Tomáš Opatrný

Generating Schrodinger-cat-like states by means of conditional measurements on a beam splitter

A scheme for generating Schrodinger-cat-like states of a single-mode optical field by means of conditional measurement is proposed. Feeding a squeezed vacuum into a beam splitter and counting the photons in one of the output channels, the conditional states in the other output channel exhibit a number of properties that are very similar to those of superpositions of two coherent states with opposite phases. We present analytical and numerical results for the photon-number and quadrature-component distributions of the conditional states and their Wigner and Husimi functions. Further, we discuss the effect of realistic photocounting on the states. @S1050-2947~97!06404-4#

Improvement on teleportation of continuous variables by photon subtraction via conditional measurement

We show that the recently proposed scheme of teleportation of continuous variables [S.L. Braunstein and H.J. Kimble, Phys. Rev. Lett. 80, 869 (1998)] can be improved by a conditional measurement in the preparation of the entangled state shared by the sender and the recipient. The conditional measurement subtracts photons from the original entangled two-mode squeezed vacuum, by transmitting each mode through a low-reflectivity beam splitter and performing a joint photon-number measurement on the reflected beams. In this way the degree of entanglement of the shared state is increased and so is the fidelity of the teleported state.

Electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy of nitric oxide

A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.

Entropic uncertainty relations for a quantum oscillator

We calculated the Shannon entropy of position and momentum for the stationary quantum states of the harmonic oscillator as a function of its energy and determined the corresponding entropic uncertainty relations for them. We found an approximate phenomenological function for the dependence of position and momentum entropies on the large quantum numbers and the corresponding asymptotic entropy - energy relation for the stationary harmonic oscillator. We also studied the time evolution of the position and momentum entropies of the non-stationary harmonic oscillator for the coherent states, squeezed vacuum and Schrodinger cat states.

II Homodyne Detection and Quantum-State Reconstruction

Publisher Summary This chapter discusses the homodyne detection and quantum-state reconstruction. The chapter reviews the experimental schemes that have been considered for quantum-state measurement on optical fields, with special emphasis on optical homodyning. Homodyne detection has been a powerful method for measuring phase sensitive properties of traveling optical fields suitable for quantum-state reconstruction. The typical methods for reconstruction of quantum-state representations and specific quantum-statistical properties of optical fields from sets of measurable quantities carrying the complete information on the quantum state are also reviewed in the chapter. The problem of quantum-state measurement and reconstruction has been studied for various matter systems. Different matter systems require different detection schemes for measuring specific quantities that carry the full information on the quantum state of the system. The chapter summarizes the methods for measurement and reconstruction of quantum-state representations of matter system, with special emphasis on optical methods and experimental demonstrations.

Adiabatic tracking of quantum many-body dynamics

The nonadiabatic dynamics of a many-body system driven through a quantum critical point can be controlled using counterdiabatic driving, where the formation of excitations is suppressed by assisting the dynamics with auxiliary multiple-body nonlocal interactions. We propose an alternative scheme which circumvents practical challenges to realize shortcuts to adiabaticity in mesoscopic systems by tailoring the functional form of the auxiliary counterdiabatic interactions. A driving scheme resorting in few-body short-range interactions is shown to generate an effectively adiabatic dynamics.

Partial suppression of nonadiabatic transitions

The adiabatic following of eigenstates of time-varying Hamiltonians can serve as a useful tool in preparing or manipulating quantum states. If the time variation is not sufficiently slow, however, nonadiabatic transitions to unwanted states occur. Recently, it has been shown that the adiabatic following can be perfectly restored if the original Hamiltonian is complemented with an additional term. Although there is an explicit formula for this compensating term, typically one may not always be able to construct it in an experiment. Here we present a straightforward approach for a partial suppression of the nonadiabatic transitions applicable for any set of available Hamilton operators. We illustrate the method on several examples including interacting spin systems, interacting bosons in a double-well potential, a particle in an expanding box and a system of atoms interacting via a Rydberg-blockade. Whenever suitable compensating operators are available, the system may be evolved faster or with higher fidelity along an eigenstate of the original time-dependent Hamiltonian.

Coupled cavities for enhancing the cross-phase-modulation in electromagnetically induced transparency

We propose an optical double-cavity resonator whose response to a signal is similar to that of an electromagnetically induced transparency (EIT) medium. A combination of such a device with a four-level EIT medium can serve for achieving large cross-Kerr modulation of a probe field by a signal field. This would offer the possibility of building a quantum logic gate based on photonic qubits. We discuss the technical requirements that are necessary for realizing a probe-photon phase shift of

Mode structure and photon number correlations in squeezed quantum pulses

\ensuremath{\pi}

Least-squares inversion for density-matrix reconstruction

caused by a single-photon signal. The main difficulty is the requirement of an ultralow reflectivity beam splitter, and we must be able to operate a sufficiently dense cool EIT medium in a cavity.