Photons with well-defined energy are highly non-localised and occupy all
available space in one dimension. On the other hand, optical elements, like
mirrors, are highly-localised objects. To model the electromagnetic field in
the presence of optical elements, we therefore introduce annihilation operators
for highly-localised field excitations. These are different from
single-particle annihilation operators and arise naturally, if we quantise the
negative as well as the positive solutions of Maxwell's equations. Moreover,

Matrix product state (MPS) belongs to the most important mathematical models
in, for example, condensed matter physics and quantum information sciences.
However, to realize an $N$-qubit MPS with large $N$ and large entanglement on a
quantum platform is extremely challenging, since it requires high-level qudits
or multi-body gates of two-level qubits to carry the entanglement. In this
work, an efficient method that accurately encodes a given MPS into a quantum
circuit with only one- and two-qubit gates is proposed. The idea is to

Quantum computers provide an opportunity to efficiently sample from
probability distributions that include non-trivial interference effects between
amplitudes. Using a simple process wherein all possible state histories can be
specified by a binary tree, we construct an explicit quantum algorithm that
runs in polynomial time to sample from the process once. The corresponding
naive Markov Chain algorithm does not produce the correct probability
distribution and an explicit classical calculation of the full distribution

Selected states of the $EF\ ^1\Sigma_\mathrm{g}^+$ electronic manifold of the
hydrogen molecule are computed as resonances of the four-body problem.
Systematic improvement of the basis representation for the variational
treatment is achieved through an energy-tracking optimization procedure. The
resulting non-relativistic energy is converged within a few nano Hartree, while
the predissociative width is found to be negligible at this level of accuracy.
The four-particle non-relativistic energies are appended with relativistic and

We employ quantum optimal control theory to realize quantum gates for two
protected superconducting circuits: the heavy-fluxonium qubit and the 0-$\pi$
qubit. Utilizing automatic differentiation facilitates the simultaneous
inclusion of multiple optimization targets, allowing one to obtain
high-fidelity gates with realistic pulse shapes. For both qubits, disjoint
support of low-lying wave functions prevents direct population transfer between
the computational-basis states. Instead, optimal control favors dynamics

Most quantum processors requires pulse sequences for controlling quantum
states. Here, we present an alternative algorithm for computing an optimal
pulse sequence in order to perform a specific task, being an implementation of
a quantum gate or a quantum state preparation. In our method, we reduced
drastically the number of parameters to be fitted, by using a limited number of
functions as the modulations for the amplitude and phase of the radio-frequency

The recently developed Wigner functional theory is used to formulate an
evolution equation for arbitrary multi-photon states, propagating through a
turbulent atmosphere under arbitrary conditions. The resulting evolution
equation, which is obtained from an infinitesimal propagation approach, is in
the form of a Fokker-Planck equation for the Wigner functional of the state and
therefore incorporates functional derivatives. We show consistency with
previously obtained solutions from different approaches and consider possible

The mass-correction function is evaluated for selected excited states of the
hydrogen molecule within a single-state non-adiabatic treatment. Its
qualitative features are studied under the avoided crossing of the $EF$ with
the $GK$ state and also for the outer well of the $H\bar{H}$ state. For the
$H\bar{H}$ state, a negative mass correction is obtained for the vibrational
motion near the outer minimum, which accounts for most of the deviation between
experiment and earlier theoretical work.

In Gaussian quantum key distribution eavesdropping attacks are conventionally
modeled through the universal entangling cloner scheme, which is based on the
premise that the whole environment is under control of the adversary, i.e., the
eavesdropper purifies the system. This assumption implies that the eavesdropper
has either access to an identity (noiseless) channel or infinite amount of
entanglement in order to simulate such an identity channel. In this work, we
challenge the necessity of this assumption, and we propose a

We show that a continuous range of nonclassical states of light can be
generated using conditional measurements on the idler mode of an optical
parametric amplifier. The output state is prepared by introducing a coherent
state in the signal mode of the amplifier with a single photon in the idler
mode, followed by a conditional measurement of a single photon in the output
idler mode. By varying the gain of the amplifier, this approach can produce a
coherent state, a photon-added state, a displaced number state, or a continuous