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double condensate SYMPATHETIC COOLING a process by which particles of one type cool particles of another type, has been demonstrated for the first time with neutral atoms. Using a combination of lasers and magnetic fields, Christopher Myatt and his colleagues at NIST and the University of Colorado (303-492-2548) trapped a group of rubidium-87 atoms each having one of two possible values for spin, a quantity that describes how a particle responds to a magnetic field. Atoms with one spin value are less tightly bound in the trap's magnetic fields and can be used to cool atoms with the other spin value since the weakly confined atoms could more easily escape the trap and carry away the energy given up by the second species during collisions. Applying this technique to the two rubidium spin species, the researchers have created, for the first time, two overlapping clouds of Bose-Einstein condensates, the new state of matter in which a group of atoms falls into exactly the same quantum state. They also observed that the BECs of the two rubidium species repelled each other. Sympathetic cooling may help enable Bose-Einstein condensation for rare isotopes, and may greatly facilitate comparative studies between fermions and bosons.

A RUDIMENTARY ATOM LASER has been created at MIT, promising significant improvements in high precision measurements with atoms and offering the prospect of future nanotechnology applications, such as atom lithography, in which lines are drawn on integrated circuits (by directly depositing atoms) with greater precision than ever before. In an atom laser the output beam consists of a single coherent atom wave, just as in a regular laser the beam consists of a coherent light wave. The working substance for the atom laser is a Bose-Einstein condensate (BEC) of sodium atoms, cooled and contained within an atom trap by a shaped magnetic field. BEC itself was achieved for the first time only as recently as 1995 (see Update 233). It is a condition in which atoms are chilled to such low energies that, in a wavelike sense, the atoms begin to overlap and enter into a single quantum state. Wolfgang Ketterle and his colleagues at MIT make their claim of producing the first atom laser on the basis of two experimental developments, as reported in two journals this week. In the first effort (M.-O.Mewes et al., Physical Review Letters, 27 January 1997) a portion of a sodium condensate was successfully extracted under controlled conditions. They achieve "output coupling" by applying radiofrequency radiation to the BEC; this "tips" the atoms' spins by an adjustable amount, putting the atoms in a superposition of quantum states. Thereafter some of the atoms feel the effect of the surrounding magnetic field in a different way and are able to leave the atom trap. It is these departing atoms, still enjoying the coherent properties of the BEC state, that constitute an atom laser beam. Pulled downward by gravity, the beam was observed over a distance of millimeters, although in principle it could travel further in an undisturbed vacuum environment. The second development was to verify that the atom waves are indeed coherent (M.R. Andrews et al., Science, 31 January 1997). At the time of the original BEC discovery, many physicists expected the atoms in the condensate to fall into a single quantum state; some hypothesized that it could take a time equal to the age of the universe for true coherence to come about. The MIT group addressed this issue by creating two BEC clouds in a special trap. Turning off the trap allows the clouds to expand, overlap, and interfere, producing a pattern of light and dark fringes. The observed patterns (viewed with an electronic camera) could only exist if each BEC was an intense coherent wave. The MIT team determined that the atom wave associated with each BEC had a wavelength of 30 microns, a million times larger than the wavelengths associated with room-temperature atoms. In addition to coherence, the atom laser waves are analogous to the light waves in an optical laser in another respect as well. Just as a laser beam is more intense than an equivalent stream of light from the Sun, the MIT atom beam is also more intense (for a given beam spotsize) than ordinary atom beams (whose atoms possess a variety of energies) since it delivers a powerful, directional stream of atoms in a single quantum state. In other ways, the atom lasers and light lasers are different. According to Ketterle, "Photons can be created but not atoms. The number of atoms in an atom laser is not amplified. What is amplified is the number of atoms in the lowest-energy quantum state, while the number of atoms in other states decreases."

long-lived atomic state AN EXCITED ATOMIC STATE WITH A 10-YEAR LIFETIME has been discovered in the ytterbium atom, raising hopes for atomic clocks 1000 times more accurate than now possible. The Heisenberg uncertainty principle states that the longer a system can be observed, the smaller the uncertainty in its energy can be; therefore, it is extremely desirable to tune an atomic clock to a long-lived high-energy (excited) state. Researchers at the National Physical Laboratory in the UK laser cool and trap a single ytterbium ion. They then use a laser photon to boost the atom's outermost electron to the long-lived state. With additional laser light, the researchers subsequently induce the electron to return to its lowest-energy (ground) state. By noting the characteristics of the laser light interacting with the electron, the researchers determine a 3700-day lifetime for the state. In addition to being the longest living excited energy state yet detected in an atom, it is the first observed "octupole" transition, a very rare transition in which the electron changes its angular momentum by a relatively large amount of three units. Once in this state, the electron (in the absence of external perturbations) can only decay via the octupole transition, which is why the state lasts so long. An atomic clock based on the transition would be very precise but requires much additional development.

