  The quantum behavior of superconducting qubits coupled to resonators is very
similar to that of atoms in optical cavities [1, 2], in which the resonant
cavity confines photons and promotes strong light-matter interactions. The
cavity end-mirrors determine the performance of the coupled system, with higher
mirror reflectivity yielding better quantum coherence, but higher mirror
transparency giving improved measurement and control, forcing a compromise. An
alternative is to control the mirror transparency, enabling switching between
long photon lifetime during quantum interactions and large signal strength when
performing measurements. Here we demonstrate the superconducting analogue,
using a quantum system comprising a resonator and a qubit, with variable
coupling to a measurement transmission line. The coupling can be adjusted
through zero to a photon emission rate 1,000 times the intrinsic photon decay
rate. We use this system to control photons in coherent states as well as in
non-classical Fock states, and dynamically shape the waveform of released
photons. This has direct applications to circuit quantum electrodynamics [3],
and may enable high-fidelity quantum state transfer between distant qubits, for
which precisely-controlled waveform shaping is a critical and non-trivial
requirement [4, 5].
