  The concept of entanglement, in which coherent quantum states become
inextricably correlated, has evolved from one of the most startling and
controversial outcomes of quantum mechanics to the enabling principle of
emerging technologies such as quantum computation and quantum sensors. The use
of entangled particles in measurement permits the transcendence of the standard
quantum limit in sensitivity, which scales as N^1/2 for N particles, to the
Heisenberg limit, which scales as N. This approach has been applied to optical
interferometry using entangled photons and spin pairs for the measurement of
magnetic fields and improvements on atomic clocks. Here, we demonstrate
experimentally an 9.4-fold increase in sensitivity to an external magnetic
field of a 10-spin entangled state, compared with an isolated spin, using
nuclear spins in a highly symmetric molecule. This approach scales in a
favourable way compared to systems where qubit loss is prevalent, and paves the
way for enhanced precision in magnetic field sensing

Quantum Physics