Switching element

Abstract

According to an embodiment, a switching element (100) comprises an input signal line (101) for providing a common pulse sequence to the switching element (100); an output signal line (102) for providing the common pulse sequence to a qubit from the switching element (100); a control signal line (103) for providing a control signal to the switching element (100); and at least one superconducting controllable connecting element electromagnetically coupled to the control signal line (103) and configured to control transfer of the common pulse sequence from the input signal line (101) to the output signal line (102) based on the control signal.

Description

SWITCHING ELEMENTTECHNICAL FIELD

[0001] The present disclosure relates to a quantum computing, and more particularly to a switching element , a qubit control arrangement , a quantum computing arrangement , and a method .BACKGROUND

[0002] Many implementations of quantum computing can require high speed bit pattern generation by cryogenic electronics for direct qubit driving . However, such approaches can suffer from fabrication limits and accomplishing sufficiently high speed, high drive power, low noise and dissipation is very challenging .SUMMARY

[0003] This summary is provided to introduce a selection of concepts in a s implif ied form that are further described below in the detailed description . This summary is not intended to identify key features or essential features of the claimed subj ect matter, nor is it intended to be used to limit the scope of the claimed subj ect matter .

[0004] It i s an obj ective to provide a switching el ement , a qubit control arrangement , a quantum computing arrangement , and a method . The foregoing and other obj ectives are achieved by the features of the independentclaims . Further implementation forms are apparent from the dependent claims , the description and the figures .

[0005] According to a first aspect , a switching element comprises an input signal line for providing a common pulse sequence to the switching element ; an output signal line for providing the common pulse sequence to a qubit from the switching element ; a control signal line for providing a control signal to the switching element ; and at least one superconducting controllable connecting element electromagnetically coupled to the control signal line and configured to control transfer of the common pulse sequence from the input signal line to the output signal line based on the control signal .

[0006] In an implementation form of the first aspect , the at least one superconducting controllable connecting element comprises at least one superconducting interference device ( SQUID) .

[0007] In another implementation form of the first aspect , the control signal line is configured to control a Josephson inductance of the at least one SQUID based on the control signal and the at least one SQUID is configured to control the transfer of the common pulse sequence from the input signal line to the output signal line based on the Josephson inductance of the at least one SQUID .

[0008] In another implementation form of the first aspect , the control signal line is configured to control the Josephson inductance of the at least one SQUID viacontrolling a magnetic flux through the at least one SQUID based on the control signal .

[0009] According to a second aspect , a qubit control arrangement comprises : a qubit ; a plurality of switching elements , wherein each switching element in the plurality of switching elements comprises a switching element according to the first aspect and the output signal line of each switching element is electromagnetically coupled to the qubit ; and a control unit electromagnetically coupled to the control signal line of each switching element in the plurality of switching elements and configured to provide the control signal to each control signal line .

[0010] In an implementation form of the second aspect , the control unit comprises an adiabatic quantum-flux- parametron (AQFP) based control logic .

[0011] In another implementation form of the second aspect , the control unit is configured to provide the control signal to each control signal line in such a manner that a critical current of the at least one SQUID of each switching element is not exceeded .

[0012] In another implementation form of the second aspect , a clock rate of the control unit i s conf igured to be less than 1 gigahertz .

[0013] In another implementation form of the second aspect , the control signal is synchroni zed with the common pulse sequence .

[0014] In another implementation form of the second aspect , the control unit is configured to perform single-qubit gates using the qubit via the control signals .

[0015] According to a third aspect , a quantum computing arrangement comprises : a plurality of qubit control arrangements , wherein each qubit control arrangement in the plurality of qubit control arrangements comprises a qubit control arrangement according to the second aspect ; and a plurality of signal sources , wherein each signal source in the plurality of signal sources i s electromagnetically coupled to the input signal line of a corresponding switching element in each qubit control arrangement in the plurality of qubit control arrangements and configured to provide a corresponding common pulse sequence to the input signal line of the corresponding switching element in each qubit control arrangement in the plurality of qubit control arrangements .

[0016] In an implementation form of the third aspect , the plurality of signal sources comprises a first signal source configured to provide a first common pulse sequence and a second signal source configured to provide a second common pulse sequence , wherein a phase difference between the first common pulse sequence and the second common pulse sequence is substantially 90 degrees .

[0017] In another implementation form of the third aspect , the plurality of signal sources further comprises a third signal source configured to provide athird common pulse sequence and a phase dif ference between the first common pulse sequence and the third common pulse sequence is substantially 180 degrees and / or the plurality of signal sources further comprises a fourth signal source configured to provide a fourth common pulse sequence and a phase difference between the second common pulse sequence and the fourth common pulse sequence is substantially 180 degrees .

[0018] In another implementation form of the third aspect , the plurality of qubit control arrangements are arranged inside a cryostat and the plurality of signal sources are arranged outside the cryostat .

[0019] In another implementation form of the third aspect , each signal source in the plurality of signal sources is configured to provide a scalable leakage optimi zed pulse sequence .

