Quantized vortices may be created in conventional experiments with liquid helium and ultracold dilute gases to undertake basic and comparative research of various superfluids. In a recent article published in Science Advances, Ivan Gnusov and a research team in photonics and physics in the U.K., Russia and Iceland produced a “spinning bucket” experiment to optically trap the quantum fluid of light.
The experiments rely on exciton-polariton Bose-Einstein condensates inside the semiconductor microcavity. The researchers exploited the beating note of two frequency stabilised single-mode lasers and developed an asymmetric time-periodic rotating non-resonant excitation profile. They next analysed the influence of the rotation frequency to discover a range of stirring frequencies that promoted quantized vortex generation. The findings may aid the study of polariton superfluidity to comprehend the influence of optics on structured nonlinear light.
The ‘spinning bucket’ experiment
Orbital angular momentum (OAM) in optical vorticity is crucial to encode and decode optical information; the phenomenon has led to the creation of microlasing devices. Optical vortices substantially vary from conventional vortices found inside interacting fluids. For instance, conventional vortices are frequently found in nature, ranging from gigantic vortex storms inside the plasma belts of Jupiter to the minuscule micron-scale quantum vortices in macroscopic quantum systems such as superconductors, superfluids and Bose-Einstein condensates. While optical vortices are geometric in origin, the vortices in superfluids and Bose-Einstein condensates are conceived of as topological flaws.
Despite major progress in the science of polaritonics, researchers have failed to comprehend vortex generation in a stirred polariton condensate or a “spinning bucket” experiment with liquid helium or diluted quantum gases. To induce the phenomena, scientists employed external electric forces or magnetic fields. In this study, Gnusov and colleagues transformed the spinning bucket experiment in a polariton condensate or bosonic quasiparticles that reside within semiconductor microcavities by utilising a cylindrically asymmetric optical apparatus. They then produced an excitation pattern by pounding the note of two frequency detuned single-mode lasers with opposing orbital angular momentum to form a composite beam.
The pumping setup in the rotating bucket experiment and numerical modelling
During the tests, the researchers optically injected the polariton condensate into an inorganic microcavity featuring Bragg reflectors with quantum wells embedded inside the intracavity light field. They next held the sample in a cold-finger cryostat at 4 K. Thereafter, the researchers overlaid two spatially modulated lasers on a non-polarizing beam-splitter to generate a rotating dumbbell-shaped excitation pattern, where the direction and frequency of the rotation were calculated from an earlier study.
For zero-frequency detuning between the two lasers, the scientists reported the production of a static dumb-bell shaped hot exciton reservoir to partly confine polaritons inside the excitation profile owing to the repulsive interaction between excitons and polaritons. They quantitatively recreated the findings through numerical modelling by employing a generalised 2D Gross-Pitaevskii equation. The rivalry between the gain and losses resulted in a quantum vortex that co-rotated with the exciton reservoir. Aside from the ability to mimic the generation of quantum vortices in spinning polariton fluids, the structured light sources with controlled charge presented applications in classical and quantum communications.
Frequency-dependent quantum vortex generation
Gnusov and colleagues were especially interested in the dynamics of the spinning bucket experiment related to its dependency on the corresponding frequency during quantum vortex production. By changing the rotation frequency of the excitation pattern with a diameter of 14 μm, the researchers detected quantum vortex production between 1 and 4 GHz.
The scientists recorded the interface for each frequency and retrieved the real-space phase distribution for 100 “single-shot” realisations. They then created a vortex sorting method to distinguish the quantum vortex states throughout the experiment. Yet again, the researchers incorporated numerical models to quantitatively corroborate the experimental data and quantum vortices as a function of rotation frequency.
Outlook
In this approach, Ivan Gnusov and colleagues examined quantum vortex formation in ultracold quantum gases and liquid helium to comprehend the intriguing basic and comparative studies of superfluids. The researchers accomplished the production of quantum vortex states in the lab using the rotating bucket experiment based on Bose-Einstein condensates of polaritons. The fundamental physics of the polariton shifts needed stirring frequencies in the gigahertz region.
Due to the existing capacity to rapidly create extended polariton networks, this method will enable researchers to engineer of vortex arrays and study the complex interplay of polarisation, orbital angular momentum and linear momentum degrees of freedom in large-scale driven-dissipative quantum fluids. The experimental demonstrations offer a source of optical vortices to enable applications in classical and quantum computing with potential to explore the transport of quantum fluids.
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