Dissipative production of a maximally entangled steady state of two quantum bits

Abstract

Entangled states are a key resource in fundamental quantum physics, quantum cryptography and quantum computation1. Introduction of controlled unitary processes—quantum gates—to a quantum system has so far been the most widely used method to create entanglement deterministically2. These processes require high-fidelity state preparation and minimization of the decoherence that inevitably arises from coupling between the system and the environment, and imperfect control of the system parameters. Here we combine unitary processes with engineered dissipation to deterministically produce and stabilize an approximate Bell state of two trapped-ion quantum bits (qubits), independent of their initial states. Compared with previous studies that involved dissipative entanglement of atomic ensembles3 or the application of sequences of multiple time-dependent gates to trapped ions4, we implement our combined process using trapped-ion qubits in a continuous time-independent fashion (analogous to optical pumping of atomic states). By continuously driving the system towards the steady state, entanglement is stabilized even in the presence of experimental noise and decoherence. Our demonstration of an entangled steady state of two qubits represents a step towards dissipative state engineering, dissipative quantum computation and dissipative phase transitions5,6,7. Following this approach, engineered coupling to the environment may be applied to a broad range of experimental systems to achieve desired quantum dynamics or steady states. Indeed, concurrently with this work, an entangled steady state of two superconducting qubits was demonstrated using dissipation8.

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Figure 1: Energy levels and entanglement preparation scheme.
Figure 2: Steady-state entanglement.
Figure 3: Entanglement with stepwise scheme.

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Acknowledgements

This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA). All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of IARPA or the ODNI. This work was also supported by ONR, by the NIST Quantum Information Program, and by the European Union’s Seventh Framework Program through SIQS (grant no. 600645) and through the ERC grant QIOS (grant no. 306576). We thank D. Allcock and B. Sawyer for comments on the manuscript and E. Knill for conversations. F.R. acknowledges conversations with B. Lanyon, R. Blatt and J. Home and support from the Studienstiftung des deutschen Volkes. This Letter is a contribution of NIST and is not subject to US copyright.

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Y.L. and J.P.G. performed the experiments, analysed the data and developed the numerical model. F.R. proposed the entanglement scheme and developed the analytic rate model described in Supplementary Information under the guidance of A.S.S. T.R.T. contributed to the numerical model and the experimental apparatus. R.B. contributed to the experimental apparatus. D.L. and D.J.W. directed the experiments. All authors provided important suggestions for the experiments, discussed the results and contributed to the manuscript.

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Correspondence to Y. Lin.

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The authors declare no competing financial interests.

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This file contains Supplementary Text, Supplementary Figures 1-2 and additional references. (PDF 212 kb)

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Lin, Y., Gaebler, J., Reiter, F. et al. Dissipative production of a maximally entangled steady state of two quantum bits. Nature 504, 415–418 (2013). https://doi.org/10.1038/nature12801

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