  Quantum fluctuations of the electromagnetic vacuum are responsible for
physical effects such as the Casimir force and the radiative decay of atoms,
and set fundamental limits on the sensitivity of measurements. Entanglement
between photons can produce correlations that result in a reduction of these
fluctuations below the vacuum level allowing measurements that surpass the
standard quantum limit in sensitivity. Here we demonstrate that the radiative
decay rate of an atom that is coupled to quadrature squeezed electromagnetic
vacuum can be reduced below its natural linewidth. We observe a two-fold
reduction of the transverse radiative decay rate of a superconducting
artificial atom coupled to continuum squeezed vacuum generated by a Josephson
parametric amplifier, allowing the transverse coherence time T_2 to exceed the
vacuum decay limit of 2T_1. We demonstrate that the measured radiative decay
dynamics can be used to tomographically reconstruct the Wigner distribution of
the the itinerant squeezed state. Our results are the first confirmation of a
canonical prediction of quantum optics and open the door to new studies of the
quantum light-matter interaction.