[0020] According to a fourth aspect , a method for performing single-qubit gates using a qubit and a plurality of switching elements , wherein each switching element in the plurality of switching elements comprises a switching element according to the first aspect and the output signal line of each switching element is elec- tromagnetically coupled to the qubit , the method comprising performing single-qubit gates using the qubit by controlling transfer of common pulse sequences to the qubit via the plurality of switching elements by providing a control signal to the control signal line of each switching element .

[0021] Many of the attendant features wil l be more readily appreciated as they become better understood by reference to the following detailed description considered in connection with the accompanying drawings .DESCRIPTION OF THE DRAWINGS

[0022] In the following, example embodiments are described in more detail with reference to the attached figures and drawings , in which :

[0023] Fig . 1 illustrates a schematic representation of a switching element according to an embodiment ;

[0024] Fig . 2 illustrates a schematic representation of a switching element according to another embodiment ;

[0025] Fig . 3 illustrates a schematic representation of a switching element according to another embodiment ;

[0026] Fig . 4 illustrates a schematic representation of a switching element according to another embodiment ;

[0027] Fig . 5 illustrates a schematic representation of qubit control arrangement according to an embodiment ;

[0028] Fig . 6 illustrates a schematic representation of quantum computing arrangement according to an embodiment ;

[0029] Fig . 7 illustrates a schematic representation of quantum computing arrangement according to another embodiment ;

[0030] Fig . 8 illustrates a flow chart representation of a method according to an embodiment ;

[0031] Fig . 9 illustrates a plot representation of simulation results according to an embodiment ;

[0032] Fig . 10 illustrates a schematic representation of a switching element according to another embodiment ;

[0033] Fig . 11 illustrates a schematic representation of an AQFP buffer coupled to a switching element according to an embodiment ;

[0034] Fig . 12 illustrates a schematic representation of an AQFP buffer coupled to a switching element according to another embodiment ;

[0035] Fig . 13 illustrates a schematic representation of an AQFP buffer coupled to a switching element according to another embodiment ; and

[0036] Fig . 14 illustrates a schematic representation of a switching element according to another embodiment .

[0037] In the following, like reference numerals are used to designate like parts in the accompanying draw- rngs .DETAILED DESCRIPTION

[0038] In the following description, reference is made to the accompanying drawings , which form part of the disclosure , and in which are shown, by way of illustration, specific aspects in which the present disclosure may be placed . I t i s understood that other aspects may be utilised, and structural or logical changes may be made without departing from the scope of the present disclosure . The following detailed description, therefore , is not to be taken in a limiting sense , as thescope of the present disclosure is defined be the appended claims .

[0039] For instance , it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa . For example , if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or il lustrated in the figures . On the other hand, for example , if a specific apparatus is described based on functional units , a corresponding method may include a step performing the described functionality, even if such step is not explicitly described or illustrated in the figures . Further, it is understood that the features of the various example aspects described herein may be combined with each other, unless specifically noted otherwise .

[0040] Fig . 1 illustrates a schematic representation of a switching element according to an embodiment .

[0041] According to an embodiment , a switching element 100 comprises an input signal line 101 for providing a common pulse sequence to the switching element 100 .

[0042] Any signal line disclosed herein may comprise , for example , a coaxial cable , any type of transmiss ion line , a transmission line on a printed circuit board, or similar .

[0043] The common pulse sequence may comprise , for example , a sequence of pulses that can be used to controla qubit . The pulses may comprise voltage pulses . For example, in some embodiments , the common pulse sequence may comprise trains of single flux quantum ( SFQ) pulses .

[0044] The common pulse sequence may also be referred to as a common pulse signal , a common pulse sequence signal , a common pulse pattern, or similar .

[0045] The switching element 100 may further comprise an output signal line 102 for providing the common pul se sequence to a qubit from the switching element 100 .

[0046] The providing the common pulse sequence to the qubit from the switching element 100 may comprise providing the common pul se sequence based on the functionality of the switching element 100 . For example , the output signal line 102 may provide the common pulse sequence to the qubit only when the switching element 100 i s configured to provide the common pulse sequence to the qubit, such as when the switching element 100 is in an on-state , and / or i f the switching element 100 is configured to modulate the common pulse sequence in some manner, the output signal line 102 may provide the modulated common pulse sequence to the qubit .

[0047] The switching element 100 may further comprise a control signal line 103 for providing a control signal to the switching element 100 .

[0048] The control signal may comprise , for example , a current and / or a voltage that can be used to control the switching element 100 . The control signal may also be referred to as a modulation signal , an envelope signal , or similar .

[0049] The switching element 100 may further comprise at least one superconducting controllable connecting element electromagnetically coupled to the control signal line and configured to control transfer of the common pulse sequence from the input signal line to the output signal line based on the control signal .

[0050] The at least one superconducting controllable connecting element may be , for example , inductively, capacitively, and / or galvanically coupled to the control signal line .

[0051] The control signal may comprise a time-dependent control signal . Thus , the switching element 100 can be used to modulate the common pulse sequence based on the time-dependent control signal before the common pulse sequence is provided to a qubit from the switching element 100 .

