Experimental investigation of the uncertainty principle in the presence of quantum memory

TL;DR

This paper experimentally tests a generalized quantum uncertainty relation involving quantum memory, demonstrating that entanglement can significantly reduce measurement uncertainties and serve as a witness for entanglement detection.

Contribution

First experimental verification of the entropic uncertainty relation with quantum memory using entangled photons and optical delay lines.

Findings

Uncertainty is reduced when quantum memory is entangled with the system.

Experimental results agree with quantum theory predictions.

The inequality can be used to witness entanglement.

Abstract

Heisenberg's uncertainty principle provides a fundamental limitation on an observer's ability to simultaneously predict the outcome when one of two measurements is performed on a quantum system. However, if the observer has access to a particle (stored in a quantum memory) which is entangled with the system, his uncertainty is generally reduced. This effect has recently been quantified by Berta et al. [Nature Physics 6, 659 (2010)] in a new, more general uncertainty relation, formulated in terms of entropies. Using entangled photon pairs, an optical delay line serving as a quantum memory and fast, active feed-forward we experimentally probe the validity of this new relation. The behaviour we find agrees with the predictions of quantum theory and satisfies the new uncertainty relation. In particular, we find lower uncertainties about the measurement outcomes than would be possible without the entangled particle. This shows not only that the reduction in uncertainty enabled by entanglement can be significant in practice, but also demonstrates the use of the inequality to witness entanglement.