Systems, methods and apparatus for active compensation of quantum processor elements
Apparatus and methods enable active compensation for unwanted discrepancies in the superconducting elements of a quantum processor. A qubit may include a primary compound Josephson junction (CJJ) structure, which may include at least a first secondary CJJ structure to enable compensation for Josephson junction asymmetry in the primary CJJ structure. A qubit may include a series LC-circuit coupled in parallel with a first CJJ structure to provide a tunable capacitance. A qubit control system may include means for tuning inductance of a qubit loop, for instance a tunable coupler inductively coupled to the qubit loop and controlled by a programming interface, or a CJJ structure coupled in series with the qubit loop and controlled by a programming interface.
1. A method of operation of a quantum processor that comprises a plurality of qubits and, for each of the qubits, a respective plurality of programming interfaces that are inductively coupled to respective qubits of the plurality of qubits, the method comprising:
applying a control signal to a first programming interface of the respective plurality of programming interfaces to tune at least one parameter of a first one of the qubits;
matching the at least one parameter between the first one of the qubits and at least a second one of the qubits, wherein the matching comprises selectively operating the respective plurality of programming interfaces for at least the first one and the second one of the qubits to synchronize a behavior of at least the first one and the second one of the qubits before executing the quantum computation; and
executing a quantum computation via the quantum processor using the first one of the qubits and the at least a second one of the qubits with matched parameters.
2. The method of claim 1 wherein selectively operating the respective plurality of interfaces for at least the first one and second one of the qubits comprises selectively operating the respective plurality of programming interfaces for all of the qubits of the plurality of qubits to synchronize a behavior of all of the qubits of the plurality of qubits before executing the quantum computation.
3. The method of claim 1 wherein applying a control signal to a first programming interface of the respective plurality of programming interfaces to tune at least one parameter of a first one of the qubits comprises selectively operating the first programming interface for the first one of the qubits to compensate, at least in part, for Josephson junction asymmetry.
4. The method of claim 3 , wherein selectively operating the first programming interface for the first one of the qubits to compensate, at least in part, for Josephson junction asymmetry comprises selectively operating a programming interface inductively coupled to a secondary compound Josephson junction interrupting a first one of two electrically parallel current paths within a primary compound Josephson junction structure of the first one of the qubits.
5. The method of claim 4 , wherein selectively operating a programming interface inductively coupled to a secondary compound Josephson junction interrupting one of two electrically parallel current paths within a primary compound Josephson junction structure comprises coupling a control signal from the programming interface into the secondary compound Josephson junction and tuning a characteristic of the secondary compound Josephson junction to match a characteristic of a single Josephson junction in a second one of two electrically parallel current paths within the primary compound Josephson junction structure of the first one of the qubits.
6. The method of claim 4 , wherein selectively operating a programming interface inductively coupled to a secondary compound Josephson junction interrupting one of two electrically parallel current paths within a primary compound Josephson junction structure comprises coupling a control signal from the programming interface into the secondary compound Josephson junction and tuning a characteristic of the secondary compound Josephson junction in association with tuning a characteristic of a third compound Josephson junction interrupting a second one of two electrically parallel current paths within the primary compound Josephson junction structure of the first one of the qubits to reduce asymmetry in the primary compound Josephson junction structure.
7. The method of claim 1 wherein applying a control signal to a first programming interface of the respective plurality of programming interfaces to tune at least one parameter of a first one of the qubits comprises selectively operating the first programming interface for the first one of the qubits to generate, at least in part, a Josephson junction asymmetry.
8. The method of claim 1 wherein applying a control signal to a first programming interface of the respective plurality of programming interfaces to tune at least one parameter of a first one of the qubits comprises selectively operating the first programming interface for the first one of the qubits to tune a capacitance and thereby change a tunneling rate of the first one of the qubits.
9. The method of claim 8 , wherein selectively operating the first programming interface for the first one of the qubits to tune a capacitance comprises selectively operating the respective plurality of programming interfaces for all of the qubits in the plurality of qubits to tune a capacitance of each of the qubits in the quantum processor before executing the quantum computation.
10. The method of claim 1 wherein applying a control signal to a first programming interface of the respective plurality of programming interfaces to tune at least one parameter of a first one of the qubits comprises tuning a capacitance of a superconducting flux qubit comprising a qubit loop and a first compound Josephson junction, tuning the capacitance comprising:
coupling a series LC-circuit in parallel with the first compound Josephson junction, wherein the series LC-circuit comprises at least one capacitance coupled in series with a second compound Josephson junction;
inductively coupling a programming interface configured to receive control signals to the second compound Josephson junction; and
adjusting a control signal applied to the programming interface to tune the capacitance of the series LC-circuit.
11. The method of claim 10 wherein coupling a series LC-circuit in parallel with the first compound Josephson junction comprises coupling a series LC-circuit comprising at least one capacitance and a second compound Josephson junction structure coupled in series with the at least one capacitance, the second compound Josephson junction structure formed by a second pair of parallel superconducting current paths, wherein each superconducting current path in the second pair of parallel superconducting current paths is interrupted by at least one Josephson junction.
12. The method of claim 10 wherein adjusting a control signal applied to the programming interface to tune the capacitance of the series LC-circuit comprises adjusting a control signal applied to the programming interface to tune an inductance of the second compound Josephson junction, thereby controlling an impedance of the series LC-circuit and tuning a portion of the impedance of the series LC-circuit that is capacitive.
13. The method of claim 12 wherein coupling a series LC-circuit in parallel with the first compound Josephson junction comprises coupling a series LC-circuit characterized in part by a resonant frequency and wherein adjusting the control signal to tune an inductance of the second compound Josephson junction further comprises varying the resonant frequency of the series LC-circuit with the capacitance of the series LC-circuit relative to a plasma frequency that characterizes the qubit loop.
14. The method of claim 1 wherein applying a control signal to a first programming interface of the respective plurality of programming interfaces to tune at least one parameter of a first one of the qubits comprises selectively operating the first programming interface for the first one of the qubits to tune an inductance.
15. The method of claim 14 , wherein selectively operating the first programming interface for the first one of the qubits to tune an inductance comprises selectively operating the respective plurality of programming interfaces for all of the qubits in the plurality of qubits to tune an inductance of each of the qubits in the quantum processor before executing the quantum computation.
16. The method of claim 1 , wherein applying a control signal to a first programming interface of the respective plurality of programming interfaces to tune at least one parameter of a first one of the qubits comprises tuning an inductance of a superconducting qubit comprising a qubit loop and a first compound Josephson junction, tuning the inductance comprising:
coupling at least a second compound Josephson junction to the qubit loop of the superconducting qubit; and
tuning the inductance of the at least a second compound Josephson junction using a programming interface that is configured to inductively couple control signals to the at least a second compound Josephson junction, thereby tuning the inductance of the qubit loop.
17. The method of claim 16 wherein coupling at least a second compound Josephson junction to the qubit loop of the superconducting qubit comprises coupling at least a second compound Josephson junction galvanically in series in the qubit loop.
18. The method of claim 16 wherein coupling at least a second compound Josephson junction to the qubit loop of the superconducting qubit comprises coupling at least a second compound Josephson junction inductively to the qubit loop.
19. The method of claim 18 wherein coupling at least a second compound Josephson junction inductively to the qubit loop comprises coupling a coupler comprising the at least a second compound Josephson junction inductively to the qubit loop, the coupler configured to communicably couple solely to the qubit loop and not substantially to any other qubits.