Nature 457, 859-862 (12 February 2009) | doi:10.1038/nature07751; Received 28 October 2008; Accepted 22 December 2008

Tunable delay of Einstein–Podolsky–Rosen entanglement

A. M. Marino1, R. C. Pooser1, V. Boyer1,2 & P. D. Lett1

  1. Joint Quantum Institute, National Institute of Standards and Technology and University of Maryland, Gaithersburg, Maryland 20899, USA
  2. MUARC, School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Correspondence to: A. M. Marino1 Correspondence and requests for materials should be addressed to A.M.M. (Email: alberto.marino@nist.gov).

Entangled systems display correlations that are stronger than can be obtained classically. This makes entanglement an essential resource for a number of applications, such as quantum information processing, quantum computing and quantum communications1, 2. The ability to control the transfer of entanglement between different locations will play a key role in these quantum protocols and enable quantum networks3. Such a transfer requires a system that can delay quantum correlations without significant degradation, effectively acting as a short-term quantum memory. An important benchmark for such systems is the ability to delay Einstein–Podolsky–Rosen (EPR) levels of entanglement and to be able to tune the delay. EPR entanglement is the basis for a number of quantum protocols, allowing the remote inference of the properties of one system (to better than its standard quantum limit) through measurements on the other correlated system. Here we show that a four-wave mixing process based on a double-lambda scheme in hot 85Rb vapour allows us to obtain an optically tunable delay for EPR entangled beams of light. A significant maximum delay, of the order of the width of the cross-correlation function, is achieved. The four-wave mixing also preserves the quantum spatial correlations of the entangled beams. We take advantage of this property to delay entangled images, making this the first step towards a quantum memory for images4.