Using continuous radio-frequency waves a coherent beam of atoms can be extracted from a magnetically trapped gaseous Bose-Einstein condensate. These novel atomic beams have matter-wave properties that are very similar to those of laser light in classical optics and are therefore called atom lasers. Furthermore, coupling out atoms from a Bose gas allows one to probe the dramatic changes in the phase coherence properties of the trapped atomic gas above and below the condensation temperature.
Using continuous radio-frequency waves a coherent beam of atoms can be extracted from a magnetically trapped gaseous Bose-Einstein condensate. These novel atomic beams have matter-wave properties that are very similar to those of laser light in classical optics and are therefore called atom lasers. Furthermore, coupling out atoms from a Bose gas allows one to probe the dramatic changes in the phase coherence properties of the trapped atomic gas above and below the condensation temperature.
We investigate the coherence properties of an atomic beam evaporatively cooled in a magnetic guide, assuming thermal equilibrium in the quantum degenerate regime. The gas experiences two-dimensional, transverse Bose-Einstein condensation rather than a full three-dimensional condensation because of the very elongated geometry of the magnetic guide. First order and second order correlation functions of the atomic field are used to characterize the coherence properties of the gas along the axis of the guide. The coherence length of the gas is found to be much larger than the thermal de Broglie wavelength in the strongly quantum degenerate regime. Large intensity fluctuations present in the ideal Bose gas model are found to be strongly reduced by repulsive atomic interactions; this conclusion is obtained with a one-dimensional classical field approximation valid when the temperature of the gas is much higher than its chemical potential, kBT ≫ |µ|. Pacs: 03.75.Fi, 42.50.-p The first experimental achievements of Bose-Einstein condensates in atomic vapors [1, 2, 3, 4] have opened promising perspectives for atom optics: condensates constitute indeed atomic waves sources of much better coherence properties than the usual ‘thermal ’ sources like the standard magneto-optical trap. These
The transition of atoms from the disorder of the gaseous phase into the “disciplined” behavior of the condensate can be compared with the transformation of chaotic incoherent light into a laser light beam. This analogy leads directly to the definition of the atom laser: a device that produces a beam of boson atoms that have “escaped” from a trapped Bose–Einstein condensate. The operation of our well-known optical laser requires essentially three things: an optical cavity, an active medium, and a mechanism that allows light to escape from the cavity. Within the cavity, a huge number of atoms of the active medium interact with photons. The cavity consists essentially of two mirrors with very high reflectivity onto which the photons are reflected many times. Multiple reflections cause photons to interact continuously with atoms in the cavity, leading to a continuous generation of new photons by stimulated emission. The result is an increase in the number of photons, i.e., amplification of the beam traveling within the cavity. All of the photons have the same frequency and phase (i.e., they oscillate synchronously). Since one of the mirrors is not perfectly reflective, the photons manage to escape from the cavity, resulting in the well-known, monochromatic, and directional laser beam.
Summary form only given. Bose-Einstein condensed atomic gases are a new class of quantum fluids with many novel properties. We have studied collective modes and dissipation by perturbing the condensate with focused laser beams. The scattering of photons imparts momentum to the condensate and creates excitations. Consequently, the coherence and collective behavior of the condensate can strongly affect optical properties. We used light scattering to demonstrate that the coherence length of a condensate is equal to its physical size, and obtained dramatic evidence for the presence of correlated momentum excitations in the many-body condensate wavefunction. At higher laser intensity, a new form of superradiance was found which provided a gain mechanism for the generation of directed beams of atoms and light. This process amplified matter waves in a phase coherent way.
Atom lasers have been demonstrated in both pulsed and semi-continuous modes. The dynamics of these devices is predicted to be dramatically different to the optical laser due to fundamental differences between the two types of boson fields, however to date it is the similarities that have been emphasized. Here we show how the atom laser dynamics are strongly affected by coupling of the output beam back to the condensate. Typically research in the field of Bose-Einstein condensation (BEC) 1 relates to studies of fundamental properties, such as collective modes 2 or to the response of the BEC to external stimuli 3. In contrast, the atom laser is one of the most promising technologies to arise from BEC, being the di-rect atomic equivalent of the optical laser 4. Mewes et al. 5 demonstrated the first atom laser, based on the application of pulsed radio-frequency (RF) fields to induce controlled spin flips from magnetically trapped to un-trapped states of a Bose-Einstein condensate. Later it was shown by Hagley et al. 6 that a pulsed Raman out-coupling could be used to achieve a quasi-continuous multi-state atomic beam. Bloch et al. 7 achieved con-tinuous RF out-coulping for up to 100ms, producing a single state atom laser beam, and showed that this beam could be coherently manipulated in direct analogy to the optical laser 8. These experiments, and others 9,10, have conclusively demonstrated that the outcoupling process can be made coherent. They have also demonstrated a number of other similarities be-tween the optical and atomic lasers such as high flux and low divergence of the output beam. In all these works, the similarities between the two sys-tems have been emphasized, however, ultimately it is the differences such as mass and wavelength that will lead to applications of the atom laser. 1
We review experimental progress on atom lasers out-coupled from Bose-Einstein condensates, and consider the prop-erties of such beams in the context of precision inertial sensing. The atom laser is the matter-wave analog of the optical laser. Both devices rely on Bose-enhanced scattering to produce a macroscopically populated trapped mode that is output-coupled to produce an intense beam. In both cases, the beams often display highly desirable properties such as low divergence, high spectral flux and a simple spatial mode that make them useful in practical applications, as well as the potential to perform measurements at or below the quantum projection noise limit. Both devices display similar second-order correlations that differ from thermal sources. Because of these properties, atom lasers are a promising source for application to precision inertial measurements.
An atom laser is a coherent state of propagating atoms. They are created out of a Bose Einstein Condensation (BEC) of atoms which are output coupled using various techniques. An optical laser or conventional laser generates a coherent beam of light waves where as an atom laser produces a coherent beam of matter waves. An atom laser will have a major impact on the fields of atom optics, atom lithography and precision measurements.The Himalayan Physics Vol. 4, No. 4, 2013 Page: 99-101 Uploaded date: 12/23/2013