Demonstrating W-type entanglement of Dicke states in resonant cavity quantum electrodynamics

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J. A. Mlynek, A. A. Abdumalikov, Jr., J. M. Fink, L. Steffen, M. Baur, C. Lang, A. F. van Loo, and A. Wallraff


Phys. Rev. A 86, 053838 (2012)

Nonlinearity and entanglement are two important properties by which physical systems can be identified as nonclassical. We study the dynamics of the resonant interaction of up to N=3 two-level systems and a single mode of the electromagnetic field sharing a single excitation dynamically. We observe coherent vacuum Rabi oscillations and their nonlinear √N speedup 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 3-tangle 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%.