Superradiant phenomena - Lessons from and for Bose-Einstein condensates

The work of this thesis is guided by the Analogue Gravity research programme, in which condensed matter systems are used as analogues of the physics of curved spacetimes to obtain new perspectives on open problems of gravitational physics. Here we use this idea to investigate the phenomenon of superradiance, most famously occurring in rotating black hole spacetimes, using as an analogue system atomic Bose-Einstein condensates (BECs). Superradiance is a radiation enhancement phenomenon in which waves of different kind are scattered with an increased amplitude by extracting energy from the object they are scattering on. In this thesis on the one hand we use the gravitational analogy to understand better superradiance starting from easier analogue setups, and on the other hand we use concepts coming from superradiance to learn something about the physics of BECs. We first present a (possibly realizable) toy model, built using the tools of synthetic gauge fields for neutral atoms, to provide a new and conceptually simple illustration of superradiant scattering. This toy model allows to disentangle the different elements at play and highlight the basic mechanisms of superradiance and has also the interesting feature of being exactly mappable to a scattering problem of a charged scalar field on an electrostatic potential. We also show how at the quantum level, superradiance implies the spontaneous emission of pairs of excitations. The low temperatures of atomic condensates can make these quantum features visible and we propose a way of detecting them via correlation measurements. Another realization of this toy model can also be built using periodic trapping potentials for the atoms. By changing the boundary conditions of the acoustic excitations of the condensate we show how superradiance can give rise to dynamical instabilities. Our toy model gives a simple illustration of superradiant instabilities occurring in rotating gravitational spacetimes, in particular ergoregion instabilities and black hole bombs. It also provides a realization of the analogous instabilities involving a charged scalar field, called the Schiff-Snyder-Weinberg effect. Our approach naturally shows how amplified scattering can also occur in the presence of dynamical instabilities, a point often object of confusion in the literature. Moreover, we add an acoustic horizon to our toy model and show that, differently from what happens in general relativity, horizons do not always prevent the presence of ergoregion instabilities. We then apply these concepts to the study of the stability of quantized vortices in two-dimensional BECs. With a careful account of boundary conditions, we show that the dynamical instability of multiply quantized vortices in trapped condensates persists in untrapped, spatially homogeneous geometries and has an ergoregion nature with some modification due to the peculiar dispersion of Bogoliubov sound. Our results open new perspectives to the physics of vortices in trapped condensates, where multiply quantized vortices can be stabilized by interference effects and singly charged vortices can become unstable in suitably designed trap potentials. We show how superradiant scattering can be observed also in the short-time dynamics of dynamically unstable systems, providing an alternative point of view on dynamical (in)stability phenomena in spatially finite systems. Finally we consider the equivalent of a shear layer between parallel flows in hydrodynamics, but in a BEC. In the present case the shear layer is constituted by and array of quantized vortices that are shown to develop an instability analogous to the Kelvin-Helmholtz instability. When the relative velocity between the two parallel flow is sufficiently large however, this instability is quenched and substituted by a slower instability that has the features of the superradiant instabilities we studied. Differently from superradiant instabilities, this one also remains with open boundary conditions on the two sides of the shear layer, and manifests itself as a continuous emission of phonons in both directions; we call this new regime radiative instability.