Coupling of two individual magnon resonators via a superconducting microwave resonator operating in the strong coupling regime

In the quest to build the computers of the future, scientists are looking for ways to store and move information using the tiniest possible units of energy. One promising candidate is a particle of light trapped inside a tiny electrical circuit, known as a microwave photon. Another is a collective ripple of magnetism moving through a solid material, called a magnon. When these two very different things are forced to interact strongly, they can exchange energy back and forth so quickly that they become a single, hybrid system. This strong connection is essential for quantum information processing, acting as a bridge that allows magnetic memory to talk to superconducting circuits. However, controlling these interactions is difficult. Usually, to tune a magnetic material so it resonates at the right frequency to talk to a photon, researchers must apply a magnetic field. If they want to control two separate magnetic pieces at the same time, they typically need two separate, complex sets of coils to generate individual fields for each piece. This requirement for bulky, local equipment makes it hard to shrink these experiments down to the size of a microchip, which is necessary for practical technology.

A team of researchers in Germany has found a simpler way to solve this problem by changing the shape of the magnetic materials themselves. Instead of using two separate coils to tune two magnetic stripes, they placed two long, thin strips of a magnetic metal called permalloy on top of a single superconducting wire. These strips were not placed parallel to each other; instead, they were angled at fifty degrees, forming a V-shape. The researchers exploited a natural property of these thin strips: because they are long and narrow, their internal magnetism prefers to align along their length. By rotating the entire device inside a single, uniform magnetic field, the researchers could change how the field interacted with each strip differently. When the field was aligned with one strip, that strip would resonate at a specific frequency. By turning the field, they could shift that frequency up or down without needing any extra wires or coils.

The experiment was conducted at extremely cold temperatures, near absolute zero, to ensure the superconducting wire carried electricity without any resistance. The team first tested a single magnetic strip on the wire and confirmed that it could indeed couple strongly with the microwave signals traveling through the circuit. They observed a clear exchange of energy where the magnetic vibration and the electrical wave merged, creating a distinct gap in the signal that proved they were talking to each other. This confirmed that their setup worked as a strong coupling system. They then moved to the more complex setup with two strips. By carefully adjusting the angle of the external magnetic field, they could tune the two strips to vibrate at the exact same frequency, or at two completely different frequencies, simply by rotating the field.

When the two strips were tuned to the same frequency, they did not just act as two separate things; they acted as a single, unified system connected through the superconducting wire. The researchers observed a complex pattern in the data where the two magnetic strips and the wire created three distinct states of interaction. Two of these states were bright and easy to detect, while a third state, where the two strips vibrated in opposite phases, remained invisible to their sensors. This invisible state is known as a dark mode, and its presence confirmed that the two strips were communicating with each other through the wire. The strength of this connection was significant, with the energy exchange happening fast enough to overcome the natural losses in the system.

The most important result of this work is the method of control. The researchers demonstrated that by arranging two magnetic elements at an angle and using a single rotating magnetic field, they could independently tune each element's behavior. This approach removes the need for the complicated, local bias coils that have made previous experiments difficult to scale. The team showed that this geometry allows for precise control over the magnetic resonance, enabling the system to switch between having two separate resonances and a single, merged resonance. This simplicity suggests a clear path forward for integrating these quantum components into planar devices, where multiple magnetic elements could be controlled on a single chip without the clutter of individual wiring for each one. The work proves that shape and orientation can be used as powerful tools to manage quantum interactions, offering a cleaner, more scalable route to building hybrid quantum systems.