Physics:Quantum Berry phase

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Quantum Berry phase is the geometric part of the phase acquired by a quantum state when the parameters of a system are carried around a closed path. It is a special case of the more general geometric phase: a phase shift determined by the path through parameter space rather than only by the time elapsed or the energy of the state.

The effect is central to quantum dynamics because it shows that the history of a state can be physically visible even when the system returns to the same observable configuration. In interference experiments, two states with the same local energy history can still differ by a phase set by the geometry and topology of the route they followed.

Geometric phase

A wave is described by both amplitude and phase. If the external parameters of a wave system are changed slowly and then returned to their initial values, the amplitude may come back to its starting form while the phase does not. The remaining phase difference is the geometric phase.

In quantum mechanics this occurs most cleanly during adiabatic evolution. A Hamiltonian changes slowly enough that a system prepared in one instantaneous eigenstate remains in the corresponding eigenstate. The final state then contains two conceptually different phase factors:

• a dynamical phase, determined by the energy and elapsed time;
• a geometric phase, determined by how the eigenstate changes along the path in parameter space.

For a closed loop, the geometric contribution cannot in general be removed by redefining the local phase convention of the eigenstates. It becomes a gauge-invariant observable modulo $2\pi$.

Berry phase

Michael Berry's 1984 formulation made the quantum geometric phase a general tool for adiabatic cyclic evolution. Earlier related ideas appeared in S. Pancharatnam's work on classical optics and in molecular physics through studies of conical intersections.

If a Hamiltonian $H(𝐑)$ depends on slowly varying parameters $𝐑$, an eigenstate $|n(𝐑)⟩$ transported around a closed curve $C$ can acquire the Berry phase

$${\gamma }_n(C)=i{\oint }_C⟨n(𝐑)|{\mathrm{\nabla }}_{𝐑}n(𝐑)⟩\cdot d𝐑.$$

This expression emphasizes that the result depends on the geometry of the path in parameter space. The integrand is often called the Berry connection, and its curl is the Berry curvature.

Physical examples

The geometric phase appears in several quantum and wave phenomena:

• the Aharonov-Bohm effect, where an enclosed magnetic flux changes the phase of charged-particle wavefunctions;
• molecular conical intersections, where electronic and nuclear coordinates create singular structures in parameter space;
• polarization optics, where Pancharatnam's phase describes phase changes of polarized light;
• band theory of solids, where Berry curvature helps describe anomalous velocity, topological bands, and quantum Hall physics.

The common feature is a loop in parameter space around a singularity, degeneracy, or topological obstruction. The phase is therefore not just a dynamical delay, but a record of the route taken by the state.

Relation to topology

Berry phase is closely related to parallel transport, holonomy, and gauge structure. A useful analogy is transporting a vector around a curved surface: after returning to the starting point, the vector may point in a different direction because the surface is curved. In quantum mechanics the transported object is a phase convention for the state vector, and the measurable change is a phase shift.

This connection makes Berry phase important in modern condensed matter physics. Berry curvature in momentum space is used to describe electronic bands, topological phases of matter, and robust transport effects.

See also

• Physics:Quantum Time evolution
• Physics:Quantum Adiabatic theorem
• Physics:Quantum Aharonov-Bohm effect
• Physics:Quantum Hall effect
• Physics:Quantum Topological phases of matter

References

Author: Harold Foppele

Source attribution: Physics:Quantum Berry phase