Frequency Fluctuation of Transmon from a Charged Two-level System: Part I*

The coupling of superconducting qubits to microscopic two-level system (TLS) is a prominent decoherence channel. Understanding the nature and the property of TLS is important to improve qubit coherence. In part I, we report unambiguous observation of the discrete charge states of a single TLS strongly coupled to an offset-charge sensitive transmon qubit. We measure an offset charge of 0.072e associated with the two TLS eigenstates. Combining measurements in the strong dispersive regime and the resonant regime, we quantify both transverse and longitudinal couplings of the TLS-qubit interaction Hamiltonian.