Thursday, November 25, 2010

A time-symmetric formulation of quantum mechanics

Yakir Aharonov, Sandu Popescu, and Jeff Tollaksen

Quantum mechanics allows one to independently select both the initial and final states of a single system. Such pre- and postselection reveals novel effects that challenge our ideas about what time is and how it flows.

Thursday, November 18, 2010

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 |GHZright fence = (|000right fence+|111right fence)/ and |Wright fence = (|001right fence+|010right fence+|100right fence)/ . Here we demonstrate the operation of three coupled superconducting phase qubits7 and use them to create and measure |GHZright fence and |Wright fence 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.