Black holes are famous for swallowing everything that crosses their event horizon—even light. Yet, in 1974, Stephen Hawking made a surprising prediction: Black holes are not completely black [1]. Quantum fluctuations near the event horizon should cause black holes to emit a faint stream of particles.
There is just one problem: observing Hawking radiation from astrophysical black holes is virtually impossible. The expected signal is so weak that even the largest astrophysical black holes produce emissions far below the limit of our current technology. Rather than waiting for the universe to reveal Hawking radiation, we decided to build our own event horizon on an optical table. In our recent work published in Nature Communications [2], our team opened a new frontier in analogue gravity: analogue Hawking radiation stimulated by a single photon.
A decade-long quest
Science rarely moves in a straight line, and this project is a testament to patience. The story began in 2015 at the annual meeting of the Quantum Information Division in Ensenada, Mexico. After my talk on analogue gravity, Professors Karina Garay-Palmett and Alfred U’Ren, who specialise in quantum optics, approached me with a proposal: Could we combine our expertise to study gravitational analogues using quantum optics?
A year later, in 2016, we met at the Institute of Nuclear Sciences (ICN) of UNAM in Mexico City. By the end of that brainstorming session, we had the blueprint for the project that would eventually become this paper. Little did we know that it would take a decade before that initial idea became a published experiment.
The project also became a training ground for students and collaborators [3,4]. In particular, my PhD student Rodrigo Felipe-Elizarraras devoted his doctoral research to overcoming the theoretical and experimental challenges that emerged along the way. His persistence paid off: Rodrigo defended his PhD in February 2024 and now continues this line of research as a postdoctoral researcher at ICN-UNAM. With the active participation of academic technician Hector Cruz-Ramirez, our team across Mexican institutions (Cinvestav, UNAM, and CICESE) slowly turned a theoretical concept into a working experiment.
Bringing the event horizon to the lab
The concept of gravitational analogues dates back to 1981, when William Unruh [5] realised that sound waves moving through a supersonic fluid behave similarly to light trapped by a black hole. Since then, researchers have adapted this concept to Bose-Einstein condensates [6], water tanks [7], microcavity polaritons [8], superconducting circuits [9], and optical fibres [10].
However, all previous optical experiments operated in the classical regime [4,10,11,12], relying on intense, high-power pulses of light. We wanted to push this idea into the quantum realm. Using a specialised photonic-crystal fibre (PCF), we designed an experiment in which the same pump pulse generated the quantum seed and produces the optical horizon. We generated correlated photon pairs using a quantum process called spontaneous four-wave mixing (SFWM). Detecting one photon served as a precise signal, allowing us to prepare clean single-photon states.
The intense pump also created a moving refractive-index gradient that served as an optical analogue of the event horizon of a black hole. When the single photon encounters this optical horizon, two outcomes become possible: it can pass through, or stimulate an analogue Hawking process, producing a frequency-shifted photon—the analogue Hawking photon.
We measured these subtle frequency shifts using single-photon detectors. The biggest challenge turned out not to be creating the optical horizon, but detecting the faint Hawking signal and proving its genuine quantum properties. Early versions of the experiment consistently buried the single-photon signal beneath noise from the intense pump, forcing us to rethink both the theoretical proposal and the detection strategy. More than once we thought we were close, only to discover that an apparently promising signal had a completely classical explanation.
The breakthrough came when we realised that using a single pump in a cross-polarised configuration—with the pump in one polarisation and the photons in the orthogonal polarisation—dramatically suppressed the background noise. Finally, by analysing photon-number correlations, we verified that the generated signal retained the quantum correlations characteristic of single-photon states. Rather than demonstrating another classical wave analogue, we observed a quantum analogue of Hawking radiation stimulated by a single photon.
From the stars to the optical table
By connecting extreme astrophysics with everyday laboratory equipment, this work opens up exciting new possibilities. Our platform allows researchers to study quantum properties—such as entanglement, quantum correlations, and energy transfer—that are inaccessible in real astrophysical environments.
Looking back, what stands out most is not only the result but the journey itself. An idea sketched during a conference conversation in 2015 evolved through years of calculations, redesigns, setbacks, and experimental refinements before finally becoming reality. Along the way, it trained students, brought together researchers from different institutions, and reminded us that answering important scientific questions often takes time. In the end, that conversation led to a platform that lets us explore one of nature's deepest mysteries using little more than a laser, a specialised optical fibre, and a single photon.
Read the full paper in https://www.nature.com/articles/s41467-026-73812-8
References:
[1] S. W. Hawking. Black hole explosions? Nature 248, 30 (1974)
[2] R. Felipe-Elizarraras, H. Cruz-Ramirez, K. Garay-Palmett, A. U’Ren, D. Bermudez. Measurement of analogue Hawking radiation stimulated by a single photon, Nature Communications 17, 7012 (2026)
[3] D. De la Torre‑Robles, F. Dominguez‑Serna, G. L. Osorio, A. U’Ren, D. Bermudez, K. Garay‑Palmett. Frequency and polarization emission properties of a photon‑pair source based on a photonic crystal fiber, Scientific Reports 11, 18092 (2021)
[4] R. Felipe-Elizarraras, H. Cruz-Ramirez, K. Garay-Palmett, A. U’Ren, D. Bermudez. Effective Michelson interference observed in fiber-optical analogue of Hawking radiation, Optics Express 30, 8063 (2022)
[5] W. G. Unruh. Experimental black-hole evaporation? Physical Review Letters 46, 1351 (1981)
[6] J. R. Munoz de Nova, K. Golubkov, V. I. Kolobov, J. Steinhauer. Observation of thermal Hawking radiation and its temperature in an analogue black hole, Nature 569, 688 (2019)
[7] S. Weinfurtner, E. W. Tedford, M. C. J. Penrice, W. G. Unruh, G. A. Lawrence. Measurement of stimulated Hawking emission in an analogue system, Physical Review Letters 106, 021302 (2011)
[8] K. Falque, A. Delhom, Q. Glorieux, E. Giacobino, A. Bramati, M. J. Jacquet. Polariton fluids as quantum field theory simulators on tailored curved spacetimes, Physical Review Letters 135, 023401 (2025)
[9] Y.-H. Shi, R.-Q. Yang, Z. Xiang, Z.-Y. Ge, H. Li, Y.-Y. Wang, K. Huang, Y. Tian, X. Song, D. Zheng, K. Xu, R.-G. Cai, H. Fan. Quantum simulation of Hawking radiation and curved spacetime with a superconducting on-chip black hole, Nature Communications 14, 3263 (2023)
[10] T. G. Philbin, C. Kuklewicz, S. Robertson, S. Hill, F. Konig, U. Leonhardt. Fiber-optical analog of the event horizon, Science 319 1367 (2008)
[11] J. Drori, Y. Rosenberg, D. Bermudez, Y. Silberberg, U. Leonhardt. Observation of stimulated Hawking radiation in an optical analogue, Physical Review Letters 122, 010404 (2019)
[12] L. M. Procopio, R. Aguero-Santacruz, D. Bermudez, U. Leonhardt. Backreaction of stimulated Hawking radiation in an optical analogue, Nature 655, 336 (2026)