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arXiv:0705.4174

Comment on ``Coupled dynamics of atoms and radiation--pressure--driven interferometers''

János K. Asbóth, P. Domokos

Published May 29, 2007• 0 citations
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Abstract

In two recent articles, Meiser and Meystre describe the coupled dynamics of a dense gas of atoms and an optical cavity pumped by a laser field. They make two important simplifying assumptions: (i) the gas of atoms forms a regular lattice and can be replaced by a fictitious mirror, and (ii) the atoms strive to minimize the dipole potential. We show that the two assumptions are inconsistent: the configuration of atoms minimizing the dipole potential is not a perfect lattice. Assumption (ii) is erroneous, as in the strong coupling regime the dipole force does not arise from the dipole potential. The real steady state, where the dipole forces vanish, is indeed a regular lattice. Furthermore, the bistability predicted by Meiser and Meystre does not occur in this system.

DOI
10.48550/arxiv.0705.4174

Related papers

Comment on “Coupled dynamics of atoms and radiation-pressure-driven interferometers” and “Superstrong coupling regime of cavity quantum electrodynamics”

János K. Asbóth, P. Domokos

In two recent articles [D. Meiser and P. Meystre, Phys. Rev. A 73, 033417 (2006); 74, 065801 (2006)], Meiser and Meystre describe the coupled dynamics of a dense gas of atoms and an optical cavity pumped by a laser field. They make two important simplifying assumptions: (i) The gas of atoms forms a regular lattice and can be replaced by a fictitious mirror and (ii) the atoms strive to minimize the dipole potential. We show that the two assumptions are inconsistent: The configuration of atoms minimizing the dipole potential is not a perfect lattice. Assumption (ii) is erroneous, as in the strong coupling regime the dipole force does not arise from the dipole potential. The real steady state, where the dipole forces vanish, is indeed a regular lattice. Furthermore, the bistability predicted by Meiser and Meystre does not occur in this system.

Physical Review A•Nov 9, 2007•8 citations

Reply to “Comment on ‘Coupled dynamics of atoms and radiation-pressure-driven interferometers’ and ‘Superstrong coupling regime of cavity quantum electrodynamics’ ”

D. Meiser, Pierre Meystre

In their Comment, Asb\'oth and Domokos question the consistency of two approximations introduced in two recent articles [D. Meiser and P. Meystre, Phys. Rev. A 73, 033417 (2006); 74, 065801 (2006)]. They show that the minimization of the atomic dipole potential does not in general give rise to a regular lattice of ultracold atoms and that the steady-state positions of atoms in a cavity are not normally characterized by a minimum of the dipole potential. Instead, the steady-state positions correspond to a vanishing force on each individual atom, the relevant force being proportional to the derivative of the optical field mode function. We agree with Asb\'oth and Domokos that the atomic lattice should be irregular. Using a microscopic model we show however that in the regime considered in our works and also in the preceding Comment the concept of force is generally not well defined, and a semiclassical theory may be inadequate for the description of the atomic motion.

Physical Review A•Nov 9, 2007•1 citation

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In a recent paper [J. K. Asboth et al., Phys. Rev. Lett. 98, 203008 (2007)] we have shown that traveling density wavelike collective oscillations can arise in an asymmetrically pumped optical lattice, and by increasing the lattice size or pump asymmetry, these waves can destabilize the structure even in the overdamped limit. The long-range interaction giving rise to collective motion stems from the back-action of the atoms on the field creating the lattice. In this paper we present a detailed description of these phenomena. We derive the force on a disk-shaped cloud of trapped particles including the back-action on the trapping light, and analyze its relation to the standard perturbative approach giving the ``dipole force'' and ``radiation pressure.'' We calculate the self-consistent lattice constant for both red and blue detuned lattices and find that it decreases---by the same amount in the two cases---as the pump asymmetry is increased. We present the detailed derivation of the lattice vibration eigenmodes using the transfer matrix method, which reveals that the instability is enhanced resonantly at certain settings of the asymmetry.

