Induced Lorentzian geometry from the fermionic quantum metric
Quantum geometry is conventionally understood in parameter or momentum space and need not be related to a geometry in configuration space. Here we show for Dirac fermions in a background electromagnetic field that an induced Lorentzian geometry is generated from the fermionic quantum metric. We study a Lorentz covariant phase space worldline path integral construction, whose application of the adiabatic theorem onto spin degrees of freedom leads to the appearance of the Berry phase at $O(\hbar)$, and the quantum metric at $O(\hbar^2)$ in the worldline action. Both the Feynman and retarded/advanced propagators are examined in 3+1 and 2+1 dimensions, whose different on-shell implementations lead to related quantum metrics; QED/QED${}_3$ and condensed matter Dirac theory are considered. An effective theory is found by integrating out the momenta degrees of freedom, which gives rise to an emergent Lorentzian metric in configuration space, whose perturbation to the flat metric is dependent on the quantum metric and the background field strength. Apart from the spin degrees of freedom encoded into the Berry phase, the effective theory describes a scalar particle moving in the induced quantum geometry. Phenomenological applications to the covariant geometry are further studied using spectral holonomy in a gapped 3-dimensional Dirac material.