New insight into light–matter thermalization could advance neutral-atom quantum computing

BUFFALO, N.Y. — Light and matter can remain at separate temperatures even while interacting with each other for long periods, according to new research that could help scale up an emerging quantum computing approach in which photons and atoms play a central role.

In a theoretical study published in Physical Review Letters, a University at Buffalo-led team reports that interacting photons and atoms don’t always rapidly reach thermal equilibrium as expected.

Thermal equilibrium is the process by which interacting particles exchange energy before settling at the same temperature, and it typically happens quickly when trapped light repeatedly interacts with matter. Under the right circumstances, however, physicists found that photons and atoms can instead settle at different — and in some cases opposite — temperatures for extended periods.

These so-called prethermal states are fleeting on human timescales, but they can last long enough to matter for neutral-atom quantum computers, which rely on interactions between photons and atoms to store and process information. 

“Thermal equilibrium alters quantum properties, effectively erasing the very information those properties represent in a quantum computer,” says the study’s lead author, Jamir Marino, PhD, assistant professor of physics in the UB College of Arts and Sciences. “So delaying thermal equilibrium between photons and atoms — even for a matter of milliseconds — offers a temporal window to preserve and process useful quantum behavior.”

All quantum computers store and process information using qubits — the most basic units of quantum information and analogous to the binary bits used in classical computers. While classical bits can exist either as a 1 or a 0, qubits have the ability to exist in a superposition of two states at once, allowing for infinitely more complex calculations. 

Qubits can take many forms. In superconducting quantum computers, a qubit is a collective quantum state involving many electrons flowing together through a superconducting circuit, such as a Josephson junction.

In neutral-atom quantum computers, each qubit is a single atom — often alkali metal atoms excited into so-called Rydberg states.

Their major advantage is simpler hardware. Instead of the complex wiring required for superconducting qubits, atomic qubits can be trapped, controlled and connected via light beams. 

However, this has raised concerns that the light will immediately thermally equilibrate with the atoms and disrupt the fragile quantum behavior required for computation.

To date, most neutral-atom quantum computing research has focused on building large arrays of Rydberg atoms. In those systems, brief laser pulses are used to trap and entangle atoms within the arrays. The light is a fleeting control tool and escapes quickly, leaving little opportunity for it to thermalize with the atoms and interfere with their delicate quantum behavior.

But looking to the future, researchers expect that truly powerful neutral-atom quantum computers will require many Rydberg arrays linked together by light. In that architecture, photons would linger and repeatedly interact with atoms, increasing the risk of rapid thermalization. 

“This would mean the light essentially destroys the very quantum information it was meant to carry,” says the study’s first author, Aleksandr Mikheev, PhD, a postdoc who worked in Marino’s lab during Marino’s time at Johannes Gutenberg University Mainz in Germany, and is now a postdoc at the University of Konstanz.

To explore whether that outcome is inevitable, Marino and his collaborators used theoretical models to simulate the quantum dynamics of photons and atoms. In their calculations, a neutral-atom array is placed inside an optical cavity — a pair of mirrors that trap light and force it to repeatedly interact with the atoms.

As the atoms interact and decay, they emit photons that become confined within the cavity. The simulations show that, after an initial burst of energy exchange, the atoms and photons can sometimes stop efficiently sharing energy, allowing them to maintain separate temperatures. In some cases, the atoms even settle at negative temperatures while the photons settle at positive temperatures. 

Eventually, as photons gradually leak out of the cavity, this prethermal state breaks down and the atoms and photons reach thermal equilibrium. 

“The modeling demonstrates it may indeed be feasible to use light to link larger neutral-atom arrays without washing away the quantum information," says co-author Hossein Hosseinabadi, PhD, a former graduate student in Marino’s lab who will soon start as an independent distinguished postdoctoral scholar at the Max Planck Institute for the Physics of Complex Systems in Germany.

“Additionally, the same light emitted by the atoms could eventually serve as the very light that connects the arrays in a full-scale neutral-atom quantum computer,” Marino adds. “This is crucial because you wouldn’t have to continuously intervene. Once the system is set up, it can naturally remain out of thermal equilibrium for a long time.”

The study was supported by the German Research Foundation and the European Union.