Preparation and measurement of three-qubit entanglement in a superconducting circuit
L. DiCarlo1, M. D. Reed1, L. Sun1, B. R. Johnson1, J. M. Chow1, J. M. Gambetta2, L. Frunzio1, S. M. Girvin1, M. H. Devoret1 & R. J. Schoelkopf1
- Departments of Physics and Applied Physics, Yale University, New Haven, Connecticut 06511, USA
- Department of Physics and Astronomy and Institute for Quantum Computing, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
Correspondence to: L. DiCarlo1 Email: leonardo.dicarlo@yale.edu
Correspondence to: R. J. Schoelkopf1 Email: robert.schoelkopf@yale.edu
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Abstract
Traditionally, quantum entanglement has been central to foundational discussions of quantum mechanics. The measurement of correlations between entangled particles can have results at odds with classical behaviour. These discrepancies grow exponentially with the number of entangled particles1. With the ample experimental2, 3, 4 confirmation of quantum mechanical predictions, entanglement has evolved from a philosophical conundrum into a key resource for technologies such as quantum communication and computation5. Although entanglement in superconducting circuits has been limited so far to two qubits6, 7, 8, 9, the extension of entanglement to three, eight and ten qubits has been achieved among spins10, ions11 and photons12, respectively. A key question for solid-state quantum information processing is whether an engineered system could display the multi-qubit entanglement necessary for quantum error correction, which starts with tripartite entanglement. Here, using a circuit quantum electrodynamics architecture13, 14, we demonstrate deterministic production of three-qubit Greenberger–Horne–Zeilinger (GHZ) states15 with fidelity of 88 per cent, measured with quantum state tomography. Several entanglement witnesses detect genuine three-qubit entanglement by violating biseparable bounds by 830 ± 80 per cent. We demonstrate the first step of basic quantum error correction, namely the encoding of a logical qubit into a manifold of GHZ-like states using a repetition code. The integration of this encoding with decoding and error-correcting steps in a feedback loop will be the next step for quantum computing with integrated circuits.
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