Quantum charge fluctuations of a proximitized nanowire

Motivated by recent experiment, we consider charging of a nanowire which is proximitized by a superconductor and connected to a normal-state lead by a single-channel junction. The charge $Q$ of the nanowire is controlled by gate voltage $e{cal N}_g/C$. A finite conductance of the contact allows for quantum charge fluctuations, making the function $Q(mathcal{N}_g)$ continuous. It depends on the relation between the superconducting gap $Delta$ and the effective charging energy $E^*_C$. The latter is determined by the junction conductance, in addition to the geometrical capacitance of the proximitized nanowire. We investigate $Q(mathcal{N}_g)$ at zero magnetic field $B$, and at fields exceeding the critical value $B_c$ corresponding to the topological phase transition. Unlike the case of $Delta = 0$, the function $Q(mathcal{N}_g)$ is analytic even in the limit of negligible level spacing in the nanowire. At $B=0$ and $Delta>E^*_C$, the maxima of $dQ/dmathcal{N}_g$ are smeared by $2e$-fluctuations described by a single-channel charge Kondo physics, while the $B=0$, $Delta<E^*_C$ case is described by a crossover between the Kondo and mixed-valence regimes of the Anderson impurity model. In the topological phase, $Q(mathcal{N}_g)$ is analytic function of the gate voltage with $e$-periodic steps. In the weak tunneling limit, $dQ/dmathcal{N}_g$ has peaks corresponding to Breit-Wigner resonances, whereas in the strong tunneling limit (i.e., small reflection amplitude $r$ ) these resonances are broadened, and $dQ/dmathcal{N}_g-e propto rcos(2pi mathcal{N}_g)$.