(19)
(11)EP 3 405 985 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
27.11.2019 Bulletin 2019/48

(21)Application number: 16826810.0

(22)Date of filing:  20.12.2016
(51)Int. Cl.: 
H01L 49/00  (2006.01)
G06N 99/00  (2019.01)
H01L 39/22  (2006.01)
(86)International application number:
PCT/US2016/067827
(87)International publication number:
WO 2017/127205 (27.07.2017 Gazette  2017/30)

(54)

TUNABLE BUS-MEDIATED COUPLING BETWEEN REMOTE QUBITS

DURCH ABSTIMMBAREN BUS VERMITTELTE KOPPLUNG ZWISCHEN ENTFERNTEN QUBITS

COUPLAGE ASSISTÉ PAR BUS RÉGLABLE ENTRE QUBITS ÉLOIGNÉS


(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(30)Priority: 21.01.2016 US 201615003232

(43)Date of publication of application:
28.11.2018 Bulletin 2018/48

(73)Proprietor: Northrop Grumman Systems Corporation
Falls Church, VA 22042-4511 (US)

(72)Inventors:
  • NAAMAN, Ofer
    Ellicott City, Maryland 21043 (US)
  • KEANE, Zachary Kyle
    Baltimore, Maryland 21217 (US)
  • STOUTIMORE, Micah
    Kensington, Maryland 20895 (US)
  • FERGUSON, David George
    Takoma Park, Maryland 20912 (US)

(74)Representative: FRKelly 
27 Clyde Road
Dublin D04 F838
Dublin D04 F838 (IE)


(56)References cited: : 
WO-A2-2014/028302
US-A1- 2010 148 853
US-B1- 7 613 764
US-A1- 2005 001 209
US-A1- 2014 235 450
  
      
    Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


    Description

    TECHNICAL FIELD



    [0001] The present invention relates generally to superconducting circuits, and more particularly to tunable bus-mediated coupling between remote qubits.

    BACKGROUND



    [0002] The fundamental challenge for quantum computation and simulation is to construct a large-scale system of highly connected coherent qubits to perform various operations. Superconducting qubits utilize macroscopic circuits to process quantum information and are a promising candidate towards this end. Recently, materials research and circuit optimization has led to significant progress in qubit coherence. Superconducting qubits can now perform hundreds of operations within their coherence times, allowing for research into complex algorithms such as error correction. In many applications, it is desirable to combine these high-coherence qubits with tunable inter-qubit coupling, since it would allow for both coherent local operations and dynamically varying qubit interactions. For quantum computation, this would provide isolation for single-qubit gates while at the same time enabling fast two-qubit gates that minimize errors from decoherence. Despite previous attempts at tunable coupling, these applications have yet to be realized due to the challenge of incorporating tunable, long-distance coupling with high coherence devices.

    [0003] US 2010/148853 is disclosing a system including an RF-SQUID having a loop of superconducting material interrupted by a compound Josephson junction, and a magnetic flux inductor.

    SUMMARY



    [0004] In one example, a tunable bus-mediated coupling system is provided that includes a first input port coupled to a first end of a variable inductance coupling element through a first resonator and a second input port coupled to a second end of the variable inductance coupling element through a second resonator. The first input port is configured to be coupled to a first qubit, and the second output port is configured to be coupled to a second qubit. A controller is configured to control the inductance of the variable inductance coupling element between a low inductance state to provide strong coupling between the first qubit and the second qubit and a high inductance state to provide isolation between the first qubit and the second qubit.

    [0005] In another example, a superconducting system is provided that comprises a first qubit system having a first qubit, and a second qubit system remote from the first qubit system and having a second qubit. A tunable bus-mediated coupler is disposed between the first qubit and the second qubit. The tunable bus-mediated coupler has a first state for strongly coupling the first qubit to the second qubit and a second state for isolating the first qubit from the second qubit.

