Removing leakage in a quantum bit

1. An apparatus, comprising:

one or more qubits, wherein each qubit facilitates occupation of at least one of a plurality of qubit levels, the qubit levels including two computational qubit levels and one or more non-computational qubit levels that are each higher than the computational qubit levels, and wherein the qubit facilitates transitions between qubit levels associated with a corresponding transition frequency;

a cavity, wherein the cavity defines a cavity frequency;

one or more couplers coupling each qubit to the cavity; and

one or more couplers coupling the cavity to an environment external to the one or more qubits and the cavity;

a frequency controller that controls the frequency of qubits, wherein for each qubit that the frequency controller controls, the frequency controller is configured to iteratively perform downward qubit level transitions until a population of a non-computational level is transferred to the cavity.

2. The apparatus of claim 1 , wherein iteratively performing downward qubit level transitions comprises:

aligning the relevant qubit transition frequency to the cavity frequency; and

maintaining the alignment for a predetermined amount of time, wherein maintaining the alignment for the predetermined amount of time causes a population transfer to the cavity.

3. The apparatus of claim 1 , wherein the environment is formed by a wire which connects to a load, optionally wherein the wire comprises a readout line.

4. The apparatus of claim 1 , wherein the cavity comprises a readout resonator.

5. The apparatus of claim 1 , wherein the one or more qubits exhibit weak non-linearity.

6. A method, comprising:

accessing a quantum system that comprises:

one or more qubits, wherein each qubit facilitates occupation of at least one of a plurality of qubit levels, the qubit levels including two computational qubit levels and one or more non-computational qubit levels that are each higher than the computational qubit levels, and wherein the qubit facilitates transitions between qubit levels associated with a corresponding transition frequency;

a cavity, wherein the cavity defines a cavity frequency;

one or more couplers coupling each qubit to the cavity; and

one or more couplers coupling the cavity to an environment external to the one or more qubits and the cavity;

a frequency controller that controls the frequency of qubits, wherein for each qubit that the frequency controller controls, the frequency controller is configured to iteratively perform downward qubit level transitions until a population of a non-computational level is transferred to the cavity; and

controlling the frequency of each qubit such that, for each qubit, downward qubit level transitions are iteratively performed such that a population of a non-computational level is transferred to the cavity.

7. The method of claim 6 , wherein iteratively performing downward qubit level transitions comprises:

aligning the relevant qubit transition frequency to the cavity frequency; and

maintaining the alignment for a predetermined amount of time, wherein maintaining the alignment for the predetermined amount of time causes a population transfer to the cavity.

8. The method of claim 6 , wherein the quantum system is provided for use in a quantum computation.

9. The method of claim 6 , wherein the quantum system is provided for use in a quantum computation, and wherein the method of claim 6 is repeatedly performed after one or more computation operations in the quantum computation.

10. The method of claim 9 , wherein a final step of the one or more computation operations comprises a measurement operation.

11. The method of claim 8 , wherein the method is repeatedly performed immediately before a measurement operation associated with one or more computation operations in the quantum computation.

12. The method of claim 6 , wherein prior knowledge of a population of each of the qubit states is not needed.

13. A method, comprising:

in response to performing a computation in a quantum system that comprises:

one or more qubits, wherein each qubit facilitates occupation of at least one of a plurality of qubit levels, the qubit levels including two computational qubit levels and one or more non-computational qubit levels that are each higher than the computational qubit levels, and wherein the qubit facilitates transitions between qubit levels associated with a corresponding transition frequency;

a cavity, wherein the cavity defines a cavity frequency; and

a frequency controller that controls the frequency of qubits, wherein for each qubit that the frequency controller controls, the frequency controller is configured to iteratively perform downward qubit level transitions until a population of a non-computational level is transferred to the cavity;

controlling the frequency of each qubit such that, for each qubit, downward qubit level transitions are iteratively performed such that a population of a non-computational level is transferred to the cavity.

14. The method of claim 13 , wherein iteratively performing downward qubit level transitions comprises:

aligning the relevant qubit transition frequency to the cavity frequency; and

maintaining the alignment for a predetermined amount of time, wherein maintaining the alignment for the predetermined amount of time causes a population transfer to the cavity.