Light and Sound Reach Strong Coupling in an Ordinary Fibre Without Any Cavity

Physicists have long relied on optical cavities to make light and matter exchange information coherently. Mirrors bounce photons back and forth millions of times, letting the light build up enough intensity to couple strongly to a mechanical vibration before the whole system decoheres. Now, a team at the Max Planck Institute for the Science of Light has shown that this critical regime can be reached in an optical fibre after a single pass of light through the waveguide, with no cavity at all. Writing in Nature Physics, Laura Blázquez Martínez, Changlong Zhu and Birgit Stiller report strong coupling between optical photons and travelling acoustic phonons in a highly nonlinear fibre cooled to cryogenic temperature, demonstrating that the waveguide equivalent of an avoided crossing can be observed in a solid-state continuum of modes.

Strong coupling is the regime in which two oscillators swap energy coherently before either one loses it to dissipation. In quantum electrodynamics, it is the gateway to coherent information transfer between light and matter, and it underpins applications ranging from molecular chemistry and condensed matter physics to atomic clocks, nanoscience, photonics and biomedicine. In optomechanics, where electromagnetic radiation couples to mechanical vibrations, strong coupling has been achieved with micromechanical resonators, Bose–Einstein condensates and membranes, enabling the generation of non-classical mechanical states, quantum-enhanced sensing and control of phonon thermodynamics. Until now, however, every experimental demonstration of Brillouin-based strong coupling shared one feature: the light circulated inside a cavity, gaining effective interaction time with each round trip.

The effect exploited here is Brillouin–Mandelstam scattering, the interaction between light and travelling acoustic waves that has recently found uses in sensing, lasing and integrated photonics. In backward scattering, an optical pump generates anti-Stokes photons that are scattered by counter-propagating thermal acoustic phonons. A key property of this process in a waveguide is automatic phase matching: the pump is always resonant with a subset of the acoustic continuum, described by a Lorentzian gain spectrum centred at the Brillouin frequency, so the system is always in resonance as long as the pump wavelength lies within the optical transparency window of the material. Shifting the pump wavelength merely shifts which acoustic modes are addressed, changing the measured resonance frequency according to the acoustic velocity and the refractive index.

Reaching strong coupling without the power enhancement of a cavity demands an unusually favourable balance of gain, optical power handling and dissipation rates. The team chose a highly nonlinear fibre, similar in structure to a single-mode fibre but with a higher germanium doping in the core, and placed it in a 4-kelvin cryostat. Cryogenic temperature works in their favour in two ways. The Brillouin gain coefficient rises from about 1.41 watts to the minus one per metre at room temperature to roughly 6 watts to the minus one per metre at 4 kelvin, while the mechanical dissipation rate to the thermal bath falls from about 46 megahertz to under 10 megahertz. Combined with the fibre’s low optical loss, giving an optical dissipation rate of 13 megahertz, and its ability to handle pump powers approaching 2 watts, these material changes let the coupling strength exceed the average linewidth of the photons and phonons, the defining condition for normal-mode splitting.

The first experimental signature came from a pump–probe measurement. A high-power pump excites the Brillouin resonance, and a low-power anti-Stokes probe, counter-propagating and slightly detuned, sweeps across it. Because the anti-Stokes process acts as a phononic loss channel, the probe is depleted as it traverses the resonance, and the transmitted power versus detuning maps out the spectral response. At low pump power, the coupling strength was about 8 megahertz and the anti-Stokes resonance showed the familiar single Lorentzian centred at 9.5997 gigahertz. As the pump power increased, the coupling grew to 61 megahertz and then 75 megahertz, and the single resonance split into two peaks, reaching a maximum measured separation of 26.7 megahertz. The two peaks are asymmetric, with Lorentzian outer slopes and sharper inner slopes, a profile the authors carefully distinguish from unrelated electromagnetically induced transparency-like effects in Brillouin amplifiers.

