Particle-Hole Asymmetry of Subgap Conductances in Superconductors Without Quasiparticle Poisoning

One universal consequence of the mean-field theory of superconductivity is a precise particle-hole symmetry (PHS) of conductances into any subgap states. However, subgap conductances are often observed experimentally to be particle-hole asymmetric. The breaking of this PHS has mostly been attributed to quasiparticle poisoning (QP), a process which destroys the coherence of SC-based qubits. Here, we provide a generic mechanism for the PHS breaking without involving QP, namely the coupling of the subgap states to bosonic modes. Using the rate equation and Keldysh formalism, we show that such coupling gives rise to a particle-hole asymmetry in subgap conductances both in the weak and strong tunnel-coupling regimes. Our results therefore show that the PHS breaking of subgap conductances in SCs could be a signature of significant modification of the conductance spectra by coupling to low-frequency bosonic modes such as phonons instead of QP.