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

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