ASYMMETRIC DC SQUID WITH SIDE-CLOSED CAPACITY FOR READ AND RESET QUBITS

Abstract

Superconducting unit that exhibits the following: a group of asymmetric DC SQUIDs, wherein one asymmetric DC SQUID in the group of asymmetric DC SQUIDs has a Josephson junction, and a capacitive unit that parallels the group of asymmetric DC SQUIDs, wherein an external magnetic flux is set to a first value and applied to a tunable resonator formed by the asymmetric DC SQUIDs and the capacitive unit, wherein the first value of the external magnetic flux causes the tunable resonator to tune to a first frequency, wherein the first frequency lies within a first frequency difference to a resonant frequency of the qubit, and wherein tuning the tunable resonator to the first frequency causes an active reset of a qubit coupled to the tunable resonator. the superconducting unit further comprises the following: the external magnetic flux changed to a second value, wherein the second value of the external magnetic flux causes the tunable resonator to tune to a second frequency, the second frequency being detuned relative to the qubit's resonance frequency by at least a second frequency difference, and wherein tuning the tunable resonator to the second frequency enables a dispersive readout operation of a quantum state of the qubit to be performed.

Description

[0001] Reference is hereby made to application 11 2017 007 873.3, the contents of which are hereby incorporated into this application. TECHNICAL AREA

[0002] The present invention relates generally to a superconducting unit, a fabrication method, and a fabrication system for reading out a state of a superconducting qubit and resetting the superconducting qubit to a ground state. In particular, the present invention relates to a unit, a method, and a system for a tunable asymmetric DC SQUID for reading out and resetting qubits. BACKGROUND

[0003] In the following, a "Q" prefix in a word or phrase indicates a reference of that word or phrase to a quantum data processing context, unless explicitly distinguished from this in the context of the respective usage.

[0004] Molecules and subatomic particles follow the laws of quantum mechanics, a branch of physics that explores how the physical world works at its most fundamental levels. At this level, particles exhibit unusual behavior, such as existing in more than one state simultaneously and interacting with other particles that are very far away. Quantum computing uses these quantum phenomena for information processing.

[0005] The computers used today are considered classical computers (also referred to here as "conventional" computers or conventional nodes or "CN" (conventional nodes)). According to the so-called von Neumann architecture, a conventional computer uses a conventional processor manufactured using semiconductor materials and semiconductor technology, semiconductor main memory, and a magnetic or solid-state storage unit. In particular, the processors in conventional computers are binary processors, i.e., processors that work with binary data represented by 1s and 0s.

[0006] A quantum processor (Q-processor) utilizes the unique nature of entangled qubit units (referred to here as "qubits" or, in the plural, "qubits") to perform computational tasks. In the specific realms where quantum mechanics operates, matter particles can exist in multiple states, such as an "on" state, an "off" state, or both an "on" and an "off" state simultaneously. While binary data processing using semiconductor processors is limited to the use of only the "on" and "off" states (corresponding to 1 and 0 in binary code), a quantum processor uses these quantum states of matter to output signals that can be used in data processing.

[0007] Conventional computers encode information in bits. Each bit can take the value 1 or 0. These ones and zeros act as on / off switches, ultimately controlling computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two fundamental principles of quantum physics: superposition and entanglement. Superposition means that each qubit can simultaneously represent both a 1 and a 0. Entanglement means that qubits in a superposition can be correlated in a non-classical way. That is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of the other, and more information can be obtained about the two qubits in their entangled state than by considering them individually.

[0008] Using these two principles, qubits function as more complex information processors, giving quantum computers capabilities that allow them to solve difficult problems that conventional computers cannot. IBM has successfully built a quantum processor using superconducting qubits and demonstrated its functionality (IBM is a registered trademark of International Business Machines Corporation in the United States and other countries).

[0009] A superconducting qubit contains a Josephson junction. A Josephson junction is formed by separating two superconducting metal thin-film layers with a non-superconducting material. When the metal in the superconducting layers is made superconducting—for example, by lowering the metal's temperature to a predetermined cryogenic temperature—electron pairs from one superconducting layer can tunnel through the non-superconducting layer into the other superconducting layer. In a qubit, the Josephson junction—which acts as a dispersive nonlinear inductor—is electrically connected in parallel with one or more capacitive units, forming a nonlinear microwave oscillator. The oscillator has a resonant / transition frequency determined by the values ​​of the inductance and capacitance in the qubit circuit.Any reference to the term "qubit" refers to switching technology made from superconducting qubits that uses a Josephson junction, unless explicitly distinguished from this in the context of the respective use.

[0010] The information processed by qubits is transported or transmitted in the form of microwave signals / photons in the microwave frequency range. These microwave signals are captured, processed, and analyzed to decode the quantum information encoded within them. A readout circuit is a circuit connected to the qubit to capture, read, and measure its quantum state. The output of the readout circuit is information that can be used by a Q-processor to perform calculations.

[0011] A superconducting qubit possesses two quantum states – |0> and |1>. These two states can be two energy states of atoms, for example, the ground state (|g>) and the first excited state (|e>) of a superconducting artificial atom (superconducting qubit). Other examples include the up and down directions of nuclear or electron spin, two positions of a defect in a crystal, or two states of a quantum dot. Since it is a quantum system, every combination of the two states is possible and valid.