[0052] The at least one superconducting controllable connecting element may comprise any element that can be used to control the transfer of the common pulse sequence from the input s ignal line to the output signal line based on the control signal . Examples of such superconducting controllable connecting element are disclosed in the embodiments herein .

[0053] The at least one superconducting controllable connecting element may also be referred to as an active superconducting element , a superconducting controllable connector, or similar .

[0054] For example , in some embodiments , the at least one superconducting controllable connecting element maycomprise at least one Josephson j unction and the control signal line 103 can be configured to control / modu- late / modify the Josephson inductance of the at least one Josephson j unction . Thus , as the Josephson inductance of the at least one Josephson j unction changes , the transfer of the common pulse sequence from the input signal line to the output signal line can be controlled .

[0055] The switching element 100 may not be able to total ly stop the trans fer of the common pulse sequence from the input signal line to the output signal line when the switching element is switched off but there may be some about of leakage . Instead, the switching element 100 may, for example , modulate the transfer of the common pulse sequence from the input signal line to the output signal line based on the control signal .

[0056] The switching element 100 may be embodied in, for example , a quantum computing device . Such a quantum computing device may comprise a plurality of qubits for performing quantum computation . A switching element 100 can be used to , for example , control the transfer of a common pulse sequence to each qubit .

[0057] The arrangement 100 may be reali zed, for example , in a superconducting circuit architecture .

[0058] The switching element 100 can enable gate level control of qubits , which can lead to a reduction in the number of coaxial cables needed in a quantum computing arrangement .

[0059] The switching element 100 can enable improving quantum processing unit (QPU) control electronics scalability and reducing the number of coaxial cables that contribute to the heat load of the QPU .

[0060] Fig . 2 illustrates a schematic representation of a switching element according to another embodiment .

[0061] According to an embodiment , the at least one superconducting controllable connecting element comprises at least one superconducting interference device ( SQUID) .

[0062] The at least one SQUID can be used to implement a f lux-tuneable fi lter, which can be used to implement a low-loss or even lossless microwave switch as the switching element 100 .

[0063] The at least one SQUID may also be referred to as at least one SQUID loop or similar .

[0064] The at least one SQUID may comprise , for example , a direct current (de ) SQUID and / or an alternating current ( ac) SQUID . An ac-SQUID may also be referred to as a radio frequency (RF) SQUID .

[0065] According to an embodiment , the control signal line is configured to control a Josephson inductance of the at least one SQUID based on the control s ignal and the at least one SQUID is configured to control the transfer of the common pulse sequence from the input signal l ine to the output signal line based on the Josephson inductance of the at least one SQUID .

[0066] For example , the at least one SQUID can be used to implement such a circuit topology that the Josephsoninductance of the at least one SQUID affects the transfer of the common pul se sequence from the input signal line 101 to the output signal line 102 .

[0067] According to an embodiment , the control signal line 103 is configured to control the Josephson inductance of the at least one SQUID via controlling a magnetic flux through the at least one SQUID based on the control signal .

[0068] The controlling the Josephson inductance of the at least one SQUID via control ling a magnetic flux through the at least one SQUID may be referred to as flux tuning .

[0069] For example , in the embodiment of Fig . 2 , the switching element 100 comprises a SQUI D 201 and an inductive element 202 inductively coupled to the SQUID 201 as indicated by the double headed arrow in Fig . 2 . The inductive element 202 can be , for example , galvanically coupled to the control signal line 103 . Thus , the control signal in the control signal line 103 can be used to modulate the magnetic flux through the SQUID 201 . The magnetic flux through the SQUID 201 can in turn modulate the Josephson inductance of the Josephson j unctions 203 of the SQUID 201 . The change in the Josephson inductance can affect the transfer of the common pulse sequence from the input signal line 101 to the output signal line 102 .

[0070] Although some embodiments disclose herein may illustrate a SQUID 201 comprising two Josephson j unctions 203 , a SQUID may also comprise a different numberof Josephson junctions. For example, an ac-SQUID may comprise one Josephson junction. A SQUID comprising two Josephson junctions may be referred to as a dc-SQUID.

[0071] Fig. 3 illustrates a schematic representation of a switching element according to another embodiment.

[0072] In the embodiment of Fig. 3, the switching element 100 comprises a plurality of SQUIDs 201. Each SQUID can be inductively coupled to a corresponding inductive element 202. Each inductive element 202 can be, for example, galvanically coupled to the control signal line 103. Thus, the control signal in the control signal line 103 can be used to modulate the magnetic flux through each SQUID 201.

[0073] By increasing the number of SQUIDs 201, the magnitude of the switching action performed by the switching element 100 can be increased.

[0074] Another reason for increasing the number of SQUIDs and / or Josephson junctions in the switching element 100 may be to increase the amplitude range of currents that can be switched while keeping the microwave design parameters substantially constant. Seriesparallel circuits can be used to increase current / power handling of the circuit without exceeding the critical current of any Josephson junction. Increasing the number of Josephson junctions may also be necessary for wider bandwidth devices.

[0075] In other embodiments, the switching element may comprise any number of SQUIDs 201. For example, thecircuit topology illustrated in the embodiment of Fig. 3 may be expanded for any number of SQUIDs 201.

