# Quantum entanglement between an atom and a molecule

## Abstract

Conventional information processors convert information between different physical carriers for processing, storage and transmission. It seems plausible that quantum information will also be held by different physical carriers in applications such as tests of fundamental physics, quantum enhanced sensors and quantum information processing. Quantum controlled molecules, in particular, could transduce quantum information across a wide range of quantum bit (qubit) frequencies—from a few kilohertz for transitions within the same rotational manifold1, a few gigahertz for hyperfine transitions, a few terahertz for rotational transitions, to hundreds of terahertz for fundamental and overtone vibrational and electronic transitions—possibly all within the same molecule. Here we demonstrate entanglement between the rotational states of a 40CaH+ molecular ion and the internal states of a 40Ca+ atomic ion2. We extend methods used in quantum logic spectroscopy1,3 for pure-state initialization, laser manipulation and state readout of the molecular ion. The quantum coherence of the Coulomb coupled motion between the atomic and molecular ions enables subsequent entangling manipulations. The qubit addressed in the molecule has a frequency of either 13.4 kilohertz1 or 855 gigahertz3, highlighting the versatility of molecular qubits. Our work demonstrates how molecules can transduce quantum information between qubits with different frequencies to enable hybrid quantum systems. We anticipate that our method of quantum control and measurement of molecules will find applications in quantum information science, quantum sensors, fundamental and applied physics, and controlled quantum chemistry.

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## Data availability

The data that support the findings of this work are available from the corresponding author upon reasonable request.

## Code availability

The computer code used to analyse the data is available from the corresponding author upon reasonable request.

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## Acknowledgements

We thank J. Du, J. J. Bollinger and A. L. Collopy for reading and providing feedback on this manuscript, and C. Kurz for help on the experimental setup. This work was supported by the US Army Research Office (grant number W911NF-19-1-0172). Y.L. acknowledges support from the National Key R&D Program of China (grant number 2018YFA0306600), the National Natural Science Foundation of China (grant number 11974330) and Anhui Initiative in Quantum Information Technologies (grant number AHY050000).

## Author information

Authors

### Contributions

Y.L., C.-w.C., D.R.L. and D.L. conceived and designed the experiments and contributed to the development of experimental methods and pulse sequences. C.-w.C., D.R.L. and D.L. developed components of the experimental apparatus. Y.L. collected and analysed the data. Y.L., C.-w.C. and D.L. wrote the manuscript. All authors provided suggestions for the experiments, discussed the results and contributed to the editing of the manuscript.

### Corresponding author

Correspondence to Yiheng Lin.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Lawrence Cheuk, Barak Dayan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Reprints and Permissions

Lin, Y., Leibrandt, D.R., Leibfried, D. et al. Quantum entanglement between an atom and a molecule. Nature 581, 273–277 (2020). https://doi.org/10.1038/s41586-020-2257-1