752 research outputs found
Generalized rotating-wave approximation for arbitrarily large coupling
A generalized version of the rotating-wave approximation for the single-mode
spin-boson Hamiltonian is presented. It is shown that performing a simple
change of basis prior to eliminating the off-resonant terms results in a
significantly more accurate expression for the energy levels of the system. The
generalized approximation works for all values of the coupling strength and for
a wide range of detuning values, and may find applications in solid-state
experiments.Comment: 4 pages, 2 figs, REVTeX
Oscillator tunneling dynamics in the Rabi model
The familiar Rabi model, comprising a two-level system coupled to a quantum
harmonic oscillator, continues to produce rich and surprising physics when the
coupling strength becomes comparable to the individual subsystem frequencies.
We construct approximate solutions for the regime in which the oscillator
frequency is small compared to that of the two-level system and the coupling
strength matches or exceeds the oscillator frequency. Relating our fully
quantum calculation to a previous semi-classical approximation, we find that
the dynamics of the oscillator can be considered to a good approximation as
that of a particle tunneling in a classical double-well potential, despite the
fundamentally entangled nature of the joint system. We assess the prospects for
observation of oscillator tunneling in the context of nano- or micro-mechanical
experiments and find that it should be possible if suitably high coupling
strengths can be engineered.Comment: 25 pages, 5 figures, preprint forma
Polaritonic characteristics of insulator and superfluid phases in a coupled-cavity array
Recent studies of quantum phase transitions in coupled atom-cavity arrays
have focused on the similarities between such systems and the Bose-Hubbard
model. However, the bipartite nature of the atom-cavity systems that make up
the array introduces some differences. In order to examine the unique features
of the coupled-cavity system, the behavior of a simple two-site model is
studied over a wide range of parameters. Four regions are identified, in which
the ground state of the system may be classified as either a polaritonic
insulator, a photonic superfluid, an atomic insulator, or a polaritonic
superfluid.Comment: 7 pages, 9 figures, 1 table, REVTeX 4; published versio
Ground-State Entanglement in a Coupled-Cavity Model
Bipartite entanglement entropies are calculated for the ground state of the
two-excitation subspace in a two-site coupled cavity model. Each region in the
phase diagram (atomic insulator, polaritonic insulator, photonic superfluid,
and polaritonic superfluid) is found to be characterized by unique entanglement
properties. In particular, the polaritonic superfluid region exhibits
multipartite entanglement among the two atoms and two cavity fields. This
system provides a toy model in which a number of intriguing aspects of
entanglement can be studied, such as the relationship of entanglement to phase
transitions, entanglement of particles with different dimensionality, and the
connection between experimentally accessible local observables and entanglement
entropies.Comment: 5 pages, 4 figure
Dynamics in a coupled-cavity array
The dynamics of a system composed of two coupled optical cavities, each
containing a single two-level atom, is studied over a wide range of detuning
and coupling values. A description of the field in terms of delocalized modes
reveals that the detuning between the atoms and these modes is controlled by
the coupling between the cavities; this detuning in turn governs the nature of
the dynamics. If the atoms are highly detuned from both delocalized field
modes, the dynamics becomes dispersive and an excitation may be transferred
from the first atom to the second without populating the field. In the case of
resonance between the atoms and one of the delocalized modes, state transfer
between the atoms requires intermediate excitation of the field. Thus the
interaction between the two atoms can be controlled by adjusting the coupling
between the cavities.Comment: 11 pages, 3 figure
Coherent exciton dynamics in a dissipative environment maintained by an off-resonant vibrational mode
This work was supported by the Leverhulme Trust (RPG-080) and the EPSRC (EP/G03673X/1).The interplay between an open quantum system and its environment can lead to both coherent and incoherent behavior. We explore the extent to which strong coupling to a single bosonic mode can alter the coherence properties of a two-level system in a structured environment. This mode is treated exactly, with the rest of the environment comprising a Markovian bath of bosonic modes. The strength of the coupling between the two-level system and the single mode is varied for a variety of forms for the bath spectral density in order to assess whether the coherent dynamics of the two-level system are modified. We find a clear renormalization of the site population oscillation frequency that causes an altered interaction with the bath. This leads to enhanced or reduced coherent behavior of the two-level system, depending on the form of the spectral density function. We present an intuitive interpretation, based on an analytical model, to explain the behavior.Publisher PDFPeer reviewe