[0076] Fig. 4 illustrates a schematic representation of a switching element according to another embodiment.

[0077] In the embodiment of Fig. 4, the switching element 100 comprises a T-circuit between the input signal line 101 and the output signal line 102. The bottom branches of the T-circuit comprise Josephson junctions 401 and the control signal line 103 is galvanically coupled to the Josephson junctions 401 via inductive elements 402.

[0078] In other embodiments, the switching element 100 may comprise any number of Josephson junctions.

[0079] Using a galvanic connection for the control signal line 103 may be beneficial if, for example, efficient magnetic coupling is difficult to arrange otherwise due to, for example, fabrication or geometry related issues.

[0080] Coupling between the input signal line 101 and the output signal line 102 can be proportional to the current of the control current signal flowing through the Josephson junction 401. Current in the Josephson junctions in turn corresponds to modulation of the Josephson inductance of the Josephson junction.

[0081] In the embodiments of Figs. 2 and 3, the control signal can be magnetically coupled to the at least one SQUID 201 without galvanic connections in order to tune the coupling between the input signal line 101 and the output signal line 102. In the embodiment of Fig.4 , a nonmagnetic galvanic coupling can also accomplish a similar functionality .

[0082] One benefit of the magnetic coupl ing of the embodiments of Figs . 2 and 3 may be that the current of the control signal and the current of the common pulse sequence can be orthogonal so the paths do not disturb each other directly .

[0083] The modulated common pulse sequence may also flow through the Josephson j unctions ( s ) 401 and the common pulse sequence may need to be small compared to the control signal so that the switching action of the switching element 100 remains approximately linear .

[0084] Fig . 5 illustrates a schematic representation of qubit control arrangement according to an embodiment .

[0085] According to an embodiment , a qubit control arrangement 500 comprises a qubit 501 .

[0086] The qubit 501 may comprise , for example , a superconducting qubit , a charge qubit , a flux qubit , a split-Cooper-pair-box charge qubit , a unimon qubit , a transmon qubit , and / or any other type of qubit .

[0087] The qubit control arrangement 500 may further comprise a plurality of switching elements 100 , wherein each switching element in the plurality of switching elements 100 is electromagnetically coupled to the qubit 501 .

[0088] Each switching element in the plurality of switching elements 100 may be , for example , galvanically, capacitively, and / or inductively coupled to the qubit 501 . The type of the electromagnetic coupling maydepend on, for example , the structure / type of the qubit 501 .

[0089] The output signal line of each switching element in the plurality of switching elements 100 can be electromagnetically coupled to the qubit 501 .

[0090] The qubit control arrangement 500 may further comprise a control unit 502 electromagnetically coupled to the control signal line 103 of each switching element 100 in the plurality of switching elements and configured to provide the control signal to each control signal line 103 .

[0091] Thus , the control unit 502 can control how each common pulse sequence is modulated by each switching element 100 .

[0092] According to an embodiment , the control unit 502 is configured to modulate a common pulse sequence of each input signal line using a corresponding switching element in the plurality of switching elements .

[0093] The control unit 502 may comprise at least one processor . The at least one processor may comprise , for example , one or more of various processing devices , such as a co-processor, a microprocessor, a control unit 502 , a digital signal processor ( DSP) , a processing circuitry with or without an accompanying DSP, or various other processing devices including integrated circuits such as , for example , an application specific integrated circuit (AS IC) , a field programmable gate array ( FPGA) , a microprocessor unit (MCU) , a hardware accelerator, a special-purpose computer chip, or the like .

[0094] The control unit 502 may further comprise a memory . The memory may be configured to store , for example , computer programs and the like . The memory may comprise one or more volatile memory devices , one or more non-volatile memory devices , and / or a combination of one or more volatile memory devices and non-volatile memory devices . For example , the memory 802 may be embodied as magnetic storage devices ( such as hard disk drives , floppy disks , magnetic tapes , etc . ) , optical magnetic storage devices , and semiconductor memories ( such as mask ROM, PROM (programmable ROM) , EPROM (erasable PROM) , flash ROM, RAM ( random access memory) , etc . ) .

[0095] The control unit 502 may further comprise other components . The control unit 502 may comprise , for example , an input / output bus for connecting the control unit 502 to other units / devices . Further, a user may control the control unit 502 via the input / output bus . The user may, for example , control quantum computation operations performed by the qubit control arrangement 500 via the control unit 502 and the input / output bus .

[0096] When the control unit 502 is configured to implement some functionality, some component and / or components of the control unit 502 , such as the at least one processor and / or the memory, may be configured to implement this functionality . Furthermore , when the at least one processor is configured to implement some functionality, this functionality may be implementedusing program code comprised, for example , in the memory .

[0097] The control unit 502 may be implemented using, for example , a computer, an AS IC, a microcontroller, some other computing device , or similar .

[0098] For example , in the embodiment of Fig . 5 , a qubit control arrangement 500 comprises a qubit 501 and two switching elements 100 each electromagnetically coupled to the qubit 501 . A control unit 502 is electromagnetically coupled to the control signal l ine 103 of each switching element 100 and configured to provide the control signal to each control signal line 103 . Thus , the control unit 502 can modulate common pulse sequences provided via the input signal lines 101 of the switching elements 100 before the common pulse sequences are fed into the qubit 501 .

