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Journal of the Optical Society of America B

Is a quantum standing wave composed of two traveling waves?

Bruce W. Shore, Pierre Meystre, Stig Stenholm

Published Apr 1, 1991• 45 citations
Publisher

Abstract

We compare the scattering of an atom by two different quantized standing-wave configurations. The first one is established in a cavity by a pair of fixed mirrors. The other consists of two independent counterpropagating traveling waves, as could occur in a ring configuration. We show that in the quantum regime (of small photon numbers) atoms are scattered differently by a true standing wave than by a superposition of two counterpropagating waves of equal amplitudes and opposite directions. This behavior is a manifestation of momentum conservation. In the case of traveling waves each wave depletes its momentum independently, whereas the standing wave that is fixed in space acts as a potentially infinite sink or source for momentum.

DOI
10.1364/josab.8.000903

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arXiv:quant-ph/0112164•Dec 26, 2001•0 citations
We compare the scattering of an atom by two different quantized standing-wave configurations. The first one is established in a cavity by a pair of fixed mirrors. The other consists of two independent counterpropagating traveling waves, as could occur in a ring configuration. We show that in the quantum regime (of small photon numbers) atoms are scattered differently by a true standing wave than by a superposition of two counterpropagating waves of equal amplitudes and opposite directions. This behavior is a manifestation of momentum conservation. In the case of traveling waves each wave depletes its momentum independently, whereas the standing wave that is fixed in space acts as a potentially infinite sink or source for momentum.
Atoms undergo two types of momentum changes as they pass across a near-resonant standing wave. First, their transverse momentum spreads. Second, if the atoms cross the field at an angle, they undergo Bragg scattering from the periodic structure provided by the wave. Classically, this behavior can be understood in terms of traveling waves: the atom catalyzes the transfer of momentum from one traveling wave to the other, thereby changing its own transverse momentum. In the quantum regime, however, there is an essential difference between scattering by two running waves and by a standing wave. With running waves, we can in principle know which of the two waves has exchanged a unit of momentum with the atom. In contrast, a standing wave is an inseparable quantum unit whose average momentum remains zero at all times. This unity is imposed by the fixed mirrors that establish the standing wave. These act as infinite sinks of momentum. Quantum mechanics forbids one even in principle to determine, via a field measurement from which traveling wave the atom picked up its momentum. To demonstrate this point, we discuss the diffraction and deflection of an atom in both a standing wave and a superposition of two counter-propagating running waves.
We study the diffraction of quantum-degenerate fermionic atoms off of quantized light fields in an optical cavity. We compare the case of a linear cavity with standing-wave modes to that of a ring cavity with two counterpropagating traveling wave modes. It is found that the dynamics of the atoms strongly depends on the quantization procedure for the cavity field. For standing waves, no correlations develop between the cavity field and the atoms. Consequently, standing-wave Fock states yield the same results as a classical standing wave field while coherent states give rise to a collapse and revivals in the scattering of the atoms. In contrast, for traveling waves the scattering results in quantum entanglement of the radiation field and the atoms. This leads to a collapse and revival of the scattering probability even for Fock states. The Pauli exclusion principle manifests itself as an additional dephasing of the scattering probability.
The scattering of an atom in the field of counterpropagating light waves is studied under conditions such that the state of the atom is a superposition of the ground and excited states. For the case in which this superposition is created by the field of a traveling wave, the momentum distribution function of the atom after scattering by a standing wave is found analytically in the approximation of a short interaction time, when the atom’s motion can be neglected. Longer interactions of the atom with the field are studied numerically. We also consider the case of counterpropagating light waves consisting of Gaussian or supergaussian pulses.
We investigate the diffraction of a three-level atom from a "walking" light wave composed of counterpropagating traveling waves of different frequencies. The coupling of the two nondegenerate internal ground states by means of an off-resonant Raman transition resonantly couples the initial external momentum state to only one other momentum state. This happens even in the Raman-Nath regime, where the interaction time is short enough that momentum transfer is not constrained by phase-matching conditions. The final momentum state depends on the initial internal state of the atom and the relative directions of the two traveling waves. By using multiple diffraction regions, this process can be used to coherently split a beam of atoms into two beams that are widely separated in momentum space. This type of beam splitter would be useful in the building an atom interferometer.
The eigenstate problem of the Jaynes–Cummings model on the basis of a complete Hamiltonian, including the centre-of-mass kinetic energy operator, is considered. The energy spectrum and wavefunctions in the standing-wave (SW) and counterpropagating-wave (CPW) cases are calculated and compared with each other. It is shown that in the CPW case: (i) the atomic momentum distribution is asymmetric; (ii) the concept of quasimomentum is not applicable and instead the problem concerns the ordinary momentum; (iii) atomic and photonic state distributions are self-consistent; and as a consequence (iv) the mean number of photons in counterpropagating travelling waves and mean atomic momentum are matched. Explicit analytic expressions are proposed to describe the transition from counterpropagating quantized waves to a standing wave. It is also shown that, if the recoil energy is taken into account, the Doppleron resonance is split into two branches, one of which diverges to Bragg-like resonance in high-order range.
The eigenstate problem of the Jaynes-Cummings model on the basis of complete Hamiltonian, including the center-of -mass kinetic energy operator, is treated. The energy spectrum and wave functions in standing-wave (SW)- and counterpropagating waves (CPW)- cases are calculated and compared with each other. It is shown that in CPW-case i) the atomic momentum distribution is asymmetric and somewhat narrower in general; ii) the concept of quasimomentum is not applicable and instead the ordinary momentum concerns the problem; iii) atomic and photonic state distributions are self-consistent, and, in consequence iiii) mean number of photons in the counterpropagating traveling waves and mean atomic momentum match. Explicit analytic expressions for energy eigenvalues and eigenfunctions are found in Tavis -Cummings-type approximation [Phys. Rev. 170, 379(1968)] and is pointed, that it implies only the bounded-like states for atomic center-of-mass motion. It is also shown that if the recoil energy is taken into account, the Doppleron resonance is split into two branches, one of which diverges to Bragg-like resonance in the high-order range.