Dynamics of a two-level system strongly coupled to a high-frequency quantum oscillator
Recent experiments on quantum behavior in microfabricated solid-state systems
suggest tantalizing connections to quantum optics. Several of these experiments
address the prototypical problem of cavity quantum electrodynamics: a two-level
system coupled to a quantum harmonic oscillator. Such devices may allow the
exploration of parameter regimes outside the near-resonance and weak-coupling
assumptions of the ubiquitous rotating-wave approximation (RWA), necessitating
other theoretical approaches. One such approach is an adiabatic approximation
in the limit that the oscillator frequency is much larger than the
characteristic frequency of the two-level system. A derivation of the
approximation is presented and the time evolution of the two-level-system
occupation probability is calculated using both thermal- and coherent-state
initial conditions for the oscillator. Closed-form evaluation of the time
evolution in the weak-coupling limit provides insight into the differences
between the thermal- and coherent-state models. Finally, potential experimental
observations in solid-state systems, particularly the Cooper-pair
box--nanomechanical resonator system, are discussed and found to be promising.Comment: 16 pages, 11 figures; revised abstract; some text revisions; added
two figures and combined others; added references. Submitted to Phys. Rev.
Defining the semiclassical limit of the quantum Rabi Hamiltonian
The crossover from quantum to semiclassical behavior in the seminal Rabi
model of light-matter interaction still, surprisingly, lacks a complete and
rigorous understanding. A formalism for deriving the semiclassical model
directly from the quantum Hamiltonian is developed here. Working in a displaced
Fock-state basis , the semiclassical limit is
obtained by taking and the coupling to zero.
This resolves the discrepancy between coherent-state dynamics and semiclassical
Rabi oscillations in both standard and ultrastrong coupling/driving regimes.
Furthermore, it provides a framework for studying the quantum-to-classical
transition, with potential applications in quantum technologies.Comment: 6 pages, 1 figure; 6 pages Supplementary Materia
Grey and white matter correlates of recent and remote autobiographical memory retrieval:Insights from the dementias
The capacity to remember self-referential past events relies on the integrity of a distributed neural network. Controversy exists, however, regarding the involvement of specific brain structures for the retrieval of recently experienced versus more distant events. Here, we explored how characteristic patterns of atrophy in neurodegenerative disorders differentially disrupt remote versus recent autobiographical memory. Eleven behavioural-variant frontotemporal dementia, 10 semantic dementia, 15 Alzheimer's disease patients and 14 healthy older Controls completed the Autobiographical Interview. All patient groups displayed significant remote memory impairments relative to Controls. Similarly, recent period retrieval was significantly compromised in behavioural-variant frontotemporal dementia and Alzheimer's disease, yet semantic dementia patients scored in line with Controls. Voxel-based morphometry and diffusion tensor imaging analyses, for all participants combined, were conducted to investigate grey and white matter correlates of remote and recent autobiographical memory retrieval. Neural correlates common to both recent and remote time periods were identified, including the hippocampus, medial prefrontal, and frontopolar cortices, and the forceps minor and left hippocampal portion of the cingulum bundle. Regions exclusively implicated in each time period were also identified. The integrity of the anterior temporal cortices was related to the retrieval of remote memories, whereas the posterior cingulate cortex emerged as a structure significantly associated with recent autobiographical memory retrieval. This study represents the first investigation of the grey and white matter correlates of remote and recent autobiographical memory retrieval in neurodegenerative disorders. Our findings demonstrate the importance of core brain structures, including the medial prefrontal cortex and hippocampus, irrespective of time period, and point towards the contribution of discrete regions in mediating successful retrieval of distant versus recently experienced events
- …