For the first time, quantum physicists at Delft University of Technology demonstrated that it is possible to control and manipulate spin waves on a chip using superconductors. In the future, these tiny waves in magnets could provide an alternative to electronics, useful for energy-efficient information technology or connecting pieces in a quantum computer, for example. The discovery, which was published in Science, primarily provides physicists with new insights into the interaction of magnets and superconductors.
Energy-efficient substitute
“Spin waves are waves in a magnetic material that we can use to transmit information,” explains experiment leader Michael Borst. “Because spin waves can be a promising building block for an energy-efficient replacement for electronics, scientists have been searching for an efficient way to control and manipulate spin waves for years.”
Metal electrodes should control spin waves, according to theory, but physicists have yet to see such effects in experiments. “The breakthrough of our research team is that we show that we can indeed control spin waves properly if we use a superconducting electrode,” says Toeno van der Sar, Associate Professor in the Department of Quantum Nanoscience.
We use electrons in diamond as sensors for the magnetic fields of the spin waves. Our lab is pioneering that technique. The cool thing about it is that we can look through the opaque superconductor at the spin waves underneath, just like an MRI scanner can look through the skin into someone’s body.
Van der Sar
Superconducting mirror
A spin wave generates a magnetic field, which in turn generates a supercurrent in the superconductor. The supercurrent acts as a mirror for the spin wave: the magnetic field is reflected back to the spin wave by the superconducting electrode. The superconducting mirror causes spin waves to move more slowly up and down, making the waves more controllable. “When spin waves pass under the superconducting electrode, their wavelength changes completely!” says Borst. And by slightly varying the temperature of the electrode, we can fine-tune the magnitude of the change.”
“We started with a thin magnetic layer of yttrium iron garnet (YIG), known as the best magnet on Earth. On top of that we laid a superconducting electrode and another electrode to induce the spin waves. By cooling to -268 degrees, we got the electrode into a superconducting state,” Van der Sar says. “It was amazing to see that the spin waves got slower and slower as it got colder. That gives us a unique handle to manipulate the spin waves; we can deflect them, reflect them, make them resonate and more. But it also gives us tremendous new insights into the properties of superconductors.”
Unique sensor
The researchers imaged the spin waves by measuring their magnetic field with a unique sensor, something that was essential to the experiment. Van der Sar: “We use electrons in diamond as sensors for the magnetic fields of the spin waves. Our lab is pioneering that technique. The cool thing about it is that we can look through the opaque superconductor at the spin waves underneath, just like an MRI scanner can look through the skin into someone’s body.”
New circuits
“Spin wave technology is still in its infancy,” Borst said. “For example, in order to build energy-efficient computers with this technology, we must first build small circuits to perform calculations.” Our discovery opens up a new avenue: superconducting electrodes enable a plethora of new and energy-efficient spin-wave circuits.”
“We can now design devices based on spin waves and superconductors that produce little heat and sound waves,” Van der Sar said. “Consider the spintronics equivalent of frequency filters or resonators, which can be found in electronic circuits such as cell phones.” Or circuits in a quantum computer that can act as transistors or connectors between qubits.”
