Historical research guide · 0498 · 2026
Backreaction of stimulated Hawking radiation in an optical analogue
Lorenzo M. Procopio · Raul Aguero-Santacruz · David Bermudez · Ulf Leonhardt
In one minute
Hawking’s 1974 result says the horizon of a black hole should glow, and almost nobody expects to catch that glow in the sky. Ulf Leonhardt’s group builds horizons in glass instead. A very short, very bright laser pulse running down an optical fibre raises the local refractive index and slows the light behind it, so a weaker probe pulse that catches up can never pass — an event horizon made of light. Procopio, Aguero-Santacruz, Bermudez and Leonhardt report two things. First they identify the actual mechanism: one simple pair-creating term in the interaction, rather than the cascade of steps the field had assumed, turns pump photons into a Hawking pair, one partner in the infrared and its negative-frequency twin deep in the ultraviolet at 233 nanometres. Second, they measure the kick back onto the pump. The Hawking signal grows with probe power, the backreaction with its square, and both carry the same temperature to within two percent.
Connection
This is the vacuum working as a fluid: a laboratory horizon that is not a metaphor but the same wave equation as curved spacetime, with quanta drawn out of the vacuum and the energy books balanced against the field that made the horizon. It also names the mechanism, which is what chapter 4 needs from any analogue — not that something happened, but which term in the interaction did it.
From the source
Hawking radiation in the fibre-optical analogue is created directly by one simple pair-creation Hamiltonian proportional to the pump intensity times the square of the probe amplitude plus its conjugate, and not by the complicated cascaded process previously believed.Main text, Eq. (3) and the two paragraphs following it; Methods Sec. B
Because that Hamiltonian is proportional to the pump intensity it does not change the total photon number of the pump field but redistributes pump photons to a different frequency, so the energy of the Hawking pair comes from the field that establishes the horizon.Main text, backreaction paragraph; Methods Sec. B, Eq. (B.3)
The backreaction of the stimulated Hawking radiation onto the pump was observed as an asymmetry of the spectral sidebands, shifted further into the ultraviolet by 0.5 to 1 nanometre in wavelength, and it grows with the square of the probe power while the Hawking radiation grows linearly.Main text, paragraphs on sidebands and asymmetry; Figs. 3 and 4
Hawking radiation and its backreaction both show thermal spectra at the same temperature: straight-line fits with relative root-mean-squares of 0.015 and 0.010 and a slope ratio of 1.02, equal within 2 percent, even in the regime of strong dispersion where the notion of the event horizon loses sense.Main text, thermality paragraph; Fig. 5; Methods Sec. D, Eqs. (D.2) and (D.3)
The interaction that makes Hawking radiation is bi-quadratic, the same structure that appears in Bose-Einstein condensates and, through the tetrad formalism in which the metric tensor is written as a quadratic form, in gravity itself — which suggests astrophysical black holes may radiate by an equally direct process and that tetrads may be the right starting point for an effective theory of quantum gravity.Main text, closing paragraphs; Methods Sec. E
The measurement is of classical stimulated Hawking radiation, not the fully quantum spontaneous emission; a systematic study of signal against probe power, requiring acquisition runs of up to 12 hours, and probes made of heralded single photons would settle the interpretation and test the entanglement of the Hawking partners.Main text, paragraph beginning ‘In the absence of a systematic study’; closing paragraph on nonclassical light
The way in
https://doi.org/10.1038/s41586-026-10720-3How to cite it
Lorenzo M. Procopio, Raul Aguero-Santacruz, David Bermudez, Ulf Leonhardt (2026) Backreaction of stimulated Hawking radiation in an optical analogue doi:10.1038/s41586-026-10720-3