The physics at the event horizon resembles the behavior of waves in moving media. Horizons are formed where the local speed of the medium exceeds the wave velocity. We use ultrashort pulses in microstructured optical fibers to demonstrate the formation of an artificial event horizon in optics. We observed a classical optical effect, the blue-shifting of light at a white-hole horizon. We also show by theoretical calculations that such a system is capable of probing the quantum effects of horizons, in particular Hawking radiation.

Hawking radiation, despite being known to theoretical physics for nearly forty years, remains elusive and undetected. It also suffers, in its original context of gravitational black holes, from practical and conceptual difficulties. Of particular note is the trans-Planckian problem, which is concerned with the apparent origin of the radiation in absurdly high frequencies.In order to gain better theoretical understanding and, it is hoped, experimental verification of Hawking radiation, much study is being devoted to laboratory systems which use moving media to model the spacetime geometry of black holes, and which, by analogy, are also thought to emit Hawking radiation. These analogue systems typically exhibit dispersion, which regularizes the wave behaviour at the horizon at the cost of a more complicated theoretical framework. This tutorial serves as an introduction to Hawking radiation and its analogues, developing the moving medium analogy for black holes and demonstrating how dispersion can be incorporated into this generalized framework.

Hawking's 1974 prediction that black holes radiate and evaporate has been hinting at a hidden connection between general relativity, quantum mechanics and thermodynamics. Recently, laboratory analogues of the event horizon have reached the level where tests of Hawking's idea are possible. In this paper we show how to go beyond Hawking's theory in such laboratory analogues in a way that is experimentally testable.Black-hole evaporation by Hawking radiation has been known to theoretical physics for nearly 40 years [1,2]. Yet it cannot claim to be understood, because Hawking radiation should originate from vacuum modes at frequencies well beyond the Planck scale [3, 4]-well beyond the limit of our physical understanding. Unruh [5] realized that the Hawking effect is not restricted to gravity, but arises for waves in moving media in analogues of the event horizon [6-8], allowing tests of Hawking's prediction [1] by experiments [9][10][11][12][13][14][15][16]. Here the trans-Planckian problem does not appear, because of dispersion [3] that tunes waves out of the horizon's grip. Despite the drastic alteration to wave propagation, Hawking radiation is 5 Both authors contributed equally to this work.

We study the spectrum and entanglement of phonons produced by temporal changes in homogeneous one-dimensional atomic condensates. To characterize the experimentally accessible changes, we first consider the dynamics of the condensate when varying the radial trapping frequency, separately studying two regimes: an adiabatic one and an oscillatory one. Working in momentum space, we then show that in situ measurements of the density-density correlation function can be used to assess the nonseparability of the phonon state after such changes. We also study time-of-flight (TOF) measurements, paying particular attention to the role played by the adiabaticity of opening the trap on the nonseparability of the final state of atoms. In both cases, we emphasize that commuting measurements can suffice to assess nonseparability. Some recent observations are analyzed, and we make proposals for future experiments.