  The control of light-matter interaction at the quantum level usually requires
coherent laser fields. But already an exchange of virtual photons with the
electromagnetic vacuum field alone can lead to quantum coherences, which
subsequently suppress spontaneous emission. We demonstrate such spontaneously
generated coherences (SGC) in a large ensemble of nuclei operating in the x-ray
regime, resonantly coupled to a common cavity environment. The observed SGC
originates from two fundamentally different mechanisms related to cooperative
emission and magnetically controlled anisotropy of the cavity vacuum. This
approach opens new perspectives for quantum control, quantum state engineering
and simulation of quantum many-body physics in an essentially decoherence-free
setting.
