892 research outputs found
Thermodynamic stability criteria for a quantum memory based on stabilizer and subsystem codes
We discuss and review several thermodynamic criteria that have been
introduced to characterize the thermal stability of a self-correcting quantum
memory. We first examine the use of symmetry-breaking fields in analyzing the
properties of self-correcting quantum memories in the thermodynamic limit: we
show that the thermal expectation values of all logical operators vanish for
any stabilizer and any subsystem code in any spatial dimension. On the positive
side, we generalize the results in [R. Alicki et al., arXiv:0811.0033] to
obtain a general upper bound on the relaxation rate of a quantum memory at
nonzero temperature, assuming that the quantum memory interacts via a Markovian
master equation with a thermal bath. This upper bound is applicable to quantum
memories based on either stabilizer or subsystem codes.Comment: 23 pages. v2: revised introduction, various additional comments, and
a new section on gapped hamiltonian
Correlation energy in a spin polarized two dimensional electron liquid in the high density limit
We have obtained an analytic expression for the ring diagrams contribution to
the correlation energy of a two dimensional electron liquid as a function of
the uniform fractional spin polarization. Our results can be used to improve on
the interpolation formulas which represent the basic ingredient for the
constructions of modern spin-density functionals in two dimensions.Comment: 3 pages, 1 figur
High density limit of the two-dimensional electron liquid with Rashba spin-orbit coupling
We discuss by analytic means the theory of the high-density limit of the
unpolarized two-dimensional electron liquid in the presence of Rashba or
Dresselhaus spin-orbit coupling. A generalization of the ring-diagram expansion
is performed. We find that in this regime the spin-orbit coupling leads to
small changes of the exchange and correlation energy contributions, while
modifying also, via repopulation of the momentum states, the noninteracting
energy. As a result, the leading corrections to the chirality and total energy
of the system stem from the Hartree-Fock contributions. The final results are
found to be vanishing to lowest order in the spin-orbit coupling, in agreement
with a general property valid to every order in the electron-electron
interaction. We also show that recent quantum Monte Carlo data in the presence
of Rashba spin-orbit coupling are well understood by neglecting corrections to
the exchange-correlation energy, even at low density values.Comment: 11 pages, 5 figure
Polaritonic stop-band transparency via exciton-biexciton coupling in CuCl
Radiation is almost completely reflected within the exciton-polariton stop band of a semiconductor, as in the typical case of CuCl. We predict, however, that a coherently driven exciton-biexciton transition allows for the propagation of a probe light beam within the stop band. The phenomenon is reminiscent of electromagnetically induced transparency effects occurring in three-level atomic systems, except that it here involves delocalized electronic excitations in a crystalline structure via a frequency and wave-vector selective polaritonic mechanism. A well-developed transparency, favored by the narrow linewidth of the biexciton, is established within the stop band where a probe pulse may propagate with significant delays. The transparency window can be controlled via the pump beam detuning and intensity
Exciton–biexciton quantum coherence and polaritonic stop-band transparency in CuCl.
A coherently driven exciton-biexciton transition in CuCl enables one to propagate a probe light beam within the exciton-polariton stop-band where radiation is otherwise completely reflected. The stop-band transparency window can be controlled via the pump beam frequency and intensity. The phenomenon is reminiscent of quantum coherence effects occurring in three-level atomic systems, except that it here involves delocalized electronic excitations in a crystal via a frequency and wave-vector selective polaritonic mechanism. Both a free standing slab and a microcavity configuration are theoretically studied
Multicentre narrative research on major depression to integrate the experiences of patients, their caregivers and healthcare providers in Italy
OBJECTIVE: This research aimed to investigate the experience of major depression by integrating the perspectives of patients, caregivers and healthcare providers through narrative-based medicine (NBM) to provide new insights to improve care relationships. DESIGN AND SETTING: The research was conducted in 2019 and involved five Italian psychiatric centres and targeted adult patients, their caregivers and healthcare providers to address data source triangulation. A sociodemographic survey and a narrative plot, based on Natural Semantic Metalanguage were collected. Narratives were analysed through NBM classifications, NVivo software and interpretative coding. PARTICIPANTS: Thirty-six patients with a diagnosis of major depression, 27 caregivers and 33 healthcare providers participated in the research. RESULTS: Among the 96 collected narratives, 'lonely' was the word patients used most frequently, while 'sad' and 'lifeless' were used most respectively by caregivers and healthcare providers. A positive care relationship was crucial for 84% of patients in relation to their care pathway, and nature (36%) and the arts (28%) were the most frequent resources. Caregivers expressed feelings of powerlessness and inadequacy, and 21% of them reported a declining social life while providing care to a loved one with depression. Thirty-one percent of mental health professionals experienced difficulties in their first encounter with patients; however, their emotions progressively moved towards trust and satisfaction. Furthermore, 89% of patients and healthcare providers and 58% of caregivers evaluated writing the narrative to be a positive experience. CONCLUSION: Findings suggested the possible role for language in understanding major depression, thereby improving care relationships between patients and physicians. Care pathways might also be more attentive to caregivers, to reduce their risk of burnout. Finally, narrative medicine could be integrated with the care pathway as an additional space of expression, dialogue, reflection and development of empathy
Efficient creation of multipartite entanglement in flux qubits
We investigate three superconducting flux qubits coupled in a loop. In this
setup, tripartite entanglement can be created in a natural, controllable, and
stable way. Both generic kinds of tripartite entanglement -the W type as well
as the GHZ type entanglement- can be identified among the eigenstates. We also
discuss the violation of Bell inequalities in this system and show the impact
of a limited measurement fidelity on the detection of entanglement and quantum
nonlocality.Comment: 15 pages, 7 figures; extended sections on coupling strength, system
preparation, and entanglement detectio
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