[0099] According to an embodiment , the control unit 502 comprises an adiabatic quantum-flux-parametron (AQFP) based control logic .

[0100] The AQFP control logic may be configured to provide the control signal to each control signal l ine 103 .

[0101] According to an embodiment , the control unit 502 is configured to provide the control signal to each control signal line in such a manner that a critical current of the at least one SQUID of each switching element 100 is not exceeded .

[0102] Herein, a critical current of a SQUID 201 may correspond to the critical current of the at least one Josephson j unction 203 of the SQUID 201 .

[0103] By not exceeding the critical current of the at least one SQUI D, phase slip associated noi se can be reduced or even eliminated . Further, by not exceeding the critical current of the at least one SQUID, the switching element 100 can be insensitive to reflections in comparison to , for example , active switches where reflections can cause additional switching and phase slips .

[0104] According to an embodiment , a clock rate of the control unit is configured to be less than 1 gigahertz .

[0105] In other embodiments , the clock rate of the control unit 502 may be configured to be less than 900 megahert z (MHz ) , les s than 800 MHz , les s than 500 MHz , less than 300 MHz , less than 200 MHz , or less than 100 MHz .

[0106] The clock rate of the control unit 502 can comprise an AQFP clock rate . Reducing the AQFP clock rate can reduce dissipation .

[0107] Reducing the clock rate can enable control through twisted pairs with microcontrollers instead of , for example , coaxial cables and FPGAs .

[0108] Reducing the clock rate can also considerably reduce AQFP dissipation and can be beneficial for operation at the millikelvin temperatures in the mixing chamber of a dilution refrigerator .

[0109] According to an embodiment, the control signal is synchronized with the common pulse sequence.

[0110] The clock rate of the control unit 502, such as the AQFP clock rate, and the modulation frequency of the control signal may need to be such that it comprises an integer number of qubit cycles so that they can be synchronized. The AQFP clock rate can run at the same or higher rate as the targeted gate rate of the qubit 501. In some embodiments, one AQFP clock cycle may correspond to one qubit gate. In other embodiments, the AQFP clock rate can be greater than the gate rate of the qubit 501.

[0111] According to an embodiment, the control unit 502 is configured to perform single-qubit gates using the qubit 501 via the control signals.

[0112] The control unit can, for example, modulate the common pulse sequences using the switching elements 100 in order to control what type of single-qubit gate is performed on the qubit 501.

[0113] Herein single-qubit gates may also be referred to as qubit rotations or similar.

[0114] Fig. 6 illustrates a schematic representation of quantum computing arrangement according to an embodiment .

[0115] According to an embodiment, quantum computing arrangement 600 comprises a plurality of qubit control arrangements 500.

[0116] The quantum computing arrangement 600 may further comprise a plurality of signal sources 601, 602,wherein each signal source in the plurality of signal sources is electromagnetically coupled to the input signal line 101 of a corresponding switching element 100 in each qubit control arrangement 500 in the plurality of qubit control arrangements and configured to provide a corresponding common pulse sequence to the input signal line 101 of the corresponding switching element in each qubit control arrangement 500 in the plurality of qubit control arrangements.

[0117] According to an embodiment, the plurality of signal sources 601, 602 comprises a first signal source 601 configured to provide a first common pulse sequence and a second signal source 602 configured to provide a second common pulse sequence, wherein a phase difference between the first common pulse sequence and the second common pulse sequence is substantially 90 degrees.

[0118] Herein, substantially 90 degrees may mean, for example, 89 - 91 degrees, 89.5 - 90.5 degrees, 89.9 - 90.1 degrees or 89.99 - 90.01 degrees.

[0119] For example, in the embodiment of Fig. 6, the quantum computing arrangement 600 comprises two qubit control arrangements 500 and two signal sources 601, 602. Each qubit control arrangement 500 comprises two switching elements 100 and each switching element 100 is electrically coupled to a corresponding signal source 601, 602.

[0120] In the embodiment of Fig. 6, a first signal source 601 may provide a +X common pulse sequence and a second signal source 602 may provide a +Y common pulsesequence. Thus, the +X and +Y common pulse sequences can be utilized for both qubit control arrangements 500 and the control unit 502 of each qubit control arrangement 500 can modulate each common pulse sequence before the common pulse sequences are fed into the qubits 501. For example, by modulating the common pulse sequences, the control units 502 can control what single-qubit gates are executed on the qubits 501. Thus, each qubit control arrangement 500 can perform single-qubit gates independently of each other while using the same common pulse sequences.

[0121] Fig. 7 illustrates a schematic representation of quantum computing arrangement according to another embodiment .

[0122] According to an embodiment, the plurality of signal sources further comprises a third signal source 701 configured to provide a third common pulse sequence and a phase difference between the first common pulse sequence and the third common pulse sequence is substantially 180 degrees and / or the plurality of signal sources further comprises a fourth signal source 702 configured to provide a fourth common pulse sequence and a phase difference between the second common pulse sequence and the fourth common pulse sequence is substantially 180 degrees.

