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Author = EASTHAM, PAUL;
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Displaying Results 1  25 of 47 on page 1 of 2
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Adiabatic state preparation of interacting twolevel systems
(2012)
EASTHAM, PAUL
Adiabatic state preparation of interacting twolevel systems
(2012)
EASTHAM, PAUL
http://hdl.handle.net/2262/69193
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Amplitudemode dynamics of polariton condensates
(2011)
EASTHAM, PAUL
Amplitudemode dynamics of polariton condensates
(2011)
EASTHAM, PAUL
Abstract:
We study the stability of collective amplitude excitations in nonequilibrium polariton condensates. These excitations correspond to renormalized upper polaritons and to the collective amplitude modes of atomic gases and superconductors. They would be present following a quantum quench or could be created directly by resonant excitation. We show that uniform amplitude excitations are unstable to the production of excitations at finite wave vectors, leading to the formation of densitymodulated phases. The physical processes causing the instabilities can be understood by analogy to optical parametric oscillators and the atomic Bose supernova.
http://hdl.handle.net/2262/60056
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BCSBEC crossover in a system of microcavity polaritons
(2005)
EASTHAM, PAUL
BCSBEC crossover in a system of microcavity polaritons
(2005)
EASTHAM, PAUL
Abstract:
We investigate the thermodynamics and signatures of a polariton condensate over a range of densities, using a model of microcavity polaritons with internal structure. We determine a phase diagram for this system including fluctuation corrections to the meanfield theory. At low densities the condensation temperature Tc behaves like that for point bosons. At higher densities, when Tc approaches the Rabi splitting, Tc deviates from the form for point bosons, and instead approaches the result of a BCSlike meanfield theory. This crossover occurs at densities much less than the Mott density. We show that current experiments are in a density range where the phase boundary is described by the BCSlike meanfield boundary. We investigate the influence of inhomogeneous broadening and detuning of excitons on the phase diagram.
http://hdl.handle.net/2262/49383
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Bose condensation in a model microcavity
(2000)
EASTHAM, PAUL
Bose condensation in a model microcavity
(2000)
EASTHAM, PAUL
Abstract:
We study the equilibrium properties of a system of dipoleactive excitons coupled to a single photon mode at fixed total excitation. Treating the presence or absence of a trapped exciton as a twolevel system produces a model, the Dicke model, that is exactly soluble. It gives a simple description of polariton condensation in optical cavities, beyond the lowdensity bosonic regime.
http://hdl.handle.net/2262/49391
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Bose condensation of cavity polaritons beyond the linear regime: The thermal equilibrium of a model microcavity
(2001)
EASTHAM, PAUL
Bose condensation of cavity polaritons beyond the linear regime: The thermal equilibrium of a model microcavity
(2001)
EASTHAM, PAUL
Abstract:
We consider a generalization of the Dicke model. This model describes localized, physically separated, saturable excitations, such as excitons bound on impurities, coupled to a single longlived mode of an optical cavity. We consider the thermal equilibrium of this model at a fixed total number of excitons and photons. We find a phase in which both the cavity field and the excitonic polarization are coherent. This phase corresponds to a Bose condensate of cavity polaritons, generalized to allow for the fermionic internal structure of the excitons. It is separated from the normal state by an unusual reentrant phase boundary. We calculate the excitation energies of the model, and hence the optical absorption spectra of the cavity. In the condensed phase the absorption spectrum is gapped. The presence of this gap distinguishes the polariton condensate from the normal state and from a conventional laser, even when the inhomogeneous linewidth of the excitons is so large that there is no ...
http://hdl.handle.net/2262/49390
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Breakdown of counterflow superfluidity in a disordered quantum Hall bilayer
(2011)
EASTHAM, PAUL
Breakdown of counterflow superfluidity in a disordered quantum Hall bilayer
(2011)
EASTHAM, PAUL
Abstract:
We present a theory for the regime of coherent interlayer tunneling in a disordered quantum Hall bilayer at total filling factor one, allowing for the effect of static vortices. We find that the system consists of domains of polarized superfluid phase. Injected currents introduce phase slips between the polarized domains which are pinned by disorder. We present a model of saturated tunneling domains that predicts a critical current for the breakdown of coherent tunneling that is extensive in the system size. This theory is supported by numerical results from a disordered phase model in two dimensions. We also discuss how our picture might be used to interpret experiments in the counterflow geometry and in twoterminal measurements.