    [0006] In yet a further example, a superconducting system is provided that comprises a first qubit system comprising a first qubit, a second qubit system remote from the first qubit system and comprising a second qubit, and a tunable bus-mediated coupler disposed between the first qubit and the second qubit, The tunable bus-mediated coupler comprises a first input port coupled to a first end of a Josephson junction through a first resonator and a second input port coupled to a second end of the Josephson junction through a second resonator. The first input port is coupled to the first qubit and the second output port is coupled to the second qubit. The tunable bus-mediated coupler comprises a first termination inductor coupled between the first resonator and the Josephson junction on a first end and ground on a second end, and a second termination inductor coupled between the second resonator and the Josephson junction on a first end and ground on a second end, wherein the first termination inductor, the Josephson junction and the second termination inductor form an RF-Squid. A bias inductor is inductively coupled to one of the first termination inductor and the second termination inductor, wherein an amount of current through the bias inductor controls the coupling strength between the first and the second qubit. A controller controls an amount of current through the bias inductor inductively coupled to one of the first and the second termination inductors to control the inductance of the Josephson junction between a low inductance state to provide strong coupling between the first qubit and the second qubit and a high inductance state to provide isolation between the first qubit and the second qubit.

    [0007] The invention is as defined in the independent claim 1. Further aspects of the invention are as set out in the dependent claims.

    BRIEF DESCRIPTION OF THE DRAWINGS



    [0008] 

    FIG. 1 illustrates a block diagram of an example of a superconducting system.

    FIG. 2 illustrates a schematic of an example of a tunable bus-mediated coupler that could be employed in FIG. 1.

    FIG. 3 illustrates a graph of the voltage along the length of combined coupled-resonator system showing the even (dashed) and odd (solid) modes of oscillation.

    FIG. 4 is a schematic level diagram showing the hybridized left and right resonators of FIG. 2 producing frequency-split even and odd modes

    FIG. 5 illustrates a graphical panel showing results of a simulation for a particular flux setting.

    FIG. 6 illustrates a graphical panel showing the frequency splitting of the even and odd bus modes due to the flux-dependent coupling.

    FIG. 7 illustrates a graph of simulation results showing the dependence of the bus mode splitting and the qubit-qubit bus-mediated coupling as a function of the junction flux-dependent critical current.


    DETAILED DESCRIPTION



    [0009] The present disclosure relates generally to superconducting circuits, and more particularly to tunable bus-mediated coupling (or coupler) between remote qubits. In one example, a variable inductance coupling element is placed between two qubits that may reside in separate remote superconducting systems. The variable inductance coupling element can be adjusted between a strongly coupled state and a decoupled (or isolation) state between qubits in addition to various states of intermediate coupling strengths in between. In this manner, manipulation can be performed on state information of isolated qubits in a decoupled state, while this state information can be exchanged between qubits during a strongly coupled state. Furthermore, state information can be manipulated and passed between qubits without destroying the state information of the originating qubit in an intermediate coupling strength state between qubits. In one example, the variable inductance coupling element can be a Josephson junction. A variable inductance coupling element can be arranged as a single Josephson junction or series array of N Josephson junctions, each having a critical current N times larger than the original Josephson junction.

    [0010] In another example, an RF-SQUID tunable coupler includes a Josephson junction embedded in the middle of a half-wave resonator bus. The RF-SQUID facilitates bus-mediated dispersive interaction between the qubits for coupling. The advantage of bus-mediated coupling is that the qubits can be physically placed remotely from each other, for example, in separate circuit blocks on the quantum processor chip. The advantage of a tunable coupler, which can essentially be turned off when desired, is a reduction in frequency crowding and unwanted residual interactions between the qubits. Furthermore, the interaction strength can be calibrated and trimmed in the field to compensate for variability in the manufacturing process, and can be controlled in real time as part of the computation protocol.

    [0011] The Josephson junction can have a first inductance when no current or a low current is induced in the SQUID, and a second inductance when a current or a higher current is induced in its respective SQUID that is at a predetermined threshold that generates or induces a flux, for example, greater than about 0.1 Φ0 and less than about 0.45 Φ0, where Φ0 is equal to a flux quantum. The first inductance (e.g., ℏ/2e* 1/lC, where ℏ is Planck's constant divided by 2π, e is electron charge and IC is the critical current of the Josephson junction) can provide coupling between the first and second qubits. The second inductance (e.g., large inductance value) can provide decoupling between the first and second qubits.