[0123] Herein, substantially 180 degrees may mean, for example, 179 - 181 degrees, 179.5 - 180.5 degrees, 179.9180.1 degrees or 179.99 - 180.01 degrees.

[0124] For example , in the embodiment of Fig . 7 , the quantum computing arrangement 700 comprises three qubit control arrangements 500 and four signal sources 601 , 602 , 701 , 702 . Each qubit control arrangement 500 comprises four switching elements 100 and each switching element is electrically coupled to a corresponding signal source .

[0125] In the embodiment of Fig . 7 , a first signal source 601 may provide a +X common pulse sequence , a second s ignal source 602 may provide a +Y common pulse sequence , a third signal source 701 may provide a -X common pulse sequence , and a fourth signal source 702 may provide a -Y common pulse sequence . Thus , the +X, +Y, -X, and -Y common pulse sequences can be utili zed in all three qubit control arrangements 500 and the control unit 502 of each qubit control arrangement 500 can modulate each common pulse sequence before the common pulse sequences are fed into the qubits 501 . For example , by modulating the common pulse sequences , the control units 502 can control what single-qubit gates are executed on the qubits 501 .

[0126] Using common pulse sequences with a 180 degree phase difference can compensate for leakage in the switching elements 100 since such common pulse sequences can interfere destructively . Here , leakage refers to any signal that passes through the switching elements 100 when the switching elements 100 are switched off by the control signal .

[0127] According to an embodiment , the plurality of qubit control arrangements are arranged inside a cryostat and the plurality of signal sources are arranged outside the cryostat .

[0128] Since the plurality of qubit control arrangements can be used to modulate the common pulse sequences while the plurality of signal sources are arranged outside the cryostat , noi se and j itter issues that the plurality of signal sources could cause inside the cryostat can be reduced .

[0129] Since each common pulse sequence may need to be transmitted via a coaxial cable from room temperature into the cryostat , the quantum computing arrangement can reduce the number of coaxial cables needed, s ince each common pulse sequence can be used for a plurality of qubits 501 by modulating the common pulse sequence using the qubit control arrangements 500 in the cryostat .

[0130] According to an embodiment , each signal source in the plurality of signal sources is configured to provide a scalable leakage optimi zed pulse sequence .

[0131] Each signal source in the plurality of signal sources may be configured to provide the scalable leakage optimi zed pulse sequence as the common pulse sequence .

[0132] A scalable leakage optimi zed pulse sequence ( SCALLOPS ) can be built up from short subsequences comprising, for example , 35-55 class ical bits that are repeatedly provided . Leakage can be minimi zed at the subsequence level . Because the subsequences can be short ,it is possible to perform efficient search over the subsequence space in order to optimi ze gate fidelity .

[0133] For example , 1 -bit SCALLOPS like bit patterns can be generated using the signal sources at room temperature where generation of high power, high signal- to-noise ratio ( SNR) s ignals can be easier, and no low latency, very high speed memories or parallel-to-serial converters need to be designed for the AQFP control logic . The common pulse sequences can be split for a plurality of switching elements in the cryostat and by using, for example , baluns differential output can be made . Individual qubit drivers in the cryostat may each comprise , for example , four switching elements that allow individual bit patterns to pass to the qubit or be blocked . Thus , drive lines and room temperature electronics costs can be reduced .

[0134] The common pulse sequences , such as SCALLOPS like bit patterns , have an intrinsic periodicity that may need to be synchroni zed with the AQFP envelope modulator for the leakage minimi zation mechanism to be most effective . Common clocking even at 100 MHz can accomplish sufficient phase accuracy with 25 Gbps common pulse sequences .

[0135] Each modulation pulse of the control signal should pick an equal amount of energy from the common pulse sequence and the modulation should not cause spurious frequency content to be generated and it should preserve deterministic phase . This can be accomplishedeven if there is a slow on / off transition in the control signal .

[0136] Fig . 8 illustrates a flow chart representation of a method according to an embodiment .

[0137] According to an embodiment , a method 800 for performing single-qubit gates us ing a qubit and a plurality of switching elements , wherein and the output signal line of each switching element is electromag- netically coupled to the qubit , comprises performing 801 single-qubit gates using the qubit by controlling transfer of common pulse sequences to the qubit via the plurality of switching elements by providing a control signal to the control signal line of each switching element .

[0138] The method 800 may be performed by, for example , the control unit 502 .

[0139] Fig . 9 illustrates a plot representation of simulation results according to an embodiment .

[0140] In the embodiment of Fig . 9 , curves 901 and902 corresponds to an S-parameter from the input signal line to the output signal line of the switching element 100 with the control current on and the control current off , respectively . The frequency illustrated on the hori zontal axis of the figure correspond to tuning the magnetic flux through the SQUIDs of the switching element from 0 to 0.25<I)o, where (Pois the magnetic flux quantum .

[0141] Fig . 10 illustrates a schematic representation of a switching element according to another embodiment .