[0012] For quantum data processing using qubits to be reliable, quantum circuits—such as the qubits themselves, the associated readout circuitry, and other parts of the quantum processor—must not significantly alter the energy states of the qubit (e.g., by adding or removing energy) or influence the relative phase between the |0> and |1> states of the qubit. This operating condition, which applies to any circuit working with quantum information, requires special consideration in the fabrication of semiconductor and superconductor structures used in such circuits.

[0013] A qubit reset is the process by which the qubit's energy state returns from an excited state to a ground state. A time constant (T1) characterizes the exponential decay from the qubit's excited energy state to its ground state as plotted over time.

[0014] Basically, a superconducting quantum interference device (SQUID) is used as a highly sensitive magnetometer capable of measuring extremely weak magnetic fields. SQUIDs possess sufficient sensitivity to detect fields as low as 5 attotesla (5 × 10⁻⁶). -18 to measure aT). For comparison: a normal refrigerator magnet generates 0.01 Tesla (10 -2 T).

[0015] There are two basic types of SQUIDs: direct current (DC) SQUIDs and high frequency (HF) SQUIDs.

[0016] A DC SQUID is based on the DC Josephson effect and has two parallel Josephson junctions in a superconducting ring. In the absence of an external magnetic field, the input current divides equally between the two segments, each leading to one of the Josephson junctions in the ring. If a weak external magnetic field is applied to the superconducting ring, a shielding current begins to circulate within the ring, generating a magnetic field that cancels out the applied external flux. The induced current flows in the same direction as the external flux in one segment of the superconducting ring, and in the opposite direction in the other segment. The total current is in one segment and in the other. Once the current in one segment exceeds the critical current I, the SQUID is switched off. cWhen the flux exceeds the threshold of the Josephson junction in this segment, a voltage is generated at this junction. If the external flux is further increased until it exceeds the threshold, or half a magnetic flux quantum, the SQUID, instead of shielding the flux, now energetically prefers to increase it by one flux quantum, since the flux surrounded by the superconducting ring must be an integer number of flux quanta. The shielding current now flows in the opposite direction. Thus, the shielding current changes direction every time the flux increases by half-integer multiples of the flux quantum. The critical current therefore oscillates in the superconducting ring of the DC SQUID depending on the applied flux.

[0017] A significant amount of time is typically lost waiting for the qubit to reset. Long-lived qubits generally decay slowly from their excited state to their ground state. Consequently, the reset process, which relies on waiting for the qubit to decay to its ground state, is also slow. The longer the qubit's T1, the longer the idle time.

[0018] This wasted time has a direct negative impact on the speed of quantum calculations possible using superconducting qubits. Since the reset procedure is typically passive, meaning it involves no circuitry other than the qubit itself, other quantum circuitry, such as the readout circuit typically used to read qubits, plays no role in the qubit reset process.

[0019] An active method for resetting the qubit is therefore desirable. In the active method, a quantum circuit located outside the qubit works in such a way that it forces the qubit into the ground energy state, accelerates the qubit's decay to the ground energy state, or a combination of both. A quantum circuit that can perform multiple operations, for example, both read and reset operations on a single qubit, is also highly desirable.

[0020] US 9,509,274 B2 describes a superconducting phase-shifting system. The system includes an all-pass filter with at least one variable-inductance element. The all-pass filter can be configured to receive an input signal and provide the input signal as an output signal that is phase-shifted relative to the input signal based on a variable inductance provided by each of the at least one variable-inductance element. The system can further include a phase controller configured to provide a phase-control current to control the variable inductance of the at least one variable-inductance element based on a characteristic of the phase-control current.

[0021] US Patent 2006 / 0225165A1 describes analog processors for solving various computational problems. These analog processors comprise a plurality of quantum devices arranged in a lattice, along with a plurality of coupling devices. The analog processors further include bias control systems, each configured to apply a local effective bias to a corresponding quantum device. One set of coupling devices within the plurality is configured to couple nearest neighbors among the quantum devices in the lattice. Another set of coupling devices is configured to couple beyond nearest neighbors of the quantum devices.The analog processors further comprise a variety of coupling control systems, each configured to adjust the coupling value of a corresponding coupling element within the multitude of coupling elements. The quantum processors further comprise a series of readout elements, each configured to measure the information from a corresponding quantum element within the multitude of quantum elements.

[0022] WO 2016 / 183213A1 describes a superconducting input and / or output system that uses at least one superconducting microwave resonator. The superconducting microwave resonator(s) can be communicatively coupled to a microwave transmission line. Each superconducting microwave resonator can include a first and a second DC-SQUID connected in series with each other and with an inductor, as well as a capacitor connected in parallel with the first and second DC-SQUIDs and the inductor. Appropriate inductive interfaces can apply a forward bias to control the DC-SQUIDs. The second DC-SQUID can be coupled to a quantum flux parameter (QFP), for example, as the last element in a shift register. A superconducting parallel-plate capacitor structure and a method for its fabrication are also described.