    [0012] FIG. 1 illustrates a block diagram of an example of a superconducting system 10. The superconducting system includes a first qubit system 12 coupled to a second qubit system 16 through a tunable coupler system 14. The first qubit system 12 includes a plurality of qubits labeled, qubit (1,1) to qubit (1,N), and the second qubit system16 includes a plurality of qubits labeled, qubit (2,1) to qubit (2,N), such that (X,N) provides X which represents the qubit system and N represents a qubit number within the qubit system, where N is an integer greater than or equal to one. The first qubit system 12 and the second qubit system 16 can be separate logical blocks that perform different logical operations such as different gate operations, error correction operations, memory operations, or any of a variety of other superconducting operations. The first qubit system 12 and second qubit system 16 can also include various additional qubits and other superconducting elements that are not coupled to qubits in the other qubit system, but may be coupled to other qubits in its respective system for performing a variety of qubit and other superconducting operations.

    [0013] Each qubit in the first qubit system 12 is coupled to a respective qubit in the second qubit system 16 by a respective tunable coupler of the tunable coupler system 14 having N tunable couplers, labeled tunable coupler 1 through tunable coupler N. Each tunable coupler includes a variable inductance coupling element that can be adjusted to allow for control of the coupling strength between two independent qubits of the opposing qubit systems 12 and 16, respectively. The variable inductance coupling element can be disposed between two resonators to allow for remote coupling of the two independent qubits via a tunable bus-mediated coupler. In one example, the variable inductance coupling element is a Josephson junction that resides in a RF SQUID disposed between two resonators. The superconducting switching system 10 also includes a swich controller 20 and bias elements 18. The variable inductance coupling elements are controlled by magnetic flux via the bias elements 18 and the switch controller 20 to couple, decouple and to control the coupling strength of the coupling between respective independent qubits in opposing qubit systems 12 and 16.

    [0014] FIG. 2 illustrates a schematic of an example of a tunable bus-mediated coupler 30 that could be employed in FIG. 1. The tunable bus-mediated coupler 30 is composed of a first quarter-wave transmission line resonator TL1 and a second quarter-wave transmission line resonator TL2. A first coupling capacitor C1 couples a first port (Port 1) to a first end of the first quarter-wave transmission line resonator TL1, and a second coupling capacitor C2 couples a second port (Port 2) to a first end of the second quarter-wave transmission line resonator TL2. The first port (Port 1) can be coupled to a first qubit and the second port (Port 2) can be coupled to a second qubit. A second end of the first quarter-wave transmission line resonator TL1 is shorted to ground via a first terminating inductor (L1) and a second end of the second quarter-wave transmission line resonator TL2 is shorted to ground via a second terminating inductor L2. A Josephson junction (J1) is further connected between the termination inductors L1 and L2, so that the Josephson junction J1 together with termination inductors L1 and L2, form an RF-SQUID 32.

    [0015] The RF-SQUID 32 functions as a variable transformer, controlled by a magnetic flux Φe induced in the RF-SQUID loop via a mutual inductance M induced by a current flowing between a third port (Port 3) and a fourth port (Port 4) through a bias inductance L3. When the flux enclosed in the RF-SQUID 32, Φe, is an appreciable fraction of Φ0/2, as determined by the ratio of the Josephson junction J1 to linear inductances L1 and L2 in the RF-SQUID 32, the effective mutual coupling between the two resonators TL1 and TL2 is essentially zero. When the enclosed flux is close to zero or an integer multiple of Φ0, the effective mutual coupling between the resonators TL1 and TL2 is appreciable, and equals to Meff=L1*L2/(LJ1+L1+L2). Close to an enclosed flux of Φ0/2, the effective mutual coupling is appreciable and negative. Therefore, the effective mutual coupling Meffe) between the two resonators TL1 and TL2 is a function of the applied flux. The flux can be varied between zero and Φ0/2 by changing the current through bias inductance L3 to provide varying strengths of the effective coupling between the first and second qubits coupled to the first port (Port 1) and the second port (Port 2), respectively.

