  Low-dimensional copper oxide lattices naturally manifest electronic states
with strong short range quantum entanglement, which are known to lead to
remarkable emergent material properties. However the nanometer scale many-body
wavefunction is challenging to measure or manipulate in a simple way. In this
study, X-ray induced $dd$ electronic transitions are used to suppress spin
entanglement across a single lattice site in the spin-1/2 antiferromagnetic
chain compound SrCuO$_2$, revealing a class of cuprate magnetic excitations
that result from breaking the spin chain. These measurements are the first to
employ two closely spaced X-ray resonances of copper (M$_2$ and M$_3$) as a
form of natural 2-slit interferometer to distinguish between different types of
electronic transition and resolve how they influence the dynamics of nearby
spin-entangled electrons.
