Witnessing Entanglement in Quantum Spin Liquids
Snigdh Sabharwal
In 1935, as part of a broader debate about the interpretation of quantum mechanics, Schrödinger introduced the term “verschränkung” (entanglement) to describe the way in which mutually-exclusive experimental outcomes could be woven together in a single quantum mechanical state. Ninety years later, entanglement is no longer a philosophical curiosity but a technological resource, central to the pursuit of quantum advantage sought in the entanglement of a few hundred qubits. In the context of condensed matter physics, the most celebrated example of entanglement arises in quantum spin liquids (QSL), exotic magnetic phases where, at comparable temperatures, billions of spins can be entangled. The sharpest distinctions between conventional magnetic phases, QSL, and states driven by disorder, are expected to be found in their entanglement properties. Yet exactly what these distinctions are, or how they might be used in experiments, remains poorly understood. In this thesis, we explore how material-based imperfections such as bond disorder and finite temperature affect experimentally accessible entanglement-based diagnostics in models of quantum spin liquids (QSLs). We ask: (1) Can spin liquid states be reliably distinguished from disorder-driven phases using such diagnostics? (2) What can these experimentally accessible entanglement witnesses reveal about the microscopic structure of entanglement in QSLs? In particular, we examine the robustness of these quantum correlations under realistic conditions where thermal fluctuations are unavoidable. In addressing (1), we find that measures like concurrence become unreliable in disordered systems. In contrast, the finite temperature scaling of multipartite diagnostics such as the quantum Fisher information (QFI) provides a more effective means of distinguishing QSLs from competing disorder-dominated phases. For (2), we investigate how the finite temperature behavior of multipartite entanglement measures obtained from semidefinite programming (SDP) methods supports a two step picture for how a high temperature paramagnet crosses over into a QSL ground state. In particular, these results provide a conceptual framework that places quantitative bounds on the temperature window over which candidate materials can be expected to exhibit the characteristic physics of the QSL ground state.
•May 31, 2026•0 citations