Noise and coherence in the Kitaev-transmon qubit

The so-called Kitaev-transmon qubit (Kitmon), based on a Josephson junction between two Kitaev chains embedded in a superconducting circuit, has emerged as a promising platform for Majorana-based quantum computation. The minimal version of this device consists of N = 2 quantum dots (QDs) in each chain. Here we investigate the extension (N > 2) of such a device by increasing the number N of QDs in each junction segment, a modification predicted a priori to enhance the robustness of Majorana zero modes (MZMs). However, the even-odd effect inherent to finite-size Kitaev chains has drastic implications for the existence and stability of MZMs, leading to fundamentally different behavior for qubits with even and odd N. We construct effective low-energy models for devices with incresing N, and analyze their spectral and parity properties in both charging and transmon regimes. Furthermore, we estimate the qubit dephasing times as a function of junction parameters, elucidating the critical impact of the even-odd effect on the potential coherence of these qubits.