Entropy as a probe of quasiparticles in mesoscopic quantum systems: prospects and challenges

Measurements of quasiparticle entropy in quantum electronic circuits have the potential to reveal characteristics of those quasiparticles that are very difficult—or in some cases impossible—to discern using conventional transport. Numerous proposals now suggest identifying exotic emergent quasiparticles—such as Majorana fermions localized in quantum-dot-based geometries—through their characteristic entropy. Most rely on a Maxwell relation, such as ∂S/∂N = -∂μ/∂T or ∂S/∂μ = ∂N/∂T, to infer entropy changes of the thermodynamic system from its response to a local control parameter.

In this talk, I will describe a series of entropy-sensitive measurements performed by our group in quantum circuits whose quasiparticles are relatively well understood, from isolated spins to Kondo-screened impurities to coupled quantum dots. These experiments demonstrate that the key technical ingredients required for Maxwell-relation entropy detection are now in hand, showing that entropy-based searches for more exotic quasiparticles are realistic. They also reveal a number of experimental complications—some still unexplained—that complicate robust extraction of quasiparticle entropy, and make it difficult to interpret results that do not agree with theoretical expectations. I will discuss lessons learned from these systems and implications for applying entropy-based probes to more complex platforms.