Non-trivial fusion of Majorana zero modes in interacting quantum-dot arrays

Abstract

Motivated by recent experimental reports of Majorana zero modes (MZMs) in quantum-dot systems at the ``sweet spot'', where the electronic hopping $t_h$ is equal to the superconducting coupling $\Delta$, we study the time-dependent spectroscopy corresponding to the non-trivial fusion of MZMs. The expression non-trivial refers to the fusion of Majoranas from different original pairs of MZMs, each with well-defined parities. For the first time, we employ an experimentally accessible time-dependent real-space local density-of-states (LDOS) method to investigate the non-trivial MZMs fusion outcomes in canonical chains and in a Y-shape array of interacting electrons. In the case of quantum-dot chains where two pairs of MZMs are initially disconnected, after fusion we find equal-height peaks in the electron and hole components of the LDOS, signaling non-trivial fusion into both the vacuum $I$ and fermion $\Psi$ channels with equal weight. For $\pi$-junction quantum-dot chains, where the superconducting phase has opposite signs on the left and right portions of the chain, after the non-trivial fusion, we observed the formation of an exotic two-site MZM near the center of the chain, coexisting with another single-site MZM. Furthermore, we also studied the fusion of three MZMs in the Y-shape geometry. In this case, after the fusion we observed the novel formation of another exotic multi-site MZM, with properties depending on the connection and geometry of the central region of the Y-shape quantum-dot array.