[0142] In the embodiment of Fig . 10 , the switching element 100 comprises two SQUIDs 201 , each SQUID 201 inductively coupled to the input signal line 103 . Any disclosure herein in relation to the embodiments of Figs . 2 and 3 may also apply to the embodiment of Fig . 10 .

[0143] Fig . 11 illustrates a schematic representation of an AQFP buffer coupled to a switching element according to an embodiment .

[0144] Fig . 12 illustrates a schematic representation of an AQFP buffer coupled to a switching element according to another embodiment .

[0145] Fig . 13 illustrates a schematic representation of an AQFP buffer coupled to a switching element according to another embodiment .

[0146] In the embodiments of Figs . 11 - 13 , an AQFP buffer 1101 comprises a clock signal source 1102 and an input 1103 . The AQFP buffer 1101 can be embodied in, for example , the control unit 502 . For example , the AQFP buffer 1101 can be part of AQFP control logic of the control unit 502 . The input 1103 of the AQFP buffer can be used to modulate the clock signal provided by the signal source 1102 and the modulated signal can be used as the control signal for the switching element 100 .

[0147] In the embodiments of Figs . 11 and 12 , the AQFP buffer 1101 is magnetically / inductively coupled 1104 to the control signal l ine 103 . In the embodiment of Fig . 13 , the AQFP buffer 1101 is galvanically coupled to the control signal line 103 . In the embodiment of Fig . 11 ,the AQFP buffer 1101 comprises a further magnetic / in- ductive coupling 1105 .

[0148] Fig . 14 illustrates a schematic representation of a switching element according to another embodiment .

[0149] According to an embodiment , the switching element 100 comprises a first signal path and a second signal path between the input signal line 101 and the output signal line 102 and the at least one superconducting controllable connecting element is configured to control the transfer of the common pulse sequence from the input signal line 101 to the output signal line 102 based on the control s ignal by control ling a phase difference between the first signal path and the second signal path .

[0150] According to an embodiment , the at least one superconducting controllable connecting element is configured to control the phase difference between the first signal path and the second signal path by modifying an inductance of the first signal path and / or of the second signal path based on the control signal .

[0151] For example , in the embodiment of Fig . 14 , the switching element 100 comprises a first signal path 1411 and a second signal path 1412 . The input signal line 101 is coupled to both paths via a transformer 1401 . The transformer 1401 is configured to couple the common pulse sequence from the input signal line 101 to the first signal path 1411 and the second s ignal path 1412 with a phase difference . The phase difference can be , for example , substantially 180 degrees . The first signalpath 1411 comprises a SQUI D 201 inductively coupled to the control signal l ine 103 and the second signal path 1412 comprises an inducive element 1402 . Any disclosure herein in relation to the SQUID 201 may also apply to this embodiment . I f the inductance of the SQUID 201 is tuned to substantially match the inductance of the inductive element 1402 , the signals from the first signal path 1411 and the second signal path 1412 can interfere destructively and thus isolation between the input signal line 101 and the output signal line 102 can be achieved . I f the inductance of the SQUID 201 is tuned to not substantially match the inductance of the inductive element 1402 , the signals from the two paths do not interfere destructively and the common pulse sequence can propagate from the input signal line 101 to the output signal line 102 . Thus , the switching element 100 can be switched on and off using the control signal .

[0152] In other embodiments , the phase difference between the f irst s ignal path 1411 and the second signal path 1412 can be something other than substantially 180 degrees . For example , in some embodiments , the common pulse sequence can be coupled in the same phase into the first signal path 1411 and the second signal path 1412 . The SQUID 201 can then be used to tune the signals in the two paths to destructively interfere when isolation is needed . Alternatively, the phase difference can be something other than 180 degrees and the inductance of the SQUID 201 can be tuned to achieve destructive interference to switch the switching element 100 into an off state .

[0153] In other embodiments , the SQUID 201 can be replaced with a Josephson j unction and the inductance of the Josephson j unction can be tuned similarly to , for example , the embodiment of Fig . 4 . In some embodiments the SQUIDs 201 can be replaced with a plurality of SQUIDs in series and the control signal line 103 can be coupled to each SQUID in the plurality of SQUIDs .

[0154] In some embodiments , the transformer 1401 can be replaced with any other component , such as an LC circuit , that can achieve coupling to the two signal paths .

[0155] The switching element 100 illustrated in the embodiment of Fig . 14 can be combined with any other type of switching element disclosed herein . Combining switching elements of different types may be useful or necessary to achieve sufficiently high on / off ratio .

[0156] Any range or device value given herein may be extended or altered without losing the effect sought . Also any embodiment may be combined with another embodiment unless explicitly disallowed .

[0157] Although the subj ect matter has been described in language specific to structural features and / or acts , it is to be understood that the subj ect matter defined in the appended claims is not necessarily limited to the specific features or acts described above . Rather, the specific features and acts described above are disclosed as examples of implementing the claims and other equivalent features and acts are intended to be within the scope of the claims .

[0158] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments . The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benef its and advantages . It wi ll further be understood that reference to ' an ' item may refer to one or more of those items .

