Academic Year 2016

Solid-state quantum emitters, in particular color centers in diamond such as nitrogen-vacancy (NV) centers, are promising candidates for single photon sources. We provide an elementary description of the dynamics of defect centers in terms of a quantum optical master equation which includes spontaneous decay and a simplified vibronic interaction with lattice phonons [1]. We present the general solution of the dynamical equation by means of the eigensystem of the Liouville operator and exemplify the usage of this damping basis to calculate the dynamics of the electronic and vibrational degrees of freedom and to provide an analysis of the spectra of scattered light. The dynamics and spectral features are discussed with respect to the applicability for color centers.

A system of Na atoms of n levels interacting with l modes of radiation is considered. The energy surface is constructed from the direct product of coherent states of U(n) for the matter times coherent states of l-modes of the electromagnetic field. A variational analysis shows that the collective region is divided into monochromatic zones. For three-level atoms in the ladder-configuration in the presence of two modes of radiation, the variational calculation has been compared with the exact quantum solution showing that both are in agreement.

We present what we believe is the first study of squeezing (and phase- dependent fluctuations, in general) of the light emitted by a blinking atomic system. We use conditional homodyne detection (CHD) for that purpose; as CHD calculates a correlation of third order in the field amplitude it is more sensitive to nonlinearities, so it reaches fluctuations beyond squeezing. We obtain simple mathematical expressions for the spectra and variance.

It is well known that environmental noise usually hinders the efficiency of charge transport through coherent quantum systems; an exception is dephasing-assisted transport (DAT). In this talk I will show that linear triple quantum dots in a transport configuration and subjected to pure dephasing exhibit DAT if the coupling to the drain reservoir exceeds a threshold. DAT occurs for arbitrarily weak dephasing and the enhancement can be controlled by the coupling to the drain. I also identify the quantum Zeno effect and long-distance tunneling as underlying dynamic processes involved in dephasing-assisted and - suppressed transport.

The coexistence of regularity and chaos in quantum systems has been often analyzed comparing Poincaré sections of its semi-classical Hamiltonian with their quantum counterpart using Peres lattices and Husimi functions, which are qualitative, and with the fluctuations in the energy spectrum, which describe generic properties of whole energy regions. In this contribution we introduce a quantitative measure of quantum chaos, which can be employed for any given energy and for each point in the semiclassical phase space. It is the participation ratio (PR) of a coherent state spanned in the eigenstate basis. It is shown that the PR quantifies quantum chaos as much as the Lyapunov exponent does in the classical case. As an example we study the Dicke Hamlltonian, which describes a system of N two-level atoms interacting with a single monochromatic electromagnetic radiation mode within a cavity. It exhibits a second- order quantum phase transition (QPT) in the thermodynamic limit, and an excited-state quantum phase transition (ESQPT) along the energy spectrum, for fixed values of the Hamiltonian parameters, manifested by singularities in the level density, order parameters, and wave function properties. Mapping the phase space for fixed Hamiltonian parameters and for different excitation energies, the Lyapunov exponents and the PR are found to have critical behavior around the QPT. For any atom-field coupling, a low energy regime with regular states is always present, and coexistence regions emerge around the ESQPT.

In this work we study the temporal evolution of a coherent state under the action of a parametric oscillator and a non-linear Kerr medium. We make use of the interaction picture representation and use an exact time evolution operator for the time-independent part of the Hamiltonian. We approximate the interaction picture Hamiltonian in such a way as to make it a member of a Lie algebra. The corresponding time evolution operator behaves as a squeezing operator due to the temporal dependence of the oscillator's frequency. We analyze the probability amplitude, the auto correlation function, the Husimi distribution function and the Wigner function for different Hamiltonian parameters and find good agreement between our approximate results and converged numerical calculations.

R. Román-Ancheyta, M. Berrondo, J. Récamier JOSAB 32 (8) 1651-1655 (2015).

We propose a procedure to achieve high fidelity conversion between a photonic qubit encoded in a single photon wave packet and a matter qubit encoded in the atomic level structure of a single atom in free space with a high success probability. This procedure makes use of electromagnetically induced transparency in order to control the interaction between the atom and the radiation field. Thereby, a matter qubit can be converted into a photonic qubit stored in a timesymmetric single photon wave packet which can be absorbed almost perfectly by a second atom due to time reversal symmetry. By absorbing such a single photon wave packet, the photonic qubit can be converted back into a matter qubit thereby achieving a high fidelity quantum state transfer between distant matter qubits. In contrast to already known schemes, the protocol proposed in this article does not rely on high finesse cavities and optical fibers and is compatible with a free space communication channel