Extrapolation and emulation techniques for few-body resonances

Quantum resonances, i.e., metastable states that decay over time, are a fascinating phenomenon relevant in different areas of physics. In nuclear physics, they not only appear as excited states of various atomic nuclei, but they can also constitute the ground state of exotic nuclei near the edges of nuclear stability. Characterized by being strongly coupled to the continuum, resonances are notoriously challenging to describe theoretically and to com­pute numerically, especially when they appear as genuine few-body states. This article summarizes how the complex-augmented eigenvector continuation (CA-EC) method can be used to track resonance states as the underlying interaction governing them is varied. In particular, CA-EC is able to robustly extrapolate states all the way from being bound to becoming resonances. Such extrapola­tions were originally demonstrated to work well for two-body states, and recent work established that this remains true for few-body systems. In particular, CA-EC can be combined with different techniques used to construct non-Hermitian Hamiltonians, and it therefore provides a path towards scalable resonance cal­culations for nuclear many-body systems.