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.

# All

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