Physical Review A•Jun 30, 2008•47 citations
In two recent articles, Meiser and Meystre describe the coupled dynamics of a dense gas of atoms and an optical cavity pumped by a laser field. They make two important simplifying assumptions: (i) the gas of atoms forms a regular lattice and can be replaced by a fictitious mirror, and (ii) the atoms strive to minimize the dipole potential. We show that the two assumptions are inconsistent: the configuration of atoms minimizing the dipole potential is not a perfect lattice. Assumption (ii) is erroneous, as in the strong coupling regime the dipole force does not arise from the dipole potential. The real steady state, where the dipole forces vanish, is indeed a regular lattice. Furthermore, the bistability predicted by Meiser and Meystre does not occur in this system.
In two recent articles [D. Meiser and P. Meystre, Phys. Rev. A 73, 033417 (2006); 74, 065801 (2006)], Meiser and Meystre describe the coupled dynamics of a dense gas of atoms and an optical cavity pumped by a laser field. They make two important simplifying assumptions: (i) The gas of atoms forms a regular lattice and can be replaced by a fictitious mirror and (ii) the atoms strive to minimize the dipole potential. We show that the two assumptions are inconsistent: The configuration of atoms minimizing the dipole potential is not a perfect lattice. Assumption (ii) is erroneous, as in the strong coupling regime the dipole force does not arise from the dipole potential. The real steady state, where the dipole forces vanish, is indeed a regular lattice. Furthermore, the bistability predicted by Meiser and Meystre does not occur in this system.
In their Comment, Asb\'oth and Domokos question the consistency of two approximations introduced in two recent articles [D. Meiser and P. Meystre, Phys. Rev. A 73, 033417 (2006); 74, 065801 (2006)]. They show that the minimization of the atomic dipole potential does not in general give rise to a regular lattice of ultracold atoms and that the steady-state positions of atoms in a cavity are not normally characterized by a minimum of the dipole potential. Instead, the steady-state positions correspond to a vanishing force on each individual atom, the relevant force being proportional to the derivative of the optical field mode function. We agree with Asb\'oth and Domokos that the atomic lattice should be irregular. Using a microscopic model we show however that in the regime considered in our works and also in the preceding Comment the concept of force is generally not well defined, and a semiclassical theory may be inadequate for the description of the atomic motion.
We consider the motion of the end mirror of a cavity in whose standing-wave mode pattern atoms are trapped. The atoms and the light field strongly couple to each other because the atoms form a distributed Bragg mirror with a reflectivity that can be fairly high. We analyze how the dipole potential in which the atoms move is modified due to this back action of the atoms. We show that the position of the atoms can become bistable. These results are of a more general nature and can be applied to any situation where atoms are trapped in an optical lattice inside a cavity and where the back action of the atoms on the light field cannot be neglected. We analyze the dynamics of the coupled system in the adiabatic limit where the light field adjusts to the position of the atoms and the light field instantaneously and where the atoms move much faster than the mirror. We calculate the sideband spectrum of the light transmitted through the cavity and show that these spectra can be used to detect the coupled motion of the atoms and the mirror.
We investigate the steady-state properties of a hybrid system where many cold atoms are confined in an optical cavity with a movable end mirror. The confined cold atoms interact with an external pump field and an internal cavity field in the three-level $\ensuremath{\Lambda}$ configuration. The internal cavity field is excited by an external driving field and exerts a radiation pressure upon the movable end mirror. Then the coupled atom-light and optomechanical interactions offer an efficient nonlinear feedback mechanism permitting the observation of three steady-state solutions for relevant variables near one atomic resonance. We find, in particular, that the weak-cavity-field assumption may result in very large errors for the first and second solutions when the number of cavity photons is only about 106{10}^{6}106. The main reason is that a typically weak light field can result in a sufficiently strong atom-light interaction in a very small cavity. Going beyond the weak-cavity-field assumption also allows us to observe an otherwise absent bistable behavior for the first solution in terms of mirror displacement and cavity photons, which is easy to control by modulating the pump and driving fields.
Lasing [1, 2] and superradiance [3‐5] phenomena are currently enjoying a renaissance as research topics in atomic physics. Fundamentally based on the interaction of matter with an electromagnetic (light) field, the atomic dipoles in a “lasing medium” collectively emit into selected modes of the electromagnetic field and thereby create an amplified and coherent source of light. Present-day lasers use di erent types of gain media and cover a wide range of output frequencies for application in both, fundamental research and industry. A recent string of papers has reported on superradiance [6, 7] and lasing-type instabilities using Bose gases of various type (thermal atoms [8, 9], photon gases [10] and Bose-Einstein condensed atoms [11‐14]) in optical cavities. Buoyed by experimental control over essentially single excitation quanta of atoms and photons [15‐19] in cavity quantum electrodynamics, these works o er intriguing prospects for the next wave of experiments on strongly interacting light-mattter systems including a new regime where unexpected many-body and quantum fluctuation e ects of the lasing medium may become visible. Bohnet et al. [20] recently operated a superradiant “bad-cavity” laser in the few-photon regime based on coherent oscillations of the inter-atomic, collective dipole of the ultracold atoms in the cavity [21]. In this paper, we point out that a quantum degenerate Fermi gas has remarkable features as a lasing medium in a transversally driven cavity as sketched in Fig. 1. We show that the quantum statistics of the Fermi gas puts the coupled cavity+atoms system generically close to superradiance threshold as a consequence of the Pauli exclusion principle (“Pauli pressure”). Physically, this arises from enhanced scattering of fermions between di erent points on the Fermi surface with radiusjkFj by exchanging relatively large photon momenta Q from the cavity as illustrated in Fig. 1. By changing the power of the pump laser at a given density of the Fermi gas, the lasing transition is concomitant with the spontaneous formation of a charge density wave oscillating with 1=Q in real space. Tuning the ratiojkFj=jQj allows to generate a cavity spectrum with “broadband light” in certain spectral ranges or, for energies outside the particle-hole continuum of the fermions, light with relatively narrow linewidth. The normal mode splitting of the cavity by a single atom is completely replaced by many-body Ω |kF| Q
In a recent paper [J. K. Asboth et al., Phys. Rev. Lett. 98, 203008 (2007)] we have shown that traveling density wavelike collective oscillations can arise in an asymmetrically pumped optical lattice, and by increasing the lattice size or pump asymmetry, these waves can destabilize the structure even in the overdamped limit. The long-range interaction giving rise to collective motion stems from the back-action of the atoms on the field creating the lattice. In this paper we present a detailed description of these phenomena. We derive the force on a disk-shaped cloud of trapped particles including the back-action on the trapping light, and analyze its relation to the standard perturbative approach giving the ``dipole force'' and ``radiation pressure.'' We calculate the self-consistent lattice constant for both red and blue detuned lattices and find that it decreases---by the same amount in the two cases---as the pump asymmetry is increased. We present the detailed derivation of the lattice vibration eigenmodes using the transfer matrix method, which reveals that the instability is enhanced resonantly at certain settings of the asymmetry.