    [0016] FIG. 3 illustrates a graph 40 of the voltage along the length of combined coupled-resonator of FIG. 2 showing the even (dashed) and odd (solid) modes of oscillation. Because of the coupling between the two transmission line resonators TL1 and TL2, the combined system exhibit two oscillating eigen-modes having different frequencies. A first odd mode having a frequency Ωο close to the half-wave frequency of the combined system, and in which the voltages at the ends of the transmission lines oscillate 180 degrees out of phase, and an even mode having a different frequency Ωe in which the voltages at the ends of the transmission lines oscillate in phase. When the coupling is positive, the even mode frequency is greater than the odd mode frequency. When the coupling is negative, the even mode frequency is lower than the odd mode. In either case the even and odd modes are split in frequency by an amount 2gc, proportional to the effective mutual Meffe).

    [0017] Qubits that are connected to the two ports of the coupled-resonator bus via capacitors C1 and C2 of FIG. 2, each interact with both even and odd modes of the bus. In the dispersive regime, when the qubit frequencies are sufficiently detuned from the bus frequencies, an effective bus-mediated interaction between the qubits exists. However, the sign of the mediated interaction due to the even mode is opposite to that of the interaction due to the odd mode, and therefore the total effective mediated coupling can be determined as a balance of the coupling due to the two bus modes. In particular, the two contributions can be made equal in magnitude and opposite in sign, resulting in a cancellation of the coupling. FIG. 4 is a schematic level diagram 50, showing the hybridized left and right resonators producing the frequency-split even and odd modes, and the left and right qubits each at a respective detuning ΔL,Ro from the odd mode, and ΔL,Re from the even mode.

    [0018] The overall bus-mediated coupling between the qubits in the dispersive regime, geff, as a function of the detuning is given by:

    where gL,R are the fixed coupling strengths of the qubit to the respective resonators via capacitors C1 and C2 in FIG. 2. When the frequencies of the two qubits are equal so that ΔLeRe =Ae and ALo=ARoo, the expression for the effective bus-mediated qubit-qubit coupling simplifies to:

    where geff is dependent on the flux Φe via gc and, implicitly, via Δe and Ao, which are all flux-dependent.

    [0019] An Agilent's Advanced Design Simulation (ADS) tool simulation was performed with the junction approximated with a linear inductor whose value was changed from the nominal zero-current Josephson inductance up to a value 50 times greater. The results of the simulation for a particular flux setting are shown in a panel 60 of FIG. 5 showing the splitting in the qubit spectrum due to the effective interaction geff, and a panel 70 of FIG. 6 showing the frequency splitting of the even and odd bus modes due to the flux-dependent coupling gc. The simulations confirm the functional dependence of geff on gc, and confirm the expected dependence of gc on the flux-tunable inductance of the Josephson junction.

    [0020] FIG. 7 illustrates a graph 80 of simulation results showing the dependence of the bus mode splitting gc and the qubit-qubit bus-mediated coupling geff as a function of the junction flux-dependent critical current for a certain value of the qubit-bus frequency detuning. While in the examples shown, the bus frequency is higher than the qubit frequencies, the same behavior is replicated when the bus frequency is lower than the qubit frequencies.

    [0021] To summarize, an RF-SQUID tunable coupler embedded between two quarter wave resonators such that the combined system forms a quantum bus having two modes that contribute with opposite signs to a mediated qubit-qubit interaction. The total effective interaction between the qubits is thus tunable with flux as a balance between the contributions to the mediated coupling from the two bus modes. The advantage of a tunable coupling, which can essentially be turned off when desired, is a reduction in frequency crowding and unwanted residual interactions between the qubits. Furthermore, the interaction strength can be calibrated and trimmed in the field to compensate for variability in the manufacturing process, and can be controlled in real time as part of the computation protocol.

    [0022] What have been described above are examples of the invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the invention are possible. Accordingly, the invention is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims.


