6.2 Bose–Einstein condensate experiments

We have already extensively discussed the theoretical aspects of Bose–Einstein-condensate–based analogue models. Regarding the actual experimental possibility of generating BEC-based acoustic black holes, several options have been envisaged in the literature. In particular, commonly proposed settings are long thin condensates in a linear or circular trap [231, 232] as well as Laval-nozzle–shaped traps [48, 539, 228, 476]. However, it is only very recently that an experiment aimed at the formation of a sonic black hole in a BEC has been set up, and the creation of a sonic horizon has been convincingly argued for [369Jump To The Next Citation Point]. In this case, a sonic horizon was achieved by a counterintuitive effect of “density inversion”, in which a deep potential minimum creates a region of low density, as would a potential maximum. This low density region corresponds to a slower-than-normal speed of sound, and hence to the possibility for the flow speed to exceed the speed of sound and generate sonic horizons at the crossing points (where the speed of the flow and that of sound coincide). The density inversion is achieved by overlapping a low-frequency (broad) harmonic potential and a high-frequency (narrow) Gaussian potential generated via an elongated laser. In this manner a sonic black hole was generated, and kept stable for about 8 ms. The Hawking radiation predicted for the system as realised has a temperature of about 0.20 – 0.35 nK; unfortunately, one order of magnitude smaller than the lowest temperature allowed by the size of the system. (Lower temperatures for the condensate permit longer-wavelength characteristic Hawking quanta, which must still fit into the condensate. So there is a trade-off between Hawking temperature and physical size of the condensate.) However, higher densities could allow one to increase the Hawking temperature, and TH ≈ a few nK seems within experimental reach. Given that 8 ms would correspond to one cycle of 6 nK Hawking radiation, it appears that increases in TH together with amelioration of the lifetime of the sonic black hole might put the detection of the analogue spontaneous quantum Hawking effect within experimental reach (via correlation experiments) in the near future.
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