Watching a Bose-Einstein condensate crystallize

In 1954 Princeton University’s Robert Dicke predicted a remarkable phenomenon: A dense cloud of excited atoms in a light field, he argued, could decay by spontaneously emitting coherent and highly polarized photons—an effect he termed superradiance. By subtly altering the Hamiltonian, researchers in the early 1970s realized that the phenomenon need not be restricted to transient pulses, and they made their own prediction: When light and matter interact strongly enough, even at zero temperature, they can exhibit a steady-state superradiant phase. By confining a Bose--Einstein condensate of some 10⁵ rubidium atoms driven by a standing-wave laser beam in an optical cavity, Tilman Esslinger and his colleagues at ETH Zürich have now observed the predicted quantum phase transition. As shown in the sketch of their experiment, if the laser light exciting the BEC is intense enough to spawn superradiant photons along the cavity axis, the photons’ repeated reflections establish a field that interferes with the laser field to form a square-patterned potential. Because the condensate atoms produce the superradiant light, they are active participants, since atoms and photons dynamically influence each other’s motion through the coherent exchange of momentum. The upshot is that above some critical laser power, the atoms still exhibit superfluid behavior but become self-ordered into a crystalline lattice. Interestingly, although the Zürich group’s system is open, laser driven, and dissipative—far from the closed equilibrium system that Dicke considered—his Hamiltonian still captures the essential physics. (K. Baumann et al., Nature 464, 1301, 2010 .)—R. Mark Wilson