Atom laser

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A device that generates an intense coherent beam of atoms through a stimulated process. It does for atoms what an optical laser does for light. The atom laser emits coherent matter waves, whereas the optical laser emits coherent electromagnetic waves. Coherence means, for instance, that atom laser beams can interfere with each other. See also Coherence.

Laser light is created by stimulated emission of photons, a light amplification process. Similarly, an atom laser beam is created by stimulated amplification of matter waves. The conservation of the number of atoms is not in conflict with matter-wave amplification: The atom laser takes atoms out of a reservoir and transforms them into a coherent matter wave similar to the optical laser, which converts energy into coherent electromagnetic radiation (but, in contrast, the number of photons need not be conserved). See also Laser.

Elements

A laser requires a cavity (resonator), an active medium, and an output coupler (see table).

*Based on evaporative cooling.

Cavity

Various analogs of laser cavities for atoms have been realized. The most important ones are magnetic traps (which use the force of an inhomogeneous magnetic field on the atomic magnetic dipole moment) and optical dipole traps (which use the force exerted on atoms by focused laser beams). See also Particle trap.

Active medium

The active medium is a reservoir of atoms which are transferred to one state of the confining potential, which is the analog of the lasing mode. The reservoir can be atoms confined in other quantum states of the atom cavity or an ultraslow atomic beam. The atoms are transferred to the lasing mode either by collisions or by optical pumping. The transfer of atoms is efficient only for an ultracold sample, which is prepared by laser cooling or evaporative cooling. This cooling ensures that the atoms in the reservoir occupy only a certain range of quantum states which can be efficiently coupled to the lasing mode.

Output coupler

The output coupler extracts atoms from the cavity, thus generating a pulsed or continuous beam of coherent atoms. A simple way to accomplish this step is to switch off the atom trap and release the atoms. This method is analogous to cavity dumping for an optical laser, and extracts all the stored atoms into a single pulse. A more controlled way to extract the atoms requires a coupling mechanism between confined quantum states and propagating modes.

Such a beam splitter for atoms can be realized by applying the Stern-Gerlach effect to atoms in a magnetic trap. Initially, all the atoms have their electron spin parallel to the magnetic field, say spin up, and in this state they are confined in the trap. A short radio-frequency pulse rotates (tilts) the spin of the atoms by a variable angle. Quantum-mechanically, a tilted spin is a superposition of spin up and spin down. Since the spin-down component experiences a repulsive magnetic force, the cloud of atoms is split into a trapped cloud and an out-coupled cloud. By using a series of radio-frequency pulses, a sequence of coherent atom pulses can be formed. These pulses are accelerated downward by gravity and spread out. See also Quantum mechanics.

The illustration shows such a sequence of coherent pulses. In this case, sodium atoms are coupled out from a magnetic trap by radio-frequency pulses every 5 ms. The atom pulses are observed by illuminating them with resonant laser light and imaging their shadows, which are caused by absorption of the light. Each pulse contains 10⁵–10⁶ sodium atoms.

Pulsed atom laser in operation, with pulses of coherent sodium atoms coupled out from a Bose-Einstein condensate that is confined in a magnetic trap.

Potential applications

Although a basic atom laser has now been demonstrated, major improvements are necessary before it can be used for applications, especially in terms of increased output power and reduced overall complexity. The atom laser provides ultimate control over the position and motion of atoms at the quantum level, and might find use where such precise control is necessary, for example, for precision measurements of fundamental constants, tests of fundamental symmetries, atom optics (in particular, atom interferometry and atom holography), and precise deposition of atoms on surfaces. See also Fundamental constants; Nanotechnology; Symmetry laws (physics).

An atom laser is a coherent state of propagating atoms. They are created out of a Bose–Einstein condensate of atoms that are output coupled using various techniques. Much like an optical laser, an atom laser is a coherent beam that behaves like a wave. There has been some argument that the term "atom laser" is misleading. Indeed, "laser" stands for "Light Amplification by Stimulated Emission of Radiation" which is not particularly related to the physical object called an atom laser, and if at all describes more accurately the Bose–Einstein condensate (BEC). The terminology most widely used in the community today is to distinguish between the BEC, typically obtained by evaporation in a conservative trap, from the atom laser itself, which is a propagating atomic wave obtained by extraction from a previously realized BEC. Some ongoing experimental research tries to obtain directly an atom laser from a "hot" beam of atoms without making a trapped BEC first.

Contents 1 Introduction 2 Physics 3 Applications 4 See also 5 References 6 External links

Introduction

The first pulsed atom laser was demonstrated at MIT by Professor Wolfgang Ketterle et al. in November 1996.^[1] Ketterle used an isotope of sodium and used an oscillating magnetic field as their output coupling technique, letting gravity pull off partial pieces looking much like a dripping tap (See movie in External Links).

From the creation of the first atom laser there has been a surge in the recreation of atom lasers along with different techniques for output coupling and in general research. The current developmental stage of the atom laser is analogous to that of the optical laser during its discovery in the 1960s. To that effect the equipment and techniques are in their earliest developmental phases and still strictly in the domain of research laboratories.

Physics

The physics of an atom laser is similar to that of an optical laser. The main differences between an optical and an atom laser are that atoms interact with themselves, cannot be created as photons can, and possess mass whereas photons do not (they therefore propagate at a speed below that of light).^[2] The van der Waals interaction of atoms with surfaces makes it difficult to make the atomic mirrors, typical for conventional lasers.

A continuously operating atom laser was demonstrated for the first time by researchers at the Max Planck Institute for Quantum Optics in Munich^[3]. Its predecessor produced pulses of atoms, rather than continuous beams. In addition, the atoms emitted from the pulsed atom laser quickly spread out in a moon-like crescent, instead of forming a more desirable narrow beam.

Applications

This section requires expansion.

Atom lasers are critical for atom holography. Similar to conventional holography atom holography uses the diffraction of atoms. The De Broglie wavelength of the atoms is much smaller than the wavelength of light, so atom laser can create much higher resolution holographic images. Atom holography might be used to project complex integrated-circuit patterns, just a few nanometres in scale, onto semiconductors. Another application, which might also benefit from atom lasers, is atom interferometry. In an atom interferometer an atomic wave packet is coherently split into two wave packets that follow different paths before recombining. Atom interferometers, which can be more sensitive than optical interferometers, could be used to test quantum theory, and have such high precision that they may even be able to detect changes in space-time.^[4] This is because the de Broglie wavelength of the atoms is much smaller than the wavelength of light, the atoms have mass, and because the internal structure of the atom can also be exploited.

See also

• Bose-Einstein condensate
• List of laser articles

References

1. ^ MIT (1997) "MIT physicists create first atom laser", http://web.mit.edu/newsoffice/1997/atom-0129.html accessed Jul. 31, 2006.
2. ^ MIT's Center for Ultracold Atoms "The Atom Laser", http://cua.mit.edu/ketterle_group/Projects_1997/atomlaser_97/atomlaser_comm.html accessed Jul. 31, 2006.
3. ^ Bloch, Immanuel; Hänsch, Theodor; Esslinger, Tilman (1999). "Atom Laser with a cw Output Coupler". Physical Review Letters 82 (15): 3008. arXiv:cond-mat/9812258. Bibcode 1999PhRvL..82.3008B. doi:10.1103/PhysRevLett.82.3008. 
4. ^ Stanford (2003) The Second Orion Workshop "Hyper precision cold atom interferometry in space", http://www-conf.slac.stanford.edu/orion/PAPERS/D02.PDF

External links

• Atom Laser Movie
• Atom lasers at physicsweb.org
• Research groups working with atom traps

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