The natural environment of a localized quantum system is the continuum of scattering wavefunctions into which the system is embedded. It can be changed by external fields, however never be deleted. The control of the system’s properties by varying a certain parameter provides us information on the system. It is, in many cases, counterintuitive and points to the same phenomena in different systems in spite of the specific differences between them. In our paper, we use a schematic model in order to simulate the main features of small open quantum systems. At low level density, the system is described well by standard Hermitian quantum physics while fundamental differences appear at high level density due to the non-Hermiticity of the Hamiltonian which cannot be neglected under this condition. The influence of exceptional points, the phase rigidity of the wavefunctions and the nonlinearities in the equations are discussed by means of different numerical and (when possible) analytical results. The transition from a closed system at low level density to an open one at high level density occurs smoothly.

http://arxiv.org/abs/1409.1149
Quantum Physics (quant-ph)

We introduce a recursive bosonic quantization technique for generating classical PT photonic structures that possess hidden symmetries and higher order exceptional points. We study light transport in these geometries and we demonstrate that perfect state transfer is possible only for certain initial conditions. Moreover, we show that for the same propagation direction, left and right coherent transports are not symmetric with field amplitudes following two different trajectories. A general scheme for identifying the conservation laws in such PT-symmetric photonic networks is also presented.

http://arxiv.org/abs/1408.1561
Optics (physics.optics); Quantum Physics (quant-ph)

Exceptional points (EPs) determine the dynamics of open quantum systems and cause also PT symmetry breaking in PT symmetric systems. From a mathematical point of view, this is caused by the fact that the phases of the wavefunctions (eigenfunctions of a non-Hermitian Hamiltonian) relative to one another are not rigid when an EP is approached. The system is therefore able to align with the environment to which it is coupled and, consequently, rigorous changes of the system properties may occur. We compare analytically as well as numerically the eigenvalues and eigenfunctions of a 2×2 matrix that is characteristic of either open quantum systems at high level density or of PT symmetric optical lattices. In both cases, the results show clearly the influence of the environment onto the system in the neighborhood of EPs. Although the systems are very different from one another, the eigenvalues and eigenfunctions indicate the same characteristic features.

http://arxiv.org/abs/1311.6320
Dynamical Systems (math.DS); Quantum Physics (quant-ph)