    Claims

    1. A superconducting system (30) comprising:

    a first qubit system (12) comprising a first qubit;

    a second qubit system (16) remote from the first qubit system (12) and comprising a second qubit;

    a tunable bus-mediated coupler (30) disposed between the first qubit and the second qubit, the tunable bus-mediated coupler comprising:

    a first port coupled to a first end of a Josephson junction (J1) through a first quarter-wave transmission line resonator (TL1), the first port being coupled to the first qubit;

    a second port coupled to a second end of the Josephson junction through a second quarter-wave transmission line resonator (TL2), the second port being coupled to the second qubit;

    a first termination inductor (L1) coupled between the first resonator and the Josephson junction on a first end and ground on a second end, and

    a second termination inductor (L2) coupled between the second resonator and the Josephson junction on a first end and ground on a second end, wherein the first termination inductor, the Josephson junction and the second termination inductor form an RF-SQUID (32);

    a bias inductor inductively coupled to one of the first termination inductor and the second termination inductor, wherein an amount of current through the bias inductor controls the coupling strength between the first and the second qubit; and

    a controller (20) that controls an amount of current through the bias inductor (L3) inductively coupled to one of the first (L1) and the second (L2) termination inductors to control the inductance of the Josephson junction (J1) between a low inductance state to provide strong coupling between the first qubit and the second qubit and a high inductance state to provide isolation between the first qubit and the second qubit.


     
    2. The system of claim 1, further comprising a first coupling capacitor (C1) coupled between the first qubit and the first resonator (TL1), and a second coupling capacitor (C2) coupled between the second resonator (TL2) and the second qubit.
     
    3. The system of claim 1, wherein the controller (20) provides a current through the bias inductor between no current that induces no net flux in the RF-SQUID (32) allowing for coupling between the first and second qubit and a current that induces a net flux in the RF-SQUID of about 0.1 Φο to about 0.45 Φο, where Φο is equal to a flux quantum providing isolation between the first and second qubit.
     
    4. The system of claim 1, wherein the effective mutual coupling between the two resonators (TL1, TL2) is a function of flux applied to the RF-SQUID (32).
     
    5. The system of claim 1, wherein the effective mutual coupling between the resonators (TL1, TL2) is equal to the product of the inductances of the termination inductors (L1, L2) divided by the sum of the inductances of the Josephson junction (J1) and the termination inductors (L1, L2) when the flux enclosed in the RF-SQUID (32) is close to zero or an integer multiple of Φο.
     
    6. The system of claim 1, wherein the effective mutual coupling between the resonators (TL1, TL2) is appreciable and negative when the flux enclosed in the RF-SQUID (32) is close to Φο/2.
     
    7. The system of claim 1, wherein the system exhibits two oscillating eigen-modes having different bus frequencies, the two modes comprising an odd mode having a bus frequency close to the half-wave frequency of the system in which the voltages at the ends of the resonators (TL1, TL2) oscillate 180 degrees out of phase, and an even mode having a different bus frequency in which the voltages at the ends of the resonators oscillate in phase.
     
    8. The system of claim 7, wherein the effective mutual coupling between the resonators (TL1, TL2) is equal to the product of the inductances of the termination inductors (L1, L2) divided by the sum of the inductances of the Josephson junction (J1) and the termination inductors (L1, L2) when the flux enclosed in the RF-SQUID (32) is close to zero or an integer multiple of Φο, and wherein the even and odd modes are split in frequency by an amount proportional to the effective mutual coupling.
     
    9. The system of claim 7, wherein there is an effective bus-mediated interaction between the qubits in a dispersive regime characterized by the frequencies of the qubits being detuned from the bus frequencies.
     
    10. The system of claim 9, further comprising a first coupling capacitor (C1) coupled between the first qubit and the first resonator (TL1), and a second coupling capacitor (C2) coupled between the second resonator (TL2) and the second qubit, and wherein the coupling strength of the effective bus-mediated interaction between the qubits in the dispersive regime is equal to the product of the fixed coupling strengths of the qubits to the respective resonators via the coupling capacitors times twice the bus mode splitting divided by the product of the detuning from the odd mode and detuning from the even mode, when the frequencies of the two qubits are equal.
     


