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Europhysics Letters (EPL)

Frequency-independent laser cooling based on interferometry

Martin Weitz, Theodor W. Hänsch

Published Feb 1, 2000• 12 citations
Publisher

Abstract

We propose an atomic or molecular interferometer based on a series of femtosecond laser pulses, that allows the measurement and control of particle velocities. The probability for a particle to absorb a net photon momentum depends on the particle velocity and on relative phases of the laser pulses, but not on the absolute laser detuning from an optical transition. The scheme has prospects for the laser cooling of atoms and molecules.

DOI
10.1209/epl/i2000-00149-4

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The laser cooling of atoms to ultracold temperatures has propelled many groundbreaking advances in fundamental research and precision measurement, through such applications as quantum simulators and interferometric sensors. Laser cooling remains, however, highly species-selective, and techniques for its application to molecules are still in their infancy. This thesis broadly concerns the development of laser cooling schemes, based on sequences of coherent optical pulses, which can in principle be applied to a wide range of species. We describe a cooling scheme, in which a velocity-selective impulse analogous to that in Doppler cooling is generated by a light-pulse Ramsey interferometer, and present a proof-of-concept demonstration of the scheme using ultracold rubidium-85 atoms as a test-bed. We realise an interferometer for the atoms, as they are in free-fall after release from a magneto-optical trap, by inducing stimulated Raman transitions between their ground hyperfine states. We provide a comprehensive characterisation of these Raman light-pulse interferometer optics, where particular attention is paid to light shift effects. Raman pulses, and indeed coherent operations in any quantum control system, unavoidably suffer from systematic errors in the control field intensity and frequency, and these lead to reductions in pulse fidelity and readout contrast. In parallel to the work on interferometric cooling in this thesis, we report our preliminary investigations into composite pulses, whereby 'naive' single pulses are replaced by sequences of rotations with tailored durations and phases, for improving pulse fidelity in the presence of inhomogeneities. We find that composite pulses can indeed be highly effective in our cold atom system, and propose that their application in such devices as interferometric sensors is a promising prospect.
Laser-cooling techniques offer novel opportunities in the generation and manipulation of entangled quantum states. Atoms trapped in infrared mesoscopic optical lattices are presented as attractive candidates for both the production of highly entangled states and quantum logic experiments. Recent experimental work on the realization of such a lattice, based on atoms trapped in the antinodes of an infrared standing wave near 10.6 /spl mu/m, is reviewed. This culminates in the individual lattice sites being resolved with an optical microscope. Finally, with the aim of achieving greater control over molecules, a matter-wave interferometer is proposed, in which the probability for absorption of photon momenta depends on the particle velocity, rather than on the absolute laser detuning from a particular optical transition. This scheme has prospects for the laser cooling of both atoms and molecules.
We demonstrate matter-wave interference in a warm vapor of rubidium atoms. Established approaches to light-pulse atom interferometry rely on laser cooling to concentrate a large ensemble of atoms into a velocity class resonant with the atom optical light pulse. In our experiment, we show that clear interference signals may be obtained without laser cooling. This effect relies on the Doppler selectivity of the atom interferometer resonance. This interferometer may be configured to measure accelerations, and we demonstrate that multiple interferometers may be operated simultaneously by addressing multiple velocity classes.
We demonstrate matter-wave interference in a warm vapor of rubidium atoms.Established approaches to light-pulse atom interferometry rely on laser cooling to concentrate a large ensemble of atoms into a velocity class resonant with the atom optical light pulse.In our experiment, we show that clear interference signals may be obtained without laser cooling.This effect relies on the Doppler selectivity of the atom interferometer resonance.This interferometer may be configured to measure accelerations, and we demonstrate that multiple interferometers may be operated simultaneously by addressing multiple velocity classes.
We report the 1D cooling of ^{85}Rb atoms using a velocity-dependent optical force based upon Ramsey matter-wave interferometry. Using stimulated Raman transitions between ground hyperfine states, 12 cycles of the interferometer sequence cool a freely moving atom cloud from 21 to 3 μK. This pulsed analog of continuous-wave Doppler cooling is effective at temperatures down to the recoil limit; with augmentation pulses to increase the interferometer area, it should cool more quickly than conventional methods and be more suitable for species that lack a closed radiative transition.
Atom interferometry is a highly versatile experimental technique for precision measurement, with applications in inertial sensing and tests of fundamental physics. Such interferometers rely on high fidelity transfer of atoms between internal states, which can be challenging when working with thermal atom clouds, where a range of resonance frequencies exists. Inhomogeneities in the spatial intensity distribution of the manipulation lasers can also give rise to a range of coupling strengths; both of these effects serve to reduce the fidelity of coherent manipulation operations. Composite pulses offer one route to high fidelity coherent manipulations. In this thesis we investigate the performance of composite pulses in a thermal cloud of Rubidium 85. We find remarkable agreement between theory and experimental investigation of composite inversion pulses, and build on this to model the performance of such pulses in atom interferometers. We also find excellent agreement between our model and the first experimental demonstration of an application of interferometry in the cooling of atoms; this is the first step towards implementing this cooling method in complex structures like molecules.