A ‘Quantum Bath’ Keeps Distant Qubits Entangled Without Active Control

Distributed quantum computers will need qubits in separate modules to share entanglement reliably. Most existing approaches create those links through precisely timed control pulses, repeated measurements or post-selection. Physicists at the Institute of Science and Technology Austria (ISTA) have now demonstrated a different route: let a deliberately engineered environment create and continuously stabilize the entangled state.

Turning the environment into a resource

Noise from the environment normally destroys fragile quantum states. In the new experiment, however, two isolated superconducting qubits were connected to a common “quantum bath”—a continuous stream of correlated microwave photons. Rather than directly exchanging a controlled photon or waiting for a successful measurement event, both qubits relaxed toward a shared entangled ground state.

The method bridges two kinds of quantum information. Correlated continuous-variable light can be generated efficiently, while many processors use stationary, discrete qubits. The bath converts the readily available photon correlations into stable entanglement between those qubits without active feedback or repeated measurement.

A two-decade-old prediction becomes hardware

The experiment, reported in Physical Review X, confirms in hardware an idea proposed more than 20 years ago: a common reservoir can create entanglement between systems that do not interact directly. Quantum tomography measurements taken over tens of nanoseconds allowed the researchers to reconstruct the qubit states and verify the shared correlations.

An important feature is persistence. The bath continuously replenishes the entangled state, allowing it to remain available beyond the qubits’ unassisted lifetime and potentially be retrieved when needed. This reservoir-engineering approach could complement active protocols in modular processors or quantum networks.

Why this is not yet a quantum-network solution

The device is a laboratory proof of concept involving two superconducting qubits and microwave photons. The researchers report that it transfers only about 10% of the entanglement available in the bath; active-control methods are currently more efficient. Extending the approach to many qubits may introduce crosstalk, loss, calibration problems and fabrication variation.

Microwave photons are suitable inside cryogenic superconducting hardware but do not travel efficiently over ordinary optical fiber. Practical long-distance networks would probably require high-efficiency microwave-to-optical conversion, another technically difficult component. The experiment therefore establishes a new autonomous mechanism and validates important physics, but substantial improvements in fidelity, efficiency, distance and scale are required before it can support fault-tolerant computing.