Microwave-based reset of persistent current qubits

Abstract

A system and method are provided for resetting a qubit including a superconducting loop and a compound Josephson junction. A first bias magnetic flux is supplied to the superconducting loop. A second bias magnetic flux is supplied to the compound Josephson junction. Each of the first bias magnetic flux and the second bias magnetic flux is supplied such that a given excited state of the qubit is near the top of a potential barrier associated with the potential of the qubit. A continuous microwave signal having a frequency equal to the transition frequency between the other excited state of the qubit and the given excited state is generated.

Description

Technical Field

[0001] The present invention relates to quantum systems, and more particularly to microwave-based reset of persistent-current qubits. (Government Interests) The present invention was made under a government contract. Accordingly, the United States government has certain rights in the invention as specified by that contract.

Background Art

[0002] Preparing qubits in a well-defined initial state is one of the important requirements 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 ancilla qubits in various operations. Current methods for preparing ground-state persistent-current qubits involve applying a large direct current (DC) magnetic flux shift to destabilize the excited state and waiting for it to decay to the ground state. Unfortunately, this requires a relatively high-bandwidth magnetic flux bias line to apply the destabilizing pulse to the persistent-current qubit. The addition of this high-bandwidth control line to the circuit introduces broadband noise that causes decoherence.

Summary of the Invention

[0003] 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 is selected in response to system control to select a first value for the first bias and a second value for the second bias. 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 related to 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.

[0004] 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 a 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.

[0005] 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. [Brief explanation of the drawing]

[0006] [Figure 1]This figure shows an example of a system for resetting a persistent current qubit to its ground state. [Figure 2] This figure shows an example of a flux qubit that can be reset using a microwave-based reset process. [Figure 3] Figure 2 is a chart showing the potential of the magnetic flux qubit. [Figure 4] This figure shows a method for resetting a flux qubit that includes a superconducting loop and a composite Josephson junction. [Figure 5] This figure shows another method for resetting a flux qubit, including a superconducting loop and a composite Josephson junction. [Modes for carrying out the invention]

[0007] Where used herein, the term “includes” means to include but not limited to, and the term “including” means to include but not limited to. The term “based on” means to be based 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 as 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, a first excited state is a qubit state immediately above the ground state.

[0008] 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 a persistent current qubit from an excited state through an intermediate state to a 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 decoherence.

[0009] 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, for example, as a flux qubit. The persistent current qubit 110 includes a superconducting loop 112 that is blocked by a composite Josephson junction 114. In one implementation, the composite Josephson junction 114 is implemented as a direct current superconducting quantum interference device (DC SQUID). A first bias source 116 supplies a first bias to the superconducting loop, and a second bias source 118 supplies a second bias to the composite Josephson junction. In one implementation, each of the first bias source 116 and the second bias source 118 can supply flux. In one example, the qubit 110 can be tuned via the first and second bias sources 116 and 118 to take a double-well potential, where the energy states of the device include states in two potential wells separated by a potential barrier. In this example, the first bias source 116 controls the symmetry of the two wells, i.e., the depth of the wells relative to each other, and the second bias source 118 controls the height of the potential barrier between the two wells.

[0010] The first bias source 116 and the second bias source 118 are configured, in response to system control 120, to select a first value for the first bias source and a second value for the second bias source. In one implementation, the values ​​of the first and second biases can be selected from a range between 0 flux quanta and 1 flux quanta. The microwave source 122 generates a microwave signal with a frequency equal to the transition frequency between the excited state just above the ground state and the selected excited state of the qubit assembly, in response to system control 120.

[0011] When a qubit reset is desired, the system control 120 can instruct the first bias source 116 and the second bias source 118 to select values ​​for the first and second biases to provide a multi-level quantum system within the potential well. In one example, the value of the second bias is set between 3 / 5 and 4 / 5 of a flux quantum during the reset process. In particular, the height of the potential barrier and the asymmetry between the wells can be tuned such that the ground state in one well is near the bottom of the well and the excited state is near the top of the potential barrier. "Near the top of the potential barrier" means that the excited state has a significant probability amplitude in both wells of the potential. The microwave source 122 can then be instructed to generate a microwave signal at a frequency equal to the transition frequency between the excited state immediately above the ground state and the selected excited state. This has the effect of moving the population of the excited state immediately above the ground state to the selected excited state. From there, the occupied state relaxes back to the excited state just above the ground state, or relaxes back to the ground state, so that it can be transitioned back to the excited state selected by microwave excitation. By maintaining the excitation for a sufficient amount of time, any level of occupied state can be forced back to the ground state, depending on the desired fidelity of the reset to the ground state. In one example, time is predetermined as a function of the relaxation time from the selected excited state to the ground state.

[0012] Generally, the selected excited state is an excited state near the top of the potential barrier, but it will be understood that a multi-step method can be used in which a first microwave signal is applied to excite the occupied state of an excited state just above the ground state to the first selected excited state, and a second microwave signal is used to excite the occupied state of the first selected excited state to the second excited state. In this case, the first microwave signal may have a first frequency equal to the transition frequency between the excited state just above the ground state and the first selected excited state, and the second microwave signal may have a second frequency equal to the transition frequency between the first selected excited state and the second selected excited state. Furthermore, it will be understood that the selected excited state does not necessarily have to be immediately higher than the excited state just above the ground state, and the microwave signal may be selected so that the occupied state is excited to a state two or more levels higher than the excited state just above the ground state.

