Nonclassical Quantum States Research Articles

Quantum-state extraction from high-Qcavities

The problem of extraction of a single-mode quantum state from a high-$Q$ cavity is studied for the case in which the time of preparation of the quantum state of the cavity mode is short compared with its decay time. The temporal evolution of the quantum state of the field escaping from the cavity is calculated in terms of phase-space functions. A general condition is derived under which the quantum state of the pulse built up outside the cavity is a nearly perfect copy of the quantum state the cavity field was initially prepared in. The results show that unwanted losses prevent the realization of a nearly perfect extraction of nonclassical quantum states from high-$Q$ optical microcavities with presently available technology.

Nonclassicality of Quantum States: A Hierarchy of Observable Conditions

A necessary and sufficient hierarchy of conditions is derived that is completely equivalent to the failure of the Glauber-Sudarshan P function to be a probability density. The conditions are formulated in terms of experimentally accessible characteristic functions of quadratures.

Quantum channel of continuous variable teleportation and nonclassicality of quantum states

Noisy teleportation of nonclassical quantum states via a two-mode squeezed-vacuum state is studied with the completely positive map and the Glauber-Sudarshan P-function. Using the nonclassical depth as a measure of transmission performance, we compare the teleportation scheme with the direct transmission through a noisy channel. The noise model is based on the coupling to the vacuum field. It is shown that the teleportation channel has better transmission performance than the direct transmission channel in a certain region. The bounds for such a region and for obtaining the nonvanished nonclassicality of the teleported quantum states are also discussed. Our model shows a reasonable agreement with the teleportation fidelity observed in the experiment performed by Furusawa et al (1998 Science 282 706). We finally mention the required conditions for transmitting nonclassical features in real experiments.

Stationary source of nonclassical or entangled atoms.

A scheme for generating continuous beams of atoms in nonclassical or entangled quantum states is proposed and analyzed. For this the recently suggested transfer technique of quantum states from light fields to collective atomic excitation by stimulated Raman adiabatic passage [M. Fleischhauer and M. D. Lukin, Phys. Rev. Lett. 84, 5094 (2000)] is employed and extended to matter waves.

Preparation of nonclassical states of a cavity field based on a nonlinear Jaynes-Cummings model

It is proven in this paper that some kinds of nonclassical quantum states of the cavity field, such as Schrödinger cat state (amplitude cat or phase cat), sub-Poissonian photon distribution and Fock state, etc., can be generated by conditional measurements on atoms in the two-photon Jaynes-Cummings model with a Kerr-like medium. The nonlinear constant plays the key modulation role in the preparation of the quantum states, which gives a new controllable parameter for further experimental researches in this field.

Gravitation at the Mesoscopic Scale

Free fall experiments are discussed by using test masses associated to quantum states not necessarily possessing a classical counterpart. The times of flight of the Galilean experiments using classical test masses are replaced in the quantum case by probability distributions which, although still not defined in an uncontroversial manner, become manifestly dependent upon the mass and the initial state. Such a dependence is also expected in non-inertial frames of reference if the weak equivalence principle still holds. The latter could be tested, merging recent achievements in mesoscopic physics, by using cooled atoms in free fall and accelerated frames initially prepared in nonclassical quantum states.

Measure of the nonclassicality of nonclassical states.

A continuous parameter introduced into the convolution transformation between P and Q functions leads to a measure of how nonclassical quantum states are with values ranging from 0 to 1: For photon-number states, the value is 1, the maximum possible. For squeezed vacuum states, it is a monotonically increasing function of the squeeze parameter with values varying from 0 to 1/2. This measure is identical to the minimum number of thermal photon necessary to destroy whatever nonclassical effects existing in the quantum states.

On far-field quantum states in optical communication (Corresp.)

The recent issues surrounding the feasibility of greatly improved far-field optical communication via the use of nonclassical quantum states are analyzed.