Scientists tested a small quantum circuit that lets microwave signals pass between two paths. It could become a building block for future quantum connections.
A tiny connection
Researchers in Germany built and tested a device that links two microwave paths through one qubit. A qubit is a quantum bit: a small system that can hold quantum information. The study appeared in npj Quantum Information on October 29, 2026. 1
The paths are called waveguides. They guide electromagnetic waves, including microwaves. The qubit is a transmon, a type of electrical circuit that can take in microwave energy and release it again. It interacts with both waveguides, allowing signals to pass between them. 1
This is not just a wire joining two paths. The qubit's interaction with the signal helps control what passes through. The team measured signals traveling along each path and crossing to the other one. 1
Testing very weak signals
The device uses superconducting circuits, which can carry current without ordinary electrical resistance. The researchers tested it at 10 millikelvin, just one-hundredth of a degree above absolute zero. Absolute zero is the lowest possible temperature. Their microwave signals were near six gigahertz, meaning about six billion wave cycles each second. 1
The weak input signal contained approximately one photon on average per interaction cycle. A photon is a tiny packet of electromagnetic energy. An average of one does not mean that every pulse contained exactly one photon. 1
The team also studied how quickly the qubit lost energy and how its quantum behavior became disturbed. They changed the temperature, the magnetic setting and the signal strength. These tests revealed limits on how the connection works. 1
A signal is not the whole message
Getting a signal to another path is only part of moving quantum information. Its quantum state—the physical condition that carries that information—must also survive.
A separate 2018 experiment shows why these are different questions. Researchers transferred quantum states between qubits on separate chips. They reported about 98% photon absorption, but about 80% transfer-process fidelity. Fidelity measures how closely the transfer preserves the intended quantum information. These numbers describe that older experiment, not the new device. 2
Why this matters
The new work tests a possible building block for future quantum routers, devices that direct quantum signals. It does not demonstrate a completed network or perfect transfer of any quantum state. 1
Microwave routing is not new; another team demonstrated it in 2011. This study's value is measuring this particular two-path connection and its limits. Those measurements can help researchers judge how to use such parts in larger systems. 1 3
Read the research
[1] Experimental characterization of the qubit-mediated coupling between two waveguides
[2] Deterministic quantum state transfer and remote entanglement using microwave photons
[3] Demonstration of a Single-Photon Router in the Microwave Regime
The journal first published this study on October 29, 2026, and released its final edited version on November 3, 2026.