# All

We show how charged levitated nano- and micro-particles can be cooled by

interfacing them with an $RLC$ circuit. All-electrical levitation and cooling

is applicable to a wide range of particle sizes and materials, and will enable

state-of-the-art force sensing within an electrically networked system.

Exploring the cooling limits in the presence of realistic noise we find that

the quantum regime of particle motion can be reached in cryogenic environments

both for passive resistive cooling and for an active feedback scheme, paving

A quantum network consists of independent sources distributing entangled

states to distant nodes which can then perform entangled measurements, thus

establishing correlations across the entire network. But how strong can these

correlations be? Here we address this question, by deriving bounds on possible

quantum correlations in a given network. These bounds are nonlinear

inequalities that depend only on the topology of the network. We discuss in

detail the notably challenging case of the triangle network. Moreover, we

The computational efficiency of quantum mechanics can be defined in terms of

the qubit circuit model, which is characterized by a few simple properties:

each computational gate is a reversible transformation in a connected matrix

group; single wires carry quantum bits, i.e. states of a three-dimensional

Bloch ball; states on two or more wires are uniquely determined by local

measurement statistics and their correlations. In this paper, we ask whether

other types of computation are possible if we relax one of those

What is the quantum state of the universe? Although there have been several

interesting suggestions, the question remains open. In this paper, I consider a

natural choice for the universal quantum state arising from the Past

Hypothesis, a boundary condition that accounts for the time-asymmetry of the

universe. The natural choice is given not by a wave function (representing a

pure state) but by a density matrix (representing a mixed state).

Einstein-Podolsky-Rosen steering is operationally defined as an entanglement

verification task between two parties when one of their measurement devices is

untrusted. Recent progress shows that the trustness of the other device can

even be removed by preparing a set of tomographically complete quantum states

along with it, in which the scheme is dubbed a measurement-device-independent

(MDI) scenario. A benefit of the MDI scheme is that the original trusted

measurement device does not need to perform quantum state tomography to

We study the effect of local decoherence on arbitrary quantum states.

Adapting techniques developed in quantum metrology, we show that the action of

generic local noise processes -- though arbitrarily small -- always yields a

state whose Quantum Fisher Information (QFI) with respect to local observables

is linear in system size N, independent of the initial state. This implies that

all macroscopic quantum states, which are characterized by a QFI that is

We study the informational underpinnings of thermodynamics and statistical

mechanics, using an abstract framework, general probabilistic theories, capable

of describing arbitrary physical theories. This allows one to abstract the

informational content of a theory from the concrete details of its formalism.

In this framework, we extend the treatment of microcanonical thermodynamics,

namely the thermodynamics of systems with a well-defined energy, beyond the

known cases of classical and quantum theory, formulating two necessary

Motivated by recent proposals of Majorana qubits and the read-out of their

quantum state we investigate a qubit setup formed by two parallel topological

wires shunted by a superconducting bridge. The wires are further coupled to two

quantum dots, which are also linked directly, thus creating an interference

loop. The transport current through this system shows an interference pattern

which distinguishes two basis states of the qubit in a QND measurement. We

analyze various properties of the interference current and the read-out

Bell's inequality was originally derived under the assumption that

experimenters are free to select detector settings independently of any local

"hidden variables" that might affect the outcomes of measurements on entangled

particles. This assumption has come to be known as "measurement independence"

(also referred to as "freedom of choice" or "settings independence"). For a

two-setting, two-outcome Bell test, we derive modified Bell inequalities that

relax measurement independence, for either or both observers, while remaining

Fracton topological phases possess a large number of emergent symmetries that

enforce a rigid structure on their excitations. Remarkably, we find that the

symmetries of a quantum error-correcting code based on a fracton phase enable

us to design highly parallelized decoding algorithms. Here we design and

implement decoding algorithms for the three-dimensional X-cube model where

decoding is subdivided into a series of two-dimensional matching problems, thus