  The properties of quantum systems interacting with their environment,
commonly called open quantum systems, can be strongly affected by this
interaction. While this can lead to unwanted consequences, such as causing
decoherence in qubits used for quantum computation, it can also be exploited as
a probe of the environment. For example, magnetic resonance imaging is based on
the dependence of the spin relaxation times of protons in water molecules in a
host's tissue. Here we show that the excitation energy of a single spin, which
is determined by magnetocrystalline anisotropy and controls its stability and
suitability for use in magnetic data storage devices, can be modified by
varying the exchange coupling of the spin to a nearby conductive electrode.
Using scanning tunnelling microscopy and spectroscopy, we observe variations up
to a factor of two of the spin excitation energies of individual atoms as the
strength of the spin's coupling to the surrounding electronic bath changes.
These observations, combined with calculations, show that exchange coupling can
strongly modify the magnetic anisotropy. This system is thus one of the few
open quantum systems in which the energy levels, and not just the excited-state
lifetimes, can be controllably renormalized. Furthermore, we demonstrate that
the magnetocrystalline anisotropy, a property normally determined by the local
structure around a spin, can be electronically tuned. These effects may play a
significant role in the development of spintronic devices5 in which an
individual magnetic atom or molecule is coupled to conducting leads.