http://hdl.handle.net/2262/49583
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Build up of offdiagonal longrange order in microcavity excitonpolaritons across the parametric threshold
(2013)
EASTHAM, PAUL
Build up of offdiagonal longrange order in microcavity excitonpolaritons across the parametric threshold
(2013)
EASTHAM, PAUL
http://hdl.handle.net/2262/69199
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Chern numbers for the index surfaces of photonic crystals: conical refraction as a basis for topological materials
(2018)
Eastham, Paul
Chern numbers for the index surfaces of photonic crystals: conical refraction as a basis for topological materials
(2018)
Eastham, Paul
http://hdl.handle.net/2262/83984
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Coherence properties of exciton polariton OPO condensates in one and two dimensions
(2012)
EASTHAM, PAUL
Coherence properties of exciton polariton OPO condensates in one and two dimensions
(2012)
EASTHAM, PAUL
http://hdl.handle.net/2262/69197
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Coherence properties of the microcavity polariton condensate
(2009)
EASTHAM, PAUL
Coherence properties of the microcavity polariton condensate
(2009)
EASTHAM, PAUL
Abstract:
A theoretical model is presented which explains the dominant decoherence process in a microcavity polariton condensate. The mechanism which is invoked is the effect of selfphase modulation, whereby interactions transform polariton number fluctuations into random energy variations. The model shows that the phase coherence decay, g (1)( ), has a Kubo form, which can be Gaussian or exponential, depending on whether the number fluctuations are slow or fast. This fluctuation rate also determines the decay time of the intensity correlation function, g (2)( ), so it can be directly determined experimentally. The model explains recent experimental measurements of a relatively fast Gaussian decay for g (1)( ), but also predicts a regime, further above threshold, where the decay is much slower.
http://hdl.handle.net/2262/49587
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Coherence protection in coupled quantum systems
(2018)
EASTHAM, PAUL
Coherence protection in coupled quantum systems
(2018)
EASTHAM, PAUL
http://hdl.handle.net/2262/83985
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Condensation and lasing of microcavity polaritons: comparison between two models
(2005)
EASTHAM, PAUL
Condensation and lasing of microcavity polaritons: comparison between two models
(2005)
EASTHAM, PAUL
Abstract:
Condensation of microcavity polaritons and the substantial influence of pairbreaking disorder and decoherence leading to a laser regime has been recently considered using two different models: a model for direct two band excitons in a disordered quantum well coupled to light and a model where the cavity mode couples instead to a medium of localised excitons, represented by twolevel oscillators in the presence of dephasing processes. Even if complementary from the point of view of assumptions, the models share most of the main conclusions and show similar phase diagrams. The issue whether excitons are propagating or localised seems secondary for the polariton condensation and the way in which pairbreaking disorder and decoherence processes influence the condensation and drive the microcavity into a lasing regime is, within the approximations used in each model, generic. The reasons for the similarities between the two physical situations are analysed and explained.
