Tunable delay of Einstein–Podolsky–Rosen entanglement

Abstract

Quantum networks will require 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. Marino et al. use a process called four-wave mixing in atomic vapour cells to obtain an optically tunable delay for EPR entangled beams of light. The four-wave mixing preserves the quantum spatial correlations of the entangled beams, allowing the authors to demonstrate a rudimentary quantum memory for images. This paper uses four-wave mixing in atomic vapour cells to obtain an optically tunable delay for Einstein–Podolsky–Rosen entangled beams of light. The four-wave mixing preserves the quantum spatial correlations of the entangled beams, allowing demonstration of rudimentary quantum memory for images. 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.