Berkeley Lab observes a quantum fluid in an atomically thin chip

Researchers at Lawrence Berkeley National Laboratory have, for the first time, observed a tunable Bose-Einstein Condensate of excitons within an atomically thin semiconductor, a feat previously limited by exciton lifespans of around a billionth of a second. The findings, published in Nature, demonstrate a new platform for studying quantum fluids and reveal a surprising internal structure within the condensate that responds to magnetic fields.

“Previous studies have shown that electrons and holes can bind into excitons, but there wasn’t an easy way to determine whether those excitons formed a condensate,” said principal investigator Feng Wang, a faculty senior scientist in Berkeley Lab’s Materials Sciences Division. This ability to access and switch the condensate could advance quantum simulations and computing.

Tunable Exciton Bose-Einstein Condensate in Atomically Thin Semiconductors

The persistence of quantum order at relatively high temperatures, up to 2 Kelvin, represents a significant departure from prior Bose-Einstein Condensate (BEC) demonstrations, which typically required temperatures near absolute zero and utilized ultracold atomic gasses. This achievement bypasses the limitations of traditional BEC creation methods. The team’s work, detailed in Nature, not only confirms the formation of the exciton condensate but also reveals a previously hidden internal structure susceptible to external control.

Conventional exciton BEC experiments faced a critical obstacle: the extremely short lifespan, around a billionth of a second, of optically generated excitons. To overcome this, the Berkeley Lab team engineered a two-dimensional semiconducting device where excitons exist in their ground state, allowing them to reach equilibrium and persist as a BEC. Using magneto-optical spectroscopy under cryogenic conditions, they observed collective behavior of electrons and holes, tuning exciton density with electrical gates.

This precise control distinguishes their approach from earlier attempts and allows for detailed characterization of the condensate’s properties. “What is unusual here is that the excitons are not just short-lived particles created by light; they form an equilibrium quantum fluid in a device that we can tune electrically and magnetically,” explained Ruishi Qi, a co-first author on the paper and a former doctoral researcher in Berkeley Lab’s Materials Sciences Division.

The discovery extends beyond simply achieving a BEC; the condensate exhibits internal structures dictated by the quantum property known as “valley,” linked to the motion of electrons and holes within the semiconductor crystal. These “valley” degrees of freedom create multiple possible spin patterns for the excitons.

The researchers found the BEC comprises two distinct components, each possessing a different internal spin-valley structure, enabling multiple condensate phases controllable with a magnetic field. “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,” said Qi, highlighting the potential for manipulating quantum order within the material.

This ability to switch the condensate’s internal structure reveals a previously inaccessible feature in solid-state systems, and our work provides a way to access that hidden structure directly. The team anticipates future work will focus on developing superfluid-based quantum devices and circuits leveraging the unique properties of this exciton BEC, potentially advancing areas like coherent optoelectronics and computing.

Recent advances in condensed matter physics have focused intensely on achieving Bose-Einstein Condensates (BECs) within solid materials, offering a potential pathway toward scalable quantum technologies. Traditionally, realizing these quantum states required manipulating supercold gasses held in a vacuum, a complex and limiting process. Remarkably, the condensate signatures remained stable up to approximately 2 Kelvin, a temperature millions of times warmer than those used in traditional ultracold atomic gas BEC experiments. The researchers anticipate future work will focus on harnessing this control for advanced quantum simulations, coherent optoelectronics, and exciton-based computing technologies.