http://hdl.handle.net/2262/49382
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Conical diffraction and the dispersion surface of hyperbolic metamaterials
(2014)
DONEGAN, JOHN; BALLANTINE, KYLE; EASTHAM, PAUL
Conical diffraction and the dispersion surface of hyperbolic metamaterials
(2014)
DONEGAN, JOHN; BALLANTINE, KYLE; EASTHAM, PAUL
Abstract:
Hyperbolic metamaterials are materials in which at least one principal dielectric constant is negative. We describe the refractive index surface, and the resulting refraction effects, for a biaxial hyperbolic metamaterial, with principal dielectric constants 1 < 0, 0 < 2 = 3 . In this general case the two sheets of the index surface intersect, forming conical singularities. We derive the ray description of conical refraction in these materials and show that it is topologically and quantitatively distinct from conical refraction in a conventional biaxial material. We also develop a wave optics description, which allows us to obtain the diffraction patterns formed from arbitrary beams incident close to the optic axis. The resulting patterns lack circular symmetry and hence are qualitatively different from those obtained in conventional, positive index materials
http://hdl.handle.net/2262/73958
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Conical diffraction intensity profiles generated using a tophat input beam
(2014)
DONEGAN, JOHN; EASTHAM, PAUL; LUNNEY, JAMES; MCCLOSKEY, DAVID
Conical diffraction intensity profiles generated using a tophat input beam
(2014)
DONEGAN, JOHN; EASTHAM, PAUL; LUNNEY, JAMES; MCCLOSKEY, DAVID
http://hdl.handle.net/2262/69200
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Conical diffraction of a Gaussian beam with a two crystal cascade
(2012)
LUNNEY, JAMES; DONEGAN, JOHN; EASTHAM, PAUL; PHELAN, CIARAN; BALLANTINE, KYLE
Conical diffraction of a Gaussian beam with a two crystal cascade
(2012)
LUNNEY, JAMES; DONEGAN, JOHN; EASTHAM, PAUL; PHELAN, CIARAN; BALLANTINE, KYLE
http://hdl.handle.net/2262/66984
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Creation of entangled states in coupled quantum dots via adiabatic rapid passage
(2012)
EASTHAM, PAUL
Creation of entangled states in coupled quantum dots via adiabatic rapid passage
(2012)
EASTHAM, PAUL
http://hdl.handle.net/2262/69198
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Critical supercurrents and self organization in quantum Hall bilayers
(2010)
EASTHAM, PAUL
Critical supercurrents and self organization in quantum Hall bilayers
(2010)
EASTHAM, PAUL
Abstract:
We present a theory of interlayer tunneling in a disordered quantum Hall bilayer at total filling factor one, allowing for the effect of static vortices. In agreement with recent experiments [Phys. Rev. B 80, 165120 (2009); Phys. Rev. B 78, 075302 (2008)], we find that the critical current is proportional to the sample area and is comparable in magnitude to observed values. This reflects the formation of a Bean critical state as a result of current injection at the boundary. We predict a crossover to a critical current proportional to the squareroot of the area in smaller samples.We also predict a peak in the critical current as the electron density varies at fixed layer separation.
http://hdl.handle.net/2262/49585
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Diamagnetism and flux creep in bilayer exciton superfluids
(2012)
EASTHAM, PAUL
Diamagnetism and flux creep in bilayer exciton superfluids
(2012)
EASTHAM, PAUL
http://hdl.handle.net/2262/69196
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Finitemomentum condensation in a pumped microcavity
(2010)
EASTHAM, PAUL
Finitemomentum condensation in a pumped microcavity
(2010)
EASTHAM, PAUL
Abstract:
We calculate the absorption spectra of a semiconductor microcavity into which a nonequilibrium exciton population has been pumped. We predict strong peaks in the spectrum corresponding to collective modes analogous to the Cooper modes in superconductors and fermionic atomic gases. These modes can become unstable, leading to the formation of offequilibrium quantum condensates. We calculate a phase diagram for condensation and show that the dominant instabilities can be at a finite momentum. Thus we predict the formation of inhomogeneous condensates, similar to FuldeFerrelLarkinOvchinnikov states.
http://hdl.handle.net/2262/40450
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Finitesize fluctuations and photon statistics near the polariton condensation transition in a singlemode microcavity
(2006)
EASTHAM, PAUL
Finitesize fluctuations and photon statistics near the polariton condensation transition in a singlemode microcavity
(2006)
EASTHAM, PAUL
Abstract:
We consider polariton condensation in a generalized Dicke model, describing a singlemode cavity containing quantum dots, and extend our previous meanfield theory to allow for finitesize fluctuations. Within the fluctuationdominated regime the correlation functions differ from their trivial meanfield values. We argue that the lowenergy physics of the model, which determines the photon statistics in this fluctuationdominated crossover regime, is that of the quantum anharmonic oscillator. The photon statistics at the crossover are different in the hightemperature and lowtemperature limits. When the temperature is high enough for quantum effects to be neglected we recover behavior similar to that of a conventional laser. At low enough temperatures, however, we find qualitatively different behavior due to quantum effects.
