Continuous-Wave Atom Laser

Army Research Laboratory, Adelphi, Maryland

Progress has been made toward realization of a continuous-wave, phase-andamplitude- stable atom laser based on magnetic guiding, magnetic compression, and continuous distributed evaporative cooling of a sparse cloud of 87Rb atoms. This apparatus is intended to serve as a prototype of sources of coherent matter waves for future atom-interferometric field and motion sensors.

A major part of the apparatus is a magnetic guide comprising two hollow, watercooled, 3.175-mm-diameter wires (see figure) about 1.7 m long in a vacuum chamber in which the ⁸⁷Rb atoms are manipulated. In operation, the wires are excited with parallel direct currents of 300 A, thereby generating a two-dimensional quadrupole magnetic field having a minimum magnitude along the central longitudinal axis of the guide with large lateral gradient. The 87Rb atoms used in this apparatus are prepared in the ⎜F = 1, m_F = -1〉 quantum state in which the atoms have magnetic moments that cause them to be attracted to the locus of minimum magnitude of the magnetic field — that is, to the central longitudinal axis.

In a 0.2-m-long section of the guide at the input end, the wires are oriented vertically and start with a center-to-center distance of about 3.7 cm at the point where the 87Rb atoms are injected. The center-tocenter distance tapers down to 5.17 mm at the upper end of this section, where there is a bend to a horizontal orientation. The distance remains constant through the bend, then tapers along the 1.5-m-long horizontal section, to a minimum of 4.175 mm at the output end. The gradient of the magnitude of the magnetic field is about 20 G/cm at the input end and, by virtue of the aforementioned tapers, increases to 1.7 kG/cm in the bend, then gradually increases after the bend to a maximum of 2.7 kG at the output end. A crucial benefit of the increase in the gradient along the guide is that the atomic distribution becomes magnetically compressed in the transverse directions as the atoms propagate toward the output end. The compression increases the rate of collisions between atoms, thus facilitating evaporative cooling. Moreover, by use of an alternating current of frequency v coupled directly into the guide wires, atoms exceeding a transverse kinetic energy of hv (where h is Planck's constant) have been continuously and selectively removed from the atomic beam.

At the input end, ⁸⁷Rb atoms are injected continuously into the guide in a side-loading scheme that involves a sequence of two modified magneto-optical traps. An open-channel imaging method enables measurement of temperatures and flux of the beam of atoms in the guide under steady-state conditions. At the stage of development at this writing, the beam in the high-gradient portion of the guide has been found to have a transverse temperature of 420 ±40 μK, a longitudinal temperature of 1 mK, an average speed of order 1 m/s, and an atom flux of about 3×10⁷.

It is planned to incorporate a potential well into the guide near the output end, for the purpose of forming a continuous-wave (CW) Bose-Einstein condensate in the well. It is further planned to utilize quantum-mechanical tunneling to extract a coherent CW matter wave from the BEC. A Zeeman slower (a device that utilizes laser cooling and the Zeeman effect to reduce speeds of atoms) has been constructed, with the intent to eventually use it to enable highflux operation of the guide. High flux is essential to further progress toward achieving CW evaporative cooling of the beam, a CW BEC, and CW atom lasing.

This work was done by Georg Raithel of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. ARL-0003

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Continuous-Wave Atom Laser

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