Quantum fluctuations of the electromagnetic vacuum are responsible forphysical effects such as the Casimir force and the radiative decay of atoms,and set fundamental limits on the sensitivity of measurements. Entanglementbetween photons can produce correlations that result in a reduction of thesefluctuations below the vacuum level allowing measurements that surpass thestandard quantum limit in sensitivity. Here we demonstrate that the radiativedecay rate of an atom that is coupled to quadrature squeezed electromagneticvacuum can be reduced below its natural linewidth. We observe a two-foldreduction of the transverse radiative decay rate of a superconductingartificial atom coupled to continuum squeezed vacuum generated by a Josephsonparametric amplifier, allowing the transverse coherence time T_2 to exceed thevacuum decay limit of 2T_1. We demonstrate that the measured radiative decaydynamics can be used to tomographically reconstruct the Wigner distribution ofthe the itinerant squeezed state. Our results are the first confirmation of acanonical prediction of quantum optics and open the door to new studies of thequantum light-matter interaction.
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