October 28, 2024 -- Researchers from Skoltech, the University of Warsaw, and the University of Iceland have demonstrated that by optical means it is possible to excite and stir the exciton-polariton condensate, which emits the linearly polarized light with polarization axis following the stirring direction. The rotation of the linear polarization of the emitted light directly corresponds to the stirring of the polariton spin. The speed of such modulation in time can reach GHz range, thanks to ultrafast dynamics of the polariton system. Remarkably, the team found that this precession occurs only at a specific resonant condition of the external stirring and internal system parameters. The work has been published in Optica.

The external manipulation of spins through magnetic or optical fields is a foundation for a wide array of applications, ranging from magnetic resonance imaging (MRI) to coherent state control in quantum computing. One of the efficient ways of driving spins is through a Larmor precession. The precession arises for magnetic material put in the transverse magnetic field making its spins to rotate steadily around the magnetic field lines with the frequency proportional to the magnitude of the applied field. The application of the additional RF-magnetic field resonant to the precession frequency results in a resonant response of the studied system (e.g., nuclear NMR or electron EMR magnetic resonance) that can be effectively measured and utilized. A prominent example here is a visualization of the human tissues in conventional MRI machines in hospitals. Recently, physicists from the Skoltech’s Laboratory of Hybrid Photonics unveiled an effect analogous to conventional NMR in the quantum fluids of light — polariton condensates. Notably, the effect was obtained utilizing just and only optical fields instead of magnetic ones.

In more detail, Skoltech researchers discovered a resonant effect of an all-optical drive on the spin precession in microcavities at cryogenic temperatures. In prior research, the team from the Skoltech’s Laboratory of Hybrid Photonics led by Professor Pavlos Lagoudakis has already  that intrinsic energy splitting, induced by the elliptically polarized laser excitation, in microcavity polaritons works as an effective magnetic field, leading to self-induced Larmor precession of polariton condensates spin. Employing a novel technique for the GHz stirring of polariton condensate developed in the same lab they achieved GHz-driven spin precession with remarkable phase stability. Analogously to the conventional NMR, this spin precession appears only when the stirring frequency is in resonance with the self-induced Larmor precession frequency. Crucially, at the resonance, the polariton spin precession features the exceptional spin dephasing time of 174 ns (that twenty times longer than previously reported value) reflecting its remarkable stability. The resonance was observed by altering different parameters of the system, such as the stirring frequency, polarization ellipticity, and laser pump power. Scientists also developed a rigorous numerical model reproducing the experimental findings. Moreover, for the first time in polariton condensates, researchers were able to retrieve the spin coherence time T2 of 320 ps from the shape of the observed spin resonance. T2 is an important timescale for the possible applications of polaritons characterizing the possible speed of the polariton spin manipulation and allowing to compare them with other physical systems.

This newly discovered resonant mechanism heralds exciting possibilities for innovative spintronic devices capable of manipulating coherent, highly nonlinear, and twisted vectoral light sources. Moreover, it can be used as a coherent light source with the rotating at GHz linear polarization. The achieved fast spin control opens avenues for advanced sensing techniques and continuous variable quantum computing based on polariton condensates. The findings may enable coherent control of the condensate spin state in a manner analogous to conventional NMR techniques, potentially extending these capabilities even to room-temperature utilizing materials with more stable exciton resonances.

The experimental work was fully conducted in the Skoltech Photonics Center. Besides the first author of the paper, Skoltech graduate Ivan Gnusov, the research team from Skoltech included Research Scientist Stepan Baryshev, Assistant Professor Sergey Alyatkin, Junior Research Scientist Kirill Sitnik, and Professor Pavlos Lagoudakis. Strong theoretical support was provided by Dr. Helgi Sigurðsson (University of Warsaw and University of Iceland).