We study the behaviour of the fidelity and the Uhlmann connection in
two-dimensional systems of free fermions that exhibit non-trivial topological
behavior. In particular, we use the fidelity and a quantity closely related to
the Uhlmann factor in order to detect phase transitions at zero and finite
temperature for topological insulators and superconductors. We show that at
zero temperature both quantities predict quantum phase transitions: a sudden
drop of fidelity indicates an abrupt change of the spectrum of the state, while

Privacy amplification (PA) is a vital procedure in quantum key distribution
(QKD) to generate the secret key that the eavesdropper has only negligible
information from the identical correcting key for the communicating parties.
With the increase of repeat frequency of discrete-variable QKD (DV-QKD) system,
the processing speed of PA has become the bottle neck restricting DV-QKD's
secure key rate. The PA using Toeplitz-based Hash function is adopted widely

We propose to manipulate the statistic properties of the photons transport
nonreciprocally via quadratic optomechanical coupling. We present a scheme to
generate quadratic optomechanical interactions in the normal optical modes of a
whispering-gallery-mode (WGM) optomechanical system by eliminating the linear
optomechanical couplings via anticrossing of different modes. By optically
pumping the WGM optomechanical system in one direction, the effective quadratic

Multiple quantum (MQ) NMR coherence spectra, which can be obtained
experimentally in MQ NMR, can be transferred from the sender to the remote
receiver without mixing the MQ-coherences of different orders and distortions.
The only effect of such transfer is scaling of the certain blocks of sender's
density matrix (matrices of MQ-coherences of different order). Such a
block-scaled transfer is an alternative to the perfect state transfer. In
particular, equal scaling of higher order MQ-coherences matrices is possible.

We consider the communication line with two-qubit sender and receiver, the
later is embedded into the four-qubit extended receiver. Using the optimizing
unitary transformation on the extended receiver we restore the structure of the
non-diagonal part of an arbitrary initial sender's state at the remote receiver
at certain time instant. Obstacles for restoring the diagonal part are
discussed. We represent examples of such structural restoring in a
communication line of 42 spin-1/2 particles.

We derive an effective equation of motion within the steady-state subspace of
a large family of Markovian open systems (i.e., Lindbladians) due to
perturbations of their Hamiltonians and system-bath couplings. Under mild and
realistic conditions, competing dissipative processes destructively interfere
without the need for fine-tuning and produce no dissipation within the
steady-state subspace. In quantum error-correction, these effects imply that
continuously error-correcting Lindbladians are robust to calibration errors,

Vortices in electron beams can manifest several types of topological
phenomena, such as the formation of exotic structures or interactions with
topologically structured electromagnetic fields. For instance, the wavefunction
of an electron beam can acquire a phase vortex upon propagating through a
magnetic monopole, which, in practice, provides a convenient method for
generating electron vortex beams. Here, we show how an electric field must be
structured in order to achieve a similar effect. We find that, much as in the

We study the production of photons in a model of three bosonic atomic modes
non-linearly coupled to a cavity mode. In absence of external driving and
dissipation, the energy levels at different photon numbers assemble into the
steps of an energy staircase which can be employed as guidance for preparing
multi-photon states. We consider adiabatic photon production, driving the
system through a sequence of Landau-Zener transitions in the presence of
external coherent light pumping. We also analyse the non-equilibrium dynamics

This proposal investigates the photon-statistics of light emitted by a
statistical ensemble of cold atoms excited by the near-field of an optical
nanofiber tip. Dipole-dipole interactions of atoms at such short distance from
each other suppress the simultaneous emission of more than one photon and lead
to antibunching of photons. We consider a mean atom number on the order of one
and deal with a poissonian mixture of one and two atoms including dipole-dipole
interactions and collective decay. Time tracks of the atomic states are

Quantum measurements can be interpreted as a generalisation of probability
vectors, in which non-negative real numbers are replaced by positive
semi-definite operators. We extrapolate this analogy to define a generalisation
of doubly stochastic matrices that we call doubly normalised tensors (DNTs),
and formulate a corresponding version of Birkhoff-von Neumann's theorem, which
states that permutations are the extremal points of the set of doubly
stochastic matrices. We prove that joint measurability arises as a mathematical