[0023] US Patent 9,425,804 B2 describes a system and method for controlling superconducting quantum circuits. The system comprises at least one superconducting quantum circuit described by multiple quantum states and at least one single-flux quantum (SFQ) control circuit configured to generate a voltage pulse train comprising a plurality of voltage pulses separated in time by a pulse-to-pulse interval tuned to a resonance period. The system also includes at least one coupling between the at least one superconducting quantum circuit and the at least one SFQ control circuit, configured to transmit the voltage pulse train generated by the SFQ control circuit to the at least one superconducting quantum circuit.In some examples, the system further includes a control system configured to optimize the pulse-to-pulse spacing to minimize gate inaccuracy due to timing error, time jitter, and / or weak qubit anharmonicity. SUMMARY

[0024] Embodiments of the present invention provide a superconducting unit, a method, and a system for its fabrication. A superconducting unit realized according to the present invention includes a capacitive unit that connects a group of asymmetric DC SQUIDs to form a tunable resonator. Each asymmetric DC SQUID in the group of asymmetric DC SQUIDs has a first Josephson junction and a second Josephson junction, wherein the first critical current of the first Josephson junction differs from the second critical current of the second Josephson junction. The superconducting unit includes a coupling between the tunable resonator and a qubit such that the capacitively connected asymmetric DC SQUIDs can dispersively read out a quantum state of the qubit.An external magnetic flux is set to a first value and applied to the tunable resonator, wherein a first value of the external magnetic flux causes the tunable resonator to tune to a first frequency, the first frequency being within a first frequency difference to a resonant frequency of the qubit, and wherein the tuning of the tunable resonator to the first frequency results in an active reset of the qubit. The present invention thus provides a superconducting unit capable of actively resetting a qubit.

[0025] In a further embodiment of the present invention, the external magnetic flux is changed to a second value, wherein the second value of the external magnetic flux causes the SQUID to tune to a second frequency, the second frequency being detuned relative to the qubit's resonant frequency by at least a second frequency difference, and wherein tuning the tunable resonator to the second frequency enables a dispersive readout operation of the qubit's quantum state to be performed by sending a microwave signal to the qubit-resonator system at the resonant frequency of the resonator and measuring the amplitude and / or phase of the output signal. This provides a superconducting unit capable of performing both a readout and an active reset operation of a qubit.

[0026] In a further embodiment of the present invention, the second frequency is a maximum frequency in a frequency resonance range of the tunable resonator. This provides a special mode of operation for the superconducting unit to perform the readout process.

[0027] In a further embodiment of the present invention, the second frequency difference depends on a degree of asymmetry between the first Josephson junction and the second Josephson junction. This provides a special way of configuring the superconducting unit for performing the readout process.

[0028] In a further embodiment of the present invention, the first frequency offset is zero and the first frequency is the resonant frequency of the qubit. This provides a special way of operating the superconducting unit for performing the active reset process.

[0029] In a further embodiment of the invention, because the first frequency lies within the first frequency difference to the resonance frequency of the qubit, the qubit emits a photon, the emission causing the qubit to return to a ground energy state. This provides a special mechanism by which the superconducting unit performs the reset process.

[0030] In another embodiment, the qubit is forced into the ground energy state faster than the qubit's energy decay time constant. This provides a special mechanism by which the reset process is performed actively by the superconducting unit.

[0031] In a further embodiment of the present invention, the group of asymmetric DC SQUIDs comprises only the asymmetric DC SQUID. This embodiment thus provides a superconducting unit in an alternative configuration.

[0032] Another embodiment of the present invention further comprises a series connection which links several asymmetric DC SQUIDs from the group of asymmetric DC SQUIDs. This provides a superconducting unit in a further alternative configuration.

[0033] In a further embodiment of the present invention, the series circuit includes a superconducting wire. This provides a special configuration in an alternative configuration of the superconducting unit.

[0034] Another embodiment of the present invention further comprises a first plate formed on a first side of the group of asymmetric DC SQUIDs. This embodiment also includes a second plate formed on a second side of the group of asymmetric DC SQUIDs, wherein the first plate and the second plate are separated by a gap and the first plate and the second plate together form the capacitive unit. This provides a further particular configuration in an alternative configuration of the superconducting unit.

[0035] One embodiment of the present invention includes a manufacturing method for producing the superconducting unit.

[0036] One embodiment of the present invention includes a manufacturing system for producing the superconducting unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The novel features considered to characterize the invention are specified in the accompanying claims. However, the invention itself, as well as a preferred use, further objectives and advantages, are best understood by referring to the following detailed description of the illustrative embodiments together with the accompanying drawings, wherein: Fig. 1 shows some exemplary configurations of a capacitively shunted asymmetric DC SQUID according to an embodiment of the present invention; Fig. 2 shows a circuit diagram of a tunable asymmetric DC SQUID used according to an illustrative embodiment for reading and resetting qubits; Fig. Figure 3 shows two exemplary implementations of tunable resonators according to an embodiment of the present invention; Fig. Figure 4 shows an exemplary configuration of a tunable resonator for actively resetting and reading a qubit according to an embodiment of the present invention; Fig. Figure 5 shows a further exemplary configuration of a tunable resonator for actively resetting and reading out another qubit according to an embodiment of the present invention; and Fig. Figure 6 shows another exemplary configuration of a tunable resonator for actively resetting and reading out a superconducting qubit according to an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The illustrative embodiments of the present invention described above address the aforementioned need for a tunable superconducting resonator for reading and resetting qubits and offer solutions for this need. The tunable resonator can be implemented as a capacitively shunted asymmetric DC SQUID array.

[0039] One embodiment of the present invention can be implemented as a capacitively shunted asymmetric DC SQUID. A design and fabrication method for the capacitively shunted asymmetric SQUID can be implemented as a software application. The application implementing one embodiment of the present invention can be configured to operate in conjunction with an existing superconductor fabrication system, such as a lithography system.

