We have studied the effect of a non-Hermitian Bosonic bath on the dynamics of

a two-level spin system. The non-Hermitian Hamiltonian of the bath is chosen

such that it converges to the harmonic oscillator Hamiltonian when the

non-Hermiticity is switched off. We calculate the dynamics of the spin system

and found that the non-Hermiticity can have positive as well as negative

effects on the coherence of the system. However, the decoherence can be

completely eliminated by choosing the non-Hermiticity parameter and the phase

# All

The paper retraces the development from the measurement problem to the

primitive ontology programme. It assesses the contribution of the GRW theory to

this programme and discusses the pros and cons of the GRWm matter density

ontology and the GRWf flash ontology in comparison to the Bohmian particle

ontology. It thereby pursues the evaluation of the proposals for a primitive

ontology of quantum physics.

The quantum coherence and gate fidelity of electron spin qubits in

semiconductors is often limited by noise arising from coupling to a bath of

nuclear spins. Isotopic enrichment of spin-zero nuclei such as $^{28}$Si has

led to spectacular improvements of the dephasing time $T_2^*$ which,

surprisingly, can extend two orders of magnitude beyond theoretical

expectations. Using a single-atom $^{31}$P qubit in enriched $^{28}$Si, we show

that the abnormally long $T_2^*$ is due to the controllable freezing of the

The toric code is known to be equivalent to free fermions. This paper

presents explicit local unitary transformations that map the $\mathbb{Z}_2$

toric and surface code --- the open boundary equivalent of the toric code ---

to fermions. Through this construction it is shown that the surface code can be

mapped to a set of free fermion modes, while the toric code requires additional

fermionic symmetry operators. Finally, it is demonstrated how the anyonic

statistics of these codes are encoded in the fermionic representations.

We study entanglement of spin degrees of freedom with continuous one in

supersymmetric (SUSY) quantum mechanics. Concurrence is determined by mean

value of spin and is calculated explicitly for SUSY states. We show that

eigenstates of supercharges are maximally entangled. As an example the

entanglement of atom state with photon state and SUSY in Jaynes-Cummings model

are considered.

Number state filtered coherent states are a class of nonclassical states

obtained by removing one or more number states from a coherent state. Phase

sensitivity of an interferometer is enhanced if these nonclassical states are

used as input states. The optimal phase sensitivity, which is related to the

quantum Cramer-Rao bound (QCRB) for the input state, improves beyond the

standard quantum limit. It is argued that removal of more than one suitable

number state leads to better phase sensitivity. As an important limiting case

The so-called stellar formalism allows to represent the non-Gaussian

properties of single-mode quantum states by the distribution of the zeros of

their Husimi Q-function in phase-space. We use this representation in order to

derive an infinite hierarchy of single-mode states based on the number of zeros

of the Husimi Q-function, the stellar hierarchy. We give an operational

characterisation of the states in this hierarchy with the minimal number of

single-photon additions needed to engineer them, and derive equivalence classes

The L\"uders rule provides a way to define a quantum channel given a quantum

measurement. Using this construction, we establish an if-and-only-if condition

for the existence of a $d$-dimensional Symmetric Informationally Complete

quantum measurement (a SIC) in terms of a particular depolarizing channel.

Moreover, the channel in question satisfies two entropic optimality criteria.

We consider an unexplored aspect of the mass equivalence principle in the

quantum realm, its connection with atomic stability. We show that if the

gravitational mass were different from the inertial one, a Hydrogen atom placed

in a constant gravitational field would become unstable in the long term. In

contrast, independently of the relation between the two masses, the atom does

not become ionized in an uniformly accelerated frame. This work, in the line of

Inspired by the decomposition in the hybrid quantum-classical optimization

algorithm we introduced in arXiv:1902.04215, we propose here a new (fully

classical) approach to solving certain non-convex integer programs using Graver

bases. This method is well suited when (a) the constraint matrix $A$ has a

special structure so that its Graver basis can be computed systematically, (b)

several feasible solutions can also be constructed easily and (c) the objective