Quantum mechanics imposes that any amplifier that works independently on the phase of the input signal has to introduce some excess noise. The impossibility of such a noiseless amplifier is rooted into unitarity and linearity of quantum evolution. A possible way to circumvent this limitation is to interrupt such evolution via a measurement, providing a random outcome able to herald a successful -and noiseless -amplification event. Here we show a successful realisation of such an approach; we perform a full characterization of an amplified coherent state using quantum homodyne tomography, and observe a strong heralded amplification, with about 6dB gain and a noise level significantly smaller than the minimal allowed for any ordinary phase-independent device.Quantum optical detection techniques are so advanced that quantum fluctuations are the main source of noise. Therefore, when amplifying optical signals, one has to look at intrinsic limitations of the process: any amplifier cannot work independently on the phase of the input, unless some additional noise is added [1]. The origin of this limitation is that adding extra noise is needed for the output field to obey Heisenberg's uncertainty relation. Also, it is connected to the impossibility of realizing arbitrarily faithful copies of a quantum signal [2,3], and it is thus deeply rooted in the linear and unitary evolution of quantum mechanical systems.Various aspects of this limitation have been studied by using optical parametric amplifiers [4,5,6,7]. For instance, a non-degenerate optical parametric amplifier amplifies all input phases, and introduces the minimal level of added noise, which degrades the signal-to-noise ratio [1]. The same process, driven in the degenerate regime, may provide amplification preserving the signalto-noise ratio. However, this occurs in a phase-dependent fashion: only the part of the signal in phase with the pump light will be amplified, while the part which is 90 degrees out of phase with the pump will be deamplified [4,5].A more intriguing idea is to find a way to tamper with the linear evolution of quantum mechanics; this is actually possible, though non-deterministically, by conditioning our observation upon the result of a measurement [8]. Noiseless amplification can then take place, but only a fraction of the times, and the correct operation is heralded. This strategy is commonly adopted for building effective nonlinearities in linear quantum optical gates [9,10].Here we follow the proposal of Ralph and Lund [11] to demonstrate experimentally that heralded nondeterministic amplification can realise processes which would be impossible for usual amplifiers. Unlike another realisation [12], we have direct access to the output state via state tomography, so we can provide a complete description of the process, and analyse the limitations arising from non-ideal components. Our study is relevant in the long-term view of the integration of amplifiers in quantum communication lines [13].The conceptual layout of the noiseless amplifier...

'Schrödinger cat' states of light 1 , defined as quantum superpositions of quasi-classical coherent states, have recently emerged as an alternative to single-photon qubits for quantuminformation processing [2][3][4][5][6] . Their richer structure provides significant advantages for quantum teleportation, universal quantum computation, high-precision measurements and fundamental tests of quantum physics [7][8][9][10][11][12][13] . Local superpositions of free-propagating coherent states have been realized experimentally, but their applications were so far limited by their extreme sensitivity to losses, and by the lack of quantum gates for coherent qubit rotations. Here, we demonstrate a simple approach to generating strongly entangled non-local superpositions of coherent states, using a very lossy quantum channel. Such superpositions should be useful for implementing coherent qubit-rotation gates, and for teleporting these qubits over long distances. The generation scheme may be extended to creating entangled coherent superpositions with arbitrarily large amplitudes.Single-mode cat states can be considered as classical light waves with two opposite phases simultaneously, expressed as C(|α + e iφ | − α ), where |α is a coherent state containing |α| 2 photons on average and C is a normalization factor omitted in the following. The non-classical nature of such states appears most strikingly in the quantum statistics of the electric field: its quasi-probability distribution, called the Wigner function, presents quantum oscillations with negative values between the two classical states. The Wigner function W (x,p), wherexare the quadrature operators of the quantized electric field, can be reconstructed by homodyne tomography 14 from several marginal distributions P θ (x θ = x cosθ +psinθ).Besides their fundamental interest, arbitrary coherent superpositions a|α + b| − α can be used as qubits carrying quantum information, if |α and | − α are sufficiently distinguishable (|α| 2 2). They present many advantages compared with discrete-variable qubits a|0 + b|1 , enabling one to circumvent the fundamental limits of discrete-variable quantum teleportation 7,15 or to carry out loophole-free Bell tests 13 . So far, their applications suffered from two major issues. One was the difficulty to build associated logic gates: arbitrary qubit rotations were believed to require either unrealistically strong nonlinear interactions, or very resource-consuming repeated infinitesimal rotations 8,13 . The other was more fundamental: the complex structure of these states, while offering many benefits, makes them notoriously fragile. For instance, many quantuminformation processing (QIP) tasks require entangled cat states such as |ψ 0 = |α 1 | − α 2 − | − α 1 |α 2 . Theoretically, they can be obtained by splitting a single-mode cat | through lossy channels. The entanglement is created by non-local photon subtraction, by interfering small fractions R of each pulse, phase-shifted by φ, on a 50/50 beamsplitter. Then an APD photon detection in...