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arXiv:1009.2306 · New Journal of Physics

Linear amplification and quantum cloning for non-Gaussian continuous variables

Hyunchul Nha, Gerard J Milburn, H. J. Carmichael

Published Oct 5, 2010• 24 citations
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Abstract

We investigate phase-insensitive linear amplification at the quantum limit for single- and two-mode states and show that there exists a broad class of non-Gaussian states whose nonclassicality survives even at an arbitrarily large gain. We identify the corresponding observable nonclassical effects and find that they include, remarkably, two-mode entanglement. The implications of our results for quantum cloning outside the Gaussian regime are also addressed.

DOI
10.1088/1367-2630/12/10/103010

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We investigate phase-insensitive linear amplification at the quantum limit for single- and two-mode states and show that there exists a broad class of non-Gaussian states whose nonclassicality survives even at an arbitrarily large gain. We identify the corresponding observable nonclassical effects and find that they include, remarkably, two-mode entanglement. The implications of our results for quantum cloning outside the Gaussian regime are also addressed.
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We study general rules of entanglement dynamics for two-mode continuous variable states subjected to deterministic phase-insensitive attenuation and amplification. Since those processes inevitably involve quantum noises, we propose and solve the problem of finding noise levels destroying entanglement of Gaussian and non-Gaussian states. The amplification with power gain greater or equal than 2 necessarily leads to the total loss of entanglement. We consider non-Gaussian states whose entanglement is robust to arbitrary signal attenuation and amplification provided the noise is restricted by some value close to the quantum limited operation. We calculate the lower bound on distillation rate of Bell states.
We experimentally demonstrate phase-insensitive linear amplification of a continuous variable system in the optical regime, preserving the ancilla system at the output. Since our amplification operation is unitary up to small excess noise, it is reversible beyond the classical limit. Here, entanglement between the amplified output system and the ancilla system is the resource for the reversibility, and the amplification gain is G=2.0G=2.0G=2.0. In addition, combining this amplifier with a beamsplitter, we also demonstrate approximate cloning of coherent states where an anticlone is present. We investigate the reversibility by reconstructing the initial state from the output correlations, and the results are slightly beyond the cloning limit. Furthermore, full characterization of the amplifier and cloner is given by using coherent states with several different mean values as inputs. Our amplifier is based on linear optics, offline-prepared additional ancillas in nonclassical states, and homodyne measurements followed by feedforward. Squeezed states are used as the additional ancillas, and nonlinear optical effects are exploited only for their generation. They introduce nonclassicality into the amplifying operation, making entanglement at the output.
We present an experimental realization of a low-noise, phase-insensitive optical amplifier using a four-wave mixing interaction in hot Rb vapor. Performance near the quantum limit for a range of amplifier gains, including near unity, can be achieved. Such low-noise amplifiers are essential for so-called quantum cloning machines and are useful in quantum information networks and protocols. We demonstrate that amplification and ''cloning'' of one half of a two-mode squeezed state is possible while preserving entanglement. The inseparability criterion between the two original modes remains satisfied for small to large gains, while the EPR criterion is satisfied for a smaller range. This amplification of quantum correlations paves the way for optimal cloning of a bipartite entangled state.
We present an experimental realization of a low-noise, phase-insensitive optical amplifier using a four-wave mixing interaction in hot Rb vapor. Performance near the quantum limit for a range of amplifier gains, including near unity, can be achieved. Such low-noise amplifiers are essential for so-called quantum cloning machines and are useful in quantum information protocols. We demonstrate that amplification and "cloning" of one half of a two-mode squeezed state is possible while preserving entanglement.
We examine the phase-insensitive amplification of three different kinds of higher-order squeezing. In particular, we consider fourth-order squeezing, intrinsic fourth-order squeezing, and amplitude-squared squeezing. It is found that for any input state both fourth-order squeezing and intrinsic fourth-order squeezing disappear at the output if the intensity gain is greater than 2. Amplitude-squared squeezing, on the other hand, can survive amplification at gains slightly greater than 2. Therefore the photon-cloning limit (intensity gain equals 2) is not a fundamental limit to nonclassical behavior.