Long-range distribution of multiphoton entanglement

Abstract

We show that long-distance quantum communications can be enabled through resources provided by current quantum photonic technology: multiphoton bipartite entanglement and photon-numberresolved detection. Our protocol is robust to high transmission losses and facilitates distribution of near-maximally entangled states in realistic implementations. It can be realized in a delayedchoice scheme and it allows one to perform loophole-free Bell tests. The schema discussed can be employed as a versatile source of entanglement for e.g. establishing an Earth-to-space quantum channel, quantum metrology and quantum key distribution.Introduction.-Distribution of photonic entanglement is a key element to building quantum networks which facilitate secure long-distance quantum communications [1], distributed quantum computation [2] and sensing [3]. However, entanglement becomes corrupted by losses in the transmitting channels which results in low transmission rates. Since amplification of quantum signals is impossible [4], alternative remedies are on high demand. One option is to use quantum repeaters [5][6][7][8][9][10][11]. However, linking distant parties necessitates numerous intermediate stations, quantum memories and multiple two-photon Bell pairs, a resource that is often created nondeterministically. The other possibility is to use satellites and space-Earth quantum communications as shown recently [12]. In this scheme, Bell-pair entanglement was distributed over 1200 km, which was confirmed by a violation of Bell inequality yet under the fair sampling assumption. Thus, since the aforementioned solutions are challenging in implementation, it remains an open problem how to distribute entanglement in a way that is resource-efficient, verifiable and well suited to the existing quantum-photonic technology.Here we propose a protocol which allows one to establish long-distance quantum communications based on multiphoton bipartite entanglement and photon-numberresolved detection. It is robust to high symmetric transmission losses which, remarkably, deteriorate only the protocol efficiency, not the amount of generated entanglement. Furthermore, the procedure enables the parties sharing entanglement to perform a loophole-free Bell test.Bipartite entanglement between two physical systems, such as, for example, two photons is revealed by the fact that outcomes of subsequent local measurements in a certain degree of freedom on these two subsystems are random but always correlated, even when the particles are separated by a large distance [13]. Here, we focus on multi-photon bipartite entanglement. Experimentally, * magdalena.stobinska@gmail.com; Corresponding author this type of state can be created by means of spontaneous parametric down-conversion (SPDC) [14]. SPDC sources deterministically produce a two-mode squeezed vacuum (SV). This Gaussian quantum state carries entanglement between quadratures of electric field (continuous variables, CVs) but also between photon numbers in the two modes (discrete variables, DVs), which are un...