# On-demand quantum state transfer and entanglement between remote microwave cavity memories

## Abstract

Coupling isolated quantum systems through propagating photons is a central theme in quantum science1,2, with the potential for groundbreaking applications such as distributed, fault-tolerant quantum computing3,4,5. To date, photons have been used widely to realize high-fidelity remote entanglement6,7,8,9,10,11,12 and state transfer13,14,15 by compensating for inefficiency with conditioning, a fundamentally probabilistic strategy that places limits on the rate of communication. In contrast, here we experimentally realize a long-standing proposal for deterministic, direct quantum state transfer16. Using efficient, parametrically controlled emission and absorption of microwave photons, we show on-demand, high-fidelity state transfer and entanglement between two isolated superconducting cavity quantum memories. The transfer rate is faster than the rate of photon loss in either memory, an essential requirement for complex networks. By transferring states in a multiphoton encoding, we further show that the use of cavity memories and state-independent transfer creates the striking opportunity to deterministically mitigate transmission loss with quantum error correction. Our results establish a compelling approach for deterministic quantum communication across networks, and will enable modular scaling of superconducting quantum circuits.

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## Acknowledgements

The authors would like to acknowledge valuable discussions with C. Zhou, A. Narla, S. Shankar and K.W. Lehnert. This work was supported by the US Army Research Office (W911NF-14-1-0011). C.J.A. was supported by the NSF Graduate Research Fellowship (DGE-1122492); L.D.B. by the ARO QuaCGR Fellowship; W.P. by the NSF (PHY1309996) and by a fellowship instituted with a Max Planck Research Award from the Alexander von Humboldt Foundation; W.P., P.R. and M.Z. by the US Air Force Office of Scientific Research (FA9550-15-1-0015); S.M.G by the NSF (DMR-1609326); L.J. by the Alfred P. Sloan Foundation (BR2013-049) and the Packard Foundation (2013-39273). Facilities use was supported by the Yale Institute for Nanoscience and Quantum Engineering (YINQE) and the Yale SEAS cleanroom.

## Author information

Authors

### Contributions

C.J.A., L.D.B. and W.P. performed the experiment and analysed the data under the supervision of R.J.S. C.J.A. and L.F. fabricated the transmon qubits. M.Z. assisted in data analysis and ran supporting simulations. M.Z., S.M.G. and L.J. provided theory support. K.C. assisted in development of a Wigner reconstruction algorithm. P.C.-I. contributed to the experimental design under the supervision of M.H.D. P.R. developed optimal control pulse software and implemented field measurement capability. C.J.A., L.D.B., W.P. and R.J.S. wrote the manuscript with contributions from all authors.

### Corresponding authors

Correspondence to Christopher J. Axline or Luke D. Burkhart or Wolfgang Pfaff or R. J. Schoelkopf.

## Ethics declarations

### Competing interests

R.J.S., M.H.D. and L.F. are founders, and R.J.S. and L.F. are equity shareholders of Quantum Circuits, Inc.

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## Supplementary information

### Supplementary Information

Supplementary notes, Supplementary figures, Supplementary references

## Rights and permissions

Reprints and Permissions

Axline, C.J., Burkhart, L.D., Pfaff, W. et al. On-demand quantum state transfer and entanglement between remote microwave cavity memories. Nature Phys 14, 705–710 (2018). https://doi.org/10.1038/s41567-018-0115-y

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