The Atom Laser

A brief commentary by Wolfgang Ketterle
Dept. of Physics and Research Laboratory of Electronics, MIT

Recent work at MIT has realized an atom laser. In this note, the concept and properties of an atom laser are discussed, and also the techniques which were necessary to demonstrate the atom laser.

What is an atom laser?

An atom laser is analogous to an optical laser, but it emits matter waves instead of electromagnetic waves. Its output is a coherent matter wave, a beam of atoms which can be focused to a pinpoint or can be collimated to travel large distances without spreading. The beam is coherent, which means, for instance, that atom laser beams can interfere with each other. Compared to an ordinary beam of atoms, the beam of an atom laser is extremely bright. One can describe laser-like atoms as atoms "marching in lockstep". Although there is no rigorous definition for the atom laser (or, for that matter, an optical laser), all people agree that brightness and coherence are the essential features.

The parts of an atom laser

A laser requires a cavity (resonator), an active medium, and an output coupler. In the MIT atom laser, the "resonator" is a magnetic trap in which the atoms are confined by "magnetic mirrors". The active medium is a thermal cloud of ultracold atoms, and the output coupler is an rf pulse which controls the "reflectivity" of the magnetic mirrors.

The gain process in an atom laser

The analogy to spontaneous emission in the optical laser is elastic scattering of atoms (collisions similar to those between billiard balls). In a laser, stimulated emission of photons causes the radiation field to build up in a single mode. In an atom laser, the presence of a Bose-Einstein condensate (atoms that occupy a "single mode" of the system, the lowest energy state) causes stimulated scattering by atoms into that mode. More precisely, the presence of a condensate with N atoms enhances the probability that an atom will be scattered into the condensate by N+1.

In a normal gas, atoms scatter among the many modes of the system. But when the critical temperature for Bose-Einstein condensation is reached, they scatter predominantly into the lowest energy state of the system, a single one of the myriad of possible quantum states. This abrupt process is closely analogous to the threshold for operating a laser, when the laser suddenly switches on as the supply of radiating atoms is increased.

In an atom laser, the "excitation" of the "active medium" is done by evaporative cooling - the evaporation process creates a cloud which is not in thermal equilibrium and relaxes towards colder temperatures. This results in growth of the condensate. After equilibration, the net "gain" of the atom laser is zero, i.e., the condensate fraction remains constant until further cooling is applied.

Unlike optical lasers, which sometimes radiate in several modes (i.e. at several nearby frequencies) the matter wave laser always operates in a single mode. The formation of the Bose condensate actually involves "mode competition": the first excited state cannot be macroscopically populated because the ground state "eats up all the pie".

The output of an atom laser

The output of an optical laser is a collimated beam of light. For an atom laser, it is a beam of atoms. Either laser can be continuous or pulsed - but so far, the atom laser has only been realized in the pulsed mode. Both light and atoms propagate according to a wave equation. Light is governed by Maxwell's equations, and matter is described by the Schroedinger equation. The diffraction limit in optics corresponds to the Heisenberg uncertainty limit for atoms. In an ideal case, the atom laser emits a Heisenberg uncertainty limited beam.

Differences between an atom laser and an optical laser

Historical roots

The atom laser is based on the quantum-mechanical wave nature of particles. Louis Victor de Broglie, during his Ph.D. thesis in 1923, predicted that all particles have wave properties and gave a famous formula stating that the wavelength of a particle varies inversely with its speed. (The wavelength equals Planck's constant divided by the mass and the speed of the particle.) In 1917, Albert Einstein discovered theoretically the stimulated emission of light which is the basic mechanism generating laser light. In what was then unrelated work, in 1924, he and Satyendra Nath Bose predicted a novel form of matter which forms at very low temperatures which is now called a Bose-Einstein condensate.

Potential applications of an atom laser

Although an 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. Laser-like atoms exist only in an ultrahigh vacuum environment, and so it is unlikely that the atom laser will ever improve supermarket scanners or CD players! However, there are many applications in fundamental research and industry where atomic beams are used, e.g., atomic clocks, atom optics, precision measurements of fundamental constants, tests of fundamental symmetries, atomic beam deposition for chip production (atom lithography), and, more generally, nanotechnology. The atom laser may have an impact on all of these applications. Today, if you have a demanding job for light, you use a laser. In the future, if there is a demanding job for an atomic beam, you may be able to use an atom laser.

The steps from a Bose condensate to an atom laser

An important intermediate step towards the atom laser was the realization of Bose-Einstein condensation (BEC), which was achieved in 1995 by a group at Boulder and Ketterle's group at MIT. (In 1996, two more groups, a group at Rice and a second group at Boulder, observed BEC). The Bose condensate has frequently been compared to photons in a laser beam, but what was missing was a controlled way of extracting a beam of atoms and a method for determining whether the Bose condensed atoms are coherent as the photons in a laser beam. Both these steps have now been taken by the MIT team, thus realizing the atom laser.

