We report the experimental observation, and a theoretical explanation, of
collective suppression of linewidths for multiple superconducting qubits
coupled to a good cavity. This demonstrateshow strong qubit-cavity coupling
can significantly modify the dephasing and dissipation processes that might be
expected for individual qubits, and can potentially improve coherence times in
many-body circuit QED.
Nonlinearity and entanglement are two important properties by which physical
systems can be identified as non-classical. We study the dynamics of the
resonant interaction of up to N=3two-level systems and a single mode of the
electromagnetic field sharing a single excitation dynamically. We observe
coherent vacuum Rabi oscillations and their nonlinear speed up by tracking the
populations of all qubits and the resonator in time. We use quantum state
tomography to show explicitly that the dynamics generates maximally entangled
states of the W class in a time limited only by the collective interaction
rate. We use an entanglement witness and the threetangle to characterize the
state whose fidelity F=78% is limited in our experiments by crosstalk arising
during the simultaneous qubit manipulations which is absent in a sequential
approach with F=91%.