A qubit-based oscilloscope for characterizing the distortion of flux pulses for two-qubit gates in circuit QED

In circuit QED, conditional-phase gates between flux-tunable Transmon qubits can be performed in 40 ns by pulsing fast adiabatically in and out of an avoided crossing in the two-excitation manifold. However, the flux pulses required are subject to distortions due to electrical components both at room temperature and inside the fridge. The room-temperature distortions can be characterized using a conventional oscilloscope and corrected using linear filtering. Correcting the cryogenic distortions is more challenging. We demonstrate a novel protocol that uses the pulsed qubit to directly measure the flux distortions on-chip, creating a cryogenic oscilloscope. We use the cryo-scope and numerical optimization to tune conditional-phase gates to the limit set by decoherence.