 1 μm. This length scale is probably related to the diffusion length of the high energy electrons and the relaxation length of the generated quasiparticles.

[0085] The switching effects described herein are particularly useful for the fabrication of fast electrical switches with resistive and superconducting states that operate at extremely low gate currents. The very high gate impedance and low power consumption make such switches ideal for interfacing between voltage-driven transistors and current-driven superconducting circuits. The relaxation distance of the quasiparticles limits the extension of the segments that can be switched and, therefore, the highest normal state resistance of the device. In such situations, it is desirable to select a superconductor with a large diffusion distance and a large normal state resistance. Alternatively, I SD >I R By choosing I SD I R For low current bias, response times of less than 90 ns can be achieved, limited by the use of the measurement setup, as explained before. The highest switching speed is unambiguously determined by the relaxation time of the quasiparticles in the superconductor, which is typically less than 1 ns. The devices described herein can also be used to study quasiparticle physics at unprecedented energy scales, in the limit of no current flowing in the nanowire. The temperature and magnetic field studies reported in Figure 2 show that the I C This shows that the gate current required to completely suppress I is a robust property of the system, which can be used to advantage in multiple applications.

[0086] However, Figures 2A and 2B show that G The lower values ​​of I show richer physics, and as the temperature increases, the initial I C The suppression of I is shifted to increasingly higher gate voltages (see grey arrows). This behavior probably reflects a thermodynamic increase in the quasiparticle density in the wire, which CIt is necessary that more electrons be injected before a significant effect on is observed.

[0087] Other mechanisms include I C (The dissipated power V G I G The Joule heating in the barrier (due to the local temperature rise and the resulting C This contradicts the absence of temperature dependence for complete suppression of superconductivity. Additional measurements further tested the hypothesis that Joule heating does not play a significant role. Electric-field-induced suppression of superconductivity is also ruled out due to the very small gate voltages applied, the consistent correlation between gate current and critical current, and the pronounced nonlocal response that extends far beyond the electric field induced by the gate (see Figure 3).

[0088] In conclusion, the inventors were able to test several devices in which superconductivity was modulated or quenched by gate currents several orders of magnitude smaller than the source-drain critical current. The devices could be operated in a non-latching mode and on fast time scales. Due to the generality and robustness of the presented effects, as well as the easy device fabrication, such devices could be advantageously used in quantum processing equipment and other cryogenic electronics. 2.2 Method

[0089] Sample fabrication. A 20 nm TiN film was deposited on intrinsic Si. Nanowires were obtained by electron beam lithography. A 50 nm thick layer of a negative resist based on hydrogen silsesquioxane (HSQ) was used as a mask. After developing the resist, the unprotected TiN areas were removed by inductively coupled plasma etching in HBr plasma. Thus, both wires and gates were obtained from the same superconducting film in a single lithography step. After etching of TiN, the HSQ was removed by immersion in a dilute hydrofluoric acid solution. Characterization of the TiN film yielded a resistivity of 68 Ω per square, a critical temperature of 3.7 K, and a critical out-of-plane magnetic field of 3.5 T. These properties did not change in the final device. Ti wires were defined by electron beam lithography using a negative polymethylmethacrylate mask, electron beam evaporation of a 30 nm thick Ti film, and lift-off. Immediately prior to deposition, the chip was briefly immersed in buffered HF to remove the Si native oxide. -9 Base pressure in mBar, 1 nm s -1 The deposition of Ti was performed at a deposition rate of 100 μm. The high deposition rate was chosen to minimize contamination of the Ti film during deposition. For both Ti and TiN wires, regular bonding pads located approximately 100 μm away from the active area of ​​the device were defined by optical lithography and evaporation of Ti / Au.

