1. National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA
2. †Present address: Los Alamos National Laboratory P-21, MS D454, Los Alamos, New Mexico 87545, USA

Correspondence to: D. Leibfried¹ Correspondence and requests for materials should be addressed to D.L. (Email: dil@boulder.nist.gov).

Abstract

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 computing^{1, 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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