Cooling high motional states in ion trap quantum computers

1. A method for cooling motional states in an ion trap for quantum computers, comprising:

performing a first cooling followed by a second cooling to an ion in the ion trap to sweep population of non-zero motional states associated with a motional mode to a zero motional state;

applying a gate interaction on a red sideband;

detecting, in response to the application of the gate interaction on the red sideband, a population of non-zero motional states of the motional mode that remains after performing the first cooling followed by the second cooling; and

removing at least part of the population of non-zero motional states.

2. The method of claim 1 , wherein the non-zero motional states include motional states with a motional mode number n≥10, n≥20, n≥30, n≥40, or n≥50.

3. The method of claim 1 , wherein the first cooling is a Doppler cooling and the second cooling is a sideband cooling.

4. The method of claim 3 , further comprising:

pumping the ion into a dark state after performing the Doppler cooling followed by the sideband cooling,

wherein detecting the population of non-zero motional states of the motional mode includes detecting whether the ion remains in the dark state or transitioned to a bright state in response to the application of the gate interaction on the red sideband.

5. The method of claim 4 , further comprising:

proceeding to a quantum computation in response to the dark state being detected, otherwise proceeding back to subsequent Doppler cooling followed by a subsequent sideband cooling in response to the bright state being detected.

6. The method of claim 5 , further comprising:

in response to the dark state being detected, determining a probability that the ion is in a high thermal state; and

proceeding back to the pumping of the ion into the dark state in response to the probability being greater than a threshold, otherwise proceeding to use the ion in the quantum operation.

7. The method of claim 6 , wherein proceeding to use the ion in the quantum operation includes pumping the ion into the dark state prior to being used in the quantum operation.

8. The method of claim 4 , wherein a cycle or iteration includes the Doppler cooling and the sideband cooling, the pumping the ion into a dark state, the applying the gate interaction on the red sideband, and the detecting whether the ion remains in the dark state or transitioned to the bright state, and wherein the method includes performing multiple cycles or iterations.

9. The method of claim 8 , wherein each cycle or iteration is tuned to a particular motional state of the motional states associated with the motional mode, or each cycle or iteration is tuned to a different motional state of the motional states associated with the motional mode to cover a band of motional states.

10. The method of claim 1 , wherein for the ion in the ion trap, the detection of the population of non-zero motional states is performed for two or more of the three motional modes associated with the ion.

11. A method for cooling motional states in an ion trap for quantum computers, comprising:

detecting, for one or more motional modes in one or more ions in the ion trap, a population of non-zero motional states that remain after performing a Doppler cooling, a sideband cooling, and a gate interaction on a red sideband; and

removing at least part of the population of non-zero motional states.

12. The method of claim 11 , wherein for one ion in the ion trap the detection is repeated on two or more of the motional modes of the ion.

13. The method of claim 11 , wherein for two or more ions in the ion trap the two or more ions are subject to a common red sideband transition associated with the gate interaction on the red sideband for detecting the population of non-zero motional states for a single motional mode of the ions.

14. The method of claim 11 , wherein for two or more ions in the ion trap the two or more ions are individually addressed and a red sideband transition associated with the gate interaction on the red sideband on each ion is tuned for detecting the population of non-zero motional states for a particular motional mode or motional modes.

15. The method of claim 11 , wherein for two or more ions in the ion trap only a subset of the two or more ions are subject to a red sideband transition associated with the gate interaction on the red sideband.

16. The method of claim 11 , wherein two or more ions in the ion trap are different species of ions.

17. The method of claim 11 , wherein two or more ions in the ion trap are used for detecting the population of non-zero motional states in more than one motional mode.

18. The method of claim 11 , further comprising flagging data for a quantum computation to indicate that a population of non-zero motional states was detected.

19. A method for cooling motional states in an ion trap for quantum computers, comprising:

performing a Doppler cooling on an ion in the ion trap;

applying a gate interaction on a red sideband; and

detecting, in response to the application of the gate interaction on the red sideband, whether a population of non-zero motional states of the motional mode remains after performing the Doppler cooling;

redistributing the non-zero motional states by a subsequent Doppler cooling in response to a population of the non-zero motional states being detected; and

applying sideband cooling to sweep the remaining population to a zero motional state.

20. A quantum information processing (QIP) system for cooling down one or more motional modes, comprising:

an ion trap having one or more ions;

an optical controller;

an imaging system; and

a cool down component configured to:

perform, using the optical controller, a Doppler cooling followed by a sideband cooling to an ion in the ion trap to sweep population of motional states associated with a motional mode to a zero motional state;

apply, using the optical controller, a gate interaction on a red sideband;

detect, using the imaging system, in response to the application of the gate interaction on the red sideband, a population of non-zero motional states of the motional mode that remains after performing the Doppler cooling followed by the sideband cooling; and

remove, using the optical controller, at least part of the population of non-zero motional states.