Exceptional points appear in non-Hermitian systems as degeneracies, where not only eigenvalues but also eigenvectors coalesce. They are of great theoretical and experimental interest due to their exotic topological properties and enhanced sensitivity to perturbations. Experimental realizations of higher-order exceptional points, where more than two eigenvectors coalesce, rely on highly fine-tuned setups. Recently, stimulated Brillouin scattering has been employed to generate second-order exceptional points in a fabrication-free setup by leveraging off-resonant scattering. In this work we generalize this approach, and we develop an off-resonant, multimode theory for stimulated Brillouin scattering as an avenue towards realizing symmetry-induced exceptional points of any order. We present the experimental implementation of our program in an accompanying paper. Our multimode theory could also be employed in applications in optoacoustic sensing, synthetic neuromorphic computing, microwave photonic filters, and optoacoustic quantum signal processing.
Multi-dimensional parameter space of higher-order exceptional points induced by Brillouin optoacoustics
Grigorii Slinkov,
Anton Montag,
Julius Gohsrich,
Quentin Levoy,
Paulina Fuentes Rivera,
Flore K. Kunst,
Birgit Stiller
Exceptional points (EPs) are degeneracies in the spectrum of non-Hermitian systems, where both the eigenvalues and eigenvectors coalesce. In the vicinity of an n-th order EP, the eigenvalues generally show n-th-root dependence on the system parameters, making EPs potentially promising candidates for ultra-sensitive measurements. Usually EPs are implemented in precisely fabricated nano- and microstructures. In this work, we instead show the experimental implementation of a third-order EP (EP3) using the synthetic dimension in a single-mode optical fiber, leveraging multi-frequency Brillouin scattering. We perform a multi-dimensional scan of the parameter space revealing not only an EP3 but also additional topological structures connected to it. Our work paves the way toward fabrication-free realizations of exceptional points of arbitrary order.
Brillouin–Mandelstam scattering-based cooling of traveling acoustic waves from cryogenic temperatures
Lisa Fischer,
Laura Blázquez Martínez,
Changlong Zhu,
Robin Chenevière,
Johann Troles,
Birgit Stiller
Thermal phonons are a major source of decoherence in quantum mechanical systems. Operating in the quantum ground state is therefore often an experimental prerequisite. In addition to passive cooling in a cryogenic environment, active laser cooling enables the reduction of phonons at specific acoustic frequencies. Brillouin cooling has been used to show efficient reduction of the thermal phonon population in waveguides at GHz frequencies down to 74 K. In this Letter, we demonstrate the cooling of a 7.608 GHz acoustic mode by combining Brillouin active cooling with precooling from 77 K using liquid nitrogen. We show a 69% reduction in the phonon population, resulting in a final temperature of 24.3 +/- 1.9 K, 50 K lower than previously reported.
Contact
Research Group Birgit Stiller
Max Planck Institute for the Science of Light Staudtstr. 2 91058 Erlangen, Germany