[0040] For the sake of clarity and without any intended limitation of the description, the illustrative embodiments of the present invention are described using some exemplary numbers of SQUIDs. An embodiment can also be implemented with a different number of SQUIDs within the scope of the illustrative embodiments.

[0041] Furthermore, simplified diagrams of exemplary SQUIDs, qubits, and other circuit components are used in the figures. In an actual fabrication or circuit, additional structures or components not shown or described herein, or structures or components that differ from those shown and described herein, may be present without deviating from the scope of the present invention. Likewise, within the scope of the present invention, a component shown or described in the exemplary capacitively shunted asymmetric DC SQUID may be fabricated or connected differently to achieve similar operation or results as described herein.

[0042] A specific value, location, position, or dimension of a component of the capacitively shunted asymmetric DC SQUID described herein shall not constitute a limitation of the present invention unless such a property is expressly described as a feature of an embodiment of the present invention. The value, location, position, dimension, or a combination thereof are chosen solely for the clarity of the drawings and description and may be increased, decreased, or otherwise modified from the actual value, location, position, or dimension that may be used in actual manufacturing or circuitry to achieve an objective according to embodiments of the present invention.

[0043] Furthermore, embodiments of the present invention are described only by way of example with respect to specific actual or hypothetical components. The steps of different embodiments of the present invention can be adapted to produce a circuit in a similar manner using a variety of components intended for similar purposes, and such adaptations are included within the scope of the present invention.

[0044] Embodiments of the present invention are described only by way of example with regard to certain types of materials, electrical properties, structures, configurations, steps, operations, dimensions, numerical data, frequencies, circuits, components, and applications. Specific manifestations of these and other similar artifacts are not intended to limit the invention. Within the scope of the present invention, any suitable manifestation of these and other similar artifacts may be selected.

[0045] The examples described herein are used solely for the sake of clarity and are not intended to limit the embodiments of the present invention. Any advantages listed herein are merely examples and are not intended to limit the present invention. Further or different advantages may be realized through specific embodiments of the present invention. Furthermore, a particular embodiment of the present invention may exhibit some, all, or none of the advantages listed above.

[0046] It will now be on Fig. Reference is made to Figure 1, which shows some exemplary configurations of a capacitively shunted asymmetric DC SQUID according to the invention. Configuration 100 is an exemplary configuration of a capacitively shunted asymmetric DC SQUID in which a single (N=1) asymmetric DC SQUID 102 is used.

[0047] Josephson junction 102-1 and Josephson junction 102-2 are shown in their respective positions merely as non-restrictive examples. Provided that the input current I is supplied via a wire centered between Josephson junctions 102-1 and 102-2, the positions of the Josephson junctions in the superconducting ring of SQUID 102 are interchangeable.

[0048] The asymmetric DC SQUID 102 is a modification of a symmetric DC SQUID in that the two Josephson junctions 102-1 and 102-2 in the SQUID 102 have different critical currents (generally different areas), as indicated by the different sizes (not to scale) of the symbols (cross) representing the Josephson junctions in the SQUID 102. The Josephson junction 102-1 has a critical current I c1 The Josephson junction 102-2 possesses a critical current I c2In preferred embodiments of the present invention, the Josephson junctions 102-1 and 102-2 advantageously have different sizes to allow a limited band of tunable frequencies (frequency resonance range) of 100 or 150 when shunted through a capacitor. For example, if the frequency of the qubit to be read or reset (not shown) is in the range of 4.7 gigahertz (GHz), a desirable limited band of tunable frequencies can be in the range of 4.5 GHz to 8.5 GHz. If the sizes of the two Josephson junctions in the SQUID 102 were identical or nearly identical, the lower limit of the frequency range of the capacitively shunted SQUID would approach zero frequencies, provided the upper limit of the range remained unchanged. As a person skilled in the art understands, a larger frequency band leads to steeper slopes between the maxima and minima of the frequency tunability graph.The significance of limiting the frequency band directly below the frequency of the qubit and the resulting lower slopes in the frequency tuning curve of the capacitively closed-loop SQUID are explained in the present. Fig. 6 to 6 more clearly visible. With suitable selection, the asymmetric Josephson junctions 102-1 and 102-2 enable a stable frequency minimum of the frequency range of the capacitively shunted asymmetric DC SQUID 102.

[0049] The asymmetric DC SQUID 102 is connected in parallel (electrically in parallel) with the capacitive unit 104. The parallel connection of the capacitor 104 with the inductance of the asymmetric DC SQUID 102 forms the capacitively connected asymmetric DC SQUID 100, which functions as a tunable resonator. The tunable resonator 100 operates as a resonator whose frequency can be adjusted by tuning or setting an external magnetic flux (Φ) applied to the tunable SQUID 100. ext ) is tunable or variable. It is assumed that the inductance of the superconducting rings of the SQUIDs 102 is small and the inequality L(I) c1 +I c2 ) << Φ extThis assumption is fulfilled. This basic assumption greatly simplifies the analysis and calculation of the unit's response behavior, as it implies that the total flux flowing through the ring of the DC SQUID is equal to or nearly equal to the applied external flux Φ. ext is.

