Recovery of quantum coherence by spatial propagation

The preservation of coherence in quantum systems is a primary concern in many quantum communication and computation applications [1-4]. Unfortunately, a given quantum system can undergo dramatic coherence loss following even a fleeting interaction - or entanglement - with another, ancillary quantum system. This decrease in coherence inevitably leads to a loss of quantum information from the system, provided that one cannot access or measure both the system and ancilla simultaneously, and has been subject the subject of major research efforts [5]. Here, we demonstrate significant coherence loss from the orbital angular momentum (OAM) degree of freedom of an entangled photon pair, due to its entanglement with the photons´ radial profile upon spatial propagation. The entangled photons are generated from the spontaneous parametric down conversion of an OAM-carrying pump photon, and the degree of coherence loss experienced by the OAM degree of freedom is assessed via tomographic analysis. Crucially, we show also that the coherence lost from the OAM space under these conditions can be reversed with a carefully designed optical system. We refer to the recovery of coherence in the OAM space as recoherence. The recoherence effect demonstrated in this work may be exploited to allow for the propagation of quantum signals without the otherwise inescapable loss of coherence that accompanies the entanglement of OAM and radial profile Hilbert spaces.