[0090] Electrical measurements. Electrical measurements were performed in a dilution refrigerator with a reference temperature of 10 mK using standard lock-in techniques. Low-pass and high-frequency filters were installed along each line. A DC source-drain current I was superimposed on a small AC component of 30 nA and 113 Hz through a large bias resistor. SD was applied to the nanowire. Then, using a lock-in amplifier with an input impedance of 10 MΩ, the AC differential voltage V across the nanowire was recorded and the differential resistance dV / dI SD was used to calculate

[0091] From positive to negative values, I SD Measurements were recorded with the fast axis at 1000 V. This allowed the wire to be initialized to the superconducting state before starting each sweep. The gate voltage was applied using a Keysight B2902A source measure unit, which also recorded the current entering the gate contact. A suitability of ±100 nA was chosen to avoid damaging the device. Approximately 1 pAV associated with spurious leakage paths in the setup. -1 The linear contribution of I is explained in Section 2.1. G First, the low bias I G and V G A linear fit of the curve is then performed and the resulting line is then fitted to the measured V G To avoid contributions from displacement currents, V G with a waiting time on the order of 30 s after a step change in I G The value was recorded.

[0092] Figure 5. Measurements were taken with a resistive bias T mounted on the chip carrier, allowing DC and AC signals to be applied simultaneously. S The nanowire resistance was measured using a Zurich Instruments Ultra-High Frequency Lock-in operated at a base frequency of 100 MHz while a rapidly varying gate voltage was applied via a Keysight signal generator with a bandwidth of 80 MHz. An AC current of 100 nA RMS amplitude was passed through the sample and the voltage drop across the nanowire was input to the lock-in amplifier, demodulated and plotted on a Keysight DSOX2024A oscilloscope with a bandwidth of 200 MHz. The lock-in was set to the shortest possible time constant (30 ns) corresponding to a measurement bandwidth of 14 MHz.

[0093] Although the present invention has been described with reference to a limited number of embodiments, variants, and accompanying drawings, it will be understood by those skilled in the art that various modifications can be made and equivalents can be substituted without departing from the scope of the present invention. In particular, features listed in a particular embodiment, variant, or shown in the drawings (such as, for example, a device or method) can be combined with or substituted for other features in another embodiment, variant, or drawing without departing from the scope of the present invention. Accordingly, various combinations of features described with respect to any of the above embodiments or variants can be contemplated while remaining within the scope of the appended claims. In addition, many minor modifications can be made to adapt a particular situation or material to the content of the invention without departing from the scope of the invention. Therefore, it is intended that the present invention is not limited to the particular embodiments disclosed, but that the present invention includes all embodiments falling within the scope of the appended claims. In addition, many variations other than those explicitly mentioned above can be contemplated. For example, materials other than those explicitly mentioned can be contemplated.

Claims

1. 1. A method for operating a superconducting channel, comprising the steps of: An electric circuit including an inductor, a channel defined by a potential superconducting material and configured as a wire; a gate electrode disposed adjacent to the channel such that an end surface of the gate electrode faces a portion of the channel; and an electrically insulating medium disposed to electrically insulate the gate electrode from the channel; a control unit connected to the gate electrode and the channel; providing a device comprising: cooling the device to cause the channel to superconduct; and applying, by the control unit, a voltage difference between the gate electrode and the channel to inject electrons into the channel through the electrically insulating medium, thereby generating a gate current between the gate electrode and the channel, the electrons being injected having sufficient average energy to modify the critical current of the channel; and adjusting, by the control unit, a kinetic inductance of the wire to adjust one of a resonant frequency or a cutoff frequency of the electric circuit.

2. The method of claim 1 , wherein said applying said voltage difference reduces said critical current to zero, thereby creating an electrical resistance within said channel locally at least in said portion of said channel.

3. The method further comprising: The method of claim 2 , further comprising varying a magnitude of the voltage difference to controllably expand a region of the channel where the electrical resistance occurs.

4. The method further comprising:

4. The method of claim 1, further comprising applying a signal to the channel and modifying the applied signal by altering the critical current during operation of the device.

