量子物理学：国際宇宙ステーションの実験室で観測されたエキゾチック物質

The generation of the fifth state of matter, Bose–Einstein condensates, onboard the International Space Station is reported in Nature this week. Exploiting the microgravity environment of space will allow scientists to explore fundamental physics in this exotic form of matter.

A Bose–Einstein condensate is a state of matter formed when a gas of bosons (such as rubidium atoms) is cooled down close to absolute zero. At this low temperature, the atoms become a single entity with quantum properties. Bose–Einstein condensates straddle the boundary between the microscopic world, governed by quantum mechanics, and the macroscopic world, governed by classical physics. As such, they may offer fundamental insights into quantum mechanics, but measuring them precisely is hampered by gravity.

To overcome these limitations, Robert Thompson and colleagues report the launch and successful operation of the Cold Atom Lab aboard the International Space Station. They describe the preparation of Bose–Einstein condensates in microgravity and measure differences in their properties to those observed on Earth. For example, the free-expansion time (how long atoms hover and can be measured after the confining traps have been switched off) extends beyond one second, as compared to tens of milliseconds typically achievable on Earth. Longer observation time translates into higher precision achievable in measurements. In addition, under microgravity conditions atoms can be trapped by weaker forces, making it possible to reach lower temperatures, at which exotic quantum effects become increasingly more prominent.

These initial experiments show that the space-borne laboratory can facilitate future studies of ultracold atomic gases. “The successful generation of Bose–Einstein condensates in orbit unveils new opportunities for research on quantum gases as well as for atom interferometry and paves the way for even more ambitious missions,” writes Maike Lachmann in an accompanying News & Views article.