Quantum memory for entangled continuous-variable states

Abstract

A quantum memory for light is a key element for the realization of future quantum information networks 1-3. Requirements for a good quantum memory are versatility (allowing a wide range of inputs) and preservation of quantum information in a way unattainable with any classical memory device. Here we demonstrate such a quantum memory for continuousvariable entangled states, which play a fundamental role in quantum information processing 4-6. We store an extensive alphabet of two-mode 6.0 dB squeezed states obtained by varying the orientation of squeezing and the displacement of the states. The two components of the entangled state are stored in two room-temperature cells separated by 0.5 m, one for each mode, with a memory time of 1 ms. The true quantum character of the memory is rigorously proved by showing that the experimental memory fidelity 0.52 ± 0.02 significantly exceeds the benchmark of 0.45 for the best possible classical memory for a range of displacements. The continuous-variable regime represents one of the principal avenues towards the realization of quantum information processing and communication 4-6. In the optical domain it operates with well-known optical modulation and detection techniques and allows for deterministic quantum operations. In the atomic domain it has been developed on the platform of atomic ensembles 2,3,7. Advances in the realization of continuous-variable quantum protocols include unconditional quantum teleportation involving light 8 and atoms 9 , a number of results on memory 2,3,10,11 and quantum key distribution 12. Hybrid continuous/discrete-variable operations 13-15 paving the road towards continuous-variable quantum computation 16,17 have also been reported. However, the ability to store non-classical continuous-variable states of light is crucial to enable further progress, in particular, for continuous-variable linear optics quantum computing with offline resources 17 , continuous-variable quantum repeaters 18,19 , entanglement-enhanced quantum metrology, iterative continuousvariable entanglement distillation 20 , continuous-variable clusterstate quantum computation 21 , communication/cryptography protocols involving several rounds 22 and quantum illumination 23. Compared with a number of impressive results reporting discrete-variable quantum memories at the single-photon level (see reviews 1-3 and references therein), there have been very few experiments towards quantum memory for continuousvariable non-classical states. A fractional, 20 nsec, delay of 50 nsec pulsed continuous-variable entangled states in the atomic

Sequential measurement-based quantum computing with memories

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Scheme for Implementing Controlled Quantum Teleportation in QDS-Cavity System

“…in the past years. Therefore, the principles of quantum mechanics have supplied much theoretical support in the field of quantum information, such as quantum communication and computation (Ralph and Lam 2009;Reid et al 2009;Lund, Ralph, and Haselgrove 2008;Gu et al 2009;Lloyd 2008), quantum key distribution and secret sharing (Grosshans et al 2003), quantum error correction (Steane 1996;Shor 1995), quantum teleportation (Sherson et al 2006;Hu and Rarity 2011) and quantum memory (Jensen et al 2011;Lvovsky, Sanders, and Tittel 2009) and so on. Especially, quantum teleportation is an important branch of quantum information and it is a technique for transferring quantum states from one place to another without moving through the intervening space.…”

Quantum-teleportation benchmarks for independent and identically distributed spin states and displaced thermal states

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