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# Realizing coherently convertible dual-type qubits with the same ion species

## Abstract

Trapped ions constitute one of the most promising systems for implementing quantum computing and networking1,2. For large-scale ion-trap-based quantum computers and networks, it is critical to have two types of qubit: one for computation and storage, and another for auxiliary operations such as qubit detection3, sympathetic cooling4,5,6,7 and entanglement generation through photon links8,9. Although the two qubit types can be implemented using two different ion species3,10,11,12,13, this approach introduces substantial complexity into creating and controlling each qubit type14,15. Here we resolve these challenges by implementing two coherently convertible qubit types using one ion species. We encode the qubits into two pairs of clock states of the 171Yb+ ions, and achieve microsecond-level conversion rates between the two types with one-way fidelities of 99.5%. We further demonstrate that operations on one qubit type, including sympathetic laser cooling, single-qubit gates and qubit detection, have crosstalk errors less than 0.06% on the other type, which is below the best-known error threshold of ~1% for fault-tolerant quantum computing using the surface code1,16. Our work establishes the feasibility and advantages of using coherently convertible dual-type qubits with the same ion species for large-scale quantum computing and networking.

## Relevant articles

• ### Erasure conversion for fault-tolerant quantum computing in alkaline earth Rydberg atom arrays

Nature Communications Open Access 09 August 2022

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## Data availability

The data that support the findings of this study are available from the authors upon request. Source data are provided with this Paper.

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## Acknowledgements

This work was supported by the Tsinghua University Initiative Scientific Research Program and the Ministry of Education of China through its fund to the IIIS.

## Author information

Authors

### Contributions

L.-M.D. proposed and supervised the experiment. H.-X.Y., J.-Y.M., Y.W., M.-M.C., W.-X.G., Y.-Y.H., L.F., Y.-K.W. and Z.-C.Z. carried out the experiment. H.-X.Y., J.-Y.M., Y.-K.W. and L.-M.D. wrote the manuscript.

### Corresponding author

Correspondence to L.-M. Duan.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

## Peer review

### Peer review information

Nature Physics thanks Cornelius Hempel and Christof Wunderlich for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Fig. 1 Detailed time sequence for measuring the crosstalk of S-qubit operations on the F-qubit.

Detailed time sequence for measuring the crosstalk of S-qubit operations on the F-qubit. a, Time sequence for the S-qubit and the F-qubit (which also starts from an S-qubit). During the verification step of the F-qubit, resonant 370-nm laser is applied, so the S-qubit needs to be reinitialized after that. b, Specific S-qubit operations for Fig. 3b to measure crosstalk errors of Raman Rabi oscillation using 355-nm laser. c, Specific S-qubit operations for Fig. 3c to measure crosstalk errors of preparation and detection of $$\left|0\right\rangle$$. d, Specific S-qubit operations for Fig. 3d to measure crosstalk errors of preparation and detection of $$\left|1\right\rangle$$.

### Extended Data Fig. 2 Carrier Rabi oscillation of the 411-nm laser and 3,432-nm laser.

Carrier Rabi oscillation of a, the 411-nm laser and b, 3,432-nm laser. The 411-nm laser has an optical power of about 0.8 mW and a beam diameter of about 8 µm, which generates a Rabi frequency of about 2π × 859.4 kHz. The 3,432-nm laser has an optical power of about 0.5 mW and a beam diameter of about 73 µm, which gives a Rabi frequency of about 2π × 1.2 MHz.

### Extended Data Fig. 3 Randomized benchmarking of the microwave-driven single-qubit gates.

Randomized benchmarking of the microwave-driven single-qubit gates for a, the S-qubit and b, the F-qubit. The average gate fidelity is (99.98 ± 0.04)% for the S-qubit and (99.99 ± 0.04)% for the F-qubit.

### Extended Data Fig. 4 Carrier Rabi oscillation of 411-nm laser after 500 µs sympathetic cooling.

Carrier Rabi oscillation of 411-nm laser after 500 µs sympathetic cooling. The fitted effective temperature is (9.2 ± 0.2) mK.

## Source data

### Source Data Fig. 2

Statistical source data.

### Source Data Fig. 3

Statistical source data.

### Source Data Fig. 4

Statistical source data.

### Source Data Extended Data Fig. 2

Statistical source data.

### Source Data Extended Data Fig. 3

Statistical source data.

### Source Data Extended Data Fig. 4

Statistical source data.

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Reprints and Permissions

Yang, HX., Ma, JY., Wu, YK. et al. Realizing coherently convertible dual-type qubits with the same ion species. Nat. Phys. (2022). https://doi.org/10.1038/s41567-022-01661-5

• Accepted:

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• DOI: https://doi.org/10.1038/s41567-022-01661-5

• ### Erasure conversion for fault-tolerant quantum computing in alkaline earth Rydberg atom arrays

• Yue Wu
• Shimon Kolkowitz
• Jeff D. Thompson

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• ### Two qubits for the price of one ion

• Cornelius Hempel

Nature Physics (2022)