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Optics Express

Feedback control of an atom laser

R. G. Dall, C. J. Dedman, Andrew G. Truscott

Published Sep 4, 2008• 1 citation
Publisher

Abstract

We report the first real-time feedback control of an atom laser. The unique feature of metastable helium atoms, the production of ions in the atom laser outcoupling process, is exploited to actively control the spatial location inside the Bose-Einstein condensate where outcoupling occurs. Unlike alkali atom lasers, this provides almost instantaneous feedback which reduces frequency, amplitude and spatial mode fluctuations in the atom laser beam.

DOI
10.1364/oe.16.014716

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Quantum-Atom Optics Downunder (QAO)•Jan 1, 2007•0 citations
We report the first real-time feedback control of an atom laser. The unique feature of metastable helium atoms, the production of ions in the atom laser outcoupling process, is exploited to actively control the spatial location inside the Bose-Einstein condensate where outcoupling occurs. Unlike alkali atom lasers, this provides almost instantaneous feedback which reduces frequency, amplitude and spatial mode fluctuations in the atom laser beam.
We present a quantum control scheme in which two different feedback methods are combined to produce a stable single mode atom laser. A difficulty of producing an atom laser comes from the nonlinear effect of Bose-Einstein Condensates which is described by Gross-Pitaevskii type of equations. One feedback method is used to eliminate the nonlinear effect of atom-atom interactions. Another difficulty comes from nonlinear interactions with an optical field which is introduced to detect the BEC state for feedback. The other feedback method is used to suppress fluctuations due to this interaction and keep the coherent properties of the atom laser.
A continuous atom laser will almost certainly have a linewidth dominated by the effect of the atomic interaction energy, which turns fluctuations in the condensate atom number into fluctuations in the condensate frequency. These correlated fluctuations mean that information about the atom number could be used to reduce the frequency fluctuations, by controlling a spatially uniform potential. We show that feedback based on a physically reasonable quantum nondemolition measurement of the atom number of the condensate in situ can reduce the linewidth enormously.
Using the unique detection properties offered by metastable helium atoms we have produced high resolution images of the transverse spatial profiles of an atom laser beam. We observe fringes on the beam, resulting from quantum mechanical interference between atoms that start from rest at different transverse locations within the outcoupling surface and end up at a later time with different velocities at the same transverse position. Numerical simulations in the low output-coupling limit give good quantitative agreement with our experimental data.
A method to create paired-atom laser beams from a metastable helium atom laser via four-wave mixing is demonstrated. Radio-frequency outcoupling is used to extract atoms from a Bose-Einstein condensate near the center of the condensate and initiate scattering between trapped and untrapped atoms. The unequal strengths of the interactions for different internal states allows an energy-momentum resonance which leads to the creation of pairs of atoms scattered from the zero-velocity condensate. The resulting scattered beams are well separated from the main atom laser in the two-dimensional transverse atom laser profile. Numerical simulations of the system are in good agreement with the observed atom laser spatial profiles and indicate that the scattered beams are generated by a four-wave mixing process, suggesting that the beams are correlated.
Using the unique detection properties offered by metastable helium atoms we have produced high resolution images of the transverse spatial profiles of an atom laser beam. We observe transverse fringes on the beam, resulting from quantum mechanical interference between atoms that start from rest at different transverse locations within the outcoupling surface and end up at a later time with different velocities at the same transverse position. Further, for high outcoupling rates we see the emergence of structure on the beam profile, which is not predicted by the usual Gross-Pitaevskii equation.