Magnetic switch traps quantum information carriers in one dimension

This recently discovered capability could help enable applications in quantum computing, sensing and more. While earlier studies identified that quantum entities called excitons are sometimes effectively confined to a single line within the material chromium sulfide bromide, the new research provides a thorough theoretical and experimental demonstration explaining how this is connected to the magnetic order in the material.

Chromium sulfide bromide is exciting to quantum researchers because it can support nearly any way information is physically encoded: in electric charge, photons (light), magnetism (electron spins) and phonons (vibrations, such as sound).

"The long-term vision is, you could potentially build quantum machines or devices that use these three or even all four of these properties: photons to transfer information, electrons to process information through their interactions, magnetism to store information, and phonons to modulate and transduce information to new frequencies," said Mackillo Kira, U-M professor of electrical and computer engineering.

One of the ways chromium sulfide bromide could encode quantum information is in excitons. An exciton forms when an electron is moved out of its "ground" energy state in the semiconductor into a higher energy state, leaving behind a "hole." The electron and hole are paired together, and that collective state is an exciton.

The excitons are trapped in single layers by chromium sulfide bromide's unusual magnetic properties. The material is made up of layers just a few atoms thick, like molecular phyllo pastry. At low temperatures under 132 Kelvin (-222 Fahrenheit), the layers are magnetized -- the spins of the electrons align with one another. The direction of the magnetic field switches to the opposite direction from one layer to the next. This is an antiferromagnetic structure.

Above 132 Kelvin, the material isn't magnetized -- the heat keeps the electron spins from staying aligned, so they point in random directions. In the unmagnetized state, the excitons aren't trapped but extend over multiple atomic layers, making them three-dimensional. They can also move in any direction.

When the antiferromagnetic structure confines excitons to a single atomic layer, the excitons are further restricted to a single line -- a single dimension -- because they can easily move along only one of the two axes of the plane. In a quantum device, this confinement helps quantum information last longer because the excitons are less likely to collide with one another and lose the information they carry.

"The magnetic order is a new tuning knob for shaping excitons and their interactions. This could be a game changer for future electronics and information technology," said Rupert Huber, professor of physics at the University of Regensburg in Germany.

The experimental team, led by Huber, produced excitons inside a sample of chromium sulfide bromide by hitting it with pulses of infrared light just 20 quadrillionths of a second long. Then, they used another infrared laser with less energetic pulses to nudge the excitons into slightly higher energy states. In this way, they discovered that there are two variations of the excitons with surprisingly different energies -- when normally, they would have identical energies. This splitting of an energy state is known as fine structure.

The team also explored how the material varies in space by shooting those less energetic pulses along two different axes within the material to probe the inner structures of excitons. This approach revealed the highly direction-dependent excitons, which could either be confined to a line or expanded in three dimensions. These configurations can be adjusted based on the magnetic states, switchable through external magnetic fields or temperature changes.

"Since the electronic, photonic and spin degrees of freedom are strongly intertwined, switching between a magnetized and a nonmagnetized state could serve as an extremely fast way to convert photon and spin-based quantum information," said Matthias Florian, U-M research investigator in electrical and computer engineering and co-first author with Marlene Liebich, a Ph.D. candidate in physics at the University of Regensburg.

The theory team, led by Kira, explained these results with quantum many-body calculations. The calculations used the structure of the material to systematically predict the exceptionally large fine-structure splitting in the magnetically ordered material and the transitions between the two exciton states when the material transitioned in and out of magnetic order. They also confirmed that the transition from one-dimensional to three-dimensional excitons accounted for the substantial changes observed in how long excitons could go without colliding, as the larger and more mobile excitons have more opportunities to collide.

One of the big questions the team plans to pursue is whether these excitons embodied in charge separation can be converted to magnetic excitations embodied in electron spins. If it can be done, it would provide a useful avenue for converting quantum information between the very different worlds of photons, excitons and spins.

The research was supported by the German Research Foundation, National Science Foundation, Air Force Office of Scientific Research and U-M's Advanced Research Computing resources.

Researchers from the University of Chemistry and Technology Prague, in the Czech Republic, and Dresden University of Technology, in Germany, also contributed to the study.