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We present a novel continuous-time control strategy to exponentially
stabilize an eigenstate of a Quantum Non-Demolition (QND) measurement operator.
In open-loop, the system converges to a random eigenstate of the measurement
operator. The role of the feedback is to prepare a prescribed QND eigenstate
with unit probability. To achieve this we introduce the use of Brownian motion
to drive the unitary control actions; the feedback loop just adapts the
amplitude of this Brownian noise input as a function of the system state.

We looked into the algorithm for calculating Betti numbers presented by
Lloyd, Garnerone, and Zanardi (LGZ). We present a new algorithm in the same
spirit as LGZ with the intent of clarifying quantum algorithms for computing
Betti numbers. Our algorithm is simpler and slightly more efficient than that
presented by LGZ. We present a thorough analysis of our algorithm, pointing out
reasons that both our algorithm and that presented by LGZ do not run in
polynomial time for most inputs. However, the algorithms do run in polynomial

We derive the full linear-response theory for non-relativistic quantum
electrodynamics in the long wavelength limit, show quantum modifications of the
well-known Maxwell's equation in matter and provide a practical framework to
solve the resulting equations by using quantum-electrodynamical
density-functional theory. We highlight how the coupling between quantized
light and matter changes the usual response functions and introduces new types
of cross-correlated light-matter response functions. These cross-correlation

It is well known that the notions of spatial locality are often lost in
quantum systems with long-range interactions, as exhibited by the emergence of
phases with exotic long-range order and faster propagation of quantum
correlations. We demonstrate here that such induced ``quasinonlocal" effects do
not necessarily translate to growth of global entanglement in the quantum
system. By investigating the ground and quenched states of the variable-range,
spin-1/2 Heisenberg Hamiltonian, we observe that the genuine multiparty

Entanglement is a fundamental resource for quantum information science.
However, bipartite entanglement is destroyed when one particle is sharply
measured, which occurs in most applications. Here we experimentally show that,
if instead of sharp measurements, one performs many sequential unsharp
measurements on one particle which are suitably chosen depending on the
previous outcomes, then entanglement is preserved and can reveal quantum
correlations through measurements on the second particle. Specifically, we

Hybrid quantum-classical systems make it possible to utilize existing quantum
computers to their fullest extent. Within this framework, parameterized quantum
circuits can be thought of as machine learning models with remarkable
expressive power. This Review presents components of these models and discusses
their application to a variety of data-driven tasks such as supervised learning
and generative modeling. With experimental demonstrations carried out on actual

Colour centres with long-lived spins are established platforms for quantum
sensing and quantum information applications. Colour centres exist in different
charge states, each of them with distinct optical and spin properties.
Application to quantum technology requires the capability to access and
stabilize charge states for each specific task. Here, we investigate charge
state manipulation of individual silicon vacancies in silicon carbide, a system
which has recently shown a unique combination of long spin coherence time and

We study the subradiant collective states of a periodic chain of two-level
atoms with either transversal or longitudinal transition dipole moments with
respect to the chain axis. We show that long-lived subradiant states can be
obtained for the transversal polarization by properly choosing the chain period
for a given number of atoms in the case of no open diffraction channels. These
highly subradiant states have a linewidth that decreases with the number of

Out-of-time-order correlators (OTOCs) have been proposed as a probe of chaos
in quantum mechanics, on the basis of their short-time exponential growth found
in some particular set-ups. However, it has been seen that this behavior is not
universal. Therefore, we query other quantum chaos manifestations arising from
the OTOCs and we thus study their long-time behavior in systems of completely
different nature: quantum maps, which are the simplest chaotic one-body system

Quantum memories are an important building block for quantum information
processing. Ideally, these memories preserve the quantum properties of the
input. We present general criteria for measures to evaluate the quality of
quantum memories. Then, we introduce a quality measure based on coherence
satisfying these criteria, which we characterize in detail for the qubit case.
The measure can be estimated from sparse experimental data and may be
generalized to characterize other building blocks, such as quantum gates and

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