Millimeter Wave Readout of a Superconducting Qubit

Millimeter waves are emerging as an enabling technology for connecting and enhancing different quantum platforms such as Rydberg atoms, optomechanics, and superconducting qubits. In this work, we focus on the interaction between millimeter wave photons and conventional transmon qubits, specifically for qubit readout. We study a circuit quantum electrodynamic (cQED) system consisting of a millimeter-wave cavity at ωr=2π×34.7 GHz and a transmon qubit at ωq=2π×3.1 GHz coupled at rate g=2π×1.3 GHz. With such a large detuning between cavity and qubit, ωr/ωq>10, we are able to suppress drive induced unwanted state transitions, enabling strong drives for qubit readout. We measure no resonant state transitions up to 1,000 drive photons and readout the qubit state with more than 100 photons to achieve a measurement fidelity greater than 99% without the aid of a quantum limited amplifier.