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We propose an analytical procedure to fully solve a two-level system coupled
to phonons. Instead of using the common formulation in terms of linear and
quadratic system-phonon couplings, we introduce different phonons depending on
the system electronic level. We use this approach to recover known results for
the linear-coupling limit in a simple way. More importantly, we derive results
for the quadratic coupling induced by a phonon frequency change, a problem
considered up to now as not analytically solvable.

Finding the ground state energy of a Hamiltonian $H$, which describes a
quantum system of several interacting subsystems, is crucial as well for
many-body physics as for various optimization problems. Variety of algorithms
and simulation procedures (either hardware or software based) rely on the
separability approximation, in which one seeks for the minimal expectation
value of $H$ among all product states. We demonstrate that already for systems
with nearest neighbor interactions this approximation is inaccurate, which

A prerequisite for universal quantum computation and other large-scale
quantum information processors is the preparation of an enormously large number
of quantum states. For continuous variable approaches to quantum information
processing, squeezed states are the natural quantum resources, but most
demonstrations have been based on a limited number of squeezed state resources
due to the experimental complexity in up-scaling. The number of physical
resources can however be significantly reduced by employing the technique of

Non-Hermitian Hamiltonians, which describe a wide range of dissipative
systems, and higher-order topological phases, which exhibit novel boundary
states on corners and hinges, comprise two areas of intense current research.
Here we investigate systems where these frontiers merge and formulate a
generalized biorthogonal bulk-boundary correspondence, which dictates the
appearance of boundary modes at parameter values that are, in general,
radically different from those that mark phase transitions in periodic systems.

The no-cloning property of quantum mechanics allows unforgeability of quantum
banknotes and credit cards. Quantum credit card protocols involve a bank, a
client and a payment terminal, and their practical implementation typically
relies on encoding information on weak coherent states of light. Here, we
provide a security proof in this practical setting for semi-device-independent
quantum money with classical verification, involving an honest bank, a
dishonest client and a potentially untrusted terminal. Our analysis uses

We analyze the performance of photon-number-resolving (PNR) detectors and
introduce a figure of merit for the accuracy of such detectors. This figure of
merit is the (worst-case) probability that the photon-number-resolving detector
correctly predicts the input photon number. Simulations of various PNR
detectors based on multiplexed single-photon `click detectors' is performed. We
conclude that the required quantum efficiency is very high in order to achieve

In this brief note, I clarify the crucial differences between three different
protocols of quantum channel discrimination, after some confusion has appeared
in recent literature.

We derive an extension of the quantum regression theorem to calculate
out-of-time-order correlation functions in Markovian open quantum systems.
While so far mostly being applied in the analysis of many-body physics, we
demonstrate that out-of-time-order correlation functions appear naturally in
optical detection schemes with interferometric delay lines, and we apply our
extended quantum regression theorem to calculate the non-trivial photon
counting fluctuations in split and recombined signals from a quantum light
source.

A quantum computer (QC) can solve many computational problems more
efficiently than a classic one. The field of QCs is growing: companies (such as
DWave, IBM, Google, and Microsoft) are building QC offerings. We position that
software engineers should look into defining a set of software engineering
practices that apply to QC's software. To start this process, we give examples
of challenges associated with testing such software and sketch potential
solutions to some of these challenges.

Distributed quantum computation requires to apply quantum remote gates on
separate nodes or subsystems of network. On the other hand, Toffoli gate is a
universal and well-known quantum gate. It is frequently used in synthesis of
quantum circuits. In this paper, a general protocol for implementing a remote
n-qubit controlled-U gate is presented with minimum required resources. Then,
the proposed method is applied for implementing a Toffoli gate in bipartite and