《Measuing complete quantum states with a single observable(Xinhua Peng)》.pdf

PHYSICAL REVIEW A 76, 042117 2007 Measuring complete quantum states with a single observable Xinhua Peng,1,* Jiangfeng Du,1,2 and Dieter Suter1 1 Fachbereich Physik, Universität Dortmund, 44221 Dortmund, Germany 2 Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Received 25 April 2007; published 18 October 2007 Experimental determination of an unknown quantum state usually requires several incompatible measure- ments. However, it is also possible to determine the full quantum state from a single, repeated measurement. For this purpose, the quantum system whose state is to be determined is first coupled to a second quantum system the “assistant” in such a way that part of the information in the quantum state is transferred to the assistant. The actual measurement is then performed on the enlarged system including the original system and the assistant. We discuss in detail the requirements of this procedure and experimentally implement it on a simple quantum system consisting of nuclear spins. DOI: 10.1103/PhysRevA.76.042117 PACS numbers: 03.65.Wj, 03.67.a, 05.30. d I. INTRODUCTION ased”. This choice assures that the data redundancy is mini- mized and the information content is maximiz