Exploring superconducting systems via charge-basis tomography

The use of accurate models and parameters in superconducting circuits is essential for the understanding and control of quantum states. In this regard, traditional spectroscopic techniques are employed to extract excited state populations, transition frequencies, and rates. However, crucial information about quantum coherence can only be obtained through phase-sensitive methods such as quantum state tomography, which is usually performed in the system’s energy basis. Here we propose a complementary method for performing state tomography of the ground state (GS) of a transmon in the charge basis [1]. The expression of the GS in the basis of the Cooper pair charge can provide new insights into the quantum circuits and can assist in validating its Hamiltonian model. The technique presented here combines the flux-tuning of a split Josephson junction and adiabatic evolution to achieve the desired density matrix reconstruction of the transmon's ground state. Further possible applications of this method include the study of hybrid superconductor-semiconductor junctions [2].

[1] Lupo et al, in preparation;

[2] Dahan et al, in preparation.