Among the classes of highly entangled states of multiple quantum systems, the so-called ‘Schrödinger cat’ states are particularly useful. Cat states are equal superpositions of two maximally different quantum states. They are a fundamental resource in fault-tolerant quantum computing1,2,3 and quantum communication, where they can enable protocols such as open-destination teleportation4 and secret sharing5. They play a role in fundamental tests of quantum mechanics6 and enable improved signal-to-noise ratios in interferometry7. Cat states are very sensitive to decoherence, and as a result their preparation is challenging and can serve as a demonstration of good quantum control. Here we report the creation of cat states of up to six atomic qubits. Each qubit's state space is defined by two hyperfine ground states of a beryllium ion; the cat state corresponds to an entangled equal superposition of all the atoms in one hyperfine state and all atoms in the other hyperfine state. In our experiments, the cat states are prepared in a three-step process, irrespective of the number of entangled atoms. Together with entangled states of a different class created in Innsbruck8, this work represents the current state-of-the-art for large entangled states in any qubit system.
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This work was supported by the US National Security Agency (NSA) and Advanced Research and Development Activity (ARDA), by the Department of Defence Multidisciplinary University Research Initiative (MURI) programme administered by the Office of Naval Research and by NIST. We thank S. Glancy and J. Bollinger for comments on the manuscript. This paper is a contribution by the National Institute of Standards and Technology and not subject to US copyright.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Leibfried, D., Knill, E., Seidelin, S. et al. Creation of a six-atom ‘Schrödinger cat’ state. Nature 438, 639–642 (2005). https://doi.org/10.1038/nature04251
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