Tiny Spins, Big Quantum Effects

Researchers at the University of Regensburg discover an effective way to magnetically control Bose-Einstein condensates in semiconductors – with exciting prospects for future technologies such as quantum information processing.

Quantum technologies are considered as key technologies of the future. However, research in this field requires not only scientific expertise but sometimes also highly specialized laboratory conditions. Bose-Einstein condensates are a remarkable example, in which atoms move in lockstep, making quantum-mechanical effects visible not only at the level of individual atoms but also on a macroscopic scale. The first Bose-Einstein condensates were observed in ultracold atomic gases, at temperatures close to absolute zero. The cooling required to achieve this was technically demanding, costly, and restricted to specialized laboratories.

Today, quantum research is increasingly focusing on Bose-Einstein condensates in solid-state materials, which can also exist at moderate temperatures. Typically, electron-hole pairs in a semiconductor, known as excitons, are used for this purpose and can be selectively generated using laser pulses. Excitons, however, are too sluggish, so to speak, to move in lockstep and are therefore coupled to the light field of an optical resonator to reduce their effective mass. The result is a hybrid state of matter and light known as exciton-polaritons. They offer the best of both worlds: they can be controlled using laser pulses and, because they are significantly lighter, can more easily be brought into a collective quantum-mechanical state.

Exciton-polariton condensates as a promising platform for quantum technologies
An exciton-polariton condensate is therefore a macroscopic quantum state that emits light and can form with significantly less external cooling, in some cases even without any external cooling at all. These condensates are consequently considered a promising platform for future applications in quantum communication, quantum optics, and quantum computing. A key challenge, however, is to control the properties of the condensate and, thus, the light it emits – a challenge that has now been overcome:

An international research team led by Prof. Rupert Huber, Dr. Fabian Mooshammer and Dr. Jan Wilhelm from the University of Regensburg, Dr. Florian Dirnberger from the Technical University of Munich, and Prof. Zdeněk Sofer from the University of Chemistry and Technology Prague has shown that exciton-polariton condensates can be controlled through the magnetic properties of the material. Instead of conventional semiconductor materials, the team used a novel layered magnetic semiconductor: chromium sulfide bromide (CrSBr). The crystal consists of many atomically thin layers whose magnetic orientation alternates from one layer to the next. The origin of these magnetic properties is the so-called spin, a type of intrinsic quantum-mechanical angular momentum of electrons. The spin is associated with a magnetic dipole moment, which can be thought of as a tiny compass needle whose orientation indicates the direction of magnetization. In CrSBr, these small “compass needles” are aligned in the same direction within each atomic layer, while pointing in opposite directions in neighboring layers. Controlling this distinctive magnetic order opens up new possibilities for selectively modifying the properties of the exciton-polariton condensate.

Demonstrating exciton-polariton condensation in chromium sulfide bromide
First, however, the researchers had to demonstrate that exciton-polariton condensation actually occurs in the structures under investigation. To do so, they excited exciton-polaritons in CrSBr structures using ultrashort laser pulses. As the density of these particles is gradually increased, they eventually scooch close enough to oscillate in sync with their neighbors. “Once the condensation threshold is reached, the intensity of the emitted light suddenly increases more than a hundredfold. At the same time, the light waves become ordered and, in a sense, oscillate in step with one another. This so-called coherence provides clear evidence of condensation,” explains first author Dr. Heng Zhang.

An external magnetic field breaks the magnetic cage of excitons
The next crucial step was to determine whether the condensate could be selectively controlled by an external magnetic field. The result was striking: as the applied magnetic field increased, the energy of the light emitted by the condensate shifted. This is a consequence of the unusual magnetic order in CrSBr. Initially, the electron spins in neighboring layers of the crystal are aligned in opposite directions. This creates a kind of magnetic cage for excitons, confining them to their respective layers. An external magnetic field can break this magnetic cage by aligning the spins in the same direction. As a result, the properties of the exciton-polaritons, such as their energy, can be selectively adjusted. This novel control mechanism based on the magnetic order of the material is highly efficient, as Christian Weidgans, one of the study’s co-first authors, explains: “While previous approaches have relied, among other methods, on applying an electrical voltage, even moderate magnetic fields in CrSBr enable a shift in the energy of the emitted light that is up to ten times larger. In this way, the quantum state can be controlled directly through the magnetism of the material.”

Magnetically controlled exciton-polariton condensation lays the foundation for future quantum technologies
The results create a new interface between extended quantum states and the magnetic order of a material. “In the future, the platform could be used to directly couple the light emitted by the condensate to magnetic states and manipulate it on extremely short timescales,” explains co-first author Dr. Niloufar Nilforoushan. In this context, microwave radiation can be used to selectively influence the magnetic order of the material. This offers promising prospects for integrating the platform into magnetic memory devices as well as for the efficient conversion of microwave radiation into optical signals. Magnetically controlled exciton-polariton condensation thus opens up entirely new possibilities at the interface of quantum optics and spintronics and lays the foundation for future quantum technologies.