Geometric and topological phases

Adrien Bouhon, Tomáš Bzdušek, Robert-Jan Slager

Saikat Mondal, Subrata Pachhal, Adhip Agarwala

Transition out of a topological phase is typically characterized by discontinuous changes in topological invariants along with bulk gap closings. However, as a clean system is geometrically punctured, it is natural to ask the fate of an underlying topological phase. To understand this physics we introduce and study both short- and long-ranged toy models where a one-dimensional topological phase is subjected to bond percolation protocols. We find that non-trivial boundary phenomena follow competing energy scales even while the global topological response is governed via geometrical properties of the percolated lattice. Using numerical, analytical, and appropriate mean-field studies we uncover the rich phenomenology and the various crossover regimes of these systems. In particular, we discuss the emergence of the ``fractured topological region'' where an overall trivial system contains a macroscopic number of topological clusters. Our study shows the interesting physics that can arise from an interplay of geometrical disorder within a topological phase.

Antônio Sérgio Teixeira Pires

Péter Lévay

Vikash Mittal

Geometric phase plays a fundamental role in quantum theory and accounts for wide phenomena ranging from the Aharanov-Bohm effect, the integer and fractional quantum hall effects, and topological phases of matter, including topological insulators, to name a few. In this thesis, we have proposed a fresh perspective of geodesics and null phase curves, which are key ingredients in understanding the geometric phase. We have also looked at a number of applications of geometric phases in topological phases, quantum walks, and non-inertial quantum systems. The shortest curve between any two points on a given surface is a (minimal) geodesic. They are also the curves along which a system does not acquire any geometric phase. In the same context, we can generalize geodesics to define a larger class of curves, known as null phase curves (NPCs), along which also the acquired geometric phase is zero; however, they need not be the shortest curves between the two points. We have proposed a geometrical decomposition of geodesics and null phase curves on the Bloch sphere, which is crucial in improving our understanding of the geometry of the state space and the intrinsic symmetries of geodesics and NPCs. We have also investigated the persistence of topological phases in quantum walks in the presence of an external (lossy) environment. We show that the topological order in one and two-dimensional quantum walks persist against moderate losses. Further, we use the geometric phase to detect the non-inertial modifications to the field correlators perceived by a circularly rotating two-level atom placed inside a cavity.

Rafael Antunes Ribeiro

In 1984, Berry discovered the geometric phase arising from the geometric structure of the quantum state parameter space and has become an important quantity in Quantum Mechanics. The geometric phase carries the geometric and topological properties of the parameter space. Among the various forms of geometric phase manifested is the fractional topological phase (FTF) arising from the polar decomposition of the evolution operator of the quantum state and that carries the dimension of the Hilbert space where the state lives. In this work we demonstrate theoretically that the FTF is robust to phase¿s noise (dephasing) when inserting phase's noise on the experimental proposal "Fractional topological phase on spatially encoded photonic qudits" of Khoury et all (Phys Rev. A, 87: 042 113, 2013) and treating the problem via Kraus¿maps. This fact makes the fractional topological phase a really reliable candidate for the implementation of quantum algorithms on environments having only phase noise.