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arXiv:0906.5024 · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Low-noise amplification of a continuous-variable quantum state

Raphael C. Pooser, Alberto M. Marino, Vincent Boyer, K. M. Jones, Paul D. Lett

Published Apr 2, 2010• 3 citations
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Abstract

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.

DOI
10.1117/12.851881

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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.
This dissertation is a summary of investigations on the optical properties of a quantum-noise-limited phase-sensitive amplifier (PSA). The PSA is implemented using four-wave mixing in hot Rb 85 vapor based on a double-Lambda atomic scheme. We experimentally demonstrate the ability of a PSA to pre-amplify quantum correlations in twin light beams produced by a phase-insensitive amplifier (PIA) before degradation due to loss and detector inefficiency. By including a PSA before loss, one is able to preserve the correlations as well as the two-mode squeezing level. We compare the results to simulations employing a simple quantum-mechanical model and find a good agreement. We have demonstrated that the cross-correlation between the two modes of a bipartite entangled state can be advanced by propagation through a PIA acting as a fast-light medium. The extra noise added by the PIA has been speculated to be the mechanism that limits the advance of entanglement, preventing the mutual information from traveling superluminally. As an extension of this phase-insensitive, gain-assisted, anomalous dispersion investigation, we explore the advance and delay of information transmitted through the PSA. We start with a two-mode squeezed state created by the PIA and measure the mutual information shared by the correlated quadratures. We then pass one of these two modes through a PSA and investigate the shift of the mutual information as a function of the PSA phase. In the case of a PSA, it is well known that no extra noise will be added to the quadrature with the correct input phase (e.g., the quadrature with the maximal amplification or the maximal deamplification). We find that there is no dispersion-like behavior at these two phases, however, the peak of mutual information could either be delayed or advanced at any other phase. We also observe an almost identical behavior when we input an amplitude modulated signal to the PSA. We are able to explain the physics of this ``fast-and-slow-light'' type of behavior utilizing a model assuming imbalanced gain on the positive and negative side bands. We obtain a good agreement between the experimental results and the theoretical simulations.
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.
Quantum noise fundamentally limits the performance of fiber-optic systems beyond the standard quantum limit (SQL), restricting long-distance quantum key distribution, quantum communication, and precision quantum sensing. To overcome these limitations, quantum-squeezed states enable quadrature-dependent noise suppression, yet their benefits rapidly degrade under fiber attenuation, necessitating low-noise amplification. Since conventional phase-insensitive amplifiers (PIAs) impose a minimum 3 dB noise figure (NF) penalty and disrupt quantum correlations, phase-sensitive amplification (PSA) becomes essential. In this work, we propose a PSA based on dual-pump frequency-degenerate four-wave mixing (FWM) to amplify weak coherent squeezed states. Here, the PSA is seeded by an information-carrying single-mode squeezed state, where the information is encoded in the displacement degree of freedom, rather than in the squeezing itself. By optimizing the relative phases among the squeezed state, pump fields, and weak signal, the scheme maintains proper squeezing alignment and preserves the encoded quantum correlations during propagation. Under low-loss conditions, it is shown that the effective NF reaches −7.787 dB, demonstrating that the scheme enables quantum-limited amplification suitable for long-haul transmission and offering a viable path toward scalable fiber-based quantum technologies.
An unknown quantum state is prohibited from perfect cloning due to the principles of quantum mechanics, while approximate cloning of an unknown quantum state can be achieved by constructing quantum cloning machines. In the continuous variable regime, to effectively improve the cloning fidelity, an N + N ′ → M + M ′ phase-conjugate inputs (PCI) quantum cloning scheme is proposed based on an optical parametric amplifier and beam splitter network. However, due to the difficulties in constructing an optical parametric amplifier approaching the quantum noise limit, such quantum cloning scheme has not been experimentally realized. In this work, we present all-optical N + N ′ → M + M ′ PCI quantum cloning of coherent states by utilizing a phase-sensitive amplifier based on a four-wave mixing process. In particular, we experimentally realize deterministic all-optical 1+1→ M + M ′ ( M = M ′ =2, 4) and 2+2→ M + M ′ ( M = M ′ =3, 4, 6, 8) PCI quantum cloning. The maximum cloning fidelities experimentally achieved are 96.76 % ±0.57 % and 96.80 % ±0.58 % in the 2+2→3+3 case, which are the maximum cloning fidelities in deterministic continuous variable quantum cloning, to our knowledge. Our results provide an effective scheme for enhancing cloning fidelity and can find applications in achieving high-fidelity quantum information protocols.