[0013] Figure 2 shows an example of a persistent current qubit 200, specifically a flux qubit, that can be reset using a microwave-based reset process. The qubit 200 includes a first Josephson junction 204, a second Josephson junction 205, a first inductor 206, a second inductor 208, and a superconducting loop 202 separated by a composite Josephson junction 210. A first flux source 212 supplying flux to the composite Josephson junction 210 is connected to a first current source Φ connected to the first inductor 214. α The inductor is equipped with such a structure, and the magnetic flux generated in the inductor is a function of the magnitude of the current supplied to the first inductor. The second magnetic flux source 216 that supplies magnetic flux to the superconducting loop 202 is connected to the second current source Φ connected to the second inductor 218. Δ The inductor is equipped with such a structure, and the magnetic flux generated by the inductor is a function of the magnitude of the current supplied to the second inductor.

[0014] When a reset of the flux qubit to its ground state is desired, the two flux sources 212 and 216 can be adjusted to tune the qubit to generate a three-level quantum system. Specifically, if the parameters of the critical current of the composite Josephson junction 210 and the Josephson junction in the superconducting loop 202 are selected in the correct range, and the second bias is set to have a value or magnitude of about 7 / 10 of one flux quantum, the potential energy of the device will have a double-well characteristic as shown in Figure 3. In this bias configuration, the first flux source 212 tunes the height of the barrier between the two potential wells of the flux qubit 200, and the second flux source 216 tunes the asymmetry of the two potential wells of the flux qubit 200, effectively tilting the potential to one side so that one well is deeper than the other.

[0015] Figure 3 is a chart 300 representing the potential 302 of the flux qubit 200, where the energy is expressed in gigahertz on the vertical axis 304, or more precisely, in units of the product of h and frequency expressed in gigahertz, where h is Planck's constant, and the phase of the qubit in the second Josephson junction 205 is shown on the horizontal axis 306. The chart shows the ground state, i.e., the lowest energy state |0>, the first excited state |1>, and the second excited state |2>. In the reset, it is desirable to force the flux qubit to occupy only the lowest energy state |0>, and therefore all occupied states in the excited state |1> must be forced to transition between wells. As can be seen from Figure 3, there is a large potential barrier 308 between the two wells, which makes thermal transitions across the barrier very unlikely and the rate of quantum mechanical tunneling to the other well for the first excited state is very low. However, when the flux sources 212 and 216 are properly tuned, the potential barrier separating the wavefunction of the second excited state is significantly reduced, and therefore the second excited state spreads between the two potential wells.

[0016] To complete the reset, the flux qubit 200 undergoes a transition frequency f from |1> to |2>.12 A microwave signal resonating with |1> is provided from a microwave drive (not shown) to excite the occupied state of |1> to |2>. From there, it can either oscillate back to |1> or collapse to |0> through dissipation into the environment. While the drive is applied, all occupied states that oscillate back to |1> are continuously driven back to |2>, giving them the opportunity to collapse to |0>. The transition frequency between the first excited state and the second excited state is the same as the transition frequency f between the ground state and the second excited state. 02 Because it is different, the microwave signal does not transition the occupation state from |0> to |2>, hf 02 ≫k B As long as T is the temperature, k B (where is the Boltzmann constant), and no reoccupation of |2> occurs even by thermal processes. After a microwave drive is applied over a sufficient interval, essentially all occupied states in |1> transition to |0> through decay from |2>, and the qubit is reset to the ground state. It will be understood that the sufficient interval can be based on a known or estimated relaxation time for the second excited state.

[0017] Considering the structural and functional features described above, the exemplary method will be better understood by referring to Figures 4 and 5. For the sake of simplicity, the exemplary method in Figures 4 and 5 is shown and described as being performed sequentially, but it should be understood and acknowledged that this embodiment is not limited by the illustrated order, as in other examples, some operations may be performed multiple times and / or simultaneously in an order different from that shown and described herein. Furthermore, it is not necessary to perform all described operations in order to carry out the method.

[0018] Figure 4 shows a method 400 for resetting a qubit, including a superconducting loop and a composite Josephson junction. In 402, a first bias flux is supplied to the superconducting loop. In 404, a second bias flux is supplied to the composite Josephson junction. The first and second bias fluxes are supplied such that a given excited state of the qubit is near the top of the potential barrier associated with the qubit's potential. It should be understood that in various implementations of the flux qubit, there are practical limitations on the height of the barrier, which in turn limits the range of excited states that can be selected as a given excited state. In one example, the first and second bias fluxes are supplied such that other excited states below a given excited state have probability amplitudes divided between the first well and the second well of the qubit potential. The magnitude of the first bias flux can be selected such that the first well is deeper than the second well, and other excited states are further from the bottom of the first well than the bottom of the second well. The magnitude of the second bias flux can be chosen to tune the height of the barrier near a given excited state. In one example, the second bias flux is supplied with a magnitude between 3 / 5 and 4 / 5 of one flux quantum.

