Historical research guide · 0345 · 2016
Observation of quantum Hawking radiation and its entanglement in an analogue black hole
Jeff Steinhauer
In one minute
Jeff Steinhauer built a black hole out of sound. He chilled a cloud of rubidium atoms into a Bose-Einstein condensate — a state in which the whole cloud behaves as a single quantum wave — and swept a sharp step of laser light along it so that on one side the atoms flow faster than their own speed of sound. Sound cannot swim back out of that region, exactly as light cannot climb out of a black hole, and the flow speed together with the sound speed fixes the metric of an analogue spacetime. Hawking predicted that a horizon like this glows: the vacuum’s own fluctuations are split at the edge into a particle that escapes and a partner that falls in. Steinhauer measured that pair. From 4,600 repetitions over six days of continuous running he recovered a band of correlations linking points on opposite sides of the horizon, a Hawking temperature near 1.2 nanokelvin, and, for the short-wavelength pairs, genuine quantum entanglement between the escaping particle and its partner inside.
Connection
Chapter 5 reads the vacuum as a quantum fluid rather than a backdrop, and this is the laboratory result that puts hardware under that reading: a flowing condensate really does carry a spacetime metric for the waves inside it, and the vacuum fluctuations of that medium really do turn into real, entangled particles at a horizon. It is the site’s strongest bench evidence that ‘the metric is a property of a medium’ is an experimental statement and not only a picture.
From the source
Sound in a flowing Bose-Einstein condensate obeys the same equation as a field in curved spacetime, so the local flow velocity and the local speed of sound together determine the metric of an analogue spacetime, and a region flowing supersonically becomes a horizon that phonons cannot escape.Introduction, following Unruh’s 1981 proposal; section The experimental system
Spontaneous Hawking radiation stimulated by quantum vacuum fluctuations is observed emanating from the analogue horizon: the two-body density correlation function computed from an ensemble of 4,600 repetitions, requiring six days of continuous measurement, shows a dark band of correlations between points outside and inside the black hole at equal propagation times from the horizon.Section Observation of Hawking radiation; Figure 4a
The horizon is created by a potential step swept at constant speed, giving a flow of 0.24 mm per second against a sound speed of 0.57 mm per second outside the horizon, and a flow of 1.02 mm per second against a sound speed of 0.25 mm per second inside it — subsonic outside, supersonic inside, with the step narrower than the condensate’s healing length of 2.0 micrometres.Section The experimental system; Figure 2
The measured Hawking temperature is 0.36 times the quantity m c-out squared in units of Boltzmann’s constant, equivalent to 1.2 nanokelvin, slightly above the predicted approximate maximum range of 0.25 to 0.32, and the measured phonon population outside the horizon agrees with the thermal distribution at that temperature.Section Measuring the population of the Hawking radiation; Figure 5a and 5b
The high-energy Hawking pairs are entangled: the nonseparability measure of Equation 3, equivalent to the Peres-Horodecki criterion, is negative over a broad range of wavenumbers, which verifies the quantum nature of the radiation, while the long-wavelength pairs are not entangled — and the driven oscillating-horizon experiment shows classical correlations, as expected.Section Measuring the entanglement of the Hawking pairs; Equation 3; Figures 6a and 6b
Two numbers are left open for the next experiment: the observed correlations at long wavelengths fall below the thermal prediction by a factor of roughly 3.6, which the author suggests may mean the low-frequency waves had insufficient time to form; and the entanglement result rests on the stated assumption that excitations of different frequencies are not correlated.Abstract; discussion following Equation 1 and Equation 3; Figure 6a
The way in
https://arxiv.org/abs/1510.00621How to cite it
Jeff Steinhauer (2016) Observation of quantum Hawking radiation and its entanglement in an analogue black hole doi:10.1038/nphys3863