Realization of an output coupler for a Bose condensate

(Phys. Rev. Lett., January 27, 1997) An output coupler is one of the essential elements of a laser. It allows the controlled extraction of atoms from the Bose condensate, i.e. the generation of a (quasi-) continuous beam or multiple pulses. Before the MIT group realized an output coupler, the entire condensate was either trapped or freely expanding.

The MIT group achieved the controlled extraction of atoms in the following way: Magnetically trapped atoms can be regarded as atoms bouncing back and forth between magnetic mirrors. The magnetic mirror is 100% reflective for atoms with their magnetic moment anti-parallel to the magnetic field, and fully transmissive for the opposite orientation. The MIT group tilted the magnetic moment of the atoms by a variable angle, thus adjusting the reflectivity of the magnetic mirror. This was done by using short pulses of an oscillating magnetic field.

When the MIT group realized the output coupler in July 1996, they had all the elements for an atom laser together. However, a crucial feature of a laser had yet to be demonstrated: the coherence of the condensed atoms. This was achieved in November 1996 through the observation of high-contrast interference between two Bose condensates.

Demonstration of coherence of a Bose condensate

(Science, January 31, 1997) It should be noted that laser light has two important features: Brightness and coherence. Brightness does not necessarily mean high absolute power, but the concentration of power into the direction of propagation and in a small frequency interval (monochromatic light). This is the reason why a laser pointer is brighter than the sun! The second important feature is coherence, i.e., all the photons in a laser form one macroscopic wave (they "oscillate synchronously").

In the case of atoms, a Bose condensate is very cold and coherent. Coldness corresponds to brightness in the optical case, because a very low temperature restricts the quantum states which are accessible to the atoms to the lowest states of the system (Brightness in the optical case also means restricting the photons to a few modes of the laser resonator). It is the low energy of the condensate which was studied in previous experiments and used to identify the Bose condensate. However, although coldness and coherence are related, there has been some controversy about how coherent the atomic Bose condensate would be. It has been argued that the atoms first become very cold, but then it would take much longer (maybe forever) for the coherence to build up. Furthermore, collisions among the atoms and with background gas were predicted to destroy the coherence. The MIT results resolve these issues. They prove that a Bose condensate is coherent, and that a coherent beam of atoms can be extracted from it.

The proof of the coherence was obtained by observing a high contrast interference pattern when two Bose condensates overlapped. The MIT researchers could directly photograph this pattern which had a period of 15 micrometer, a gigantic length for matter waves. (Room temperature atoms have a matter wavelength of 0.04 nm, 400,000 times smaller). The interfering condensates were propagating with an energy of 0.5 nanokelvin - the coldest temperature ever reported. However, temperature has lost its meaning in this regime, it is only used as a measure for the residual (non-thermal) energy of the atoms.

When matter waves interfere destructively, it is as if one atom plus one atom give zero atoms! Of course, the matter is not destroyed, and the atoms appear elsewhere. Nevertheless, the interference of streams of atoms from separate sources is a dramatic phenomenon.

Other techniques used to realize the atom laser

A variety of schemes to realize an atom laser have been discussed during the last several years. The MIT group chose a particularly simple way. They cooled an atomic gas to extremely low temperatures until it spontaneously formed a Bose-Einstein condensate with "laser-like" properties, and then extracted these atoms into output pulses (see above).

The MIT work was based on powerful cooling techniques which were used to reduce the temperature of a sodium gas by a factor of a billion, from the temperature of an oven to around one microkelvin. These cooling techniques are laser cooling (the key techniques were invented at NIST (W. Phillips), Bell Labs/Stanford (S. Chu), MIT (D. Pritchard)) and evaporative cooling (developed at MIT (T. Greytak, D. Kleppner)). Many other groups in the atomic physics and condensed matter communities have contributed to these efforts (e.g., Amsterdam, Boulder, Cornell, Harvard, Paris). Between 1992 and 1995, Ketterle's group pioneered ways to combine laser cooling and evaporative cooling. The combined cooling was key to the observation of Bose-Einstein condensation in Boulder in June and at MIT in September of 1995.

In laser cooling, the atoms are bombarded with laser light. The frequencies and polarizations of the laser beams are chosen in such a way that the photons emitted by the atoms are slightly more energetic than the absorbed photons. The energy difference is responsible for the cooling effect. After absorbing and emitting about 100,000 photons, the atoms reach a temperature of about 100 microkelvin. Subsequently, the atoms are cooled to BEC using evaporative cooling. In this technique, the hottest atoms are removed from the atomic sample, thus reducing the average energy (and therefore the temperature) of the remaining atoms. The same principle cools a cup of coffee and water in a bathtub.

The atoms have not only to be cooled, but also very well insulated from the room-temperature environment. This is accomplished by purely magnetic confinement inside an ultrahigh vacuum chamber.

visit http://amo.mit.edu/~bec/atomlasr/atomlasr.html to see the figures