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Experimental deterministic correction of qubit loss

Abstract

The successful operation of quantum computers relies on protecting qubits from decoherence and noise, which—if uncorrected—will lead to erroneous results. Because these errors accumulate during an algorithm, correcting them is a key requirement for large-scale and fault-tolerant quantum information processors. Besides computational errors, which can be addressed by quantum error correction1,2,3,4,5,6,7,8,9, the carrier of the information can also be completely lost or the information can leak out of the computational space10,11,12,13,14. It is expected that such loss errors will occur at rates that are comparable to those of computational errors. Here we experimentally implement a full cycle of qubit loss detection and correction on a minimal instance of a topological surface code15,16 in a trapped-ion quantum processor. The key technique used for this correction is a quantum non-demolition measurement performed via an ancillary qubit, which acts as a minimally invasive probe that detects absent qubits while imparting the smallest quantum mechanically possible disturbance to the remaining qubits. Upon detecting qubit loss, a recovery procedure is triggered in real time that maps the logical information onto a new encoding on the remaining qubits. Although the current demonstration is performed in a trapped-ion quantum processor17, the protocol is applicable to other quantum computing architectures and error correcting codes, including leading two- and three-dimensional topological codes. These deterministic methods provide a complete toolbox for the correction of qubit loss that, together with techniques that mitigate computational errors, constitute the building blocks of complete and scalable quantum error correction.

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Fig. 1: The surface code and correction of qubit loss.
Fig. 2: Experimental realization of the 1+4-qubit algorithm aiming at loss detection and correction.
Fig. 3: Investigating the performance of the QND loss-detection unit.

Data availability

The data underlying the findings of this work are available at https://doi.org/10.5281/zenodo.3900057Source data are provided with this paper.

Code availability

All codes used for data analysis are available from the corresponding author upon reasonable request.

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Acknowledgements

We gratefully acknowledge funding by the US Army Research Office (ARO) through grant number W911NF-14-1-0103. We also acknowledge funding by the Austrian Science Fund (FWF), through the SFB BeyondC (FWF Project number F71), by the Austrian Research Promotion Agency (FFG) contract 872766, by the EU H2020-FETFLAG-2018-03 under Grant Agreement number 820495, and by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), via US ARO Grant number W911NF-16-1-0070. All statements of fact, opinions or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of ODNI, the IARPA, or the US Government. We acknowledge support from the Samsung Advanced Institute of Technology Global Research Outreach. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement number 801110 and the Austrian Federal Ministry of Education, Science and Research (BMBWF). The information provided in this Article reflects only the authors’ views; the EU Agency is not responsible for any use that may be made of this information.

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Contributions

D.V. and M. Müller derived the theory results. R.S., A.E., L.P., M. Meth, M.R., P.S. and T.M. performed the experiments. R.S. analysed the data. T.M., M. Müller and R.B. supervised the project. All authors contributed to the writing of the manuscript.

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Correspondence to Roman Stricker.

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The authors declare no competing interests.

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Peer review information Nature thanks Tom Stace and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Supplementary Information

This file contains Supplementary Sections 1–5, which contain further experimental and theoretical results and details on the detection and correction of qubit loss. It includes Supplementary Figures 1–6 and Supplementary Tables 1–3.

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Supplementary Data This file contains source data for Supplementary Figures 2–6 and Supplementary Tables 1–3.

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Stricker, R., Vodola, D., Erhard, A. et al. Experimental deterministic correction of qubit loss. Nature 585, 207–210 (2020). https://doi.org/10.1038/s41586-020-2667-0

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