Negative frequency at the horizon : scattering of light at a refractive index front

This thesis considers the problem of calculating and observing the mixing of modes of positive and negative frequency in inhomogeneous, dispersive media. Scattering of vacuum modes of the electromagnetic field at a moving interface in the refractive index of a dielectric medium is discussed. Kinematics arguments are used to demonstrate that this interface may, in a regime of linear dispersion, act as the analogue of the event horizon of a black hole to modes of the field. Furthermore, a study of the dispersion of the dielectric shows that five distinct configurations of modes of the inhomogeneous medium at the interface exist as a function of frequency. Thus it is shown that the interface is simultaneously a black- and white-hole horizon-like and horizonless emitter. The role, and importance, of negative-frequency modes of the field in mode conversion at the horizon is established and yields a calculation of the spontaneous photonic flux at the interface. An algorithm to calculate the scattering of vacuum modes at the interface is introduced. Spectra of the photonic flux in the moving and laboratory frame, for all modes and all realisable increase in the refractive index at the interface are computed. As a result of the various mode configurations, the spectra are highly structured in intervals with black-hole, white-hole and no horizon. The spectra are dominated by a negative-frequency mode, which is the partner in any Hawking-type emission. An experiment in which an incoming positive-frequency wave is populated with photons is assembled to observe the transfer of energy to outgoing waves of positive and negative frequency at the horizon. The effect of mode conversion at the interface is clearly shown to be a feature of horizon physics. This is a classical version of the quantum experiment that aims at validating the mechanism of Hawking radiation

Analytical description of quantum emission in optical analogs to gravity

Funding: UK EPSRC via Grant No. EP/L505079/1 and the IMPP.We consider a moving refractive index perturbation in an optical medium as an optical analogue to waves under the influence of gravity. We describe the dielectric medium by the Lagrangian of the Hopfield model. We supplement the field theory in curved spacetime for this model to solve the scattering problem for all modes and frequencies analytically. Because of dispersion, the kinematic scenario of the field modes may contain optical event horizons for some frequencies. We calculate the spectra of spontaneous emission in the frame co-moving with the perturbation and in the laboratory frame. We also calculate the spectrally-resolved photon number correlations in either frame. The emitted multimode field comes in different types depending on the presence of horizons. We show that these types are robust against changes in the system parameters and thus are genuine features of optical and non-optical analogues. These methods and findings pave the way to new observations of analogue gravity in dispersive systems.PostprintPeer reviewe

The next generation of analogue gravity experiments

PostprintPeer reviewe

Spectroscopic measurement of the excitation spectrum on effectively curved spacetimes in a polaritonic fluid of light

Quantum fields in regions of extreme spacetime curvature give rise to a wealth of effects, like Hawking radiation at the horizon of black holes. While quantum field theory can only be studied theoretically in black holes, it can be tested in controlled laboratory experiments. Typically, a fluid accelerating from sub- to supersonic speed will create an effectively curved spacetime for the acoustic field, with an apparent horizon where the speed of the fluid equals the speed of sound. Here we create effective curved spacetimes with a quantum fluid of light, with smooth and steep acoustic horizons and various supersonic fluid speeds. We use a recently developed spectroscopy method to measure the spectrum of acoustic excitations on these spacetimes, thus observing negative energy modes in the supersonic regions. This demonstrates the potential of quantum fluids of light for the study of field theories on curved spacetimes.Comment: 5 pages, 3 figure

Coherence, not conditional meaning, accounts for the relevance effect

Missing-link conditionals like “If bats have wings, Paris is in France” are generally felt to be unacceptable even though both clauses are true. According to the Hypothetical Inferential Theory, this is explained by a conventional requirement of an inferential connection between conditional clauses. Bayesian theorists have denied the need for such a requirement, appealing instead to a requirement of discourse coherence that extends to all ways of connecting clauses. Our experiment compared conditionals (“If A, C”), conjunctions (“A and C”), and bare juxtapositions (“A. C.”). With one systematic exception that is predicted by prior work in coherence theory, the presence or absence of an inferential link affected conditionals and other statement types in the same way. This is as expected according to the Bayesian approach together with a general theory of discourse coherence

Spectrum of collective excitations of a quantum fluid of polaritons

We use a recently developed high-resolution coherent probe spectroscopy method to investigate the dispersion of collective excitations of a polaritonic quantum fluid. We measure the dispersion relation with high energy and wavenumber resolution, which allows us to determine the speed of sound in the fluid and to evidence the contribution of an excitonic reservoir. We report on the generation of collective excitations at negative energies, on the ghost branch of the dispersion curve. Precursors of dynamical instabilities are also identified. Our methods open the way to the precise study of quantum hydrodynamics of quantum fluids of light