Berry phase and quantum structure

• The minimal structure of quantum theory is identified as a fiber bundle.
• The quantum bundle is a U(1) bundle over the projective Hilbert space.
• The Berry phase is a geometrical holonomy of the quantum bundle.
• It is neither brought about by forces nor by the geometry of spacetime.
• Structural realism about the quantum bundle is proposed.

The paper aims to spell out the relevance of the Berry phase in view of the question what the minimal mathematical structure is that accounts for all observable quantum phenomena. The question is both of conceptual and of ontological interest. While common wisdom tells us that the quantum structure is represented by the structure of the projective Hilbert space, the appropriate structure rich enough to account for the Berry phase is the U(1) bundle over that projective space. The Berry phase is ultimately rooted in the curvature of this quantum bundle, it cannot be traced back to the Hamiltonian dynamics alone. This motivates the ontological claim in the final part of the paper that, if one strives for a realistic understanding of quantum theory including the Berry phase, one should adopt a form of ontic structural realism.