    Ansprüche

    1. Supraleitendes System (30), umfassend:

    ein erstes Qubit-System (12), das ein erstes Qubit umfasst;

    ein zweites Qubit-System (16), das von dem ersten Qubit-System (12) entfernt ist und ein zweites Qubit umfasst;

    einen durch abstimmbaren Bus vermittelten Koppler (30), der zwischen dem ersten Qubit und dem zweiten Qubit angeordnet ist, wobei der durch abstimmbaren Bus vermittelte Koppler Folgendes umfasst:

    einen ersten Port, der mit einem ersten Ende eines Josephson-Übergangs (J1) über einen ersten Viertelwellen-Übertragungsleitungsresonator (TL1) gekoppelt ist, wobei der erste Port mit dem ersten Qubit gekoppelt ist;

    einen zweiten Port, der mit einem zweiten Ende des Josephson-Übergangs über einen zweiten Viertelwellen-Übertragungsleitungsresonator (TL2) gekoppelt ist, wobei der zweite Port mit dem zweiten Qubit gekoppelt ist;

    einen ersten Abschlussinduktor (L1), der zwischen dem ersten Resonator und dem Josephson-Übergang an einem ersten Ende und der Erde an einem zweiten Ende gekoppelt ist, und

    einen zweiten Abschlussinduktor (L2), der zwischen dem zweiten Resonator und dem Josephson-Übergang an einem ersten Ende und der Erde an einem zweiten Ende gekoppelt ist, wobei der erste Abschlussinduktor, der Josephson-Übergang und der zweite Abschlussinduktor ein RF-SQUID (32) bilden;

    einen Vorspannungsinduktor, der induktiv mit einem aus dem ersten Abschlussinduktor und dem zweiten Abschlussinduktor gekoppelt ist, wobei eine Strommenge durch den Vorspannungsinduktor die Kopplungsstärke zwischen dem ersten und dem zweiten Qubit steuert; und

    eine Steuerung (20), die eine Strommenge durch den Vorspannungsinduktor (L3) steuert, der induktiv mit einem aus dem ersten (L1) und dem zweiten (L2) Abschlussinduktor gekoppelt ist, um die Induktivität des Josephson-Übergangs (J1) zwischen einem Zustand niedriger Induktivität, um eine starke Kopplung zwischen dem ersten Qubit und dem zweiten Qubit bereitzustellen, und einem Zustand hoher Induktivität, um eine Isolierung zwischen dem ersten Qubit und dem zweiten Qubit bereitzustellen, zu steuern.


     
    2. System nach Anspruch 1, ferner umfassend einen ersten Kopplungskondensator (Cl), der zwischen dem ersten Qubit und dem ersten Resonator (TL1) gekoppelt ist, und einen zweiten Kopplungskondensator (C2), der zwischen dem zweiten Resonator (TL2) und dem zweiten Qubit gekoppelt ist.
     
    3. System nach Anspruch 1, wobei die Steuerung (20) einen Strom durch den Vorspannungsinduktor zwischen kein Strom, der keinen Nettofluss in dem RF-SQUID (32) induziert, was ein Koppeln zwischen dem ersten und dem zweiten Qubit ermöglicht, und einem Strom, der einen Nettofluss in dem RF-SQUID von zwischen 0,1 Φ0 bis etwa 0,45 Φ0 induziert, wobei Φ0 gleich einem Flussquant ist, was eine Isolierung zwischen dem ersten und dem zweiten Qubit bereitstellt.
     
    4. System nach Anspruch 1, wobei die effektive gegenseitige Kopplung zwischen den zwei Resonatoren (TL1, TL2) von dem Fluss abhängig ist, der auf das RF-SQUID (32) aufgebracht wird.
     
    5. System nach Anspruch 1, wobei die effektive gegenseitige Kopplung zwischen den Resonatoren (TL1, TL2) gleich dem Produkt der Induktivitäten der Abschlussinduktoren (L1, L2) dividiert durch die Summe der Induktivitäten des Josephson-Übergangs (J1) und der Abschlussinduktoren (L1, L2) ist, wenn der Fluss, der in dem RF-SQUID (32) eingebettet ist, nahe Null ist oder ein ganzzahliges Vielfaches von Φ0 ist.
     
