A. Karpati

Engineering mixed states in a degenerate four-state system

A method is proposed for preparing any pure and wide class of mixed quantum states in the decoherence-free ground-state subspace of a degenerate multilevel lambda system. The scheme is a combination of optical pumping and a series of coherent excitation processes, and for a given pulse sequence the same final state is obtained regardless of the initial state of the system. The method is robust with respect to the fluctuation of the pulse areas, as in adiabatic methods; however, the field amplitude can be adjusted in a larger range.

Nonclassical light in complex optical systems

A general method is developed for analysing the evolution of nonclassical light in complex two-mode optical systems where linear and nonlinear processes with quadratic Hamiltonians can take place simultaneously or successively. Analytic expressions are derived for the output Wigner function and the output photon number distribution for a general coherent-state superposition input that can approximate any quantum state of light with a high degree of accuracy. As an example, the evolution of amplitude squeezed light is analysed in a nonlinear coupler with degenerate and nondegenerate parametric amplification. It is shown that all-optical switching can be realized in this system.

Long-lasting molecular alignment: Fact or fiction?

It has been suggested that appropriate periodic sequences of laser pulses can maintain molecular alignment for arbitrarily long times [J. Ortigoso, Phys. Rev. Lett. 93, 073001 (2004)]. These aligned states are found among the cyclic eigenstates of truncated matrix representations of the one-period time propagator U(T,0). However, long time localization of periodic driven systems depends on the nature of the spectrum of their exact propagator; if it is continuous, eigenstates of finite-basis propagators cease to be cyclic, in the long time limit, under the exact time evolution. We show that, for very weak laser intensities, the evolution operator of the system has a point spectrum for most laser frequencies, but for the laser powers needed to create aligned wave packets it is unknown if U(T,0) has a point spectrum or a singular continuous spectrum. For this regime, we obtain error bounds on the exact time evolution of rotational wave packets that allow us to determine that truncated aligned cyclic states do not lose their alignment for millions of rotational periods when they evolve under the action of the exact time propagator.

Robust state preparation in a degenerate four-state system

A robust method utilizing a combination of optical pumping and a series of coherent excitation processes is developed for preparing any pure and a wide class of mixed quantum states in the decoherence-free ground-state subspace of a degenerate four-state system. For a given pulse sequence the same final state is obtained regardless of the initial state of the system. An example is presented where a pure state is prepared by a series of excitation processes in the two-dimensional dark subspace of the atom.

Quantum operations in probabilistic representation

Representing a single qubit by using a set of totally decohered mixed-state qubits is considered. A representation is constructed where physically realizable quantum mechanical transformations correspond approximately to unitary operations on the qubit. It is shown that the representation is valid for multiple qubit systems as well, and the precision of the approximation is sufficient for the realization of quantum algorithms.

Quantum trajectory method for determining the time evolution operator of dissipative quantum systems

The time evolution operator is obtained as a stochastic average of operators constructed from individual quantum trajectories. It is shown that simultaneously determining the time evolution of many initial states is more efficient than performing the task one by one using the conventional quantum state diffusion method.

Stochastically-induced quantum interference in coherently driven two-level atoms

It is shown that stochastic perturbation can create phase correlation in a coherently driven two-level system. This correlation results in a noise-induced interference in the two-level system. One possible manifestation of the described interference is a narrow structure in resonance fluorescence spectra. Theoretical interpretation is well confirmed by preliminary experimental results.

Phase Optimized Light in Complex Optical Systems

Evolution of phase optimized light in complex two-mode optical systems of quadratic Hamiltonian is analyzed. It is shown that amplification of phase optimized light, and enhancement of phase squeezing preserving the phase optimized property can be realized in these systems with an appropriate set of parameters.