Yes, cold-atom condensates are interesting and useful
DOI: 10.1063/PT.5.010020
When Eric Cornell and Carl Wieman made the first Bose–Einstein condensate from a gas of cold rubidium-87 atoms in 1995, I was excited and impressed. Conceptually, BECs are simple enough to have been included in my undergraduate quantum mechanics course. But making one—what a feat!
Two years later, I joined the editorial staff at Physics Today. It became my job to follow developments in all of physics, not just in my former field of high-energy astrophysics. As more groups began working on BECs and publishing results, my initial enthusiasm for condensates waned. My (no doubt mistaken) impression was that people made BECs to study them and studied them to make them.
My interest perked up when physicists began recreating in BECs phenomena that had already been observed in condensed-matter systems. Two results stand out for me. In 2002, Immanuel Bloch and his collaborators filled the egg-carton-like pockets of an optical lattice with the atoms of a BEC. By making the pockets deeper, they could observe a transition in the way each atom interacted with its nearest neighbors—just like the metal–insulator transition predicted by Nevill Mott in 1949.
The other personal standout was the observation by Zoran Hadzibabic and his collaborators of another transition : a Berezinskii-Kosterlitz-Thouless crossover in BECs that had been squashed to near two-dimensionality.
Both Bloch’s and Hadzibabic’s papers were experimental tours de force, as is the more recent work shown in the figure. Last week in Science, David Hall and his collaborators published a new method to observe quantized vortices in a BEC. The figure shows a sequence of direct, real-time images of a vortex pair in rubidium-87 condensate.
Mott transitions, Berezinskii-Kosterlitz-Thouless crossovers, vortex pairs, and some other phenomena predicted and seen in BECs are rediscoveries. But BECs have the potential for testing theories that are beyond the reach of other systems. If experimenters can cool condensates to the point that particleparticle interactions extend robustly beyond nearest neighbors, then several important models in condensed-matter physics can be vindicated or refuted. The models, like John Hubbard’s two-term Hamiltonian for electrons in solids, are outwardly simple, yet remain too difficult either to isolate in doped crystals or to fully realize in a computer simulation.
Proving that high-temperature superconductivity emerges from the Hubbard model would be a major coup. But the feat would be unlikely to excite the congressional paymasters who fund condensate research in the US. Fortunately—and surprisingly—applications for BECs are already appearing. On that topic, it’s perhaps fitting to end this entry with the words of one of the BEC discoverers, Eric Cornell. When he appeared in 2006 before the House of Representatives’ subcommittee on environment, technology, and standards, Cornell testified:
Where has Bose-Einstein condensation led us, in the ten years since we first created it? What, in particular has it been good for? BEC has found several direct applications, and in particular we and other research groups around the country are trying to develop precision accelerometers, gravitometers, and gyroscopes, to be used for remote sensing and navigation by dead reckoning. In the long run, BEC is likely to be still more important because of its role as a scientific building block, a tool to help us understand and tame quantum mechanics, and to put quantum mechanics to use on problems with relevance to our economy, our health, and our national security.
