Breakthrough? Physicists may have taken a decisive step towards novel mini-quantum computers based on magnetic qubits. In the experiment, they extended the lifespan of so-called magnon qubits to 18 microseconds for the first time – a hundred times longer than previously possible. Magnons are created by spin waves in magnetic crystal materials and have long been considered promising candidates for solid-state quantum computers that can be easily miniaturized. But their stability was not sufficient until now – that has now changed.
Quantum computers are considered to be the computers of the future, but so far their practical use is limited. On the one hand, the computing units of these quantum systems, the qubits, are extremely sensitive. Even the smallest disturbances cause the quantum physical superposition and entanglement of the qubits to collapse. On the other hand, today’s quantum computers often require complex, space-consuming cabling and cooling systems. Researchers are therefore looking for systems that are easier to handle and more compact.
Spin wave quasiparticles as qubits?
So-called solid-state platforms could offer such an alternative – magnetic materials in which so-called magnons take on the role of qubits. These quasiparticles are created by the movement of atomic spins in the material and resemble tiny waves in magnetization. Because these wave patterns can behave like particles and can only be a few nanometers small, they are considered possible carriers of quantum physics circuits.
“Magnons, especially in the gigahertz frequency range, inherently have wave-like quantum behavior, are easy to manipulate and show rich nonlinear and non-reciprocal physical phenomena,” explain Rostyslav Serha from the University of Vienna and his colleagues. In addition, magnons can couple to many other quasiparticles, which means they could also be used in hybrid quantum systems.
The catch, however, is that magnons have an extremely short lifespan. In previous systems, these quasiparticles decay again within a few hundred nanoseconds – far too short for any practical quantum calculation.
Stable for up to 18 microseconds
But now the physicists working with Serha have achieved a breakthrough: for the first time, they managed to keep magnons stable for up to 18 microseconds. “This is almost a hundred times longer than any previously observed value,” explains the team. With such a coherence time, magnons for the first time achieve a lifespan that is comparable to the superconducting qubits used in most quantum computers. “This positions magnons as practical, long-lasting information carriers for solid-state quantum computing,” the researchers write.
Physicists achieved this progress through two crucial factors. On the one hand, they only used a specific variant of the magnons, so-called short-wave dipole exchange magnons (DEM). Unlike the quasiparticles most commonly used to date, these are less sensitive to surface defects in the material and are therefore less easily destabilized. The test material also consisted of ultra-pure yttrium iron garnet (YIG) spheres measuring around 300 micrometers in size. This synthetically grown crystal material has special magneto-optical properties.
For their experiment, the physicists cooled three spheres of different degrees of purity to 30 millikelvin – just above absolute zero. This cold inhibits vibrations and thermal processes that cause magnons to decay prematurely. They then specifically excited the material using microwaves.
The purity of the material is crucial
It turned out that the lifespan of the magnons depends on the purity of the material. On the least pure, commercially available YIG crystal sphere, the magnons remained stable for only 4.5 microseconds – still far more than previously possible. “For the third, ultra-pure sphere, the measurements even showed an impressive 18 microseconds lifespan,” report Serha and his colleagues. This is far longer than any previous approach.
Although these quantum systems require deep cooling, this is also the case with superconducting quantum dots. As the experiment showed, the lifetime of the magnons then has no fixed limit, but depends primarily on the purity of the crystalline material. “Our results open up a viable path for materials research to further extend the magnon lifetime,” write the physicists.
Quantum computer the size of a cent coin
According to the team, their breakthrough paves the way to new, small, magnon-based quantum computers. “This degree of coherence transforms the magnons from lossy intermediate forms into robust quantum memories and low-loss waveguides on a chip scale,” the physicists state. According to them, solid-state systems with such quasi-particles as qubits could enable a drastic miniaturization of quantum computers – an entire computer would then be barely larger than a cent coin.
Because magnons can interact with photons, quantum dots, phonons and many other quasiparticles, magnon qubits would also be extremely compatible. They could therefore serve as universal “translators” between different quantum architectures. This would be an important advantage for hybrid or modular quantum computers, for example.
Source: Rostyslav Serha (University of Vienna) et al., Science Advances, 2026; doi: 10.1126/sciadv.aee2344