arXiv subjects

D. C. Roberts

Publications and source records attributed to D. C. Roberts.

10 recordsLinked to original sources

arXiv2008

Casimir Friction II: Casimir effect and drag in zero temperature superfluids

We discuss a force arising from zero-temperature quantum fluctuations in a weakly interacting Bose-Einstein condensate (BEC). We argue that there is a drag force on an object moving at speeds well below Landau's critical velocity, and motivate our argument by an analogy to the Casimir force in an electromagnetic vacuum. We sketch the calculation of this force for an object described by a general potential. We comment about the relevance of our results to the stability of superfluid flow, arguing that this effect is consistent with the observation of persistent currents and that this effect introduces new experimental observables. Finally, we describe a new hydryodynamic boundary condition for superfluid flow that takes into account the scattering of quantum fluctuations off a rough surface.

cond-mat.stat-mech↗

arXiv2006

Finite time collapse of N classical fields described by coupled nonlinear Schrodinger equations

We prove the finite-time collapse of a system of N classical fields, which are described by N coupled nonlinear Schrodinger equations. We derive the conditions under which all of the fields experiences this finite-time collapse. Finally, for two-dimensional systems, we derive constraints on the number of particles associated with each field that are necessary to prevent collapse.

cond-mat.other↗

arXiv2006

Force on a moving point impurity due to quantum fluctuations in a Bose-Einstein condensate

An analytic expression is derived for a force on a weak point impurity arising from the scattering of quantum fluctuations in a slow-moving, weakly interacting, three-dimensional Bose-Einstein condensate at zero temperature. In an infinitely extended geometry, this force is shown to exist at any arbitrarily small flow velocity below Landau's critical velocity. Furthermore, this force is shown to be directly proportional to the flow speed.

cond-mat.stat-mech↗

arXiv2005

Probing temperature and damping rates in Bose-Einstein condensates using ultraslow light experiments

We propose a method to probe Landau and Beliaev processes in dilute trapped atomic condensates with a multiple state structure using ultraslow light experimental configurations. Under certain conditions, damping rates from these collisional processes are directly proportional to the dephasing rates, making it possible to determine damping rates through measurement of the dephasing. In the ultraslow light systems we consider, Landau decay rates are enhanced at low momenta, which allows one to distinguish between Landau-dominated and Beliaev-dominated regimes at the same temperature. Furthermore, the enhancement of Landau rates potentially provides a way to measure low temperatures ($T \ll T_c$) in dilute condensates more accurately than current methods permit.

cond-mat.stat-mech↗

arXiv2005

Casimir-like force arising from quantum fluctuations in a slow-moving dilute Bose-Einstein condensate

We calculate a force due to zero-temperature quantum fluctuations on a stationary object in a moving superfluid flow. We model the object by a localized potential varying only in the flow direction and model the flow by a three-dimensional weakly interacting Bose-Einstein condensate at zero temperature. We show that this force exists for any arbitrarily small flow velocity and discuss the implications for the stability of superfluid flow.

cond-mat.other↗

arXiv2002

Exciting Relative Number Squeezed Particles from Condensates Using Stimulated Light Scattering

We investigate the relative particle number squeezing produced in the excited states of a weakly interacting condensate at zero temperature by stimulated light scattering using a pair of lasers. We shall show that a modest number of relative number squeezed particles can be achieved when atoms with momentum $k$, produced in pairs through collisions in the condensate, are scattered out by their interaction with the lasers. This squeezing is optimal when the momentum $k$ is larger than the inverse healing length, $k>k_0$. This modest number of relative number squeezed particles has the potential to be amplified in four-wave-mixing experiments.

quant-ph↗

arXiv2002

Probing States in the Mott Insulator Regime

We propose a method to probe states in the Mott insulator regime produced from a condensate in an optical lattice. We consider a system in which we create time-dependent number fluctuations in a given site by turning off the atomic interactions and lowering the potential barriers on a nearly pure Mott state to allow the atoms to tunnel between sites. We calculate the expected interference pattern and number fluctuations from such a system and show that one can potentially observe a deviation from a pure Mott state. We also discuss a method in which to detect these number fluctuations using time-of-flight imaging.

cond-mat↗

arXiv2002

Limitations of light delay and storage times in EIT experiments with condensates

We investigate the limitations arising from atomic collisions on the storage and delay times of probe pulses in EIT experiments. We find that the atomic collisions can be described by an effective decay rate that limits storage and delay times. We calculate the momentum and temperature dependence of the decay rate and find that it is necessary to excite atoms at a particular momentum depending on temperature and spacing of the energy levels involved in order to minimize the decoherence effects of atomic collisions.

quant-ph↗

arXiv2001

Entanglement between photons and atoms coupled out from a Bose-Einstein-Condensate

We study the limitations to the relative number squeezing between photons and atoms coupled out from a homogeneous Bose-Einstein-Condensate. We consider the coupling between the translational atomic states by two photon Bragg processes, with one of the photon modes involved in the Bragg process in a coherent state, and the other initially unpopulated. We start with an interacting Bose- condensate at zero temperature and compute the time evolution for the system. We study the squeezing, i.e. the variance of the occupation number difference between the second photon and the atomic c.m. mode. We discuss how collisions between the atoms and photon rescattering affect the degree of squeezing which may be reached in such experiments.

quant-ph↗

arXiv2000

A Solution to the Graceful Exit Problem in Pre-Big Bang Cosmology

We examine the string cosmology equations with a dilaton potential in the context of the Pre-Big Bang Scenario with the desired scale factor duality, and give a generic algorithm for obtaining solutions with appropriate evolutionary properties. This enables us to find pre-big bang type solutions with suitable dilaton behaviour that are regular at $t=0$, thereby solving the graceful exit problem. However to avoid fine tuning of initial data, an `exotic' equation of state is needed that relates the fluid properties to the dilaton field. We discuss why such an equation of state should be required for reliable dilaton behaviour at late times.

gr-qc↗