http://hdl.handle.net/2262/49381
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Generation of continuously tuneable fractional optical orbital angular momentum using internal conical diffraction
(2010)
LUNNEY, JAMES; RAKOVICH, YURY; EASTHAM, PAUL; DONEGAN, JOHN
Generation of continuously tuneable fractional optical orbital angular momentum using internal conical diffraction
(2010)
LUNNEY, JAMES; RAKOVICH, YURY; EASTHAM, PAUL; DONEGAN, JOHN
Abstract:
Abstract: When a leftcircularly polarised Gaussian light beam, which has spin angular momentum (SAM) Jsp = ?? = 1? per photon, is incident along one of the optic axes of a slab of biaxial crystal it undergoes internal conical diffraction and propagates as a hollow cone of light in the crystal. The emergent beam is a superposition of equal amplitude zero and first order Bessel like beams. The zero order beam is leftcircularly polarised with zero orbital angular momentum (OAM) Jorb = ?? = 0, while the first order beam is rightcircularly polarized but carries OAM of Jorb = 1? per photon. Thus, taken together the two beams have zero SAM and Jorb = ?? per photon. In this paper we examine internal conical diffraction of an elliptically polarised beam, which has fractional SAM, and demonstrate an alloptical process for the generation light beams with fractional OAM up to ? 1?
http://hdl.handle.net/2262/49407
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Intrinsic decoherence mechanisms in the microcavity polariton condensate
(2008)
EASTHAM, PAUL
Intrinsic decoherence mechanisms in the microcavity polariton condensate
(2008)
EASTHAM, PAUL
Abstract:
The fundamental mechanisms which control the phase coherence of the polariton BoseEinstein condensate (BEC) are determined. It is shown that the combination of number fluctuations and interactions leads to decoherence with a characteristic Gaussian decay of the firstorder correlation function. This line shape, and the long decay times ( 150 ps) of both first and secondorder correlation functions, are explained quantitatively by a quantumoptical model which takes into account interactions, fluctuations, and gain and loss in the system. Interaction limited coherence times of this type have been predicted for atomic BECs, but are yet to be observed experimentally.
http://hdl.handle.net/2262/49590
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Microcavity quantumdot systems for nonequilibrium BoseEinstein condensation
(2010)
EASTHAM, PAUL
Microcavity quantumdot systems for nonequilibrium BoseEinstein condensation
(2010)
EASTHAM, PAUL
Abstract:
We review the practical conditions required to achieve a nonequilibrium BEC driven by quantum dynamics in a system comprising a microcavity field mode and a distribution of localised twolevel systems driven to a steplike population inversion profile. A candidate system based on eight 3.8nm layers of In0.23Ga0.77As in GaAs shows promising characteristics with regard to the total dipole strength which can be coupled to the field mode.
http://hdl.handle.net/2262/49586
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Mechanism for the failure of the Edwards hypothesis in the SherringtonKirkpatrick spin glass
(2006)
EASTHAM, PAUL
Mechanism for the failure of the Edwards hypothesis in the SherringtonKirkpatrick spin glass
(2006)
EASTHAM, PAUL
Abstract:
The dynamics of the SherringtonKirkpatrick model at T=0 starting from random spin configurations is considered. The metastable states reached by such dynamics are atypical of such states as a whole, in that the probability density of site energies, p , is small at =0. Since virtually all metastable states have a much larger p 0 , this behavior demonstrates a qualitative failure of the Edwards hypothesis. We look for its origins by modeling the changes in the site energies during the dynamics as a Markov process. We show how the small p(0) arises from features of the Markov process that have a clear physical basis in the spin glass, and hence explain the failure of the Edwards hypothesis.
http://hdl.handle.net/2262/49380
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Mode locking and mode competition in a nonequilibrium solidstate condensate
(2008)
EASTHAM, PAUL
Mode locking and mode competition in a nonequilibrium solidstate condensate
(2008)
EASTHAM, PAUL
Abstract:
A trapped polariton condensate with continuous pumping and decay is analyzed using a generalized Gross? Pitaevskii model. Whereas an equilibrium condensate is characterized by a macroscopic occupation of a ground state, here the steady states take more general forms. Some are characterized by a large population in an excited state; others by large populations in several states, which may overlap spatially. In the latter case, the highly populated states synchronize to a common frequency above a critical density. At intermediate densities, they can drive condensation in other trap modes by parametric scattering. Estimates for the critical density of the synchronization transition are consistent with experiments
http://hdl.handle.net/2262/49377
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