[0050] The tunable resonator 100 can be connected via nodes 106 and 108 to a qubit (not shown) and / or an input / output transmission line (not shown) of a circuit (not shown). By adjusting the external flux, the tunable resonator 100 exhibits a resonant frequency response that lies within the frequency range defined, at least in part, by the selected asymmetrical critical currents (magnitudes) of the Josephson junctions 102-1 and 102-2 and the magnitude of the capacitor 104. This tunable frequency of the tunable resonator 100 can be tuned to be equal to or close to the qubit's transition frequency (qubit resonant frequency), or it can be tuned to be at the opposite end of the frequency range from the qubit's frequency.When the frequency of the tunable resonator 100 is tuned to be equal to or close to the frequency of the qubit, the tunable resonator 100 – which is essentially a resonator – is said to be in resonance with the qubit. Conversely, when the frequency of the tunable resonator 100 is tuned to be significantly different from the frequency of the qubit, the tunable resonator 100 is said to be out of resonance with the qubit.

[0051] By simply tuning the inductance of the SQUID 102, which in turn brings the resonator into or out of resonance with the qubit, the tunable resonator 100 based on the SQUID can be tuned to actively reset or read out the qubit. If the qubit is to be reset / set to its ground state, the frequency of the tunable resonator 100 is brought into resonance with the qubit's resonant frequency. This tuning of the tunable resonator 100 into resonance stimulates, via the Purcell effect, the emission of a photon from the qubit into the readout resonator (tunable resonator 100), thus leaving the qubit in its ground state.

[0052] If the state of the qubit is to be read out or otherwise manipulated, the frequency of the readout resonator (tunable resonator 100) is made out of resonance with the qubit's resonant frequency. In this operating mode, the qubit's state is read out using dispersive readout technology. In this case, the frequency of the tunable resonator is set at an optimal point, which corresponds to the maximum resonant frequency of the readout resonator plotted against the flux. The optimal point has a region with a slope of zero (or near zero) on the frequency tunability curve of the tunable resonator 100 plotted against the flux. The point with a slope of zero, or the region with a slope near zero, minimizes the susceptibility of the readout resonator's frequency to flux noise.

[0053] Configuration 150 represents another exemplary configuration of a tunable resonator in which several (N>1) asymmetric DC SQUIDs 152A, 152B...152N are used. Each of the asymmetric DC SQUIDs 152A-N corresponds in configuration and operation to the asymmetric DC SQUID 102 in the tunable resonator 100.

[0054] The asymmetric DC SQUIDs 152A-N are connected in series as shown. The series of asymmetric DC SQUIDs 152A-N is connected in parallel with capacitor 154. Capacitor 154 functions similarly to capacitor 104 in the tunable resonator 100, but its value can differ depending on the inductance of the series of asymmetric DC SQUIDs 152 and the desired frequency resonance range for the tunable SQUID 152. Nodes 156 and 158 correspond to nodes 106 and 108, respectively, in the tunable resonator 100.

[0055] In one embodiment of the present invention, the asymmetric DC SQUIDs 152A, 152B, and 152N are each substantially identical to one another. In a further embodiment of the present invention, the series 152 can include an asymmetric DC SQUID which has the same or different dimensions of one or both Josephson junctions compared to another asymmetric DC SQUID in the series.

[0056] It will now be on Fig. Reference is made to Figure 2, which shows a circuit diagram of a tunable asymmetric DC SQUID designed according to the invention for reading and resetting qubits. The resonator 202 can be the tunable SQUID 100 or the tunable SQUID 150. Fig. 1. In the non-restrictive example shown, the resonator 202 is the tunable SQUID 150.

[0057] Resonator 202 is configured to read and reset qubit 204. Qubit 204 is formed by Josephson junction 206 and capacitor 208. Josephson junction 206 has a critical current I eq , and capacitor 208 has a capacitance C q . Node 156 (or possibly 106) is connected via a coupling capacitor 210 with a capacitance C c to qubit 204. Node 156 (or possibly 106) is connected via a readout capacitor 214 with a capacitance C. r connected to an input / output transmission line 212. Node 158 (or 108, if applicable) is connected to qubit 204 as shown.

[0058] It will now be on Fig. Reference is made to Figure 3, which shows two tunable resonators designed according to the invention. The tunable resonator 300 is essentially similar in its function to the tunable resonator 100 of the Fig. 1. The tunable resonator 350 is essentially similar in its operation to the tunable resonator 150 of the Fig. 1.

[0059] The capacitor 104 in the tunable SQUID can be fabricated in a variety of implementation-specific ways using a variety of lithographic processes. In one of these processes, plates 304A and 304B are formed and connected to the tunable SQUID 102 via nodes 106 and 108, respectively, as shown. The plates 304A and 304B, fabricated from a superconducting material, are separated by a gap d1, forming a capacitor, namely the capacitor 104 of the tunable resonator 100. In one embodiment, the plates 304A and 304B are coplanar, i.e., they lie in the same fabrication plane.

[0060] The capacitor 154 can be manufactured similarly for the tunable SQUID 154. For example, the plates 354A and 354B, made of a superconducting material, are separated by a distance d2, thereby forming a capacitor, namely the capacitor 154 of the tunable SQUID 100. In one embodiment of the present invention, the plates 354A and 354B are coplanar.