    6. System nach Anspruch 1, wobei die effektive gegenseitige Kopplung zwischen den Resonatoren (TL1, TL2) nennenswert und negativ ist, wenn der Fluss, der in dem RF-SQUID (32) eingebettet ist, nahe Φ0/2 ist.
     
    7. System nach Anspruch 1, wobei das System zwei schwingende Eigenmoden zeigt, die verschiedene Busfrequenzen aufweisen, wobei die zwei Moden einen ungerade Mode, der eine Busfrequenz nahe der Halbwellenfrequenz des Systems aufweist, bei der die Spannungen an dem Enden der Resonatoren (TL1, TL2) um 180 Grad phasenverschoben schwingen, und einen geraden Mode umfassen, der eine andere Busfrequenz aufweist, bei der die Spannungen an den Enden der Resonatoren phasengleich schwingen.
     
    8. System nach Anspruch 7, wobei die effektive gegenseitige Kopplung zwischen den Resonatoren (TL1, TL2) gleich dem Produkt der Induktivitäten der Abschlussinduktoren (L1, L2) dividiert durch die Summe der Induktivitäten des Josephson-Übergangs (J1) und der Abschlussinduktoren (L1, L2) ist, wenn der Fluss, der in dem RF-SQUID (32) eingebettet ist, nahe Null ist oder ein ganzzahliges Vielfaches von Φ0 ist, und wobei der gerade und der ungerade Mode der Frequenz nach um eine Menge proportional zur effektiven gegenseitigen Kupplung getrennt sind.
     
    9. System nach Anspruch 7, wobei es eine effektive busvermittelte Wechselwirkung zwischen den Qubits in einem dispersiven Betrieb gibt, dadurch gekennzeichnet, dass die Frequenzen der Qubits von den Busfrequenzen verstellt sind.
     
    10. System nach Anspruch 9, ferner umfassend einen ersten Kopplungskondensator (Cl), der zwischen dem ersten Qubit und dem ersten Resonator (TL1) gekoppelt ist, und einen zweiten Kopplungskondensator (C2), der zwischen dem zweiten Resonator (TL2) und dem zweiten Qubit gekoppelt ist, und wobei die Kopplungsstärke der effektiven busvermittelten Wechselwirkung zwischen den Qubits in dem dispersiven Betrieb gleich dem Produkt der feststehenden Kopplungsstärken der Qubits mit den jeweiligen Resonatoren über die Kopplungskondensatoren mal zweimal die Busmodentrennung dividiert durch das Produkt des Verstellens von dem ungeraden Mode und des Verstellens von dem geraden Mode, wenn die Frequenzen der zwei Qubits gleich sind, ist.
     


    Revendications

    1. Système supraconducteur (30) comprenant :

    un premier système de qubit (12) comprenant un premier qubit ;

    un second système de qubit (16) éloigné du premier système de qubit (12) et comprenant un second qubit ;

    un coupleur assisté par bus accordable (30) disposé entre le premier qubit et le second qubit, le coupleur assisté par bus accordable comprenant :

    un premier port couplé à une première extrémité d'une jonction Josephson (J1) par l'intermédiaire d'un premier résonateur à ligne de transmission quart d'onde (TL1), le premier port étant couplé au premier qubit ;

    un second port couplé à une seconde extrémité de la jonction Josephson par l'intermédiaire d'un second résonateur à ligne de transmission quart d'onde (TL2), le second port étant couplé au second qubit ;

    un premier inducteur de terminaison (L1) couplé entre le premier résonateur et la jonction Josephson sur une première extrémité et la terre sur une seconde extrémité, et

    un second inducteur de terminaison (L2) couplé entre le second résonateur et la jonction Josephson sur une première extrémité et la terre sur une seconde extrémité, dans lequel le premier inducteur de terminaison, la jonction Josephson et le second inducteur de terminaison forment un SQUID radiofréquence (32) ;

    un inducteur de polarisation couplé de manière inductive à l'un du premier inducteur de terminaison et du second inducteur de terminaison, dans lequel une quantité de courant à travers l'inducteur de polarisation commande la force de couplage entre le premier et le second qubit ; et

    un dispositif de commande (20) qui commande une quantité de courant à travers l'inducteur de polarisation (L3) couplé de manière inductive à l'un des premier (L1) et second (L2) inducteurs de terminaison pour commander l'inductance de la jonction Josephson (J1) entre un état à faible inductance pour fournir un couplage fort entre le premier qubit et le second qubit et un état à haute inductance pour fournir une isolation entre le premier qubit et le second qubit.


