The issue of time travel can be reduced in quantum theory to an appropriate
Hilbert-space description of feedback loops. I show how to do it in a way that
automatically eliminates problems with chronology protection, provided all
input-output relations are given by unitary maps. Examples of elementary loops
and a two-loop time machine illustrate the construction.

Scalable quantum computing relies crucially on high-fidelity entangling
operations. Here we demonstrate that four coupled qubits can operate as a
high-fidelity two-qubit entangling gate that swaps two target qubits and adds a
relative sign on the $\lvert 11 \rangle$ state (ZSWAP). The gate operation is
controlled by the state of two ancilla (control) qubits. The system is readily
implementable with superconducting qubits, using capacitively coupled qubits

When compared to quantum mechanics, classical mechanics is often depicted in
a specific metaphysical flavour: spatio-temporal realism or a Newtonian
"background" is presented as an intrinsic fundamental classical presumption.
However, the Hamiltonian formulation of classical analytical mechanics is based
on abstract generalized coordinates and momenta: It is a mathematical rather
than a philosophical framework. If the metaphysical assumptions ascribed to
classical mechanics are dropped, then there exists a presentation in which

We experimentally demonstrate a variation on a Sisyphus cooling technique
that was proposed for cooling antihydrogen. In our implementation, atoms are
selectively excited to an electronic state whose energy is spatially modulated
by an optical lattice, and the ensuing spontaneous decay completes one Sisyphus
cooling cycle. We characterize the cooling efficiency of this technique on a
continuous beam of Sr, and compare it with the case of a Zeeman slower. We

Coherent superposition is a key feature of quantum mechanics that underlies
the advantage of quantum technologies over their classical counterparts.
Recently, coherence has been recast as a resource theory in an attempt to
identify and quantify it in an operationally well-defined manner. Here we study
how the coherence present in a state can be used to implement a quantum channel
via incoherent operations and, in turn, to assess its degree of coherence. We

This paper proposes a brain-inspired approach to quantum machine learning
with the goal of circumventing many of the complications of other approaches.
The fact that quantum processes are unitary presents both opportunities and
challenges. A principal opportunity is that a large number of computations can
be carried out in parallel in linear superposition, that is, quantum
parallelism. The challenge is that the process is linear, and most approaches
to machine learning depend significantly on nonlinear processes. Fortunately,

Role of entanglement is yet to be fully understood in quantum thermodynamics.
We shed some light upon that direction by considering the role of entanglement
for a single temperature quantum heat engine without feedback, introduced
recently by J. Yi, P. Talkner and Y. W. Kim (Phys. Rev. E 96, 022108 (2017)).
We take the working medium of the engine to be a 1-dim Heisenberg model of two
spins. We calculate the efficiency of the engine undergoing a cyclic process at

In these lecture notes we give a technical overview of tangent-space methods
for matrix product states in the thermodynamic limit. We introduce the manifold
of uniform matrix product states, show how to compute different types of
observables, and discuss the concept of a tangent space. We explain how to
variationally optimize ground-state approximations, implement real-time
evolution and describe elementary excitations for a given model Hamiltonian.
Also, we explain how matrix product states approximate fixed points of

The emergence of a special type of fluid-like behavior at large scales in
one-dimensional (1d) quantum integrable systems, theoretically predicted in
2016, is established experimentally, by monitoring the time evolution of the in
situ density profile of a single 1d cloud of $^{87}{\rm Rb}$ atoms trapped on
an atom chip after a quench of the longitudinal trapping potential. The theory
can be viewed as a dynamical extension of the thermodynamics of Yang and Yang,

Extending our previous analysis on bi-coherent states, we introduce here a new class of quantum
mechanical vectors, the bi-squeezed states , and we deduce their main mathematical properties. We
relate bi-squeezed states to the so-called regular and non regular pseudo-bosons. We show that these
two cases are different, from a mathematical point of view. Some physical examples are considered.