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Preparation and measurement of three-qubit entanglement in a superconducting circuit


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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Figure 1: Four-qubit cQED processor, and spectroscopic characterization.
Figure 2: Frequency- and time-domain characterization of two-qubit-gate primitive.
Figure 3: Building 3QE with two two-qubit gates.
Figure 4: Witnessing of 3QE using fidelity and Mermin inequalities.


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We thank M. Brink for experimental contributions, and L. S. Bishop, E. Ginossar, O. Gühne, C. Rigetti, D. I. Schuster and J. Siewert for discussions. We acknowledge support from LPS/NSA under ARO contract W911NF-05-1-0365, from IARPA under ARO contract W911NF-09-1-0369 and from the NSF under grants DMR-0653377 and DMR-0603369. Additional support was provided by CNR-Istituto di Cibernetica, Pozzuoli, Italy (L.F.), and by CIFAR, MITACS, MRI and NSERC (J.M.G.). 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 the US Government.

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L.D.C., M.D.R. and L.S. carried out measurements and data analysis. B.R.J. and J.M.C. provided further experimental contributions. J.M.G. provided theory support. L.F., L.D.C. and L.S. fabricated devices. L.D.C. and M.H.D. wrote the manuscript, with feedback from all authors. S.M.G., M.H.D. and R.J.S. designed and supervised the project.

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Correspondence to L. DiCarlo or R. J. Schoelkopf.

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The authors declare no competing financial interests.

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DiCarlo, L., Reed, M., Sun, L. et al. Preparation and measurement of three-qubit entanglement in a superconducting circuit. Nature 467, 574–578 (2010).

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