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# Control and single-shot readout of an ion embedded in a nanophotonic cavity

## Abstract

Distributing entanglement over long distances using optical networks is an intriguing macroscopic quantum phenomenon with applications in quantum systems for advanced computing and secure communication1,2. Building quantum networks requires scalable quantum light–matter interfaces1 based on atoms3, ions4 or other optically addressable qubits. Solid-state emitters5, such as quantum dots and defects in diamond or silicon carbide6,7,8,9,10, have emerged as promising candidates for such interfaces. So far, it has not been possible to scale up these systems, motivating the development of alternative platforms. A central challenge is identifying emitters that exhibit coherent optical and spin transitions while coupled to photonic cavities that enhance the light–matter interaction and channel emission into optical fibres. Rare-earth ions in crystals are known to have highly coherent 4f–4f optical and spin transitions suited to quantum storage and transduction11,12,13,14,15, but only recently have single rare-earth ions been isolated16,17 and coupled to nanocavities18,19. The crucial next steps towards using single rare-earth ions for quantum networks are realizing long spin coherence and single-shot readout in photonic resonators. Here we demonstrate spin initialization, coherent optical and spin manipulation, and high-fidelity single-shot optical readout of the hyperfine spin state of single 171Yb3+ ions coupled to a nanophotonic cavity fabricated in an yttrium orthovanadate host crystal. These ions have optical and spin transitions that are first-order insensitive to magnetic field fluctuations, enabling optical linewidths of less than one megahertz and spin coherence times exceeding thirty milliseconds for cavity-coupled ions, even at temperatures greater than one kelvin. The cavity-enhanced optical emission rate facilitates efficient spin initialization and single-shot readout with conditional fidelity greater than 95 per cent. These results showcase a solid-state platform based on single coherent rare-earth ions for the future quantum internet.

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## Data availability

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

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## Acknowledgements

This work was funded by a National Science Foundation (NSF) Faculty Early Career Development Program (CAREER) award (1454607), the AFOSR Quantum Transduction Multidisciplinary University Research Initiative (FA9550-15-1-0029), NSF 1820790, and the Institute of Quantum Information and Matter, an NSF Physics Frontiers Center (PHY-1733907) with support from the Moore Foundation. The device nanofabrication was performed in the Kavli Nanoscience Institute at the California Institute of Technology. J.G.B. acknowledges the support from the American Australian Association’s Northrop Grumman Fellowship. J.R. acknowledges the support from the Natural Sciences and Engineering Research Council of Canada (NSERC) (PGSD3-502844-2017). Y.Q.H. acknowledges the support from the Agency for Science, Technology and Research (A*STAR) and Carl & Shirley Larson as a Frederick W. Drury Jr. SURF Fellow. We thank M. Shaw, S. Woo Nam and V. Verma for help with superconducting photon detectors; A. Sipahigil for discussion; K. Schwab for help with electronics; and D. Riedel for supporting measurements.

## Author information

Authors

### Contributions

J.M.K., J.G.B. and A.F. conceived the experiments. J.R. fabricated the nanophotonic device. J.M.K., A.R. and J.G.B. performed the experiments and analysed the data. Y.Q.H. provided simulation support. J.M.K., A.R. and A.F. wrote the manuscript with input from all authors. A.F. supervised the project.

### Corresponding author

Correspondence to Andrei Faraon.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks David Hunger, Andreas Reiserer and the other, anonymous, reviewer(s) 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 figures and tables

### Extended Data Fig. 1 Extended PLE scans and expected ion distribution.

a, Photoluminescence scan over 12 GHz centred around the optical transition of the zero-nuclear-spin isotope with zero applied magnetic field. The dashed box highlights the region scanned in Fig. 2a. b, Predicted optical transition frequencies of the different Yb isotopes for $$E\parallel c\,$$ with transition strength scaled to natural abundance.

### Extended Data Fig. 2 Spin initialization.

Blue (red) scans correspond to preparation into the $${|0\rangle }_{{\rm{g}}}$$ ($${|1\rangle }_{{\rm{g}}}$$) state. a, Initialization into qubit subspace. The number of preparation pulses on the A and fe transitions is held fixed at 100 while the number of preparation pulses on transition F is varied. b, Initialization within qubit subspace. The number of preparation pulses on F is held fixed at 150 while the number of preparation pulses on transitions A and fe is varied. The observed population contrast corresponds to an initialization fidelity of >96% within the qubit subspace.

### Extended Data Fig. 3 Additional optical coherence measurements.

a, Measurement of optical T2 on ion Y using an echo sequence. The fit gives T2 = 4.1 ± 0.2 μs. b, Post-selected optical Ramsey measurement (Supplementary Section 4) with resonant excitation showing improvement in $${T}_{2}^{* }$$ for increasing numbers of photons nc detected in a subsequent probe sequence. c, Post-selected Ramsey sequence for nc = 2 with readout detuned by 1 MHz to demonstrate that decay is due to optical coherence.

### Extended Data Fig. 4 Additional CPMG measurements.

a, Scaling of coherence time extracted from CPMG envelope (Fig. 3c) for increasing numbers of rephasing pulses. Error bars represent 68% confidence intervals for CPMG coherence times. The fit gives $${T}_{2,{\rm{s}}\,}^{N}\propto {N}^{0.70\pm 0.01}$$. b, Fine-resolution CPMG scans performed with N = 1 (red) and N = 8 (blue) rephasing pulses showing periodic collapses and revivals of the spin coherence. The N = 1 scan is offset by 500 counts for clarity.

### Extended Data Fig. 5 Spin lifetime measurements.

a, Measurement of population in the $${|0\rangle }_{{\rm{g}}}$$ and $${|1\rangle }_{{\rm{g}}}$$ states after initializing into the $${|0\rangle }_{{\rm{g}}}$$ state and waiting for time τ. Exponential fit gives a lifetime of 54 ± 5 ms. The measured population difference corresponds to a device temperature of 59 ± 4 mK. b, Slow decay of population from qubit subspace into the $${|{\rm{aux}}\rangle }_{{\rm{g}}}$$ state with decay constant 26 ± 2 s.

## Supplementary information

### Supplementary Information

This file contains Supplementary Text 1-6 and Supplementary Figures S1-S10.

## Rights and permissions

Reprints and Permissions

Kindem, J.M., Ruskuc, A., Bartholomew, J.G. et al. Control and single-shot readout of an ion embedded in a nanophotonic cavity. Nature 580, 201–204 (2020). https://doi.org/10.1038/s41586-020-2160-9

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• DOI: https://doi.org/10.1038/s41586-020-2160-9

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