Abstract
Strongly-coupled quantum fields, such as multi-component atomic condensates, optical fields and polaritons, are remarkable systems where the simple dynamics of coupled oscillators can meet the intricate phenomenology of quantum fluids. When the coupling between the components is coherent, not only the particles number, but also their phase texture that maps the linear and angular momentum, can be exchanged. Here, on a system of exciton-polaritons, we have realized a so-called full-Bloch beam: a configuration in which all superpositions of the upper and the lower polariton -- all quantum states of the associated Hilbert space -- are simultaneously present at different points of the physical space, evolving in time according to Rabi-oscillatory dynamics. As a result, the light emitted by the cavity displays a peculiar dynamics of spiraling vortices endowed with oscillating linear and angular momentum and exhibiting ultrafast motion of their cores with striking accelerations to arbitrary speeds. This remarkable vortex motion is shown to result from distortions of the trajectories by a homeomorphic mapping between the Rabi rotation of the full wavefunction on the Bloch sphere and Apollonian circles in the real space where the observation is made. Such full-Bloch beams offer new prospects at a fundamental level regarding their topological properties or in the interpretation of quantum mechanics, and the Rabi-rotating vortices they yield should lead to interesting applications such as ultrafast optical tweezers.
Highlights
Some of the most counterintuitive concepts of physics arise from the representation that quantum mechanics brings to the usual notions of reality: one cannot refer to physical objects with definite properties and attributes, but only to measurements made on them
At a more fundamental level, our experiment could bring forward important developments regarding the topology of complex light or on the interpretation of quantum mechanics, providing an example of a simple and elegant description in an abstract space not immediately accessible to our physical reality, which becomes counterintuitive and bizarre in the physical space where the observation is performed
This allows us to map analytically all the trajectories for all the quantum states, including vortex cores, in any basis of observables, by means of the Möbius transformation, and track them through the stereographic projection between the sphere and the plane. Such connections illustrate how physical interpretations in terms of familiar objects, in this case photons, which are being detected, may be counterintuitive when involving extreme topological configurations such as the fullBloch beam: once emitted by the cavity, photons represent a physical field carrying an optical vortex at any time, which, from one snapshot to the can even travel superluminally, but this effect does not originate from a photon field inside the cavity since no such field exists: at all times, the pure photon state is represented by only one point in space
Summary
Some of the most counterintuitive concepts of physics arise from the representation that quantum mechanics brings to the usual notions of reality: one cannot refer to physical objects with definite properties and attributes, but only to measurements made on them. While the vorticity transfer and dynamics in coherently coupled atomic BECs has been considered theoretically [34,35], here, based on the recent progress in both the quality of samples and the level of control and detection, we experimentally study the joint polariton Rabi-oscillatory and spatiotemporal vortex dynamics It yields a spectacular phenomenology illustrating how notions of “physical objects” must be treated with care and linked to the underlying full-wave-function picture. In the light emitted by the system, we observe a vortex, being at all times accurately defined and visible, undergoing striking ultrafast rotations with large accelerations and decelerations This is achieved with a fine control over the shape of the fields formed in the microcavity and based on the coherent transfer of particles and their momentum between the exciton and photon modes, by preparing a wave function that realizes simultaneously all the possible quantum states of the polaritonic Hilbert space. IV, we discuss the concrete consequences and impact of our findings on polaritonics in particular and on the research fields that deal with dressed states of light-matter interactions and richly structured fields, in general
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