Magnon-mediated qubit coupling determined via dissipation measurements

Controlled interaction between localized and delocalized solid-state spin systems offers a compelling platform for on-chip quantum information processing with quantum spintronics. Hybrid quantum systems (HQSs) of localized nitrogen-vacancy (NV) centers in diamond and delocalized magnon modes in ferrimagnets-systems with naturally commensurate energies-have recently attracted significant attention, especially for interconnecting isolated spin qubits at length-scales far beyond those set by the dipolar coupling. However, despite extensive theoretical efforts, there is a lack of experimental characterization of the magnon-mediated interaction between NV centers, which is necessary to develop such hybrid quantum architectures. Here, we experimentally determine the magnon-mediated NV-NV coupling from the magnon-induced self-energy of NV centers. Our results are quantitatively consistent with a model in which the NV center is coupled to magnons by dipolar interactions. This work provides a versatile tool to characterize HQSs in the absence of strong coupling, informing future efforts to engineer entangled solid-state systems.

Blurring the Boundaries Between Topological and Nontopological Phenomena in Dots

We investigate the electronic and transport properties of topological and nontopological InAs_{0.85}Bi_{0.15} quantum dots (QDs) described by a ∼30  meV gapped Bernevig-Hughes-Zhang (BHZ) model with cylindrical confinement, i.e., "BHZ dots." Via modified Bessel functions, we analytically show that nontopological dots quite unexpectedly have discrete helical edge states, i.e., Kramers pairs with spin-angular-momentum locking similar to topological dots. These unusual nontopological edge states are geometrically protected due to confinement for a wide range of parameters and remarkably contrast with the bulk-edge correspondence in topological insulators, as no bulk topological invariant guarantees their existence. Moreover, for a conduction window with four edge states, we find that the two-terminal conductance G versus the QD radius R and the gate V_{g} controlling its levels shows a double peak at 2e^{2}/h for both topological and trivial BHZ QDs. This is in stark contrast to conductance measurements in 2D quantum spin Hall and trivial insulators. All of these results were also found in HgTe QDs. Bi-based BHZ dots should also prove important as hosts to room temperature edge spin qubits.