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Register a new userNegative radiation pressure on an ensemble of atoms in free space
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Light is known to exert a pushing force through the radiation pressure on any surface it is incident upon, via the transfer of momentum from the light to the surface. For an atom, the interaction with light can lead to both absorption as well as emission of photons, leading to repulsive and attractive forces, respectively. For classical light, these two processes occur at the same rates. Therefore, a thermal ensemble of atoms at a finite temperature always experiences a net pushing force. In this paper, we show that when treated quantum mechanically the pulsed electromagnetic field interacting with the thermal ensemble of atoms leads to unequal transition rates, again resulting in a non-zero net force. However, the signature and the magnitude of the force depends upon the intensity of the light, the number of atoms, and the initial temperature of the ensemble. Thus, even at finite temperature, controlling the parameters of the electromagnetic pulse and the number of particles in the ensemble, the net force can be changed from repulsive to attractive, generating negative radiation pressure in the process. Quite counterintuitively, this negative radiation pressure arising out of pure quantum character of light gets stronger for higher temperatures.
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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.
We consider a sonic analog of a black hole realized in the one-dimensional flow of a Bose-Einstein condensate. Our theoretical analysis demonstrates that one- and two-body momentum distributions accessible by present-day experimental techniques provide clear direct evidence (i) of the occurrence of a sonic horizon, (ii) of the associated acoustic Hawking radiation and (iii) of the quantum nature of the Hawking process. The signature of the quantum behavior persists even at temperatures larger than the chemical potential.
The equivalence principle is a perennial subject of controversy, especially in connection with radiation by a uniformly accelerated classical charge, or a freely falling charge observed by a supported detector. Recently, related issues have been raised in connection with the Unruh radiation associated with accelerated detectors (including two-level atoms and resonant cavities). A third type of system, very easy to analyze because of conformal invariance, is a two-dimensional scalar field interacting with perfectly reflecting boundaries (mirrors). After reviewing the issues for atoms and cavities, we investigate a stationary mirror from the point of view of an accelerated detector in Rindler space. In keeping with the conclusions of earlier authors about the electromagnetic problem, we find that a radiative effect is indeed observed; from an inertial point of view, the process arises from a collision of the negative vacuum energy of Rindler space with the mirror. There is a qualitative symmetry under interchange of accelerated and inertial subsystems (a vindication of the equivalence principle), but it hinges on the accelerated detectors being initially in its own Rindler vacuum. This observation is consistent with the recent work on the Unruh problem.
Multiparameter estimation, which aims to simultaneously determine multiple parameters in the same measurement procedure, attracts extensive interests in measurement science and technologies. Here, we propose a multimode many-body quantum interferometry for simultaneously estimating linear and quadratic Zeeman coefficients via an ensemble of spinor atoms. Different from the scheme with individual atoms, by using an $N$-atom multimode GHZ state, the measurement precisions of the two parameters can simultaneously attain the Heisenberg limit, and they respectively depend on the hyperfine spin number $F$ in the form of $Delta p propto 1/(FN)$ and $Delta q propto 1/(F^2N)$. Moreover, the simultaneous estimation can provide better precision than the individual estimation. Further, by taking a three-mode interferometry with Bose condensed spin-1 atoms for an example, we show how to perform the simultaneous estimation of $p$ and $q$. Our scheme provides a novel paradigm for implementing multiparameter estimation with multimode quantum correlated states.
I defend the extremist position that the fundamental ontology of the world consists of a vector in Hilbert space evolving according to the Schrodinger equation. The laws of physics are determined solely by the energy eigenspectrum of the Hamiltonian. The structure of our observed world, including space and fields living within it, should arise as a higher-level emergent description. I sketch how this might come about, although much work remains to be done.
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