Physicists from the Institute of Science and Technology Austria (ISTA) have, for the first time, implemented a fully autonomous method for distributed quantum entanglement using a so-called "quantum bath" composed of correlated light particles.

A New Approach to Distributed Entanglement

Future quantum computers may require connecting distant modules through distributed entanglement. Traditionally, creating such links has required active control and repeated measurements. The new method developed at ISTA is based on using a quantum bath, which automatically synchronizes remote qubits without the need for external intervention.

The experiment, published in the journal Physical Review X, is the first to confirm a theoretical prediction made over 20 years ago. This approach could become the foundation for practical quantum technologies, enabling the connection of remote quantum bits (qubits) to build scalable quantum computers and networks.

Traditional Methods and Their Limitations

Previously, there were two main strategies for entangling distant qubits:

• Transmitting a single photon from one qubit to another with active control.
• Generating photons with each qubit and then comparing them to create entanglement.

The second method was recognized with the Nobel Prize in Physics in 2022, but it also relies on repeated measurements and does not always result in successful entanglement.

Features of the New Method

The new prototype uses a common source of correlated light particles to entangle two separated qubits. The quantum bath creates and stabilizes entanglement between the qubits, acting as an environment responsible for forming and maintaining quantum coherence. This approach ensures a continuous availability of the entangled state, unlike temporary methods where entanglement exists only briefly.

Use of Microwave Photons

Microwave photons were used to connect the qubits with the source of entangled photons. These low-energy light particles are widely used for manipulating quantum information and are essential for modern superconducting qubits. Optical photons, in turn, are used in optics and atomic physics and may be important for transmitting quantum information between distant quantum computers via fiber optics.

Confirmation of Entanglement

To confirm the synchronization of the two qubits within the quantum bath, quantum tomography was used—a method that reconstructs the quantum system from multiple different measurements. The measurements lasted only 20–80 nanoseconds, allowing researchers to study the internal states of the qubits.

Prospects and Limitations

During the experiment, the team managed to create a laboratory prototype that confirms a long-standing theoretical hypothesis. The new method is currently less efficient than traditional active-control approaches, transmitting about 10% of the available entanglement from the bath. One reason for the long realization time was the difficulty in reproducing the idealized conditions assumed in the original theory.

The developed prototype could open new opportunities for experiments in quantum optics and help advance scalable quantum processors, bringing them closer to operating with error correction.