High fidelity dispersive qubit readout in circuit QED without using a Josephson Parametric Amplifier

In the cQED architecture, the Josephson parametric amplifier (JPA) enables high fidelity readout by providing minimal degradation of the signal-to-noise ratio (SNR) of the amplified readout signal. While the SNR can also be increased by increasing readout power and integration time, the fidelity typically gets limited due to unwanted transitions both within and outside the computational subspace. Here, we present and demonstrate an alternate design where qubit-cavity coupling does not rely on the dispersive approximation of the Jaynes-Cummings Hamiltonian. This multi-modal circuit is reminiscent of the quantronium qubit design but with two differences: the qubit is of the transmon type and the cavity is linear. The device is implemented in the 3D cQED architecture and we use a rectangular waveguide to couple the readout cavity to the measurement line. We achieve a readout fidelity of 97.8% for 800ns integration time with a histogram overlap error of only 0.3% without using a JPA. The best fidelity obtained with the JPA was 98.9% for a 300 ns integration time with an overlap error of less than 0.01%. We will conclude by discussing the variation of readout fidelity with measurement power and compare with a conventional transmon readout in cQED.