Closed-loop Berry phase
For an isolated normalized eigenstate taken adiabatically around a closed parameter loop C, the geometric phase is γ_n(C)=∮_C A_n(R)·dR modulo 2π, with A_n=i⟨n(R)|∇_R n(R)⟩.
Key knowledge
For an isolated normalized eigenstate taken adiabatically around a closed parameter loop C, the geometric phase is γ_n(C)=∮_C A_n(R)·dR modulo 2π, with A_n=i⟨n(R)|∇_R n(R)⟩. It depends on the path in parameter space and survives changes of traversal speed that keep the evolution adiabatic.
Values and equations
A_n(R)=i⟨n|∇_R n⟩; γ_n(C)=∮_C A_n·dR (mod 2π); A_n′=A_n−∇χ
- R,C
- Control parameters (component-dependent units) and closed path
- A_n
- Berry connection; reciprocal units of each parameter component
- γ_n,χ
- Dimensionless geometric/eigenstate gauge phases
- |n⟩
- Normalized instantaneous eigenstate
Conditions and scope
- Nondegenerate eigenline with a nonzero gap on the path, smooth phase choices patched consistently and the initial/final Hamiltonian parameters identified. Parameter components can have different physical units; A has reciprocal component units.
- Under |n⟩→e^(iχ)|n⟩, A→A−∇χ; a closed-loop phase is invariant modulo 2π when endpoint/patch phases are accounted for. A bare open-path integral is gauge dependent, and degenerate-subspace non-Abelian transport is outside this scalar formula.
Sources · 1
References
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Evidence and review
Quantum Physics III: chapter 6, adiabatic approximation (2017 notes, 2018 course): §5 equations 5.1–5.11, 5.14–5.18 and 5.22–5.23, printed pages 12–14: geometric connection, phase and closed-loop gauge invariance. Source uses ket phase e^(−iβ); χ=−β yields the stated transformation convention.
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