Universal Optical Amplification without Nonlinearity

Abstract

We propose and experimentally realize a new scheme for universal phase-insensitive optical amplification. The presented scheme relies only on linear optics and homodyne detection, thus circumventing the need for nonlinear interaction between a pump field and the signal field. The amplifier demonstrates near optimal quantum noise limited performance for a wide range of amplification factors.PACS numbers: 42.65.Yj, 42.50.Lc, Optical amplification is inevitably affected by fundamental quantum noise no matter whether it is phase sensitive or phase insensitive as stressed by Louisell et al. [1] and by Haus and Mullen [2]. The ultimate limits imposed by quantum mechanics on amplifiers was later concisely formulated by Caves [3] in fundamental theorems. This intrinsic noise, intimately linked with measurement theory and the no-cloning theorem, gives rise to many inextricable restrictions on the manipulations of quantum states. For example, microscopic quantum objects cannot be perfectly transformed, through amplification, into macroscopic objects for detailed inspection [4]: for phase insensitive amplification nonclassical features of quantum states, such as squeezing or oscillations in phase space, will be gradually washed out, and the signal to noise ratio of an information carrying quantum state will be reduced during the course of amplification. Despite these limitations, the universal phase insensitive amplifier is however rich of applications, in particular in optical communication. Amplifiers operating at the quantum noise limit are of particular importance for quantum communication where information is encoded in fragile quantum states, thus extremely vulnerable to noise.Numerous apparatuses accomplish, in principle, ideal phase insensitive amplification as, for instance, solid state laser amplifiers [5], parametric downconverters [6] and schemes based on four wave mixing processes [7]. However, to date phase-insensitive amplification at the quantum limit has been only partially demonstrated [8,9]: a number of difficulties are indeed involved in practice, especially for low gain applications. These difficulties mainly lie in the fact that the amplified field has to be efficiently coupled, mediated by a non linearity, to a pump field.Following a recent trend in quantum information science, where non linear media are efficiently replaced by linear optics [10], we show in this Letter that universal phase insensitive amplification can also be achieved using only linear optics and homodyne detection. The simplicity and the robustness of this original scheme enable us to achieve near quantum noise limited amplification of coherent states, even in the low gain regime.Let us first briefly summarize the basic formalism describing a phase-insensitive amplifier [3]. Because of the symmetry of such an amplifier, it can be described by the following input-output transformation:â out = √ Gâ in +N whereâ in(out) represent the input (output) annihilation bosonic operators, G is the power gain and N the operator associated wi...

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