On 14 July 1995 a paper in Science reported the first observation of the exotic state of matter predicted in the 1920s by Satyendra Nath Bose and Albert Einstein in an ultracold gas of rubidium atoms. It was the culmination of over a decade of refinement of atom cooling and trapping technologies. It was also one of the occasions when news pieces could be titled ‘Einstein was right’.
In fact, Einstein was wrong. In a 1925 paper, building on Bose’s ideas, Einstein developed a quantum theory to describe a monoatomic ideal gas, but concluded that the theory provided a paradox because it predicted a state with indistinguishable particles occupying the same volume. “But, this appears to be as good as impossible,” Einstein wrote on the last line of his paper. A Bose–Einstein condensate (BEC), as the collective low-energy state of bosons has come to be known, is very much possible and has been found to exist not only in ultracold atomic gases, but also at higher temperatures in materials hosting bosonic quasiparticles such as magnons, excitons and polaritons.
The first BEC, made of 87Rb atoms, was reported by Eric Cornell and Carl Wieman’s team at JILA. A few months earlier they had broken the temperature record in atom cooling, reaching 200 nK. The feat was possible thanks to a so-called time-orbiting potential trap providing a deep enough confinement to allow evaporative cooling (the process of letting hot atoms leave the trap) without losing too many atoms. The JILA team cooled the atoms further down to 20 nK, but already at 170 nK they started to see the first unmistakable signatures of the Bose–Einstein condensation among which is the narrow peak in the velocity distribution of the atoms (pictured). A few months later Wolfgang Ketterle and colleagues made the first sodium BEC. In 2001, Cornell, Wieman and Ketterle shared the Nobel Prize in Physics. Despite the recognition of the discovery it was unclear what BECs were good for.
In the early 2000s on an educational website called ‘Physics 2000’ created and originally hosted by the University of Colorado, the question was answered in a digestible way. Imagine you are someone who has never seen ice before, it would take a while before you would realize that it can be used to make ice cream.
BEC ice cream comes in many surprising flavours. “Quantum mechanics rules over the physics in two regimes: the very cold, and the very small. Insights derived from one regime can apply in the other,” notes Cornell. Over the past 25 years BECs have been the workhorse of ultracold atomic gas experiments, advancing our understanding of quantum many-body phenomena through quantum simulation (see also the Technical Review in this issue). They enabled the study of exotic systems such as the emission of Hawking radiation from an analogue event horizon and fundamental tests of quantum mechanics with macroscopic superposition states. BECs have also been used to create atom lasers, atomic clocks and gravitational, rotational or magnetic sensors with excellent sensitivity.
In terms of the experimental techniques, making atomic BECs in the lab has become routine. Today there are commercial table-top BEC systems available and there are compact systems that can be used for portable sensors or free-fall experiments. The next stage of such experiments was reported in a paper published last month: the first BEC made in space in the Cold Atom Laboratory on board the International Space Station.
“it predicted a state with indistinguishable particles occupying the same volume. “But, this appears to be as good as impossible””
The first BEC experiments 25 years ago led to many unexpected directions in terms of both the science and applications. The field of ultracold atomic gases has been expanding ever since with no sign of slowing down. As Ketterle put it “maybe the best is yet to come”.
Anderson, M. H. et al. Observation of Bose-Einstein condensation in a dilute atomic vapor. Science 269, 198–201 (1995)
Einstein, A. Quantentheorie des einatomigen idealen Gases. Sitzungsber. Preussischen Akad. Wiss. 1, 3–30 (1925)
Davis, K. B. et al. Bose-Einstein condensation in a gas of sodium atoms. Phys. Rev. Lett. 75, 3969 (1995)
Ketterle, W. Happy birthday BEC. Nat. Phys. 11, 982–983 (2015)
Aveline, D. C. et al. Observation of Bose–Einstein condensates in an Earth-orbiting research lab. Nature 582, 193–197 (2020)