[0061] It will now be on Fig. Reference is made to Figure 4, which shows an exemplary configuration of a tunable resonator for actively resetting and reading a qubit according to an embodiment of the present invention. Only as a non-limiting example, in a configured experiment, the tunable resonator 402 was used in the same way as the tunable resonator 350 of the Fig. 3 configured, with eight (N=8) similar asymmetric DC SQUIDs in series 404. For each asymmetric DC SQUID in series 404, I wasc1 400 nanoamperes (nA) and I c2 800 nA. The capacitance C between plates 306A and 306B—which function as the shunt capacitor in the tunable resonator 402—was 175 femtofarads (fF). The series inductance of the 404 series was 0.1 nanohenry (nH). The tunable resonator 402 was configured to read and reset a qubit (not shown) whose resonant frequency f q 4.73 GHz.

[0062] Graph 408 shows the change in the resonance frequency of the tunable resonator 402 when Φ ext changes, where Φ ext The applied external flux passing through the ring of each DC SQUID is the x-axis of graph 408, representing the ratio Φ. ext / Φ0, where Φ0 is the flux quantum. The y-axis of graph 408 represents the unit resonance frequency as a function of the ratio Φ. ext / Φ0 on.

[0063] As graph 408 shows, resonance frequency maxima with a slope of zero occur when Φ ext is an integer multiple of Φ0. Resonance frequency minima with a slope of zero occur when Φ ext an odd multiple of CD0 / 2 in the positive or negative direction. The tunable resonator 402 is constructed with suitable asymmetric Josephson junctions and the capacitance C such that the resonance frequency minima with slope zero are at or nearly at f q appear.

[0064] If the tunable resonator 402 is varied by changing Φ extIf the system is tuned to a resonance frequency maximum with zero slope (far from the qubit frequency), e.g., at point 410 or another point of a resonance frequency maximum with zero slope (maximum frequency), the qubit can be read out as described here. In the computational example, which uses the configuration shown, a resonance frequency maximum with zero slope was reached at 8.071 GHz.

[0065] If the tunable resonator 402 is varied by changing Φ ext If the qubit is tuned to a resonance frequency minimum with zero slope, e.g., at point 412 or another point of a resonance frequency minimum with zero slope (minimum frequency), it can be reset as described here. In the computational example, which uses the configuration shown, a resonance frequency minimum with zero slope was reached at 4.73 GHz.

[0066] It will now be on Fig. Reference is made to Figure 5, which shows a further exemplary configuration of a tunable resonator for actively resetting and reading out another qubit according to an embodiment of the present invention. The tunable resonator 502 was essentially as described in Figure 5. Fig. The 404 series is configured as described above, but with a series inductance L = 0.8 nH. The 504 series is essentially configured like the 404 series, but with a different series inductance, for example, by changing the shape, size, or material of the superconducting wires. Plates 506A and 506B form the shunt capacitor essentially like plates 406A and 406B.

[0067] The tunable resonator 502 was configured to read out and reset a qubit (not shown) whose resonant frequency f q 4.5 GHz. Graph 508 shows the change in the resonance frequency of the tunable resonator 502 as a function of Φ, corresponding to graph 408.ext .

[0068] If the adjustable SQUID 502 is varied by changing Φ ext If the qubit is tuned to a point of a maximum with zero slope (far from the qubit frequency), for example point 510 or another point of a maximum with zero slope, the qubit can be read out as described here. In the computational example, which uses the configuration shown, a resonant frequency maximum with zero slope was reached at 7.038 GHz.

[0069] If the tunable SQUID 502 is varied by changing Φ ext Once the qubit is tuned to a resonance frequency minimum with zero slope (minimum), e.g., at point 512 or another point of a resonance frequency minimum with zero slope, it can be reset as described here. In the computational example, which uses the configuration shown, a resonance frequency minimum with zero slope was reached at 4.5 GHz.

[0070] It will now be on Fig. Reference is made to Figure 6, which shows a further exemplary configuration of a tunable resonator for actively resetting and reading out a superconducting qubit according to an embodiment of the present invention. Only as a non-limiting example, in a configured experiment, the tunable resonator 602 was used in the same way as the tunable resonator 300 of the Fig. 3 configured, with a single (N=1) asymmetric DC SQUID 604. When constructing a tunable resonator with a single asymmetric DC SQUID as in this case, there is a risk of possible hybridization effects between the energy states of the qubit and the qubit-like resonator. Such a configuration should be analyzed theoretically to verify that there are no undesired effects.

[0071] For the asymmetric DC SQUID 604, I was c1 40 nA and Ic2 80 nA. The capacitance C between the plates 606A and 606B – which function as the shunt capacitor in the tunable resonator 300 – was 175 fF. The series inductance of the asymmetric DC SQUID 604 was 0.1 nH. The tunable resonator 602 was configured to read and reset a qubit (not shown) whose resonant frequency f q 4.2 GHz.

[0072] In accordance with graphs 408 and 508, graph 608 shows the change in the resonance frequency of the tunable resonator 602 when Φ is changed. ext .

[0073] If the adjustable resonator 602 is varied by changing Φ extIf the system is tuned to a resonant frequency maximum with zero slope (far from the qubit frequency), for example point 610 or another resonant frequency maximum with zero slope, the qubit can be read out as described here. In the computational example, which uses the configuration shown, a resonant frequency maximum with zero slope was reached at 7.251 GHz.

[0074] If the tunable resonator 602 is varied by varying Φ ext Once the qubit is tuned to a resonant frequency minimum with zero slope, e.g., at point 612 or another point of a resonant frequency minimum with zero slope, it can be reset as described here. In the computational example, which uses the configuration shown, a resonant frequency minimum with zero slope was reached at 4.2 GHz.

