Atomic homodyne detection of continuous-variable entangled twin-atom states C. Gross, H. Strobel, E. Nicklas, T. Zibold, N. Bar-Gill, G. Kurizki & M. K. Oberthaler Affiliations Contributions Corresponding author Nature 480,219–223(08 December 2011)doi:10.1038/nature10654Received04 March 2011Accepted18 October 2011Published online30 November 2011 Historically, the completeness of quantum theory has been questioned using the concept of bipartite continuous-variable entanglement1. The non-classical correlations (entanglement) between the two subsystems imply that the observables of one subsystem are determined by the measurement choice on the other, regardless of the distance between the subsystems. Nowadays, continuous-variable entanglement is regarded as an essential resource, allowing for quantum enhanced measurement resolution2, the realization of quantum teleportation3, 4, 5 and quantum memories3, 6, or the demonstration of the Einstein–Podolsky–Rosen paradox1, 7, 8, 9. These applications rely on techniques to manipulate and detect coherences of quantum fields, the quadratures. Whereas in optics coherent homodyne detection10 of quadratures is a standard technique, for massive particles a corresponding method was missing. Here we report the realization of an atomic analogue to homodyne detection for the measurement of matter-wave quadratures. The application of this technique to a quantum state produced by spin-changing collisions in a Bose–Einstein condensate11, 12 reveals continuous-variable entanglement, as well as the twin-atom character of the state13. Our results provide a rare example of continuous-variable entanglement of massive particles6, 14. The direct detection of atomic quadratures has applications not only in experimental quantum
