Physicists at the Institute of Science and Technology Austria (ISTA) have demonstrated a fully autonomous way to entangle distant quantum bits — a breakthrough published August 31 in Physical Review X that confirms a theoretical prediction made more than 20 years ago.
The team, led by PhD student Alejandro Andres-Juanes and Professor Johannes Fink, created a quantum bath: a shared environment filled with correlated microwave photons that automatically pushes separated qubits into an entangled state and helps keep them there. Unlike previous methods that require constant measurements and active control, this approach needs no human intervention.
Entanglement — the quantum phenomenon where particles share correlations impossible in classical physics — is essential for building larger quantum computers and future quantum networks. Creating distributed entanglement between physically separated qubits has generally required either actively sending photons between qubits or having each qubit emit photons that are then matched. The latter approach was recognized by the 2022 Nobel Prize in Physics, but still depends on repeated measurements and doesn't always succeed.
In the ISTA experiment, the researchers used microwave photons to couple qubits with an entangled photon source, creating a new ground state through a continuous stream of correlated photons. The entangled qubit state remains stable even beyond the qubits' own lifetime and stays always available as a resource for further quantum processing — unlike temporary entanglement that must be used during a brief window.
The team confirmed the entanglement using quantum tomography, performing measurements lasting only 20-80 nanoseconds to reconstruct the quantum system's behavior.
We present a relatively simple method that could be scaled up to synchronize multiple distant qubits, said Andres-Juanes. However, the method currently transfers about 10% of the bath's available entanglement, leaving room for improvement.
The researchers noted that the original theory was developed under idealized conditions difficult to reproduce experimentally, which may explain why it took over two decades to demonstrate. The prototype could provide new opportunities for quantum-optics experiments and contribute to efforts to expand quantum processors toward fault-tolerant operation.
