33 research outputs found
Entangling two superconducting LC coherent modes via a superconducting flux qubit
Based on a pure solid-state device consisting of two superconducting LC
circuits coupled to a superconducting flux qubit, we propose in this paper that
the maximally entangled coherent states of the two LC modes can be generated
for arbitrary coherent states through flux qubit controls.Comment: 5 pages, 2 figure
Transient quantum transport in double-dot Aharonov-Bohm interferometers
Real-time nonequilibrium quantum dynamics of electrons in double-dot
Aharonov-Bohm (AB) interferometers is studied using an exact solution of the
master equation. The building of the coherence between the two electronic paths
shows up via the time-dependent amplitude of the AB oscillations in the
transient transport current, and can be enhanced by varying the applied bias on
the leads, the on-site energy difference between the dots and the asymmetry of
the coupling of the dots to the leads. The transient oscillations of the
transport current do not obey phase rigidity. The circulating current has an
anti-symmetric AB oscillation in the flux. The non-degeneracy of the on-site
energies and the finite bias cause the occupation in each dot to have an
arbitrary flux dependence as the coupling asymmetry is varied.Comment: 11 pages, 5 figure
Exact wave-packet decoherence dynamics in a discrete spectrum environment
We find an exact analytical solution of the reduced density matrix from the
Feynman-Vernon influence functional theory for a wave packet influenced by an
environment containing a few discrete modes. We obtain two intrinsic energy
scales relating to the time scales of the system and the environment. Different
relationship between these two scales alters the overall form of the solution
of the system. We also introduce a decoherence measure for a single wave packet
which is defined as the ratio of Schr\"odinger uncertainty over the
delocalization extension of the wave packet and characterizes the
time-evolution behavior of the off-diagonal reduced density matrix element. We
utilize the exact solution and the docherence measure to study the wave packet
decoherence dynamics. We further demonstrate how the dynamical diffusion of the
wave packet leads to non-Markovian decoherence in such a microscopic
environment.Comment: 12 pages, 2 figure
Dynamically stabilized decoherence-free states in non-Markovian open fermionic systems
Decoherence-free subspaces (DFSs) provide a strategy for protecting the
dynamics of an open system from decoherence induced by the system-environment
interaction. So far, DFSs have been primarily studied in the framework of
Markovian master equations. In this work, we study decoherence-free (DF) states
in the general setting of a non-Markovian fermionic environment. We identify
the DF states by diagonalizing the non-unitary evolution operator for a
two-level fermionic system attached to an electron reservoir. By solving the
exact master equation, we show that DF states can be stabilized dynamically.Comment: 11 pages, 3 figures. Any comments are welcom
General non-Markovian dynamics of open quantum systems
We present a general theory of non-Markovian dynamics for open quantum
systems. We explore the non-Markovian dynamics by connecting the exact master
equations with the non-equilibirum Green functions. Environmental back-actions
are fully taken into account. The non-Markovian dynamics consists of
non-exponential decays and dissipationless oscillations. Non-exponential decays
are induced by the discontinuity in the imaginary part of the self-energy
corrections. Dissipationless oscillations arise from band gaps or the finite
band structure of spectral densities. The exact analytic solutions for various
non-Markovian environments show that the non-Markovian dynamics can be largely
understood from the environmental-modified spectra of the open systems.Comment: 6 pages, 2 figure
Exact Master Equation and Non-Markovian Decoherence for Quantum Dot Quantum Computing
In this article, we report the recent progress on decoherence dynamics of
electrons in quantum dot quantum computing systems using the exact master
equation we derived recently based on the Feynman-Vernon influence functional
approach. The exact master equation is valid for general nanostructure systems
coupled to multi-reservoirs with arbitrary spectral densities, temperatures and
biases. We take the double quantum dot charge qubit system as a specific
example, and discuss in details the decoherence dynamics of the charge qubit
under coherence controls. The decoherence dynamics risen from the entanglement
between the system and the environment is mainly non-Markovian. We further
discuss the decoherence of the double-dot charge qubit induced by quantum point
contact (QPC) measurement where the master equation is re-derived using the
Keldysh non-equilibrium Green function technique due to the non-linear coupling
between the charge qubit and the QPC. The non-Markovian decoherence dynamics in
the measurement processes is extensively discussed as well.Comment: 15 pages, Invited article for the special issue "Quantum Decoherence
and Entanglement" in Quantum Inf. Proces
Group art therapy as an adjunctive treatment for people with schizophrenia: a randomised controlled trial (MATISSE).
OBJECTIVE: To examine the clinical effectiveness and cost-effectiveness of referral to group art therapy plus standard care, compared with referral to an activity group plus standard care and standard care alone, among people with schizophrenia. DESIGN: A three-arm, parallel group, single-blind, pragmatic, randomised controlled trial. Participants were randomised via an independent and remote telephone randomisation service using permuted blocks, stratified by study centre. SETTING: Study participants were recruited from secondary care mental health and social services in four UK centres. PARTICIPANTS: Potential participants were aged 18 years or over, had a clinical diagnosis of schizophrenia, confirmed by an examination of case notes, and provided written informed consent. We excluded those who were unable to speak sufficient English to complete the baseline assessment, those with severe cognitive impairment and those already receiving arts therapy. INTERVENTIONS: Group art therapy was delivered by registered art therapists according to nationally agreed standards. Groups had up to eight members, lasted for 90 minutes and ran for 12 months. Members were given access to a range of art materials and encouraged to use these to express themselves freely. Activity groups were designed to control for the non-specific effects of group art therapy. Group facilitators offered various activities and encouraged participants to collectively select those they wanted to pursue. Standard care involved follow-up from secondary care mental health services and the option of referral to other services, except arts therapies, as required. MAIN OUTCOME MEASURES: Our co-primary outcomes were global functioning (measured using the Global Assessment of Functioning Scale - GAF) and mental health symptoms (measured using the Positive and Negative Syndrome Scale - PANSS) at 24 months. The main secondary outcomes were level of group attendance, social functioning, well-being, health-related quality of life, service utilisation and other costs measured 12 and 24 months after randomisation. RESULTS: Four hundred and seventeen people were recruited, of whom 355 (85%) were followed up at 2 years. Eighty-six (61%) of those randomised to art therapy and 73 (52%) of those randomised to activity groups attended at least one group. No differences in primary outcomes were found between the three study arms. The adjusted mean difference between art therapy and standard care at 24 months was -0.9 [95% confidence interval (CI) -3.8 to 2.1] on the GAF Scale and 0.7 (95% CI -3.1 to 4.6) on the PANSS Scale. Differences in secondary outcomes were not found, except that those referred to an activity group had fewer positive symptoms of schizophrenia at 24 months than those randomised to art therapy. Secondary analysis indicated that attendance at art therapy groups was not associated with improvements in global functioning or mental health. Although the total cost of the art therapy group was lower than the cost of the two comparison groups, referral to group art therapy did not appear to provide a cost-effective use of resources. CONCLUSIONS: Referring people with established schizophrenia to group art therapy as delivered in this randomised trial does not appear to improve global functioning or mental health of patients or provide a more cost-effective use of resources than standard care alone. TRIAL REGISTRATION: Current Controlled Trials ISRCTN 46150447. FUNDING: This project was funded by the NIHR Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 16, No. 8. See the HTA programme website for further project information