Quantum measurement based on subharmonic parametric driving

High fidelity, QND measurement is vital for performing quantum error correction and so is one of the key requirements to build a quantum computer. Previous works have shown that dispersive superconducting qubit readout is limited by multi-photon excitation of the transmon when the readout resonator is driven resonantly and strongly. In this talk, we propose an alternative way to perform qubit readout, realized by parametrically driving the qubit instead of the resonator and using its self-Kerr. By driving the coupled transmon at 1/3 of the resonator frequency, the resonator is populated through the four-wave mixing process between the far-detuned driving field and the resonator mode. This method provides new possibilities to improve qubit readout strength. We will present the characterization and tune up of the subharmonic readout and discuss its limitations due to the strong, low frequency, drive on the qubit. We will also show that the dispersive readout can be strengthened, or longitudinal readout can be realized for generic transmon/cavity parameters, by combining a far-detuned driving tone and a resonant readout tone.