All

Silicon spin qubits are a promising quantum computing platform offering long
coherence times, small device sizes, and compatibility with industry-backed
device fabrication techniques. In recent years, high fidelity single-qubit and
two-qubit operations have been demonstrated in Si. Here, we demonstrate
coherent spin control in a quadruple quantum dot fabricated using isotopically
enriched 28Si. We tune the ground state charge configuration of the quadruple

We consider a bipartite quantum conductor and analyze fluctuations of heat
quantity in a subsystem as well as self-information associated with the
reduced-density matrix of the subsystem. By exploiting the multi-contour
Keldysh technique, we calculate the R\'enyi entropy, or the information
generating function, subjected to the constraint of the local heat quantity of
the subsystem, from which the probability distribution of conditional
self-information is derived. We present an equality that relates the optimum

We present a tomographic method which requires only $4d-3$ measurement
outcomes to reconstruct \emph{any} pure quantum state of arbitrary dimension
$d$. Using the proposed scheme we have experimentally reconstructed a large
number of pure states of dimension $d=7$, obtaining a mean fidelity of $0.94$.
Moreover, we performed numerical simulations of the reconstruction process,
verifying the feasibility of the method for higher dimensions. In addition, the

Here we prove the existence and uniqueness of solutions of a class of
integral equations describing two Dirac particles in 1+3 dimensions with direct
interactions. This class of integral equations arises naturally as a
relativistic generalization of the integral version of the two-particle
Schr\"odinger equation. Crucial use of a multi-time wave function
$\psi(x_1,x_2)$ with $x_1,x_2 \in \mathbb{R}^4$ is made. A central feature is
the time delay of the interaction. Our main result is an existence and

The Aharanov-Bohm (AB) effect, which predicts that a magnetic field strongly
influences the wave function of an electrically charged particle, is
investigated in a three site system in terms of the quantum control by an
additional dephasing source. The AB effect leads to a non-monotonic dependence
of the steady-state current on the gauge phase associated with the molecular
ring. This dependence is sensitive to site energy, temperature, and dephasing,
and can be explained using the concept of the dark state. Although the phase

Suppressing undesired non-unitary effects in a quantum system is a major
challenge in quantum computation and quantum control. In this scenario, the
investigation of the adiabatic dynamics under decoherence allows for optimal
strategies in adiabatic protocols in the presence of a surrounding environment.
In this work, we address this point by theoretically and experimentally
analyzing the robustness of the adiabatic theorem in open quantum systems. More

The ZX-Calculus is a graphical language for diagrammatic reasoning in quantum
mechanics and quantum information theory. It comes equipped with an equational
presentation. We focus here on a very important property of the language:
completeness, which roughly ensures the equational theory captures all of
quantum mechanics. We first improve on the known-to-be-complete presentation or
the so-called Clifford fragment of the language - a restriction that is not
universal - by adding some axioms. Thanks to a system of back-and-forth

A phenomenological construction of quantum Langevin equations, based on the
physical criteria of (i) the canonical equal-time commutators, (ii) the Kubo
formula, (iii) the virial theorem and (iv) the quantum fluctuation-dissipation
theorem is presented. The case of a single harmonic oscillator coupled to a
large external bath is analysed in detail. This allows to distinguish a
markovian semi-classical approach, due to Bedeaux and Mazur, from a
non-markovian full quantum approach, due to to Ford, Kac and Mazur. The

We demonstrate the preparation and coherent control of the angular momentum
state of a two-ion crystal. The ions are prepared with an average angular
momentum of $7780\hbar$ freely rotating at 100~kHz in a circularly symmetric
potential, allowing us to address rotational sidebands. By coherently exciting
these motional sidebands, we create superpositions of states separated by up to
four angular momentum quanta. Ramsey experiments show the expected dephasing of

An outstanding goal in quantum optics and scalable photonic quantum
technology is to develop a source that each time emits one and only one
entangled photon pair with simultaneously high entanglement fidelity,
extraction efficiency, and photon indistinguishability. By coherent two-photon
excitation of a single InGaAs quantum dot coupled to a circular Bragg grating
bullseye cavity with broadband high Purcell factor up to 11.3, we generate
entangled photon pairs with a state fidelity of 0.90(1), pair generation rate