[0075] An adjustable resonator in the form of a capacitively switched-mode asymmetric DC SQUID according to the invention is compact and requires little space. It can be manufactured using the same process as the qubits, provides a mechanism for fast qubit reset, has a higher internal Q factor (>2 M), exhibits low surface participation, and features high-Q Josephson junctions (higher Q than superconducting meandering inductors). Furthermore, the capacitively switched-mode asymmetric DC SQUID according to the invention eliminates the need for qubit reset using a fast-read resonator, as this device is known to shorten the lifetime T1 of the qubits.Furthermore, by using a capacitively shunted asymmetric DC SQUID designed according to the invention to perform the reset process (by bringing it into resonance with the qubit), the need for qubit measurement and feedback to reset the qubit state is also eliminated.

[0076] The invention can further be described, without limitation and only by way of example, by the following embodiments. The following embodiments may include preferred embodiments. Accordingly, the term "clause" used therein may refer to such a "preferred embodiment".

[0077] Clause 1. Superconducting unit that has the following features: a tunable resonator having a capacitive unit that parallels a group of asymmetric DC SQUIDs, wherein an asymmetric DC SQUID in the group has a first and a second Josephson junction, wherein a first critical current of the first Josephson junction differs from a second critical current of the second Josephson junction, a coupling between the resonator and a qubit such that the asymmetric DC SQUIDs can dispersively read out a quantum state of the qubit, and an external magnetic flux, wherein the flux is set to a first value and applied to the tunable resonator, wherein a first value of the flux causes the resonator to tune to a first frequency, wherein the first frequency is within a first frequency difference to a resonant frequency of the qubit, and wherein tuning the resonator to the first frequency causes an active reset of the qubit. Clause 2. Unit according to Clause 2, which further includes the following: the flux changed to a second value, wherein the second value causes the resonator to tune to a second frequency, wherein the second frequency is detuned relative to the resonance frequency of the qubit by at least a second frequency difference, and wherein tuning the resonator to the second frequency enables a dispersive readout operation of the quantum state of the qubit to be performed. Clause 3. Unit according to Clause 2, wherein the second frequency is a maximum frequency in a frequency resonance range of the resonator. Clause 4. Unit according to Clause 2, wherein the second frequency difference depends on a degree of asymmetry between the first and the second Josephson transition. Clause 5. Unit according to Clause 1, where the first frequency difference is zero and the first frequency is the resonant frequency of the qubit. Clause 6. Unit according to Clause 1, wherein, due to the fact that the first frequency lies within the first frequency difference to the resonance frequency of the qubit, a photon is emitted from the qubit, the emission causing the qubit to de-excite to a ground energy state. Clause 7. Unit according to Clause 6, wherein the forcing of the qubit into the ground energy state occurs faster than an energy decay time constant of the qubit. Clause 8. Unit according to Clause 1, wherein the group includes only the asymmetric DC SQUID. Clause 9. Unit according to Clause 1, which further includes the following: a series connection connecting several of the asymmetrical DC SQUIDs from the group. Clause 10. Unit according to Clause 9, wherein the series circuit includes a superconducting wire. Clause 11. Unit according to Clause 1, which further includes the following: a first tile formed on a first side of the group and a second plate formed on a second side of the group, wherein the first plate and the second plate are separated by a distance and wherein the first plate and the second plate together form the capacitive unit. Clause 12. Procedure, comprehensive: Forming a tunable resonator by parallel connecting a group of asymmetric DC SQUIDs with a capacitive unit, wherein an asymmetric DC SQUID in the group has a first and a second Josephson junction, wherein a first critical current of the first Josephson junction differs from a second critical current of the second Josephson junction, Coupling the resonator to a qubit such that the resonator can dispersively read out a quantum state of the qubit, and Active resetting of the qubit by applying an external magnetic flux of a first value to the tunable resonator, wherein a first value of the flux causes the resonator to tune to a first frequency, the first frequency being within a first frequency difference to a resonant frequency of the qubit. Clause 13. Procedure according to Clause 12, furthermore comprehensive: Changing the flux to a second value, wherein the second value causes the resonator to tune to a second frequency, the second frequency being detuned relative to the qubit's resonant frequency by at least a second frequency difference, and Performing a dispersive readout operation of the quantum state of the qubit using the resonator tuned to the second frequency. Clause 14. Procedure according to Clause 13, wherein the second frequency is a maximum frequency in a frequency resonance region of the resonator. Clause 15. Procedure according to Clause 13, wherein the second frequency difference depends on a degree of asymmetry between the first Josephson transition and the second Josephson transition. Clause 16. Procedure according to Clause 14, wherein the first frequency difference is zero and the first frequency is the resonance frequency of the qubit. Clause 17. Procedure according to Clause 12, wherein, due to the fact that the first frequency lies within the first frequency difference to the resonance frequency of the qubit, a photon is emitted from the qubit, the emission causing the qubit to return to a ground state. Clause 18. Procedure according to Clause 17, wherein the forcing of the qubit into the ground energy state occurs faster than an energy decay time constant T1 of the qubit. Clause 19. Procedure according to Clause 12, wherein the group includes only the asymmetric DC SQUID. Clause 20. Procedure according to Clause 12, furthermore comprehensive: Forming a series circuit from several asymmetrical DC SQUIDs from the group. Clause 21. Method according to Clause 20, wherein the multiple asymmetric DC SQUIDs are connected in series by means of a superconductor. Clause 22. Procedure according to Clause 12, furthermore comprehensive: Forming a first tile on a first side of the group and Forming a second plate on a second side of the group, wherein the first plate and the second plate are separated by a distance and wherein the first plate and the second plate together form the capacitive unit. Clause 23. Superconducting manufacturing system which, when operating to produce a tunable resonator, performs the steps of a process according to any one of Clauses 12 to 22.

