We measure the population distribution in one of the atomic twin beams
generated by four-wave mixing in an optical lattice.

Although the produced two-mode squeezed vacuum state is pure, each individual
mode is described as a statistical mixture.

We confirm the prediction that the particle number follows an exponential
distribution when only one spatio-temporal mode is selected.

We also show that this distribution accounts well for the contrast of an
atomic Hong--Ou--Mandel experiment.

The quantum measurement procedure based on the Lorentz transformation
formalism and weak perturbation of the system is considered. In the simple case
of a single-qubit it turns out that one can perform 4-dimension pseudo-rotation
along with ordinary 3-dimension rotations on the Bloch sphere. These
pseudo-rotations are similar to the Lorentz transformation in special
relativity theory. The extension of the Lorentz transformation for many-qubit
systems is also considered. The quantum measurement protocols based on the

Recently, the study of non-Hermitian physics has attracted considerable
attention. The modified bulk-boundary correspondence has been proposed to
understand topological edge states in non-Hermitian static systems. Here we
report a new experimental observation of edge states in non-Hermitian
periodically driven systems. Some unconventional edge states are found not to
be satisfied with the bulk-boundary correspondence when the system belongs to
the broken parity-time (PT) symmetric phase. The experiments are performed in

We analyse quasi-periodically driven quantum systems that can be mapped
exactly to periodically driven ones and find Floquet Time Spirals in analogy
with spatially incommensurate spiral magnetic states. Generalising the
mechanism to many-body systems we discover that a form of discrete
time-translation symmetry breaking can also occur in quasi-periodically driven
systems. We construct a discrete time quasi-crystal stabilised by many-body
localisation, which persists also under perturbations that break the

Clarifying the nature of the quantum state $|\Psi\rangle$ is at the root of
the problems with insight into (counterintuitive) quantum postulates. We
provide a direct -- and math axiom-free -- empirical derivation of this object
as an element of a vector space. Establishing the linearity of this structure
-- quantum superposition -- is based on a set-theoretic creation of ensemble
formations and invokes the following three principia: ($\textsf{I}$) quantum
statics, ($\textsf{II}$) doctrine of a number in a physical theory, and

We study effects of perturbation Hamiltonian to quantum spin systems which
can include quenched disorder. Model-independent inequalities are derived,
using an additional artificial disordered perturbation. These inequalities
enable us to prove that the variance of the perturbation Hamiltonian density
vanishes in the infinite volume limit even if the artificial perturbation is
switched off. This theorem is applied to spontaneous symmetry breaking
phenomena in a disordered classical spin model, a quantum spin model without

The work is devoted to the theoretical and experimental study of quantum
states of light conditionally prepared by subtraction of a random number of
photons from the initial multimode thermal state. A fixed number of photons is
subtracted from a multimode quantum state, but only a subsystem of a lower
number of modes is registered, in which the number of subtracted photons turns
out to be a non-fixed random variable. It is shown that the investigation of

In practical implementation of quantum key distributions (QKD), it requires
efficient, real-time feedback control to maintain system stability when facing
disturbance from either external environment or imperfect internal components.
Usually, a "scanning-and-transmitting" program is adopted to compensate
physical parameter variations of devices, which can provide accurate
compensation but may cost plenty of time in stopping and calibrating processes,
resulting in reduced efficiency in key transmission. Here we for the first

We present a detailed study of the topological Schwinger model
[$\href{this http URL}{Phys. \; Rev.\; D \; {\bf
99},\;014503 \; (2019)}$], which describes (1+1) quantum electrodynamics of an
Abelian $U(1)$ gauge field coupled to a symmetry-protected topological matter
sector, by means of a class of $\mathbb{Z}_N$ lattice gauge theories. Employing
density-matrix renormalization group techniques that exactly implement Gauss'

We apply the scattering approach to the Casimir interaction between two
dielectric half-spaces separated by an electrolyte solution. We take the
nonlocal electromagnetic response of the intervening medium into account, which
results from the presence of movable ions in solution. In addition to the usual
transverse modes, we consider longitudinal channels and their coupling by
reflection at the surface of the local dielectric. The Casimir interaction
energy is calculated from the matrix describing a round-trip of coupled