Quantum Channel-Induced Geometry of the Uhlmann Phase in Qubit Systems

Abstract

We show that a cyclically controlled non-coherence-generating channel supplies a two-parameter family of closed Bloch trajectories, absent in the single-parameter (temperature) cycles of the thermal Uhlmann literature, whose associated Uhlmann phase develops a genuine vortex--antivortex structure on the channel-parameter torus. A qubit prepared in a pure state, which carries no geometric phase of its own, acquires a nontrivial Uhlmann phase, obtained here in closed form, purely from a closed loop in the space of the native quantum channel parameters. The net topological charge of these defects is constrained to zero by the Poincar\'e--Hopf theorem. As the input-state orientation approaches its critical values, the defects merge and annihilate in pairs following a square-root coalescence law. This defect dynamics drives local geometric transitions, a mechanism that contrasts with single-parameter thermal Uhlmann transitions, where a global quantized winding jumps as a bulk invariant.