The Aharanov-Bohm (AB) effect, which predicts that a magnetic field strongly
influences the wave function of an electrically charged particle, is
investigated in a three site system in terms of the quantum control by an
additional dephasing source. The AB effect leads to a non-monotonic dependence
of the steady-state current on the gauge phase associated with the molecular
ring. This dependence is sensitive to site energy, temperature, and dephasing,
and can be explained using the concept of the dark state. Although the phase

Suppressing undesired non-unitary effects in a quantum system is a major
challenge in quantum computation and quantum control. In this scenario, the
investigation of the adiabatic dynamics under decoherence allows for optimal
strategies in adiabatic protocols in the presence of a surrounding environment.
In this work, we address this point by theoretically and experimentally
analyzing the robustness of the adiabatic theorem in open quantum systems. More

The ZX-Calculus is a graphical language for diagrammatic reasoning in quantum
mechanics and quantum information theory. It comes equipped with an equational
presentation. We focus here on a very important property of the language:
completeness, which roughly ensures the equational theory captures all of
quantum mechanics. We first improve on the known-to-be-complete presentation or
the so-called Clifford fragment of the language - a restriction that is not
universal - by adding some axioms. Thanks to a system of back-and-forth

A phenomenological construction of quantum Langevin equations, based on the
physical criteria of (i) the canonical equal-time commutators, (ii) the Kubo
formula, (iii) the virial theorem and (iv) the quantum fluctuation-dissipation
theorem is presented. The case of a single harmonic oscillator coupled to a
large external bath is analysed in detail. This allows to distinguish a
markovian semi-classical approach, due to Bedeaux and Mazur, from a
non-markovian full quantum approach, due to to Ford, Kac and Mazur. The

We demonstrate the preparation and coherent control of the angular momentum
state of a two-ion crystal. The ions are prepared with an average angular
momentum of $7780\hbar$ freely rotating at 100~kHz in a circularly symmetric
potential, allowing us to address rotational sidebands. By coherently exciting
these motional sidebands, we create superpositions of states separated by up to
four angular momentum quanta. Ramsey experiments show the expected dephasing of

An outstanding goal in quantum optics and scalable photonic quantum
technology is to develop a source that each time emits one and only one
entangled photon pair with simultaneously high entanglement fidelity,
extraction efficiency, and photon indistinguishability. By coherent two-photon
excitation of a single InGaAs quantum dot coupled to a circular Bragg grating
bullseye cavity with broadband high Purcell factor up to 11.3, we generate
entangled photon pairs with a state fidelity of 0.90(1), pair generation rate

Percolation theory allows simple description of the phase transition based on
the scaling properties of the network clusters with respect to a single
parameter - site or bond occupation probability. How to design a network
exhibiting the percolation transition for a chosen occupation probability has
been an open problem. At the same time, the task to find a structurally simple
network having the desired property seemed to be impossible. I suggest a model,
where the combination of the classical and the quantum resources creates a

We consider the quantum partition function for a system of quantum spinors
and then derive an equivalent (or dual) classical partition function for some
scalar degrees of freedom. The coupling between scalars is non-trivial (e.g. a
model on 2-sphere configuration space), but the locality structure of the dual
system is preserved, in contrast to the imaginary time formalism. We also show
that the measure of integration in the classical partition function can be

The Caldeira-Leggett master equation as an example of Markovian master
equation without Lindblad form is investigated for mathematical consistency. We
explore situations both analytically and numerically where the positivity
violations of the density operator occur. We reinforce some known knowledge
about this problem but also find new surprising cases. Our analytical results
are based on the full solution of the Caldeira-Leggett master equation obtained
via the method of characteristics. The preservation of positivity is mainly