Advancements in quantum sensing and computing may arise from the findings regarding how chromium sulfide bromide reacts to magnetic fields

A quantum “wonder material” has the potential to enable magnetic switching, as demonstrated by a group of scientists from the University of Regensburg and the University of Michigan.

This newly identified ability could facilitate applications in quantum computing, sensing, and beyond. While previous research established that quantum entities known as excitons can be effectively confined to a single line within chromium sulfide bromide, the latest study offers a comprehensive theoretical and experimental analysis establishing the connection to the material’s magnetic arrangement.

Chromium sulfide bromide excites quantum scientists due to its capacity to support virtually any method of physical information encoding: in electric charge, photons (light), magnetism (electron spins), and phonons (vibrations, like sound).

“The long-term aspiration is to potentially create quantum machines or devices that leverage these three, or even all four of these traits: photons to transmit information, electrons to process information through their interactions, magnetism to store information, and phonons to modulate and transduce information to different frequencies,” stated Mackillo Kira, a professor of electrical and computer engineering at U-M.

One method by which chromium sulfide bromide might encode quantum information is through excitons. An exciton is generated when an electron transitions from its “ground” energy state in the semiconductor to a higher energy state, leaving a “hole” in its wake. The electron and hole pair together, forming an exciton.

The excitons are confined within single layers owing to the unique magnetic properties of chromium sulfide bromide. The material consists of layers only a few atoms thick, reminiscent of molecular phyllo pastry. At low temperatures, below 132 Kelvin (-222 Fahrenheit), these layers become magnetized—the electron spins align with one another. The direction of the magnetic field alternates from one layer to the next, creating an antiferromagnetic structure.

At temperatures exceeding 132 Kelvin, the material loses its magnetization—the heat disrupts the alignment of electron spins, causing them to point randomly. In this unmagnetized state, the excitons cannot be contained and extend over multiple atomic layers, rendering them three-dimensional. They are also capable of moving in any direction.

When the antiferromagnetic structure confines excitons to a singular atomic layer, they are further limited to a single line—effectively a single dimension—since they can predominantly move along just one of the two axes within the plane. This confinement within a quantum device enhances the longevity of quantum information because the excitons face reduced chances of colliding and losing the information they carry.

“The magnetic order serves as a new lever for shaping excitons and their interactions. This could revolutionize future electronics and information technology,” remarked Rupert Huber, professor of physics at the University of Regensburg, Germany.

The experimental team, spearheaded by Huber, generated excitons within a sample of chromium sulfide bromide by subjecting it to pulses of infrared light lasting merely 20 quadrillionths of a second. Subsequently, they employed another infrared laser with less energetic pulses to gently elevate the excitons to slightly higher energy levels. Through this process, they discovered the existence of two distinct variations of excitons with unexpectedly differing energies—when typically, they would possess identical energies. This separation of an energy state is referred to as fine structure.

The researchers also investigated spatial variations within the material by directing those less energetic pulses along two axes to explore the internal structures of excitons. This technique uncovered the highly direction-dependent excitons, which could either be restricted to a line or extend in three dimensions. These configurations can be modified according to the magnetic states, which can be toggled through external magnetic fields or temperature fluctuations.

“Given the strong interconnection of electronic, photonic, and spin characteristics, switching between a magnetized and a nonmagnetized state may provide an exceptionally rapid means to transition photon and spin-based quantum information,” expressed Matthias Florian, a U-M research investigator in electrical and computer engineering, who co-authored with Marlene Liebich, a doctoral candidate in physics at the University of Regensburg.

The theoretical team, also directed by Kira, clarified these findings via quantum many-body computations. The calculations utilized the material’s structure to systematically forecast the unusually large fine-structure splitting observed in the magnetically ordered substance, as well as the transitions between the two exciton states when the material fluctuated in and out of magnetic order. They also confirmed that the shift from one-dimensional to three-dimensional excitons accounted for the significant changes observed in the duration excitons could persist without colliding, as larger and more mobile excitons have increased opportunities to collide.

A pressing inquiry the team aims to explore is whether excitons characterized by charge separation can be transformed into magnetic excitations represented by electron spins. If successful, this would create a valuable pathway for transitioning quantum information between the vastly different domains of photons, excitons, and spins.

This research received support from the German Research Foundation, the National Science Foundation, the Air Force Office of Scientific Research, and U-M’s Advanced Research Computing resources.

Collaboration for the study also included researchers from the University of Chemistry and Technology Prague, Czech Republic, and Dresden University of Technology, Germany.