A non-local quantum eraser

The complementarity behavior of quantum systems is strikingly illustrated by the quantum eraser, where one can actively choose whether or not to erase which-path information of one particle by performing suitable measurements on another particle entangled with it [1-2]. Quantum mechanics predicts that this choice can be arbitrarily delayed and spatially separated from interference [1-3]. We report the first quantum eraser experiment performed under Einstein locality, i.e. under relativistic space-like separation. We employ the hybrid entanglement between path and polarization of photon pairs and distribute the photons over an optical fibre link of 55 m and, in another experiment, over a free-space link of 144 km. A complementarity inequality is measured and well fits the predictions of quantum mechanics. Our experiment represents a conclusive demonstration of the quantum eraser concept. \\[0pt] [1] M. O. Scully, K. Dr\"{u}hl, Phys. Rev. A 25, 2208 (1982). [2] J. A. Wheeler, in Quantum Theory and Measurement (1984). [3] V. Jacques, \textit{et al.}, Science 315, 966 (2007).