For more than 60 years, physicists have chased a solid-state route to macroscopic quantum coherence: a Bose-Einstein condensate (BEC) built from excitons — bound pairs of electrons and holes. A team led by Lawrence Berkeley National Laboratory has now achieved it, in an atomically thin semiconductor device, and shown that the condensate hides an internal structure that can be flipped with a magnetic field.
The findings, published in Nature, describe a tunable exciton BEC that persists at temperatures up to about 2 Kelvin — still extremely cold, yet millions of times warmer than earlier BEC demonstrations in ultracold atomic gases. The key trick: the researchers engineered the excitons into the ground state of a 2D semiconductor device rather than creating them as short-lived excited states, allowing them to reach equilibrium and survive as a condensate.
Using cryogenic magneto-optical spectroscopy, the team found the exciton fluid is not a single-flavor quantum state. Electrons and holes in atomically thin crystals carry spin and "valley" degrees of freedom, giving the condensate multiple distinct phases — up-up, down-down, up-down or down-up — that can be switched by applying a small magnetic field.
"The exciting part is that this is not just a simple condensate," said co-first author Ruishi Qi. "It has internal structures that we can control. By simply applying a small magnetic field, we can switch the same exciton fluid between different quantum states."
Beyond its fundamental appeal, the advance establishes a new solid-state platform for studying quantum fluids, with long-term implications for quantum information science, quantum simulations, coherent optoelectronics and future exciton-based devices for faster, more efficient computing. The team next hopes to demonstrate superfluid-based quantum devices and circuits built on the exciton BEC.
