Cavity-less Brillouin strong coupling demonstrated in optical fibre
Researchers achieved strong coupling between light and sound in a standard optical fibre at cryogenic temperatures without using an optical cavity, enabling new quantum signal processing applications.
Strong coupling between optical photons and travelling acoustic phonons has been achieved in a single pass through an optical waveguide. This cavity-less breakthrough, using Brillouin-Mandelstam scattering at 4 kelvin, opens a path to quantum signal processing in everyday fibre optics.
Researchers observed normal-mode splitting and avoided crossing in a highly nonlinear fibre at 4 K using pump-probe detection. The technique involved forced detuning via a pump-probe experiment combined with spectral detection. A low-power anti-Stokes probe with a specific frequency offset was used, with lock-in amplifier detection to increase the signal-to-noise ratio. As pump power increased, the optomechanical coupling strength grew, pushing the system into the strong coupling regime.
| Parameter | Value at Low Power | Value at Higher Power |
|---|---|---|
| Coupling strength (g_om) | 8 ± 3 MHz | 61 ± 2 MHz, then 75 ± 2 MHz |
| Maximum normal-mode split (2δ_split) | Not observed | 26.7 MHz |
| Anti-Stokes resonance (Ω_B/2π) | 9.5997 ± 0.0001 GHz | - |
The normal-mode splitting became observable when the coupling strength exceeded approximately 11 MHz, which is half the sum of the optical and mechanical linewidths. The shape of the split peaks was asymmetric, with outer slopes following a Lorentzian profile and inner slopes being sharper. The experimental results were confirmed by analytical theory on dynamic continuum quantum optoacoustics. An alternative measurement technique uses thermal-noise-initiated measurement with spectrometer detection of the back-reflected scattered signal.
Material and optical properties enable low-loss interaction
The germanium-doped fibre exhibits low optical loss, high Brillouin gain at cryogenic temperatures, and reduced mechanical dissipation. Its core-cladding structure is similar to standard single-mode fibre but with higher germanium doping in the core. The fibre's exceptional properties at 4 kelvin are critical for the observed effects.
| Property | Room Temperature (293 K) | Cryogenic (4 K) |
|---|---|---|
| Optical loss | 0.001 dB m⁻¹ | - |
| Brillouin gain coefficient | 1.41 ± 0.06 W⁻¹ m⁻¹ | 6 ± 1 W⁻¹ m⁻¹ |
| Mechanical dissipation rate (Γ_m) | 46.3 ± 0.2 MHz | 9.93 ± 0.12 MHz |
The optical dissipation rate was 13 ± 2 MHz. The fibre could handle pump powers approaching 2 watts, leading to a maximum estimated cooperativity of 179. The thermal phonon occupation was of the order of ten phonons, with an acoustic frequency of nearly 9.6 GHz. The experiment used a laser wavelength of 1550 nanometres.
Theoretical framework and coupling dynamics confirmed
The coupling strength follows a theoretical model and increases with pump power, transitioning from weak to strong coupling. The relationship is given by g_om ≈ √[G_B Γ_m P_pump c / (4n)], where c is the speed of light in vacuum and P_pump is the pump power. At weak coupling, around 4 MHz, the probe was barely depleted and no splitting was observed. The authors estimate that 5 watts of pump power could achieve quantum coherent coupling over the full acoustic bandwidth.
Heating-induced Brillouin frequency shifts were negligible due to automatic phase matching in the waveguide. Pump depletion could be further mitigated using lower-loss materials like chalcogenide fibres. The interaction is broadband across the silica glass transmission window, unlike the narrow resonances of optical cavities.
Potential for quantum technologies
The platform enables quantum manipulations in common optical fibre without bandwidth restrictions or dependence on fixed resonances. This may pave the way for high-capacity quantum communication systems via quantum signal processing of multiple lines in a frequency comb. Specific proposals include in-line quantum memories, repeaters, transduction, non-classical state preparation, and entanglement distribution. A possible link to neuromorphic computing, where quantum neural networks are controlled by acoustic vibrations, has also been suggested.
The research was conducted at the Max Planck Institute for the Science of Light. One of the researchers involved said, "We show the splitting of the optoacoustic spectral response and introduce a technique to measure the avoided crossing of hybrid optoacoustic modes via forced detuning." The study, by Laura Blázquez Martínez, Changlong Zhu, and Birgit Stiller, is published in Nature Physics. The platform enables quantum manipulations in common optical fibre without bandwidth restrictions or dependence on fixed resonances.
