No abstract

Entanglement distillation is a process via which the strength and purity of quantum entanglement can be increased probabilistically. It is a key step in many quantum communication and computation protocols. In particular, entanglement distillation is a necessary component of the quantum repeater, a device which counters the degradation of entanglement that inevitably occurs due to losses in a communication line. Here we report an experiment on distilling the Einstein-Podolsky-Rosen (EPR) state of light, the workhorse of continuous-variable entanglement, using the technique of noiseless amplification. In contrast to previous implementations, the entanglement enhancement factor achievable by our technique is not fundamentally limited and permits recovering an EPR state with a macroscopic level of entanglement no matter how low the initial entanglement or how high the loss may be. In particular, we recover the original level of entanglement after one of the EPR modes has passed through a channel with a loss factor of 20. The level of entanglement in our distilled state is higher than that achievable by direct transmission of any state through a similar loss channel. This is a key bench-marking step towards the realization of a practical continuous-variable quantum repeater and other CV quantum protocols.Comment: 8 pages, 5 figure

Transfer of quantum information between physical systems of a different nature is a central matter in quantum technologies. Particularly challenging is the transfer between discrete-and continuous degrees of freedom of various harmonic oscillator systems. Here we implement a protocol for teleporting a continuous-variable optical qubit, encoded by means of coherent states, onto a discretevariable, single-rail qubit -a superposition of the vacuum-and single-photon optical states -via a hybrid entangled resource. We test our protocol on coherent states of different phases and obtain the teleported qubit with a fidelity of 0.83 ± 0.04. We also find that the phase of the resulting qubit varies consistently with that of the input, having a standard deviation of 3.8 • from the target value. Our work opens up the way to wide application of quantum information processing techniques where discrete-and continuous-variable encodings are combined within the same optical circuit.Real-world application of quantum technologies in many cases requires simultaneous use of physical systems of a different nature [1] in the hybrid fashion. For example, one of the most promising platforms for quantum information processing is superconducting circuitry, best storage times are achieved in atomic systems, whereas transmission of that information in space is better realized with photons.Within quantum optics, the hybrid paradigm consists in symbiotic involvement of states and methods from discrete (DV) and continuous (CV) variable domains. Fortes of both can thereby be employed at once; for example, the continuous part takes advantage of the de- terministic state preparation and relatively simple integration with existing information technologies [2], while the discrete side benefits from a natural photodetection basis and "built-in" entanglement distillation after losses [3]. This flexibility allows hybrid quantum technologies to offer protocols which are superior to both pure CV and DV solutions [4]. For example, using hybrid methods in quantum computing [5] and error correction [6] is favorable, compared to the mainstream CV and DV protocols, in terms of the number of resources required. Long-distance distribution of CV entanglement requires hybrid processing to mitigate the effect of propagation losses [7,8].Taking advantage of these protocols however requires a procedure for transferring quantum data between continuous and discrete qubit systems [4,9]; hybrid teleportation is therefore identified as one of the three key priorities for quantum teleportation science [10]. In this work we make a step towards this goal, demonstrating quantum teleportation by means of a hybrid entangled channel. Specifically, we develop a technique to teleport a CV qubit, encoded as a superposition of coherent states, onto a DV qubit -a superposition of the vacuum and single-photon Fock states. The protocol is conceptually different from the teleportation of photonic bits by a hybrid technique [11], in which both the input and output states are discrete-...