Millimeter waves for microwave qubits

Millimeter waves are emerging as a key enabling technology for enhancing and connecting disparate quantum systems such as Rydberg atoms, optomechanics and superconducting qubits. Here we implement a cQED architecture using millimeter wave cavities and conventional microwave qubits. Specifically, we demonstrate a strong dispersive coupling (χ = 2π × 1.5 MHz) between a ω_r = 2π × 35 GHz Al cavity and a ω_q = 2π × 3 GHz transmon qubit. Recently, it has been shown that a large detuning (ω_r /ω_q > 10) between the qubit and cavity allows for strong readout without unwanted resonant state transitions. In part I we present numerical simulations motivating the use of millimeter waves for qubit readout and the device design. In part II we show experimental results for qubit coherence, millimeter wave readout, and suppression of unwanted state transitions.