Phase-space Quasiprobability Distributions Research Articles

A general theory of phase-space quasiprobability distributions

We present a general theory of quasiprobability distributions on phase spaces of quantum systems whose dynamical symmetry groups are (finite-dimensional) Lie groups. The family of distributions on a phase space is postulated to satisfy the Stratonovich-Weyl correspondence with a generalized traciality condition. The corresponding family of the Stratonovich-Weyl kernels is constructed explicitly. In the presented theory we use the concept of the generalized coherent states, that brings physical insight into the mathematical formalism.

Operational approach for reconstruction of quantum distributions in a preamplified homodyne-detection scheme

We use the operational theory of homodyne detection, introduced by Banaszek and W\'odkiewicz [Phys. Rev. A 55, 3117 (1997)], to express a generic quantum phase space quasiprobability distribution (in the sense of Cahill and Glauber) of an electromagnetic field in terms of the photocount moments. We adapt the method of degenerate parametric (pre)amplification of a signal, within the operational theory, in order to overcome the drawback of nonunit efficiency of the detector. Simulations show that as the gain parameter of the amplification is increased, the quasiprobability distribution goes closer to the original one, even for an efficiency lower than 0.5. We also express the mean value of an arbitrary observable in terms of the same photocount moments.

Hypergeometric states and their nonclassical properties

“Hypergeometric states,” which are a one-parameter generalization of binomial states of the single-mode quantized radiation field, are introduced and their nonclassical properties are investigated. Their limits to the binomial states and to the coherent and number states are studied. The ladder operator formulation of the hypergeometric states is found and the algebra involved turns out to be a one-parameter deformation of su(2) algebra. These states exhibit highly nonclassical properties, like sub-Poissonian character, antibunching, and squeezing effects. The quasiprobability distributions in phase space, namely the Q and the Wigner functions are studied in detail. These remarkable properties seem to suggest that the hypergeometric states deserve further attention from theoretical and applicational sides of quantum optics.

Quantum Superpositions of Binomial States of Light

We introduce new kinds of states of electromagnetic field, which are quantum superpositions of binomial states. They not only exhibit remarkable non-classical properties but can also interpolate between the 'Schrodinger cat' type of states and the number states in a particularly interesting way. We basically discuss some of their main statistical properties, as well as schemes for their generation. We also employ quasiprobability distributions in phase space.

Experimental determination of the quantum-mechanical state of a molecular vibrational mode using fluorescence tomography.

We present a spectroscopic method for the complete characterization of the quantum state of the vibrational mode of a molecule in terms of a phase-space quasiprobability distribution. The distribution for a molecular vibrational mode excited by a short optical pulse is reconstructed from measurements of its time-dependent spectrum of fluorescence.

Statistical and phase properties of the binomial states of the electromagnetic field.

We investigate the nonclassical properties of the single-mode binomial states of the quantized electromagnetic field. We concentrate our analysis on the fact that the binomial states interpolate between the coherent states and the number states, depending on the values of the parameters involved. We discuss their statistical properties, such as squeezing (second and fourth order) and sub-Poissonian character. We show how the transition between those two fundamentally different states occurs, employing quasiprobability distributions in phase space, and we provide, at the same time, an interesting picture for the origin of second-order quadrature squeezing. We also discuss the phase properties of the binomial states using the Hermitian-phase-operator formalism.

Correlations and squeezing of two-mode oscillations

In a squeezed state, the variance in one canonical variable may be suppressed below that normally associated with either the ground state or a coherent state, at the expense of an expansion in the variance of the conjugate variable. Squeezed states are usually discussed in the context of quantized light fields. The principal properties of squeezed states are demonstrated using the motion of quantum mechanical simple harmonic oscillators. The motion of a single oscillator is discussed to introduce the key concepts of squeezing, including quadrature operators, and the error contours of the Wigner function describing the quantum quasiprobability distributions in phase space of the oscillator motion. The main topic of two-mode squeezing is then addressed, in which the fluctuations in a system of two oscillators are tightly correlated. The existence of squeezing is demonstrated in normal mode coordinates representing motion of superpositions of the motion of the two oscillators. The fluctuations within a single oscillator are shown to increase when squeezing increases in the normal modes, generating thermal noise in individual mode subsystems.