Scientists have observed a phenomenon called Floquet rotational super-radiance, using a network of time-modulated resonators arranged in a ring rather than any physically spinning object, according to a study published in Nature.
The effect relies on so-called time-driven, or Floquet, systems, in which properties of a material are modulated in both space and time to control waves without any mechanical motion. According to the researchers, travelling-wave modulations of this kind can behave as if a medium were physically moving, producing effects such as Doppler-induced non-reciprocity, in which waves behave differently depending on the direction they travel.
A related and more elusive effect is the extraction of energy from a rotating medium. Theory has long predicted that this should happen when waves experience sufficiently large rotational Doppler shifts as they interact with something spinning. But testing this prediction experimentally has been difficult, the researchers note, because mechanically rotating systems would need to spin at extreme, largely impractical speeds to reach the necessary regime.
Simulating extreme rotation without moving parts
To get around that barrier, the team used Floquet-induced rotation — a purely spatio-temporal modulation scheme — to reach what they describe as effective superluminal spinning speeds. At these effective speeds, the underlying "space–time crystal" develops angular-momentum bandgaps in its band structure. Within these gaps, parametric processes emerge that efficiently draw energy out of the Floquet-rotating medium, producing amplification of orbital waves that is selective according to their angular momentum, shaped by the system's dissipation.
The researchers implemented this scheme experimentally in a ring network of time-modulated resonators. In that setup they observed the predicted Floquet regime of rotational super-radiance, driven by a combination of non-Hermitian and parametric dynamics in the space–time structured medium.
The authors describe the result as demonstrating a controllable platform for studying how energy can be transferred through rotation and how wave amplification can depend on angular momentum in media whose properties are modulated in both space and time. The underlying theoretical questions trace back to earlier predictions about extracting rotational energy, including from rotating bodies and, in a different physical context, from black holes, concepts the paper's references connect to decades of prior work on superradiant amplification of waves.