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QUROPE - aggregated feeds in category JournalsenarXiv.org: Quantum Physics: Hamiltonian surgery: Cheeger-type gap inequalities for nonpositive (stoquastic), real, and Hermitian matrices. (arXiv:1804.06857v1 [math.SP])
http://arxiv.org/abs/1804.06857
<p>Cheeger inequalities bound the spectral gap $\gamma$ of a space by
isoperimetric properties of that space and vice versa. In this paper, I derive
Cheeger-type inequalities for nonpositive matrices (aka stoquastic
Hamiltonians), real matrices, and Hermitian matrices. For matrices written $H =
L+W$, where $L$ is either a combinatorial or normalized graph Laplacian, each
bound holds independently of any information about $W$ other than its class and
the weighted Cheeger constant induced by its ground-state.<p><a href="http://arxiv.org/abs/1804.06857">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200arXiv.org: Quantum Physics: Entropy Dynamics in the System of Interacting Qubits. (arXiv:1804.06873v1 [quant-ph])
http://arxiv.org/abs/1804.06873
<p>The classical Second Law of Thermodynamics demands that an isolated system
evolves with a non-diminishing entropy. This holds as well in quantum mechanics
if the evolution of the energy-isolated system can be described by a unital
quantum channel. At the same time, the entropy of a system evolving via a
non-unital channel can, in principle, decrease. Here, we analyze the behavior
of the entropy in the context of the H-theorem. As exemplary phenomena, we
discuss the action of a Maxwell demon (MD) operating a qubit and the processes
of heating and cooling in a two-qubit system.<p><a href="http://arxiv.org/abs/1804.06873">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200arXiv.org: Quantum Physics: H-theorem and Maxwell Demon in Quantum Physics. (arXiv:1804.06886v1 [quant-ph])
http://arxiv.org/abs/1804.06886
<p>The Second Law of Thermodynamics states that temporal evolution of an
isolated system occurs with non-diminishing entropy. In quantum realm, this
holds for energy-isolated systems the evolution of which is described by the
so-called unital quantum channel. The entropy of a system evolving in a
non-unital quantum channel can, in principle, decrease. We formulate a general
criterion of unitality for the evolution of a quantum system, enabling a simple
and rigorous approach for finding and identifying the processes accompanied by
decreasing entropy in energy-isolated systems.<p><a href="http://arxiv.org/abs/1804.06886">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200arXiv.org: Quantum Physics: All-Optical control of linear and nonlinear energy transfer via Zeno effect. (arXiv:1804.06921v1 [quant-ph])
http://arxiv.org/abs/1804.06921
<p>Microresonator-based nonlinear processes are fundamental to applications
including microcomb generation, parametric frequency conversion, and harmonics
generation. While nonlinear processes involving either second- ($\chi^{(2)}$)
or third- $\chi^{(3)}$) order nonlinearity have been extensively studied, the
interaction between these two basic nonlinear processes has seldom been
reported. In this letter, we demonstrate a coherent interplay between second-
and third- order nonlinear processes.<p><a href="http://arxiv.org/abs/1804.06921">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200arXiv.org: Quantum Physics: Generalized spatial differentiation from spin Hall effect of light. (arXiv:1804.06965v1 [physics.optics])
http://arxiv.org/abs/1804.06965
<p>Optics naturally provides us with some powerful mathematical operations. Here
we reveal that a single planar interface can compute spatial differentiation to
paraxial coherent beams under oblique incidence. We show that intrinsically the
spatial differentiation results from the spin Hall effect of light with
preparing and postselecting polarization states, in both quantum and classical
levels.<p><a href="http://arxiv.org/abs/1804.06965">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200arXiv.org: Quantum Physics: Accounting for Errors in Quantum Algorithms via Individual Error Reduction. (arXiv:1804.06969v1 [quant-ph])
http://arxiv.org/abs/1804.06969
<p>We discuss a surprisingly simple scheme for accounting (and removal) of error
in observables determined from quantum algorithms. A correction to the value of
the observable is calculated by first measuring the observable with all error
sources active and subsequently measuring the observable with each error source
removed separately.<p><a href="http://arxiv.org/abs/1804.06969">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200arXiv.org: Quantum Physics: Influence of Vacuum modes on Photodetection. (arXiv:1804.06976v1 [quant-ph])
http://arxiv.org/abs/1804.06976
<p>Photodetection is a process in which an incident field induces a polarization
current in the detector. The interaction of the field with this induced current
excites an electron in the detector from a localized bound state to a state in
which the electron freely propagates and can be classically amplified and
detected. The induced current can interact not only with the applied field, but
also with all of the initially unpopulated vacuum modes. This interaction with
the vacuum modes is assumed to be small and is neglected in conventional
photodetection theory.<p><a href="http://arxiv.org/abs/1804.06976">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200arXiv.org: Quantum Physics: Exploring Topological Phase Transition via Quantum Walk in Coherent State Space. (arXiv:1804.06994v1 [quant-ph])
http://arxiv.org/abs/1804.06994
<p>Topological phase transition plays a significant role in modern condensed
matter physics, since it is beyond Landau symmetry-breaking theory. Quantum
walks have been demonstrated to be a powerful method in exploring topological
quantum matter. Here, we investigate a special quantum walk in a line in terms
of coherent state representation. By detecting average number of photons of the
system, quantum phase transition can be observed, and the properties of
coherent space are utilized.<p><a href="http://arxiv.org/abs/1804.06994">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200arXiv.org: Quantum Physics: Fault-tolerant quantum error correction for Steane's seven-qubit color code with few or no extra qubits. (arXiv:1804.06995v1 [quant-ph])
http://arxiv.org/abs/1804.06995
<p>Steane's seven-qubit quantum code is a natural choice for fault-tolerance
experiments because it is small and just two extra qubits are enough to correct
errors. However, the two-qubit error-correction technique, known as "flagged"
syndrome extraction, works slowly, measuring only one syndrome at a time. This
is a disadvantage in experiments with high qubit rest error rates. We extend
the technique to extract multiple syndromes at once, without needing more
qubits. Qubits for different syndromes can flag errors in each other.<p><a href="http://arxiv.org/abs/1804.06995">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200arXiv.org: Quantum Physics: Synchronization in Quantum Key Distribution Systems. (arXiv:1804.07003v1 [quant-ph])
http://arxiv.org/abs/1804.07003
<p>In the description of quantum key distribution systems, much attention is
paid to the operation of quantum cryptography protocols. The main problem is
the insufficient study of the synchronization process of quantum key
distribution systems. This paper contains a general description of quantum
cryptography principles. A two-line fiber-optic quantum key distribution system
with phase coding of photon states in transceiver and coding station
synchronization mode was examined.<p><a href="http://arxiv.org/abs/1804.07003">read more</a></p>
Fri, 20 Apr 2018 06:45:04 +0200