We present a scheme for the amplification of Schrödinger cats that collapses two smaller states onto their constructive interference via a homodyne projection. We analyze the performance of the amplification in terms of fidelity and success rate when the input consists of either exact coherent state superpositions or of photon-subtracted squeezed vacua. The impact of imprecise homodyne detection and of impure squeezing is quantified. We also assess the scalability of iterated amplifications.Coherent state superpositions, or optical Schrödinger cat states, are widely recognized as promising resources in quantum information [1][2][3][4][5], quantum metrology [6][7][8], and fundamental tests [9][10][11][12]. In the near-orthogonal basis of coherent states γ|−γ = e −2γ 2 , two particular instances for these states arewhere the sign (±) of the superposition refers to the even and odd cat state, respectively. These states exhibit quasi-probability distributions in phase space which are distinctly non-classical. This makes them all the more challenging to generate deterministically as that would require strong Kerr-type non-linearities [13][14][15].One has then to resort to heralding techniques which, though probabilistic, need only linear optics and projective measurements [16]. These state-engineering schemes are nonetheless approximative and present a limitation in the fidelity they produce with ideal cat states. Photon-subtraction of squeezed vacuum, for example, is a well-established method to generate approximations of small amplitude cat states, colloquially referred to as Schrödinger kittens [17][18][19][20][21]. Even in the best experimental conditions, the fidelity between the photon-subtracted squeezed vacuum (PSSV) and an actual odd cat state |κ − (γ) degrades markedly for γ ≥ 1.2 [22]. Yet, for these states to be reliable resources in quantum computation, their fidelity with cat states at least as large as γ = 1.2 need to be maintained at near-unit fidelity [3,23]. Single-photon subtraction is only one example of several measurement-induced schemes which have been proposed to generate kitten states [20,[24][25][26][27][28][29]. However, none of these schemes can produce arbitrarily large cats in a single run. Ways to get around this issue have been devised using the recursive amplification of small, approximate cats [30,31]. For example, it was suggested in [32] to interfere a supply of delocalized single photons followed by homodyne heralding to generate large entangled cat states. These proposals have in common that they rely on the coherent mixing of two small cats, whereupon a projective measurement collapses one of the two outputs onto a constructive interference of the inputshence the amplification. Here, we shall pursue the same idea but make use solely of homodyne heralding for its relative simplicity and high quantum efficiency. We also demonstrate that the acceptance window of homodyne heralding can be widened to increase the success rate of the amplification while at the same time maintaining a s...

We discuss and experimentally demonstrate a probabilistic Hadamard gate for coherent state qubits. The scheme is based on linear optical components, non-classical resources and the joint projective action of a photon counter and a homodyne detector. We experimentally characterize the gate for the coherent states of the computational basis by full tomographic reconstruction of the transformed output states. Based on the parameters of the experiment we simulate the fidelity for all coherent state qubits on the Bloch sphere

With the slow but constant progress in the coherent control of quantum systems, it is now possible to create large quantum superpositions. There has therefore been an increased interest in quantifying any claims of macroscopicity. We attempt here to motivate three criteria which we believe should enter in the assessment of macroscopic quantumness: The number of quantum fluctuation photons, the purity of the states, and the ease with which the branches making up the state can be distinguished

13, 053054 (2011)] revealed that a single quantum symmetrically delocalized over N modes, namely a W state, effectively allows for all-versus-nothing proofs of nonlocality in the limit of large N . Ideally, this finding opens up the possibility of using the robustness of the W states while realizing the nonlocal behavior previously thought to be exclusive to the more complex class of Greenberger-Horne-Zeilinger (GHZ) states. We show that in practice, however, the slightest decoherence or inefficiency of the Bell measurements on W states will degrade any violation margin gained by scaling to higher N . The non-statistical demonstration of nonlocality is thus proved to be impossible in any realistic experiment.

We outline a proof that teleportation with a single particle is, in principle, just as reliable as with two particles. We thereby hope to dispel the skepticism surrounding single-photon entanglement as a valid resource in quantum information. A deterministic Bell-state analyzer is proposed which uses only classical resources, namely, coherent states, a Kerr nonlinearity, and a two-level atom.