Liquid light was utilized for ultra-fast computations.

The scientific work was published in the journal Nature Materials. Exciton polaritons are quantum objects (liquid light) that represent a superposition of material quasiparticles, excitons (bound electron-hole pairs), and quanta of light, photons, placed in special semiconductor microresonators. These unique particles possess spin properties, making them ideal candidates for transmitting spin currents over long distances.

Since its inception in the early 21st century, spintronics has attracted the attention of scientists due to its potential to surpass traditional electronic technologies. However, the complex nature of spin interactions and rapid spin relaxation has made the realization of spin computing devices challenging. Previous research has shown that energy splittings related to crystal symmetry hinder the stable flow of pure spin current.

If polaritons are the spin carriers, it is essential to consider the strong effective magnetic field in polariton microresonators when creating devices, as this field rapidly rotates the spin of polaritons, complicating the use of spin current. However, the use of a superfluid polariton liquid formed in the organic-inorganic hybrid microcavity FAPbBr₃ with an isotropic cubic crystal structure eliminates this issue, enabling the generation of highly coherent spin currents. Spins in such a structure are carried by superfluid polariton flows, which also resolves the problem of rapid scattering due to thermal fluctuations, allowing the spintronic device to operate at room temperature.

A team of researchers from MIPT, Tsinghua University (Beijing), and Westlake University (Hangzhou) successfully demonstrated the optical spin Hall effect at room temperature for the first time.

The scientists also implemented two innovative spintronic devices: a NOT logic gate and a spin-polarized beam splitter. The logic gate can change right circular spin polarization to left and vice versa, while the beam splitter separates linearly polarized light into two beams with opposite spins. These devices can operate on an ultra-fast picosecond timescale, significantly outpacing current electrical devices that function on a nanosecond timescale. A nanosecond is one billionth of a second, while a picosecond is a thousand times shorter. This means that such spintronic devices could operate a thousand times faster than modern electronic ones.

The physicists conducted both theoretical calculations and experimental studies. The theoretical calculations involved solving a two-component controlled-dissipative Schrödinger equation that describes the motion of polaritons. The modeling included calculating the spin components of the wave function and examining the influence of random potential on spin states. This contributed to a deeper understanding of the processes occurring in polariton flows and enabled predictions on how they could be utilized in spintronic devices.

The modeling predicted that liquid light particles could propagate ballistically while maintaining a coherent state. The experiment demonstrated that at room temperature, polaritons could travel up to 60 micrometers without losing their state, which is more than sufficient for their use as spin current carriers in spintronic devices. This was confirmed through the observation of interference fringes.

Moreover, theoretical calculations predicted that the spin state of liquid light particles oscillates along their propagation path, allowing for manipulation and inversion of polarization using a magnetic field.
The described effect was experimentally demonstrated using laser excitation of a superfluid polariton flow. When a linearly polarized laser beam excited the microresonator, the spin states of the polaritons formed depending on the direction of their angular momentum. The observed effect allows for effective control of the direction of spin currents and their use in computations.

“It is very gratifying that our Chinese colleagues have experimentally realized the effect we proposed 19 years ago and created the first two devices based on the optical spin Hall effect. In the future, we plan to use electron beam lithography and controlled magnetic fields to develop more complex spintronic circuits. Introducing two control light beams with nonlinear interaction of polaritons may enable additional logical operations, paving the way for the creation of fully optical logic circuits based on crystals,” explains Alexey Kavokin, director of the A. A. Abrikosov International Center for Theoretical Physics at MIPT.

— Fully optical logic circuits could provide high data processing speeds with low energy consumption, which is particularly crucial in the era of big data and artificial intelligence. As a result, we can expect not only an increase in the performance of modern computing systems but also the emergence of new innovative applications in quantum computing, information processing, and data transmission. Ultimately, this could lead to the development of more compact, powerful, and resilient devices that could transform the design and architecture of future computer systems.”

This work represents a significant step forward in the fields of spintronics and polaritonics. The researchers have opened new horizons for the practical application of exciton polaritons at room temperature, which could lead to revolutionary changes in the technologies we use every day.