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The exotic physics emerging in non-Hermitian systems with balanceddistributions of gain and loss has drawn a great deal of attention in recentyears. These systems exhibit phase transitions and exceptional pointsingularities in their spectra, at which eigen-values and eigen-modes coalesceand the overall dimensionality is reduced. Among several peculiar phenomenaobserved at exceptional points, an especially intriguing property, withrelevant practical potential, consists in the inherently enhanced sensitivityto small-scale perturbations. So far, however, these principles have beenimplemented at the expenses of precise fabrication and tuning requirements,involving tailored nano-structured devices with controlled distributions ofoptical gain and loss. In this work, anti-parity-time symmetric phasetransitions and exceptional point singularities are demonstrated in a singlestrand of standard single-mode telecommunication fibre, using a setupconsisting of entirely of off-the-shelf components. Two propagating signals areamplified and coupled through stimulated Brillouin scattering, which makes theprocess non-Hermitian and enables exquisite control over gain and loss.Singular response to small variations around the exceptional point andtopological features arising around this singularity are experimentallydemonstrated with large precision, enabling robustly enhanced spectral responseto small-scale changes in the Brillouin frequency shift. Our findings openexciting opportunities for the exploration of non-Hermitian phenomena over atable-top setup, with straightforward extensions to higher-order Hamiltoniansand applications in quantum optics, nanophotonics and sensing.

Observation of anti-parity-time-symmetry, phase transitions and exceptional points in an optical fibre