Speaker: Dr. Ivan Pechenezhskiy , University of Maryland
Abstract: Several promising superconducting qubit designs, including the protected ones, require superconductors operating as perfect inductors at microwave frequencies. As the value of the inductance increases with the inductor length, so does the parasitic capacitance due to the spatial extension of the inductor. Parasitic self-resonances formed this way impede the potential applications of extremely large inductances. Even in the simplest superconducting qubit circuit, in which a Josephson junction is shunted by a hyperinductor, the distributed nature of parasitic capacitance leads to an ultrastrong coupling of the parasitic modes to the qubit. This ultrastrong coupling of the parasitic modes prevents a perturbation theory treatment of the qubit excitation spectra. While a complete quantum description of the underlying qubit circuit is computationally prohibitive, diagonalization of an effective multi-mode Hamiltonian reproduces the experimental data both below and above the lowest parasitic modes. On the experimental side, the effects of the parasitic modes can be partially mitigated by releasing the qubit circuit off the substrate. This trick enables the realization of a new qubit design — blochnium.