Observation of Braid-Protected Unpaired Exceptional Points.

Area of Science:

• Quantum physics
• Condensed matter physics
• Photonics

Background:

• Spectral degeneracies, or nodal points, are crucial for exotic light and matter properties.
• No-go theorems typically prevent unpaired band-structure degeneracies in lattice systems (both Hermitian and non-Hermitian).

Purpose of the Study:

• To investigate and leverage the non-Abelian braid topology loophole in non-Hermitian multiband systems.
• To implement and characterize an unpaired third-order exceptional point (EP3).

Main Methods:

• Utilized a non-Hermitian three-band system.
• Implemented a novel single-photon interferometry design for high-resolution spectral and eigenstate analysis.
• Experimentally demonstrated intricate braiding topology and non-Abelian fusion rules.

Main Results:

• Successfully implemented an unpaired third-order exceptional point (EP3), acting as a non-Abelian monopole.
• Explicitly demonstrated the braiding topology and non-Abelian, path-dependent fusion rules associated with the EP3.
• Achieved widely tunable parameters in a multiband system.

Conclusions:

• The study successfully bypasses traditional no-go theorems using non-Abelian braid topology.
• The findings pave the way for exploring exotic non-Abelian topology unique to non-Hermitian physics.
• Highlights the synergy between advanced experiments, theoretical concepts, and braiding principles.