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 investigate a compact source of entanglement. This device is composed of a pair of linearly coupled nonlinear
waveguides operating by means of degenerate parametric downconversion. For the vacuum state at the input
the generalized squeeze variance and logarithmic negativity are used to quantify the amount of nonclassicality
and entanglement of output beams. Squeezing and entanglement generation for various dynamical regimes of
the device are discussed. The influence of losses is investigated in detail.
We propose a setup for teleportation of continuous quantum variables which is optimized for teleportation of squeezed coherent states. Our scheme is a modified version of the original setup proposed by Braunstein and Kimble [Phys. Rev. Lett. 80, 869, 1998] and it involves an unbalanced beam splitter on the sender's side and a single mode squeezer on receiver's side. We show that the quality of the teleportation can be evaluated by means of a fidelity of entanglement swapping and we prove that our modified setup and the original Braunstein-Kimble setup are equivalent with respect to this fidelity measure.
We study teleportation of qubits with imperfect Bell analysis on the sender's side. For the chosen family of Alice's measurements we find Bob's operations maximizing the overall fidelity of the teleportation protocol. In certain cases the optimum maps turn out to be nonunitary operations. This means that decoherence can sometimes enhance the performance of quantum teleportation protocols.
We investigate quantum phase properties of two-mode optical fields whose quasi distributions have Gaussian form. We show how to simplify calculation of the joint phase distribution defined via radial integration of the quasi distribution related to s-ordering of the field operators. Introducing hyper spherical coordinates, we can carry out analytically one of two required integrations. The second integral over a finite interval is then evaluated numerically. Analytical formula for the joint phase distribution can be obtained in special case of vanishing coherent components of both modes. The general results are applied to analysis of quantum phase properties of two-mode Stokes-anti-Stokes field generated by means of Raman scattering with broad reservoir phonon system and strong coherent laser pumping.
We propose a substituting scheme for nonlinear optical couplers operating by means of degenerate parametric down- conversion with strong coherent pumping. The scheme, which provides the same unitary input-output transformation as the original coupler, consists of simple linear and nonlinear optical devices: beam splitter and optical parametric amplifiers. Using group theoretical approach, we find analytical formulas for parameters of these optical elements. The scheme allows us to get a better insight into the coupler behavior, because the complex dynamics of the coupler is transformed into a sequence of simpler evolutions governed by the beam splitter and parametric amplifiers, whose properties are well known and understood.
Quantum statistical properties of light in nonlinear coupler in which Raman or Brillouin scattering are in operation are investigated. Main attention is given to possibility of generation of nonclassical states of light. Phase mismatches and losses are included and their influence is discussed. The second part of the paper is devoted to analysis of stationary point. Coherent pumping of optical modes is assumed and stationary solution for c-number amplitudes is found and small linear operator corrections are calculated. Stability of the stationary point is discussed and it is shown that phase uncertainty induces instability.
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