[0159] The steps of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate . Additionally, individual blocks may be deleted from any of the methods without departing from the spirit and scope of the subj ect matter described herein . Aspects of any of the embodiments described above may be combined with aspects of any of the other embodiments described to form further embodiments without losing the effect sought .

[0160] The term ' comprising ' is used herein to mean including the method, blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements .

[0161] It will be understood that the above description is given by way of example only and that various modif ications may be made by those s kil led in the art . The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments . Although various embodiments havebeen described above with a certain degree of particularity, or with reference to one or more individual embodiments , those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this specification .

Claims

CLAIMS :

1. A switching element (100) comprising: an input signal line (101) for providing a common pulse sequence to the switching element (100) ; an output signal line (102) for providing the common pulse sequence to a qubit from the switching element(100) ; a control signal line (103) for providing a control signal to the switching element (100) ; and at least one superconducting controllable connecting element electromagnetically coupled to the control signal line (103) and configured to control transfer of the common pulse sequence from the input signal line(101) to the output signal line (102) based on the control signal.

2. The switching element (100) according to claim 1, wherein the at least one superconducting controllable connecting element comprises at least one superconducting interference device, SQUID, (201) .

3. The switching element (100) according to claim 2, wherein the control signal line (103) is configured to control a Josephson inductance of the at least one SQUID (201) based on the control signal and the at least one SQUID (201) is configured to control the transfer of the common pulse sequence from the input signal line (101) to the output signal line (102) based on the Josephson inductance of the at least one SQUID (201) .

4. The switching element (100) according to claim 3, wherein the control signal line (103) is configured to control the Josephson inductance of the at least one SQUID (201) via controlling a magnetic flux through the at least one SQUID (201) based on the control signal.

5. A qubit control arrangement (500) comprising: a qubit (501) ; a plurality of switching elements, wherein each switching element (100) in the plurality of switching elements comprises a switching element (100) according to any preceding claim and the output signal line (102) of each switching element is electromagnetically coupled to the qubit (501) ; and a control unit (502) electromagnetically coupled to the control signal line (103) of each switching element (100) in the plurality of switching elements and configured to provide the control signal to each control signal line (103) .

6. The qubit control arrangement (500) according to claim 5, wherein the control unit (502) comprises an adiabatic quantum-flux-parametron, AQFP, based control logic .

7. The qubit control arrangement (500) according to claim 5 or claim 6, wherein the control unit (502) is configured to provide the control signal to each controlsignal line (103) in such a manner that a critical current of the at least one SQUID (201) of each switching element (100) is not exceeded.

8. The qubit control arrangement (500) according to claim any of claims 5 - 7, wherein a clock rate of the control unit (502) is configured to be less than 1 gigahertz .

9. The qubit control arrangement (500) according to any of claims 5 - 8, wherein the control signal is synchronized with the common pulse sequence.

10. The qubit control arrangement (500) according to any of claims 5 - 9, wherein the control unit (502) is configured to perform single-qubit gates using the qubit (501) via the control signals.

11. A quantum computing arrangement (600) comprising : a plurality of qubit control arrangements, wherein each qubit control arrangement (500) in the plurality of qubit control arrangements comprises a qubit control arrangement according to any of claims 5 - 10; and a plurality of signal sources (601, 602, 701, 702) , wherein each signal source in the plurality of signal sources (601, 602, 701, 702) is electromagnetically coupled to the input signal line (101) of a corresponding switching element (100) in each qubit control arrange-ment (500) in the plurality of qubit control arrangements and configured to provide a corresponding common pulse sequence to the input signal line (101) of the corresponding switching element (100) in each qubit control arrangement (500) in the plurality of qubit control arrangements .

12. The quantum computing arrangement (600) according to claim 11, wherein the plurality of signal sources comprises a first signal source (601) configured to provide a first common pulse sequence and a second signal source (602) configured to provide a second common pulse sequence, wherein a phase difference between the first common pulse sequence and the second common pulse sequence is substantially 90 degrees.

13. The quantum computing arrangement (600) according to claim 12, wherein the plurality of signal sources further comprises a third signal source (701) configured to provide a third common pulse sequence and a phase difference between the first common pulse sequence and the third common pulse sequence is substantially 180 degrees and / or the plurality of signal sources further comprises a fourth signal source (702) configured to provide a fourth common pulse sequence and a phase difference between the second common pulse sequence and the fourth common pulse sequence is substantially 180 degrees .

14. The quantum computing arrangement (600) according to any of claims 11 - 13, wherein the plurality of qubit control arrangements are arranged inside a cryostat and the plurality of signal sources are arranged outside the cryostat.

15. The quantum computing arrangement (600) according to any of claims 11 - 14, wherein each signal source in the plurality of signal sources is configured to provide a scalable leakage optimized pulse sequence.

16. A method (800) for performing single-qubit gates using a qubit and a plurality of switching elements, wherein each switching element in the plurality of switching elements comprises a switching element according to any of claims 1 - 4 and the output signal line of each switching element is electromagnetically coupled to the qubit, the method (800) comprising performing (801) single-qubit gates using the qubit by controlling transfer of common pulse sequences to the qubit via the plurality of switching elements by providing a control signal to the control signal line of each switching element.

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