  Antiparallel spins are superior in orienteering to parallel spins. This
intriguing phenomenon is tied to entanglement associated with quantum
measurements rather than quantum states. Using photonic systems, we
experimentally realize the optimal orienteering protocols based on parallel
spins and antiparallel spins, respectively. The optimal entangling measurements
for decoding the direction information from parallel spins and antiparallel
spins are realized using photonic quantum walks, which is a useful idea that is
of wide interest in quantum information processing and foundational studies.
Our experiments clearly demonstrate the advantage of antiparallel spins over
parallel spins in orienteering. In addition, entangling measurements can
extract more information than local measurements even if no entanglement is
present in the quantum states.

Quantum Physics