Physicists Use a 'Quantum Bath' to Put Entanglement on Autopilot

Physicists at the Institute of Science and Technology Austria (ISTA) have demonstrated a fully autonomous way to create quantum entanglement between distant qubits, using a "quantum bath" made from correlated particles of light. Published in Physical Review X, the experiment is the first demonstration of an idea first proposed more than 20 years ago, and could offer a new foundation for practical quantum technologies.

Entanglement lets particles or systems share correlations that classical physics cannot explain, and creating it between physically separated qubits is important for building larger quantum computers and quantum networks. Previous methods generally sent a single, actively controlled photon between qubits, or had each qubit emit a photon that was then matched with the other -- an approach recognised by the 2022 Nobel Prize in Physics -- but both still depend on repeated measurements and do not always succeed.

PhD student Alejandro Andres-Juanes and professor Johannes Fink at ISTA, working with international collaborators, instead used a shared source of correlated light particles to entangle two separated qubits automatically. "By stabilizing the entangled states remotely, our approach is fully autonomous and requires no active control or measurement," Andres-Juanes said. Fink said the surrounding environment, or "quantum bath," itself becomes the source of entanglement, creating a state that stays available "even beyond the qubits' own lifetime" as a resource for further processing. The team used microwave photons, central to leading superconducting-qubit technology, and confirmed the synchronisation using quantum tomography, with measurements lasting just 20 to 80 nanoseconds.

The method is not yet as efficient as actively controlled techniques, transferring about 10% of the bath's available entanglement, the researchers said. They suggest the approach "could be scaled up to synchronize multiple distant qubits," and believe the original theory took more than two decades to demonstrate because it was developed under idealised conditions that were difficult to reproduce experimentally.