     
    2. Système selon la revendication 1, comprenant en outre un premier condensateur de couplage (C1) couplé entre le premier qubit et le premier résonateur (TL1), et un second condensateur de couplage (C2) couplé entre le second résonateur (TL2) et le second qubit.
     
    3. Système selon la revendication 1, dans lequel le dispositif de commande (20) fournit un courant à travers l'inducteur de polarisation entre une absence de courant qui induit une absence de flux net dans le SQUID radiofréquence (32) permettant le couplage entre le premier et le second qubit et un courant qui induit un flux net dans le SQUID radiofréquence d'environ 0,1 Φ0 à environ 0,45 φ0, où φ0 est égal à un quantique de flux fournissant une isolation entre le premier qubit et le second qubit.
     
    4. Système selon la revendication 1, dans lequel le couplage mutuel effectif entre les deux résonateurs (TL1, TL2) est fonction d'un flux appliqué au SQUID radiofréquence (32).
     
    5. Système selon la revendication 1, dans lequel le couplage mutuel effectif entre les résonateurs (TL1, TL2) est égal au produit des inductances des inducteurs de terminaison (L1, L2) divisé par la somme des inductances de la jonction Josephson (J1) et des inducteurs de terminaison (L1, L2) lorsque le flux contenu dans le SQUID radiofréquence (32) est proche de zéro ou un multiple entier de φ0.
     
    6. Système selon la revendication 1, dans lequel le couplage mutuel effectif entre les résonateurs (TL1, TL2) est sensible et négatif lorsque le flux contenu dans le SQUID radiofréquence (32) est proche de φ0/2.
     
    7. Système selon la revendication 1, dans lequel le système présente deux modes propres d'oscillation ayant des fréquences de bus différentes, les deux modes comprenant un mode impair ayant une fréquence de bus proche de la fréquence demi-onde du système dans lequel les tensions au niveau des extrémités des résonateurs (TL1, TL2) oscillent de manière déphasée de 180 degrés, et un mode pair ayant une fréquence de bus différente dans lequel les tensions au niveau des extrémités des résonateurs oscillent en phase.
     
    8. Système selon la revendication 7, dans lequel le couplage mutuel effectif entre les résonateurs (TL1, TL2) est égal au produit des inductances des inducteurs de terminaison (L1, L2) divisé par la somme des inductances de la jonction Josephson (J1) et des inducteurs de terminaison (L1, L2) lorsque le flux contenu dans le SQUID radiofréquence (32) est proche de zéro ou un multiple entier de φ0, et dans lequel les modes pair et impair sont séparés en fréquence par une quantité proportionnelle au couplage mutuel effectif.
     
    9. Système selon la revendication 7, dans lequel il y a une interaction assistée par bus effective entre les qubits dans un régime dispersif caractérisé par les fréquences des qubits qui sont désaccordées des fréquences de bus.
     
    10. Système selon la revendication 9, comprenant en outre un premier condensateur de couplage (C1) couplé entre le premier qubit et le premier résonateur (TL1), et un second condensateur de couplage (C2) couplé entre le second résonateur (TL2) et le second qubit, et dans lequel la force de couplage de l'interaction assistée par bus effective entre les qubits dans le régime dispersif est égale au produit des forces de couplage fixes des qubits jusqu'aux résonateurs respectifs via les condensateurs de couplage multiplié par deux fois la séparation de mode bus divisé par le produit du désaccord du mode impair et du désaccord du mode pair, lorsque les fréquences des deux qubits sont égales.
     




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    REFERENCES CITED IN THE DESCRIPTION



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    Patent documents cited in the description