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AIP conference proceedings

A nonlocal single-mode amplification process using linear optics

Yuli Dong, Xu‐Bo Zou, Guang‐Can Guo, Alexander I. Lvovsky

Published Jan 1, 2009• 0 citations
Publisher

Abstract

We first present a scheme to implement a nonlocal single‐mode amplification process using linear optics. Then by employing this process, cloning coherent state with known phases can be realized. With the help of ancillary squeezed states, the cloning fidelity can reach a high level.
DOI
10.1063/1.3131296

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First, we present a scheme for implementation of$N\ensuremath{\rightarrow}M$ local cloning of coherent states with known phases, based on linear optics, single-mode squeezed sources, and homodyne detection. The amplification of such cloning is accomplished with the help of a strongly single-mode squeezed source. Then we propose a scheme for $N\ensuremath{\rightarrow}M$ nonlocal cloning of such input states by employing an additional two-mode squeezed source. In the case of ideal squeezed sources, the fidelities of the two cloning machines can reach a high level.
Fundamental laws of quantum mechanics impose that arbitrary quantum states cannot be perfectly cloned or amplified without introducing some unavoidable noise in the process. The quantum noise intrinsic to the functioning of a linear phase-insensitive amplifier can however be avoided by relaxing the requirement of a deterministic operation. Non-deterministic noiseless linear amplifiers that do not violate any fundamental quantum law are therefore possible and here we present the first experimental realization of a scheme that allows noiseless amplification of coherent states at the best level of effective gain and final state fidelity ever reached. This scheme, based on a sequence of photon addition and subtraction, and characterized by a significant amplification and low distortions, may become a useful tool for quantum communications and metrology, by enhancing the discrimination between partially overlapping quantum states or by recovering the information transmitted over lossy channels.
We propose a scheme for continuous-variable quantum cloning of coherent states with phase-conjugate input modes using linear optics. The quantum cloning machine yields MMM identical optimal clones from NNN replicas of a coherent state and NNN replicas of its phase conjugate. This scheme can be straightforwardly implemented with the setups accessible at present since its optical implementation only employs simple linear optical elements and homodyne detection. Compared with the original scheme for continuous-variable quantum cloning with phase-conjugate input modes proposed by Cerf and Iblisdir [Phys. Rev. Lett. 87, 247903 (2001)], which utilized a nondegenerate optical parametric amplifier, our scheme loses the output of phase-conjugate clones and is regarded as irreversible quantum cloning.
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 address quantum state engineering of single- and two-mode states by means of nondeterministic noiseless linear amplifiers (NLAs) acting on Gaussian states. In particular, we consider the optimal implementation of a NLA proposed by Pandey et al. [Phys. Rev. A 88, 033852 (2013)] and we show that it provides an effective scheme to generate highly non-Gaussian and nonclassical states, when used outside the regime of high-fidelity amplification. Additionally, we show that in this regime also the amplification of a two-mode squeezed vacuum state (twin-beam) highly increases entanglement.