Self-consistent calculation of the electron distribution near a Quantum-Point Contact in the integer Quantum Hall Effect

In this work we implement the self-consistent Thomas-Fermi-Poisson approach to a homogeneous two dimensional electron system (2DES). We compute the electrostatic potential produced inside a semiconductor structure by a quantum-point-contact (QPC) placed at the surface of the semiconductor and biased with appropriate voltages. The model is based on a semi-analytical solution of the Laplace equation. Starting from the calculated confining potential, the self-consistent (screened) potential and the electron densities are calculated for finite temperature and magnetic field. We observe that there are mainly three characteristic rearrangements of the incompressible "edge" states, which will determine the current distribution near a QPC.Comment: 12 pages, 10 figures, submitted to Phys. Rev.

Dissipative optomechanical squeezing of light

We discuss a simple yet surprisingly effective mechanism which allows the generation of squeezed output light from an optomechanical cavity. In contrast to the well known mechanism of "ponderomotive squeezing", our scheme generates squeezed output light by explicitly using the dissipative nature of the mechanical resonator. We show that our scheme has many advantages over ponderomotive squeezing; in particular, it is far more effective in the good cavity limit commonly used in experiments. Furthermore, the squeezing generated in our approach can be directly used to enhance the intrinsic measurement sensitivity of the optomechanical cavity; one does not have to feed the squeezed light into a separate measurement device. As our scheme is very general, it could also e.g. be implemented using superconducting circuits

Arbitrarily large steady-state bosonic squeezing via dissipation

We discuss how large amounts of steady-state quantum squeezing (beyond 3 dB) of a mechanical resonator can be obtained by driving an optomechanical cavity with two control lasers with differing amplitudes. The scheme does not rely on any explicit measurement or feedback, nor does it simply involve a modulation of an optical spring constant. Instead, it uses a dissipative mechanism with the driven cavity acting as an engineered reservoir. It can equivalently be viewed as a coherent feedback process, obtained by minimally perturbing the quantum nondemolition measurement of a single mechanical quadrature. This shows that in general the concepts of coherent feedback schemes and reservoir engineering are closely related. We analyze how to optimize the scheme, how the squeezing scales with system parameters, and how it may be directly detected from the cavity output. Our scheme is extremely general, and could also be implemented with, e.g., superconducting circuits.Comment: 5 pages, 3 figures ; 6 pages supplemental informatio

Optomechanical circuits for nanomechanical continuous variable quantum state processing

We propose and analyze a nanomechanical architecture where light is used to perform linear quantum operations on a set of many vibrational modes. Suitable amplitude modulation of a single laser beam is shown to generate squeezing, entanglement, and state-transfer between modes that are selected according to their mechanical oscillation frequency. Current optomechanical devices based on photonic crystals may provide a platform for realizing this scheme.Comment: 11 pages, 5 figure

Review of The Sleeper's Almanac 8 edited by Zoe Dattner and Louise Swinn

Review of The Sleeper's Almanac 8 edited by Zoe Dattner and Louise Swin

Decoherence induced by an interacting spin environment in the transition from integrability to chaos

We investigate the decoherence properties of a central system composed of two spins 1/2 in contact with a spin bath. The dynamical regime of the bath ranges from a fully integrable integrable limit to complete chaoticity. We show that the dynamical regime of the bath determines the efficiency of the decoherence process. For perturbative regimes, the integrable limit provides stronger decoherence, while in the strong coupling regime the chaotic limit becomes more efficient. We also show that the decoherence time behaves in a similar way. On the contrary, the rate of decay of magnitudes like linear entropy or fidelity does not depend on the dynamical regime of the bath. We interpret the latter results as due to a comparable complexity of the Hamiltonian for both the integrable and the fully chaotic limits.Comment: Submitted to Phys. Rev.

Topological phase transitions and chiral inelastic transport induced by the squeezing of light

We show how the squeezing of light can lead to the formation of topological states. Such states are characterized by non-trivial Chern numbers, and exhibit protected edge modes which give rise to chiral elastic and inelastic photon transport. These topological bosonic states are not equivalent to their fermionic (topological superconductor) counterparts and cannot be mapped by a local transformation onto topological states found in particle-conserving models. They thus represent a new type of topological system. We study this physics in detail in the case of a Kagome lattice model, and discuss possible realizations using nonlinear photonic crystals or superconducting circuits.Comment: 11 pages, 4 figure

Settlement and resettlement experience from Uganda’s National parks, game reserves and forest reserves

In Uganda the need for and justification of population resettlement has risen from a number of aspects related to the country’s economic setting. The existence of agricultural land of high potential but low population densities; settlement on land cleared of tse tse infestation as a mechanism to prevent resurgence of the fly; forced or persuaded movement of people from areas of high population densities

Transverse angular momentum of photons

We develop the quantum theory of transverse angular momentum of light beams. The theory applies to paraxial and quasi-paraxial photon beams in vacuum, and reproduces the known results for classical beams when applied to coherent states of the field. Both the Poynting vector, alias the linear momentum, and the angular momentum quantum operators of a light beam are calculated including contributions from first-order transverse derivatives. This permits a correct description of the energy flow in the beam and the natural emergence of both the spin and the angular momentum of the photons. We show that for collimated beams of light, orbital angular momentum operators do not satisfy the standard commutation rules. Finally, we discuss the application of our theory to some concrete cases.Comment: 10 pages, 2 figure

Fermionic Mach-Zehnder interferometer subject to a quantum bath

We study fermions in a Mach-Zehnder interferometer, subject to a quantum-mechanical environment leading to inelastic scattering, decoherence, renormalization effects, and time-dependent conductance fluctuations. Both the loss of interference contrast as well as the shot noise are calculated, using equations of motion and leading order perturbation theory. The full dependence of the shot-noise correction on setup parameters, voltage, temperature and the bath spectrum is presented. We find an interesting contribution due to correlations between the fluctuating renormalized phase shift and the output current, discuss the limiting behaviours at low and high voltages, and compare with simpler models of dephasing.Comment: 5 pages, 3 figure