Search

JP-7857439-B2 - Microwave-based reset of persistent current qubits

JP7857439B2JP 7857439 B2JP7857439 B2JP 7857439B2JP-7857439-B2

Inventors

  • マラコフ、アレクサンドル
  • プシビシュ、アンソニー ジョセフ
  • メドフォード、ジェームズ アール．

Assignees

  • ノースロップ グラマン システムズ コーポレーション

Dates

Publication Date
20260512
Application Date
20230405
Priority Date
20220516

Claims (15)

  1. It is an assembly, A qubit containing a superconducting loop blocked by a composite Josephson junction, A first bias source that supplies a first bias to the superconducting loop, A second bias source that provides a second bias to the composite Josephson junction, wherein each of the first and second bias sources is selected in response to system control such that a first value is selected for the first bias and a second value is selected for the second bias, and each of the first and second values is selected such that a given excited state of the qubit is near the top of a potential barrier associated with the potential of the qubit, An assembly comprising: a microwave source that generates a continuous microwave signal having a frequency equal to the transition frequency between another excited state of the qubit and the given excited state.
  2. The assembly according to claim 1, wherein the first and second values are selected such that the other excited states have occupied states divided between the first well of the qubit's potential and the second well of the qubit's potential.
  3. The assembly according to claim 2, wherein the first value is selected such that the first well is deeper than the second well, and the other excited states are further from the bottom of the first well than the bottom of the second well.
  4. The assembly according to claim 1, wherein the first bias and the second bias are each supplied as magnetic flux, and the second value is selected to be between 3/5 and 4/5 of one magnetic flux quantum.
  5. The assembly according to claim 1, wherein the microwave source provides the continuous microwave signal for a predetermined time in response to the system control, the predetermined time being a function of the relaxation time of the qubit from the given excited state to the ground state.
  6. The assembly according to claim 1, wherein the microwave source is a first microwave source, the frequency is a first frequency, the continuous microwave signal is a first continuous microwave signal, and the assembly further comprises a second microwave source that generates a second continuous microwave signal having a second frequency equal to the transition frequency between an excited state immediately above the ground state of the qubit and the other excited states of the qubit.
  7. The assembly according to claim 1, wherein an intermediate excited state lies between the given excited state and the other excited state.
  8. The assembly according to claim 1, wherein the qubit is a magnetic flux qubit.
  9. A method for resetting a qubit including a superconducting loop and a composite Josephson junction, The steps include supplying a first bias magnetic flux to the superconducting loop, A step of supplying a second bias flux to the composite Josephson junction, wherein each of the first bias flux and the second bias flux is supplied such that a given excited state of the qubit is near the top of a potential barrier related to the potential of the qubit; A method comprising the step of generating a continuous microwave signal having a frequency equal to the transition frequency between another excited state of the qubit and the given excited state.
  10. The method according to claim 9, wherein the first bias flux and the second bias flux are supplied such that the other excited state has a probability amplitude divided between the first well of the potential of the qubit and the second well of the potential of the qubit.
  11. The method according to claim 10, wherein the magnitude of the first bias flux is selected such that the first well is deeper than the second well and the other excited states are further from the bottom of the first well than the bottom of the second well.
  12. The method according to claim 9 , wherein an intermediate excited state lies between the given excited state and the other excited state.
  13. The method according to claim 9, wherein the frequency is a first frequency, the continuous microwave signal is a first continuous microwave signal, and the method further comprises the step of generating a second continuous microwave signal having a second frequency equal to the transition frequency between an excited state immediately above the ground state of the qubit and the other excited state of the qubit.
  14. The method according to claim 9 , wherein the second bias magnetic flux is supplied in a magnitude between 3/5 and 4/5 of one magnetic flux quantum.
  15. The method according to claim 9, wherein the step of generating the continuous microwave signal having a frequency equal to the transition frequency between the other excited state of the qubit and the given excited state comprises generating the continuous microwave signal over a predetermined time, the predetermined time being a function of the relaxation time of the qubit from the given excited state to the ground state.

Description

This invention relates to quantum systems, and more particularly to microwave-based resetting of persistent current qubits. (Government interests) This invention was made under a government contract. Therefore, the U.S. government has rights to this invention as defined in that contract. Preparing qubits to clearly defined initial states is a crucial requirement for quantum computing algorithms. In particular, for most quantum algorithms, it is assumed that a large number of high-fidelity ground-state qubits are available to function as ancillary qubits in various operations. The current method for preparing ground-state persistent-current qubits involves applying a large DC flux shift to destabilize the excited state and waiting for it to collapse back to the ground state. Unfortunately, this requires a relatively high-bandwidth flux bias line to apply the destabilization pulse to the persistent-current qubit. Adding this high-bandwidth control line to the circuit introduces broadband noise that causes decoherence. In one example, the assembly includes a qubit containing a superconducting loop separated by a composite Josephson junction. A first bias source supplies a first bias to the superconducting loop, and a second bias source supplies a second bias to the composite Josephson junction. Each of the first and second bias sources, in response to system control, is configured such that a first value is selected for the first bias and a second value is selected for the second bias. Each of the first and second values is selected such that a given excited state of the qubit lies near the top of the potential barrier associated with the qubit's potential. A microwave source generates a continuous microwave signal having a frequency equal to the transition frequency between the given excited state and other excited states of the qubit. Another example provides a method for resetting a qubit comprising a superconducting loop and a composite Josephson junction. A first bias flux is supplied to the superconducting loop. A second bias flux is supplied to the composite Josephson junction. Each of the first and second bias fluxes is supplied such that a given excited state of the qubit is near the top of the potential barrier associated with the qubit's potential. A continuous microwave signal is generated having a frequency equal to the transition frequency between the other excited states of the qubit and the given excited state. Further examples provide a method for resetting a flux qubit, including a superconducting loop and a composite Josephson junction. A first bias flux is supplied to the superconducting loop. A second bias flux is supplied to the composite Josephson junction. Each of the first and second bias fluxes is supplied such that the second excited state of the flux qubit is near the top of the potential barrier associated with the qubit's potential. A continuous microwave signal is generated having a frequency equal to the transition frequency between the first and second excited states of the qubit. This figure shows an example of a system for resetting a persistent current qubit to its ground state.This figure shows an example of a flux qubit that can be reset using a microwave-based reset process.Figure 2 is a chart representing the potential of the magnetic flux qubit.This figure shows a method for resetting a flux qubit that includes a superconducting loop and a composite Josephson junction.This figure shows another method for resetting a flux qubit, including a superconducting loop and a composite Josephson junction. Where used herein, the term “includes” means “includes but not limited to,” and the term “including” means “includes but not limited to.” The term “based on” means “based on at least in part.” In addition, where this disclosure or claims enumerate “a,” “an,” “a first,” or “another” components, or their equivalents, it should be interpreted as including one or more such components, and not requiring or excluding two or more such components. Ordinal terms such as “first” or “second” are generally arbitrary and do not imply a particular order, except when used to describe excited states of a qubit or other quantum system. For example, the first excited state is the state of a qubit immediately above the ground state. The systems and methods described herein provide microwave-based reset or initialization of persistent current qubits. The systems and methods described herein utilize microwave tones to transition the persistent current qubit from an excited state through an intermediate state to the ground state. This microwave-based reset allows the use of only a low-pass filtered DC flux bias line for device tuning and incorporates a narrowband microwave drive for resetting, resulting in reduced noise and less decoherence. Figure 1 shows an example of a system 100 for resetting a persistent current qubit 110 to its ground state. The persistent current qubit 110 can be implemented,