[0019] At 406, a continuous microwave signal having a frequency equal to the transition frequency between another excited state of the qubit and a given excited state is generated. This causes the occupancy state to transition from another excited state to the given excited state, from where it can relax to the ground state. For this purpose, the continuous microwave signal can be generated over a predetermined time, which is a function of the relaxation time of the qubit from a given excited state to the ground state. In one example, there is an intermediate excited state between the given excited state and another excited state, and the occupancy state is excited by two or more states. In another example, a second continuous microwave signal having a second frequency equal to the transition frequency between the excited state just above the ground state of the qubit and another excited state of the qubit is used to transition the occupancy state from the excited state just above the ground state of the qubit to another excited state before and / or during the use of the first microwave signal to transition the occupancy state from another excited state to the given excited state.

[0020] FIG. 5 shows another method 500 for resetting a flux qubit including a superconducting loop and a composite Josephson junction. At 502, a first bias magnetic flux is supplied to the superconducting loop. At 504, a second bias magnetic flux is supplied to the composite Josephson junction. Each of the first bias magnetic flux and the second bias magnetic flux is supplied such that the second excited state of the flux qubit is near the top of a potential barrier associated with the potential of the flux qubit. In particular, the magnitude of each magnetic flux is selected to provide two asymmetric well potentials such that the occupancy state of the second excited state can move relatively freely between the two wells, while the occupancy state of the first excited state is split between the two wells. In one implementation, the second magnetic flux bias is supplied with a magnitude between three-fifths and four-fifths of one magnetic flux quantum.

[0021] At 506, a continuous microwave signal having a frequency equal to the transition frequency between the first excited state and the second excited state of the qubit is generated. This has the effect of shifting the occupancy state of the first excited state to the second excited state. From there, the occupancy state can relax back to the first excited state, from which it can be shifted back to the second excited state, or it can relax back to the ground state and, barring thermal processes, remain in the ground state. In one example, the continuous microwave signal is provided for a predetermined time that is a function of the relaxation time of the qubit from the second excited state to the ground state of the flux qubit.

[0022] In the foregoing description, specific details have been set forth in order to provide a thorough understanding of the exemplary implementations of the invention described in this disclosure. However, it will be apparent that various implementations may be practiced without these specific details. For example, circuits, systems, networks, processes, and other components may be shown as components in block diagram form in order not to obscure the exemplary implementations with unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the examples. The description of the exemplary implementations is provided to enable one skilled in the art to practice the examples of the invention, but it should be understood that various changes may be made in the function and configuration of the elements without departing from the spirit and scope of the invention. Accordingly, the invention is intended to embrace all such changes, modifications, and variations that fall within the scope of the appended claims. The technical concepts that can be understood from the above embodiments are described below as an addendum. [Note 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. [Note 2] The assembly according to Appendix 1, wherein the first and second values ​​are selected such that the other excited states have occupied states divided between a first well of the qubit's potential and a second well of the qubit's potential. [Note 3] The assembly as described in Appendix 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. [Note 4] The assembly as described in Appendix 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. [Note 5] The assembly as described in Appendix 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. [Note 6] The assembly according to Appendix 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. [Note 7] The assembly described in Appendix 1, wherein an intermediate excited state lies between the given excited state and the other excited states. [Note 8] The assembly described in Appendix 1, wherein the aforementioned qubit is a magnetic flux qubit. [Note 9] The assembly as described in Appendix 1, wherein the given excited state is the second excited state of the qubit, and the other excited state is the first excited state of the qubit. [Note 10] 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 associated with 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. [Note 11] The method according to Appendix 10, 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 qubit's potential and the second well of the qubit's potential. [Note 12] The method according to Appendix 11, 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. [Note 13] The method according to Appendix 10, wherein an intermediate excited state lies between the given excited state and the other excited state. [Note 14] The method according to Appendix 10, 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 states of the qubit. [Note 15] The method described in Appendix 10, wherein the second bias flux is supplied in a magnitude between 3 / 5 and 4 / 5 of one flux quantum. [Note 16] The method according to Appendix 10, 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. [Note 17] The method according to Appendix 10, wherein the given excited state is the second excited state of the qubit, and the other excited state is the first excited state of the qubit. [Note 18] A method for resetting a flux 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 the second excited state of the flux qubit is near the top of the potential barrier associated with the potential of the flux qubit; A method comprising the step of generating a continuous microwave signal having a frequency equal to the transition frequency between the first excited state and the second excited state of the magnetic flux qubit. [Note 19] The method described in Appendix 18, wherein the second bias flux is supplied in a magnitude between 3 / 5 and 4 / 5 of one flux quantum. [Note 20] The method according to Appendix 18, wherein the step of generating the continuous microwave signal having a frequency equal to the transition frequency between the first excited state and the second excited state comprises generating the continuous microwave signal over a predetermined time, the predetermined time being a function of the relaxation time of the flux qubit from the second excited state to the ground state.

Claims

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 a first well of the qubit's potential and a 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 state is 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.

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