All

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

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

Pages