A team at the Institute of Science and Technology Austria (ISTA) has entangled two superconducting transmon qubits across a metre of coaxial cable using only shared quantum-correlated microwave noise. The group reports a concurrence of 0.10 — on a scale where 1 is a perfect pair — in Physical Review X.

The result marks the first experimental realisation of a protocol proposed more than 20 years ago by Barbara Kraus and Ignacio Cirac. It removes the need for synchronised pulses, heralding, post-selection or feedback — the conventional overhead for active entanglement generation.

How does the protocol work?

A Josephson parametric converter splits each pump photon into a pair of entangled photons at gigahertz frequencies. Each photon travels down its own coaxial cable, half a metre in either direction, to one transmon qubit.

Because the noise reaching both qubits is correlated, relaxing into it does not scramble them. The shared field does not wash out the fixed phase relationship between them, as independent noise on each qubit would.

Instead the qubits settle into a superposition in which absorbing a photon from the field and emitting one into it interfere destructively. Once there, the qubits stop evolving. The field passes through unchanged.

The setup converts the continuous-variable (CV) entanglement of the two microwave beams into qubit-qubit entanglement. Roughly a tenth of the original entanglement transfers across.

What does it remove from current entanglement schemes?

Most entanglement-distribution schemes deliver entanglement as discrete events. A pulse sequence runs, a detector clicks, a pair is announced, and the state decays until the next sequence runs.

This "autonomous" version eliminates that machinery. Lead author Alejandro Andrés-Juanes of the Fink Group at ISTA told Physics World: "It removes a lot of overhead on the pulse sequences you need to run to get the entanglement."

He added: "Another differential feature of this protocol is that the entanglement is 'always on' for when you need to use it. In active protocols, you would have to reinitialize the entangled state because it would decohere after a while."

That property matters most at scale, since one correlated photon source can drive many pairs simultaneously.

What are the practical limits?

The protocol's binding constraint is temperature. Microwave frequencies stay quantum only at very low temperatures. Physical separation depends on the size of the dilution refrigerator.

The longest published link of this kind runs 30 m, at ETH Zurich. Andrés-Juanes frames the work as an intermediate-distance technology, not a long-distance one.

At optical frequencies, the thermal constraint lifts. But matching the strong qubit–waveguide coupling in an atomic system is its own engineering problem.

What comes next for the ISTA team?

The numbers remain modest:

• Two qubits, with a concurrence of 0.10
• About 10% entanglement inherited from the microwave beams
• One qubit per node

The group now builds a module with more than one qubit per node. The theoretical underpinning dates to a 2004 Physical Review Letters paper by Barbara Kraus and Ignacio Cirac. Until this work, no group had experimentally realised the scheme.

For R&D managers tracking quantum hardware roadmaps, the implication is twofold. First, entanglement-distribution overhead is now a confirmed engineering target, not just theory. Second, the ISTA group's next milestone — multi-qubit nodes fed by a single photon source — will be the key data point to watch.