Theoretically, the system is described by continuum quantum optoacoustics. At each position along the one-dimensional propagation direction, envelope bosonic operators are constructed for the anti-Stokes optical wave and the acoustic wave, integrating over the wave-number modes of the continuum. In the undepleted pump regime, the interaction Hamiltonian linearizes to a beam-splitter form with a pump-enhanced coupling strength that generates state swapping between photon and phonon modes. Normal-mode splitting becomes observable when the coupling exceeds half the sum of the optical and mechanical linewidths, about 11 megahertz in this experiment, at which point the resonance shifts to those of the hybrid optoacoustic system. The observed splitting is the waveguide equivalent of the avoided crossing familiar from cavity optomechanics, and the analytical model shows good agreement with the measured spectra.

Observing the avoided crossing itself required a new measurement technique. Standard Brillouin detection methods always measure the resonant backscattering spectrum, because each probe detuning corresponds to a single transmitted power. The team introduced a forced detuning scheme that combines the frequency scanning of a pump–probe experiment with spectral detection, allowing off-resonance spectra to be measured. A 1550-nanometre laser is divided into local oscillator, pump and probe paths; the probe is sinusoidally modulated to generate a sideband swept around the Brillouin resonance, while the pump is amplified to bring the system into the strong coupling regime. Heterodyne detection on a photodiode connected to an electrical spectrum analyser then records the anti-Stokes response.

The forced detuning measurement revealed the avoided crossing directly. As the low-power probe swept across the strongly coupled resonance, it was heavily depleted, confirming the anti-Stokes origin of the signal, and energy was transferred from the upper-frequency hybrid mode to the lower-frequency one as different off-resonance modes were probed. In the weak coupling regime, with a coupling strength of only about 4 megahertz, the probe was barely depleted, no splitting appeared and no avoided crossing was visible, exactly as the theory predicts. Closer inspection showed the expected adiabatic behaviour: for a red-detuned probe, more energy resided in the higher-frequency mode; at resonance, the two modes carried equal energy, producing symmetric peak heights; and for a blue-detuned probe, the energy transferred to the lower-frequency mode. Increasing the pump power broadened the gap between the hybrid modes at the avoided crossing, in quantitative agreement with the model.

The maximum cooperativity achieved, a figure of merit comparing coherent exchange to dissipation, was estimated at 179, and the thermal phonon occupation was of the order of ten phonons, thanks to the high acoustic frequency of nearly 9.6 gigahertz and the low sample temperature. The authors calculate that raising the pump power to around 5 watts could push the system into the quantum coherent coupling regime over the whole bandwidth of addressed acoustic modes, where the coupling strength exceeds all thermal decoherence rates. Heating-induced shifts of the Brillouin frequency can be neglected because of the automatic phase matching to the quasi-continuum of acoustic modes, an advantage over fixed-frequency optomechanical cavities. Pump depletion, which could limit access to the quantum regime, might be circumvented with samples of lower optical dissipation or lower acoustic loss, such as chalcogenide fibres, which could realize non-classical state transfer between photons and phonons at a moderate cryogenic temperature of several kelvin.

The broadband nature of the interaction is what sets this demonstration apart. Unlike the narrow optical resonances of a high-quality-factor cavity, the fibre system is always in resonance across the transmission window of silica glass, allowing the processing of frequency combs and high-dimensional entangled photons without considerable crosstalk or restrictions on frequency spacing. Strong coupling over a wide bandwidth of continuum modes, rather than between single or multiple discrete resonances, opens applications in quantum information processing, entanglement distribution, quantum computing and precision measurements. The team specifically points towards single-pass, in-line continuous-variable quantum signal processing and storage, in-line quantum memories and repeaters, quantum transduction and non-classical state preparation. Coupled with recent advances in neuromorphic computing, the findings may even enable a new class of quantum neural networks controlled by acoustic vibrations, establishing waveguide platforms as serious candidates for the next generation of quantum technologies.

Subject of Research: Cavity-less Brillouin strong coupling in a solid-state continuous system

Article Title: Cavity-less Brillouin strong coupling in a solid-state continuous system

Article References: Blázquez Martínez, L., Zhu, C., & Stiller, B. (2026). Cavity-less Brillouin strong coupling in a solid-state continuous system. Nature Physics. https://doi.org/10.1038/s41567-026-03458-2

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03458-2

Keywords: Cavity-less, Brillouin, strong, coupling, solid-state, continuous, system, scientific research