5. The method further comprising:

5. The method of claim 1, further comprising turning off the applied voltage difference so that the latent superconducting material self-resets to a fully superconducting state.

6. The method further comprising: The method of claim 4 , further comprising repeatedly applying and turning off the voltage difference to modulate the applied signal.

7. The method of claim 4 or 6, wherein the applied signal is modulated at a frequency greater than 10 MHz and less than 10 GHz.

8. 8. A method according to claim 1, wherein the magnitude of the gate current produced upon application of the voltage difference is at least one tenth of the critical current.

9. A method according to any preceding claim, wherein the magnitude of the gate current produced upon application of the voltage difference is in the range of 10 pA to 10 nA.

10. 10. The method of claim 1, wherein the voltage difference is applied such that the average energy is at least 100 times the superconducting gap of the potential superconducting material.

11. 11. The method of claim 1, wherein the applied voltage difference corresponds to a minimum value required to switch the channel to a normal conducting state and the impedance of the gate electrode determined by the electrically insulating medium is greater than 1 GΩ.

12. The method further comprising:

12. A method according to any preceding claim, further comprising varying the applied voltage difference so as to adjust a characteristic of the channel or a characteristic implied by this characteristic of the channel.

13. the device being configured as a frequency tunable coupler connected to a quantum circuit of a quantum processing device; 13. The method of claim 1, wherein the applying of the voltage difference tunes a frequency of a resonator of the frequency tunable coupler, thereby changing the coupling between the quantum circuits.

14. An apparatus comprising a device, The device, an electric circuit including an inductor; a channel defined by a potential superconducting material and configured as a wire, the device being configured to be operated at cryogenic temperatures that render the channel superconducting; a gate electrode disposed adjacent to the channel such that an end surface of the gate electrode faces a portion of the channel; an electrically insulating medium disposed to electrically insulate the gate electrode from the channel; The apparatus further comprising: an apparatus comprising: a control unit connected to the gate electrode and the channel, the control unit configured to apply a voltage difference between the gate electrode and the channel to inject electrons into the channel through the electrically insulating medium, thereby generating a gate current between the gate electrode and the channel, an average energy of the electrons being sufficient to modify a critical current of the channel during operation of the device at the cryogenic temperature, and the control unit further configured to adjust a kinetic inductance of the wire to adjust one of a resonant frequency or a cutoff frequency of the electric circuit.

15. the control unit is further connected to the channel; The apparatus of claim 14 , wherein the control unit is further configured to both apply a signal to the channel and modify the applied signal by varying the critical current during the operation.

16. The control unit: During said operation, a characteristic of said channel or a characteristic implied by said characteristic of said channel is measured; 16. The apparatus of claim 14 or 15, further configured to enable the applied voltage difference to be controllably varied in accordance with the measured characteristic so as to adjust the characteristic, or the characteristic implied by the characteristic, of the channel during said operation.

17. the device comprises a substrate forming the electrically insulating medium; 17. A device according to any of claims 14 to 16, wherein the gate electrodes and each of the wires are structured on the substrate.

18. the wire has a width in the range of 40 to 300 nm; 18. The device of claim 17, wherein a minimum gap between the edge of the gate electrode and the portion of the channel is in the range of 40 to 300 nm, the width and the minimum gap being measured parallel to a major surface of the substrate.

19. 19. Apparatus according to any of claims 14 to 18, wherein the potential superconducting material comprises at least one element selected from the group consisting of Ti, TiN, Nb, W, WSi, Al, and Mo.

20. the device is configured as a quantum processing device further including a quantum circuit; 20. Apparatus according to any of claims 14 to 19, wherein the channels of the device are connected to one or more of the quantum circuits.

21. The device, a switch for switching the radio frequency signals applied to said channels; or 21. The apparatus of claim 20, configured as one of a frequency tunable coupler for varying coupling between two or more of the quantum circuits.

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