Improved Superconducting Qubit Readout by Qubit-Induced Nonlinearities in the Straddling Regime

In dispersive readout schemes, qubit-induced nonlinearities have typically limited the measurement fidelities by reducing the signal-to-noise ratio (SNR) when the measurement power is increased [1]. However, it has been recently shown that these nonlinearities, together with the many-level system (MLS) nature of superconducting qubits, can be used to improve qubit readout in some regimes [2]. Moreover, for the transmon qubit [3], it has been shown that when the resonator's frequency sits between two of the MLS' transition frequencies -- the so-called straddling regime -- contributions of higher levels add constructively to improve the SNR [4]. In this talk, we explore the advantages of using both the qubit-induced nonlinearties and the straddling regime for qubit readout.\\[4pt] [1] Boissonneault et al, PRA 77, 060305(R) (2007)\\[0pt] [2] Reed et al, PRL 105, 173601 (2010), Bishop et al, PRL 105, 100505 (2010), Boissonneault et al, PRL 105, 100504 (2010)\\[0pt] [3] Koch et al, PRA 76, 042319 (2007)\\[0pt] [4] Srinivasan et al, V26.00006, 2010 March Meeting.