Generation of three-qubit entangled states using superconducting phase qubits

Matthew Neeley, Radoslaw C. Bialczak, M. Lenander, E. Lucero, Matteo Mariantoni, A. D. O’Connell, D. Sank, H. Wang, M. Weides, J. Wenner, Y. Yin, T. Yamamoto, A. N. Cleland & John M. Martinis

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.

**Groupmeeting by Dylan Jervis, Nov 18th, 2010**