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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.

- Read more about On Testing Quantum Programs. (arXiv:1812.09261v1 [cs.SE])
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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