In some aspects, a quantum computing system includes an electromagnetic waveguide system. The waveguide system has an interior surface that defines an interior volume of intersecting waveguides. Qubit devices are housed in the waveguide system. In some cases, the intersecting waveguides each define a cutoff frequency, and the qubit devices have qubit operating frequencies below the cutoff frequency. In some cases, coupler devices are housed in the waveguide system; each coupler device is configured to selectively couple a pair of neighboring qubit devices based on control signals received from a control source.