After the realization of BEC in your lab, and also at Rice University, Houston using ⁷Li atoms, and NIST, Boulder using ⁸⁷Rb, you decided that your next goal was to build an atom laser, in which coherent matter from BEC would replace the coherent light of an optical laser.

   Ketterle: Yes. An atom laser is a device that emits coherent matter waves. It is an intense source of coherent atoms. Bose condensate, with its coherent atoms, is an excellent starting point on the road to the atom laser. We had to take two further steps to make the atom laser. First we had to extract the atoms from the Bose condensate, so we added an output coupler to the magnetic trap in which the condensate was confined. An optical laser relies on mirrors which leak maybe 10% of the light. We made a leak in the trap by using a magnetic field to tilt the spins of the atoms. By controlling the spin angle we could make dollops of sodium condensate. The second step, which I've already mentioned, was the much harder part: we now had to show that the extracted atoms were truly coherent in that they had laser-like properties. Taken together, the two experiments realized the atom laser, as was reported in work that we published early in 1997. But that was just one approach. There are other atom laser concepts: all you need is an atom resonator or cavity, and you have to create a situation where you have a strong population of a single mode of this cavity.

   Many people said an atom laser is impossible because you cannot amplify atoms, which would violate the conservation of mass. Here’s my answer to that: an optical laser is not creating energy. Rather, it transforms an energy input into coherent radiation. Likewise the atom laser is not creating atoms; it takes atoms out of a reservoir and transfers these atoms into a single mode of the cavity. The atom laser generates coherent matter waves by transferring atoms from an incoherent reservoir into this single mode. The atom laser is definitely based on matter-wave amplification. It works by amplifying matter waves in one mode of the cavity at the expense of atoms occupying other modes.

Are there applications where a matter laser would take over from an optical laser?

   Ketterle: Well, light propagates through air whereas atoms are stuck after less than a micrometer. That means an atom laser can only operate in vacuum, so it won't lead to better CD players or supermarket scanners! Atoms strongly interact with each other—unlike photons—and they also respond to gravitational fields. So if you shoot a beam of atoms they are bent towards the Earth by gravity. In the case of light, although the same effect happens, it is usually completely negligible.

You've achieved this goal of an atom laser. When the optical laser was realized it was described as a solution in search of a problem. What are the potential applications for coherent beams of atoms?

   Ketterle: First a caveat: I like the analogy with laser light, and that is what motivates people to intensify research on the atom laser. The step from a light bulb to a laser was a major step in controlling light. Until recently in atomic physics we had only the incoherent sources. Now we have made the major step towards coherent atoms. This is important for atomic physics where we need to control light and atoms.

   On the other hand this analogy might raise certain promises: even being very optimistic I would not foresee that the atom laser will revolutionize research and technology in the same way that the optical laser did. That's because you cannot send atomic beams through air and you cannot superimpose the atomic beams, because of atom-atom scattering, in the way you can combine laser beams.

   The atom laser with long matter wavelengths can only be reached at extremely low temperatures. Personally I cannot see how the operation of an atom laser can be scaled up to work at higher temperatures. It will always be technologically demanding to operate an atom laser. In fact, since our publications early in 1997, my group has been working on fundamentally understanding BEC rather than rushing ahead with the atom laser. We have been looking at sound wave propagation in BEC, as well as collective excitations. The atom laser remains high on our priority list, and we need to increase the power by one or two orders of magnitude.

Right now we can say this has enlivened atomic physics because we can now control atoms in a new way. However, we are a long way from any devices or applications.

   Ketterle: We are in the early days. We have to learn how to control the atom laser. But I think the effect is so general it is hard to imagine that there won't be major applications in research, simply because the atom laser means we can control the position and motion of atoms at an unprecedented level: we are now down at the quantum level. We are approaching the theoretical limitations which are given by the quantum mechanical nature of matter. That's a major achievement—it means control of the elementary building blocks of nature. So I guess the atom laser will be used in those situations where you need precise control over atoms. For example, precision measurements, measurements of fundamental constants, or tests of fundamental symmetries.

   Other areas include my dreams of atom microscopy or better deposition of atoms. I think those applications are harder because they not only need a high degree of control over atoms but they also require intensity in the atomic beam at a much higher level than we have achieved. There are many challenges but few applications. Nevertheless, as a researcher you have to be optimistic and open-minded. My philosophy will be to develop this open and exciting field and not dwell on arguments why the potential might be limited.

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