Realization of a basic atom laser
The possibility of producing a coherent beam of atoms, which could be collimated to travel large distances or brought to a tiny focus like an optical laser, has sparked the imagination of atomic physicists. Such an atom laser could have a major impact on the fields of atom optics, atom lithography and precision measurements.
A Bose-Einstein condensate is a sample of coherent atomic matter and is thus a good starting point for an atom laser. The process of condensing atoms into the ground state of a magnetic trap is analogous to stimulated emission into a single mode of an optical laser and one can think of the trap as a resonator with "magnetic mirrors". An important feature of a laser is an output coupler to extract a fraction of the coherent field in a controlled way, and at MIT we recently demonstrated such a device for a trapped Bose gas.
Since a magnetic trap can only confine atoms with magnetic moments lying antiparallel to the magnetic field, we altered the "reflectivity" of the magnetic mirrors by applying a short radio-frequency pulse to tilt the magnetic moments of the atoms. The atoms extracted in this way were accelerated under gravity and observed by absorption imaging. By changing the amplitude of the radio-frequency field, the extracted fraction could be varied between 0% and 100%.
A crucial feature of a laser is the coherence of its output - in other words, the presence of a macroscopic wave. In theoretical treatments coherence has been used as the defining criterion for BEC. However, none of the measurements described so far have provided evidence for long-range order. For example, although measurements of the collective excitation frequencies agree with the solutions of the Schrödinger equation at zero temperature, similar frequencies have been predicted for a classical gas in the hydrodynamic regime.
One method to investigate coherence is to look for the effects of the phase of the condensate wavefunction. In superconductors this phase has been observed through the Josephson effect, and in liquid helium it has been inferred from the motion of quantized vortices. The phase is a complex quantity, which means it can only be detected as an interference effect between two different wavefunctions, analogous to the interference between two independent laser beams. The subject of coherence and the interference properties of trapped Bose gases has recently received considerable theoretical attention. The underlying theoretical questions are of major interest and concern. For example, does the concept of spontaneous symmetry breaking (an important concept throughout physics) apply in systems with small atom numbers, and what are the effects of the particle-particle interactions on the phase of the condensate ("phase diffusion")?
At MIT we have recently observed high-contrast interference of two independent Bose condensates, and thus showed that a condensate does indeed have a well defined phase. We formed a double-well trapping potential by focusing a sheet of light into the cloverleaf trap to repel atoms from the centre. Evaporation then produced two separate condensates. These were allowed to free-fall under gravity and expand ballistically.
After dropping 1 cm the clouds overlapped horizontally and the interference pattern was observed by absorption. The period of the fringes is 15 µm, which agrees with the estimated relative de Broglie wavelengths of the overlapping clouds (see figure 6). When the resolving power of the imaging system is taken into account, the intensity difference between the light and dark fringes implies that the modulation of the atomic interference was between 50% and 100%. An exciting feature of the final image is that it is a real-time photograph of interference. In traditional experiments that have demonstrated the wave nature of matter, the wavefront of a single particle is split and then recombined, and this is repeated many times to build up an interference pattern.
High-contrast interference was also observed between two pulses of atoms extracted from a double condensate using the radio-frequency output coupler. This demonstrates that it is possible to extract a coherent beam of atoms from a Bose condensate, and is a rudimentary realization of an atom laser.
Where do we go next?
Bose-Einstein condensation has not only provided us with a novel form of quantum matter, but also a unique source of ultracold atoms. Evaporative cooling has made nanokelvin temperatures accessible to experimenters, and with further optimization it should be possible to produce condensates of 108 atoms in timescales of 5 - 10 seconds. This production rate is comparable to the performance of a standard light trap, and indicates the potential for Bose condensation to replace the magneto-optical trap as the standard bright source of ultracold atoms for precision experiments and matter-wave interferometry.
The experiments to date have studied some of the basic properties of condensation, and have found good agreement with theory. The observation of phase coherence in BEC is a significant step that has opened the door to work with coherent atomic beams. However, a number of fundamental questions that can be addressed by experiments still remain, in particular with regard to the dynamics of Bose gases, such as the formation of vortices and superfluidity.
Another important direction is to explore other atomic systems. The work at Rice with lithium has made the first steps towards understanding Bose gases with negative scattering lengths. Recently at Boulder two different types of condensate were produced in the same trap by "sympathetic cooling". In this scheme a cloud of rubidium atoms in one energy state was cooled in the usual way with evaporation, while a second cloud of atoms in a different energy state was simultaneously cooled just by thermal contact with the first cloud. Such cooling of one species by another will make ultracold temperatures accessible for atoms that are not well suited to evaporation, such as fermionic atoms whose elastic scattering rate vanishes at low temperatures, or rare isotopes that can only be trapped in small numbers. Fermions do not condense but cooling them into the regime where quantum statistics applies could yield valuable insights into Cooper pairing and superconductivity.
It is always exciting when subfields of physics overlap. Atomic physics has traditionally dealt with individual atoms or interactions between a few atoms. The realization of BEC in dilute atomic gases now allows us to study many-body physics and quantum statistical effects with the precision of atomic physics experiments, and it is exciting to see the rapid progress that is being made in exploring these new quantum gases.