The interplay between Hamiltonian perturbations and dynamically occurring noise is considered as a system in its ground state is brought into contact with a thermal reservoir. This proves that when utilizing the toric code on N^2 qubits as a quantum memory, the information cannot be reliably stored for a time O(N). In contrast, the 2D Ising model protects classical information against the described noise model for exponentially long times. The proof constitutes a general perturbation-based technique to argue that schemes are protected, and imposes the requirement of string tension. The results also have implications for the robustness of braiding operations in topological quantum computation.

Adiabatic quantum computation (AQC) employs the ground state of time-dependent Hamiltonians for algorithm implementation. AQC initial Hamiltonians conventionally have a uniform superposition as ground state. We diverge from this practice by introducing a new strategy, in which adiabatic evolution starts with an initial guess chosen at random or following intuition about the problem, followed by a ``sombrero-like'' perturbation, hence the name sombrero AQC (SAQC). We provide a scheme to build initial Hamiltonians which encode such initial guesses in their ground states, and we present a proof of concept for SAQC by performing an exhaustive numerical study on hard-to-satisfy instances of the satisfiability problem (3-SAT). Our results show that about 35% of the initial 7 variable guesses have a significantly larger minimum gap compared to the minimum gap expected for conventional AQC (CAQC), possibly allowing for more efficient quantum algorithms. Finally, we propose serial and parallel versions of a quantum adiabatic algorithm based on SAQC.

The dynamics of a simple spin chain (2 spins) coupled to bosonic baths at different temperatures is studied. For a particular case an analytical solution for the reduced density matrix of the system is found. The dynamics and temperature dependence of spin-spin entanglement is analyzed.

A method of reversible quantum optical data storage is presented using resonant Raman field excited spin coherence, where the spin coherence is stored in an inhomogeneously broadened spin ensemble. Unlike the photon echo method, present technique uses a 2 pi Raman optical rephasing pulse, and multimode (parallel) optical channels can be utilized, where the multimode access gives a great benefit to both all-optical and quantum information processing.

Avalanche photodiodes are widely used as practical detectors of single photons.1 Although conventional devices respond to one or more photons, they cannot resolve the number in the incident pulse or short time interval. However, such photon number resolving detectors are urgently needed for applications in quantum computing,2-4 communications5 and interferometry,6 as well as for extending the applicability of quantum detection generally. Here we show that, contrary to current belief,3,4 avalanche photodiodes are capable of detecting photon number, using a technique to measure very weak avalanches at the early stage of their development. Under such conditions the output signal from the avalanche photodiode is proportional to the number of photons in the incident pulse. As a compact, mass-manufactured device, operating without cryogens and at telecom wavelengths, it offers a practical solution for photon number detection.

Stochastic and bistochastic matrices providing positive maps for spin states (for qudits) are shown to form semigroups with dense intersection with the Lie groups $IGL(n, \mathbb{R})$ and $GL(n, \mathbb{R})$ respectively. The density matrix of a qudit state is shown to be described by a spin tomogram determined by an orbit of the bistochastic semigroup acting on a simplex. A class of positive maps acting transitively on quantum states is introduced by relating stochastic and quantum stochastic maps in the tomographic setting. Finally, the entangled states of two qubits and Bell inequalities are given in the framework of the tomographic probability representation using the stochastic semigroup properties.

We report on the collapse and revival of Ramsey fringe visibility when a spatially dependent phase is imprinted in the coherences of a trapped ensemble of two-level atoms. The phase is imprinted via the light shift from a Gaussian laser beam which couples the dynamics of internal and external degrees of freedom for the atoms in an echo spectroscopy sequence. The observed revivals are directly linked to the oscillatory motion of atoms in the trap. An understanding of the effect is important for quantum state engineering of trapped atoms.

By cyclic adiabatic change of two control parameters of an optical trap one can induce a circulating current of condensed bosons. The amount of particles that are transported per period depends on the "radius" of the cycle, and this dependence can be utilized in order to probe the interatomic interactions. For strong repulsive interaction the current can be regarded as arising from a sequence of Landau-Zener crossings. For weaker interaction one observes either gradual or coherent mega crossings, while for attractive interaction the particles are glued together and behave like a classical ball. For the analysis we use the Kubo approach to quantum pumping with the associated Dirac monopoles picture of parameter space.