(Nearly) Visible Quantum Interactions

I'll make a start to the best of my understanding so that at least more learned readers can correct me. In particular the Josephson phase qubit is beyond my ken, really, so take this with a pinch of salt.

They have a lump of material which will vibrate when driven with microwaves. The mechanical energy of this vibration is, like other forms of energy, quantised. They cool it down so they can distinguish the ground and first excited states.

The oscillator is piezoelectric, so they can get a current from it that depends on the frequency of the oscillation, and this enables it to be coupled to the qubit device.

The Josephson junction has two phases, superconducting and non-superconducting. The junction in a circuit behaves like an oscillating circuit. Transitions between these phases occur at certain resonant frequencies which depend on the current applied. The resonant frequencies are quantised and you can have a superposition of the states, and that's why this is a qubit. The splitting between these quantum states depends on the current.

Coupling the piezoelectric oscillator with the qubit allowed them to drive a circuit at frequencies that could change the qubit state. Conversely, the state of the qubit can set the vibrational state of the oscillator. Since you can set the qubit in a superposition of the ground and first excited state, the ground and first excited states of the oscillator i.e. not vibrating and vibrating, are also superposed.