A collaborative study conducted by Prof. Zhao Jin, Associate Prof. Zheng Qijing from the University of Science and Technology of China (USTC), and Prof. Hrvoje Petek from the University of Pittsburgh, has revealed the mechanisms behind the transition of bright-dark excitons in anatase TiO₂. Their findings have been published in Proceedings of the National Academy of Sciences.

Understanding Excitons

Excitons, quasi-particles formed by the binding of electrons and holes in condensed matter systems via Coulomb interaction, exhibit distinct properties as bright and dark excitons. While bright excitons directly couple with light and play a pivotal role in light absorption, dark excitons, with their relatively longer lifetimes, hold significance in quantum information processing, Bose-Einstein condensation, and light-energy harvesting.

While atomic clocks are already the most precise timekeeping devices in the universe, physicists are working hard to improve their accuracy even further. One way is by leveraging spin-squeezed states in clock atoms.

Spin-squeezed states are entangled states in which particles in the system conspire to cancel their intrinsic quantum noise. These states, therefore, offer great opportunities for quantum-enhanced metrology since they allow for more precise measurements. Yet, spin-squeezed states in the desired optical transitions with little outside noise have been hard to prepare and maintain.

One particular way to generate a spin-squeezed state, or squeezing, is by placing the clock atoms into an optical cavity, a set of mirrors where light can bounce back and forth many times. In the cavity, atoms can synchronize their photon emissions and emit a burst of light far brighter than from any one atom alone, a phenomenon referred to as superradiance. Depending on how superradiance is used, it can lead to entanglement, or alternatively, it can instead disrupt the desired quantum state.

A team of physicists, including University of Massachusetts assistant professor Tigran Sedrakyan, recently announced in the journal Nature that they have discovered a new phase of matter. Called the “chiral Bose-liquid state,” the discovery opens a new path in the age-old effort to understand the nature of the physical world.

Under everyday conditions, matter can be a solid, liquid or gas. But once you venture beyond the everyday—into temperatures approaching absolute zero, things smaller than a fraction of an atom or which have extremely low states of energy—the world looks very different. “You find quantum states of matter way out on these fringes,” says Sedrakyan, “and they are much wilder than the three classical states we encounter in our everyday lives.”

Sedrakyan has spent years exploring these wild quantum states, and he is particularly interested in the possibility of what physicists call “band degeneracy,” “moat bands” or “kinetic frustration” in strongly interacting quantum matter.

The quantum ground state of an acoustic wave of a certain frequency can be reached by completely cooling the system. In this way, the number of quantum particles, the so-called acoustic phonons, which cause disturbance to quantum measurements, can be reduced to almost zero and the gap between classical and quantum mechanics bridged.

Over the past decade, major technological advances have been made, making it possible to put a wide variety of systems into this state. Mechanical vibrations oscillating between two mirrors in a resonator can be cooled to very low temperatures as far as the quantum ground state. This has not yet been possible for optical fibers in which high-frequency sound waves can propagate. Now researchers from the Stiller Research Group have taken a step closer to this goal.