Claims

[1] Superconducting unit which has the following features: a group of asymmetric DC SQUIDs, wherein one asymmetric DC SQUID in the group of asymmetric DC SQUIDs has a Josephson junction, and a capacitive unit that parallels the group of asymmetric DC SQUIDs, wherein an external magnetic flux is set to a first value and applied to a tunable resonator formed by the asymmetric DC SQUIDs and the capacitive unit, wherein the first value of the external magnetic flux causes the tunable resonator to tune to a first frequency, wherein the first frequency lies within a first frequency difference to a resonant frequency of the qubit, and wherein tuning the tunable resonator to the first frequency causes an active reset of a qubit coupled to the tunable resonator. the superconducting unit further comprises the following: the external magnetic flux changed to a second value, wherein the second value of the external magnetic flux causes the tunable resonator to tune to a second frequency, the second frequency being detuned relative to the qubit's resonance frequency by at least a second frequency difference, and wherein tuning the tunable resonator to the second frequency enables a dispersive readout operation of a quantum state of the qubit to be performed. [2] Superconducting unit according to claim 1, wherein the second frequency is a maximum frequency in a frequency resonance range of the tunable resonator. [3] Superconducting unit according to claim 1, wherein the second frequency difference depends on a degree of asymmetry between the Josephson junction and a second Josephson junction in the asymmetric DC SQUID. [4] Superconducting unit according to claim 1, wherein the first frequency difference is zero and the first frequency is the resonance frequency of the qubit. [5] Superconducting unit according to claim 1, wherein, due to the fact that the first frequency lies within the first frequency difference to the resonance frequency of the qubit, a photon is emitted from the qubit, the emission causing the qubit to de-excite to a ground energy state. [6] Superconducting unit according to claim 5, wherein the forcing of the qubit into the ground energy state occurs faster than an energy decay time constant of the qubit. [7] Superconducting unit according to claim 1, wherein the group of asymmetric DC SQUIDs includes only the asymmetric DC SQUID. [8] Superconducting unit according to claim 1, which further comprises: a series connection connecting several asymmetric DC SQUIDs from the group of asymmetric DC SQUIDs. [9] Superconducting unit according to claim 8, wherein the series connection comprises a superconducting wire. [10] Superconducting unit according to claim 1, which further comprises: a first plate formed on a first side of the group of asymmetric DC SQUIDs and a second plate formed on a second side of the group of asymmetric DC SQUIDs, wherein the first plate and the second plate are separated by a distance and wherein the first plate and the second plate together form the capacitive unit. [11] Procedure, encompassing: Forming a tunable resonator by parallel connecting a group of asymmetric DC SQUIDs with a capacitive unit, wherein one asymmetric DC SQUID in the group of asymmetric DC SQUIDs has a Josephson junction, and Active resetting of a qubit by applying an external magnetic flux of a first value to the tunable resonator, wherein the first value of the external magnetic flux causes the tunable resonator to tune to a first frequency, the first frequency being within a first frequency difference to a resonant frequency of the qubit, Changing the external magnetic flux to a second value, wherein the second value of the external magnetic flux causes the tunable resonator to tune to a second frequency, the second frequency being detuned relative to the qubit's resonant frequency by at least a second frequency difference, and Performing a dispersive readout operation of the quantum state of a qubit using the tunable resonator tuned to the second frequency. [12] Method according to claim 11, wherein the second frequency is a maximum frequency in a frequency resonance range of the tunable resonator. [13] Method according to claim 11, wherein the second frequency difference depends on a degree of asymmetry between the Josephson junction and a second Josephson junction in the asymmetric DC SQUID. [14] Method according to claim 11, wherein the first frequency difference is zero and the first frequency is the resonance frequency of the qubit. [15] Method according to claim 11, wherein, due to the fact that the first frequency lies within the first frequency difference to the resonance frequency of the qubit, a photon is emitted from the qubit, the emission causing the qubit to return to a ground state. [16] Method according to claim 15, wherein the forcing of the qubit into the ground energy state occurs faster than an energy decay time constant T1 of the qubit. [17] Method according to claim 11, wherein the group of asymmetric DC SQUIDs includes only the asymmetric DC SQUID. [18] The method of claim 11, further comprising: Forming a series circuit from several asymmetric DC SQUIDs from the group of asymmetric DC SQUIDs. [19] Method according to claim 18, wherein the multiple asymmetric DC SQUIDs are connected in series by means of a superconductor. [20] The method of claim 11, further comprising: Forming a first plate on a first side of the group of asymmetric DC SQUIDs and Forming a second plate on a second side of the group of asymmetric DC SQUIDs, wherein the first plate and the second plate are separated by a distance and wherein the first plate and the second plate together form the capacitive unit. [21] Superconducting manufacturing system which, when operated to produce a tunable resonator, performs the steps of a method according to any one of claims 11 to 20.

Images