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Author(s): Andre Schneider, Jochen Braumüller, Lingzhen Guo, Patrizia Stehle, Hannes Rotzinger, Michael Marthaler, Alexey V. Ustinov, and Martin Weides
Analyzing weak microwave signals in the GHz regime is a challenging task if the signal level is very low and the photon energy widely undefined. A superconducting qubit can detect signals in the low photon regime, but due to its discrete level structure, it is only sensitive to photons of certain en...
[Phys. Rev. A 97, 062334] Published Fri Jun 22, 2018

The principles of quantum optics have yielded a plethora of ideas to surpass
the classical limitations of sensitivity and resolution in optical microscopy.
While some ideas have been applied in proof-of-principle experiments, imaging a
biological sample has remained challenging mainly due to the inherently weak
signal measured and the fragility of quantum states of light. In principle,
however, these quantum protocols can add new information without sacrificing

We consider the general form of "Correlated Worldline" (CWL) theories of
quantum gravity. We show that one can have 2 different kinds of CWL theory, in
which the generating functional is written as either a sum or a product over
multiple copies of the coupled matter and gravitational fields. In both
versions, the paths in a functional formulation are correlated via gravity
itself, causing a breakdown of the superposition principle; however, the
product form survives consistency tests not satisfied by the summed form. To

Noisy intermediate-scale quantum computing devices are an exciting platform
for the exploration of the power of near-term quantum applications. Performing
nontrivial tasks in such a framework requires a fundamentally different
approach than what would be used on an error-corrected quantum computer. One
such approach is to use hybrid algorithms, where problems are reduced to a
parameterized quantum circuit that is often optimized in a classical feedback

Magic states can be used as a resource to circumvent the restrictions due to
stabilizer-preserving operations, and magic-state conversion has not been
studied in the single-copy regime thus far. Here we solve the question of
whether a stabilizer-preserving quantum operation exists that can convert
between two given magic states in the single-shot regime. We first phrase this
question as a feasibility problem for a semi-definite program (SDP), which
provides a procedure for constructing a stabilizer-preserving quantum operation

The entanglement Hamiltonian $H_E$, defined through the reduced density
matrix of a subsystem $\rho_A=\exp(-H_E)$, is an important concept in
understanding the nature of quantum entanglement in many-body systems and
quantum field theories. In this work, we explore a numerical scheme which
explicitly reconstructs the entanglement Hamiltonian using one entangled mode
(i.e., an eigenstate) of $\rho_A$. We demonstrate and benchmark this scheme on
quantum spin lattice models. The resulting $H_E$ bears a form similar to a

In this chapter we address the topic of quantum thermodynamics in the
presence of additional observables beyond the energy of the system. In
particular we discuss the special role that the generalized Gibbs ensemble
plays in this theory, and derive this state from the perspectives of a
micro-canonical ensemble, dynamical typicality and a resource-theory
formulation. A notable obstacle occurs when some of the observables do not
commute, and so it is impossible for the observables to simultaneously take on

We extend classical coarse-grained entropy, commonly used in many branches of
physics, to the quantum realm. We find two coarse-grainings, one using
measurements in local particle numbers and then total energy, and the second
using local energy measurements, that lead to an entropy that is defined
outside of equilibrium, is in accord with the thermodynamic entropy for
equilibrium systems, and reaches the thermodynamic entropy in the long-time
limit, even in genuinely isolated quantum systems. This answers the

We investigate the ultimate precision achievable in Gaussian quantum
metrology. We derive general analytical expressions for the quantum Fisher
information matrix and for the measurement compatibility condition, ensuring
asymptotic saturability of the quantum Cram\'er-Rao bound, for the estimation
of multiple parameters encoded in multimode Gaussian states. We then apply our
results to the joint estimation of a phase shift and two parameters
characterizing Gaussian phase covariant noise in optical interferometry. In

We design an optical feedback network making use of machine learning
techniques and demonstrate via simulations its ability to correct for the
effects of turbulent propagation on optical modes. This artificial neural
network scheme only relies on measuring the intensity profile of the distorted
modes, making the approach simple and robust. The network results in the
generation of various mode profiles at the transmitter that, after propagation
through turbulence, closely resemble the desired target mode. The corrected