1. Department of Physics, University of California, Santa Barbara, California 93106, USA


2. Green Innovation Research Laboratories, NEC Corporation, Tsukuba, Ibaraki 305-8501, Japan

Correspondence to: John M. Martinis¹ Email: martinis@physics.ucsb.edu

Abstract

Entanglement is one of the key resources required for quantum computation¹, so the experimental creation and measurement of entangled states is of crucial importance for various physical implementations of quantum computers². In superconducting devices³, two-qubit entangled states have been demonstrated and used to show violations of Bell’s inequality⁴ and to implement simple quantum algorithms⁵. Unlike the two-qubit case, where all maximally entangled two-qubit states are equivalent up to local changes of basis, three qubits can be entangled in two fundamentally different ways⁶. These are typified by the states |GHZ = (|000 + |111)/ and |W = (|001 + |010 + |100)/. Here we demonstrate the operation of three coupled superconducting phase qubits⁷ and use them to create and measure |GHZ and |W states. The states are fully characterized using quantum state tomography⁸ and are shown to satisfy entanglement witnesses⁹, confirming that they are indeed examples of three-qubit entanglement and are not separable into mixtures of two-qubit entanglement.

ftp://mail.ihim.uran.ru/localfiles/nature09418.pdf