  Quantum metrology overcomes standard precision limits and plays a central
role in science and technology. Practically it is vulnerable to imperfections
such as decoherence. Here, we demonstrate quantum metrology for noisy channels
such that entanglement with ancillary qubits enhances the quantum Fisher
information for phase estimation but not otherwise. Our photonic experiment
covers a range of noise for various types of channels, including for two
randomly alternating channels such that assisted entanglement fails for each
noisy channel individually. We have simulated noisy channels by implementing
space-multiplexed dual interferometers with quantum photonic inputs. We have
demonstrated the advantage of entanglement-assisted protocols in phase
estimation experiment run with either single-probe or multi-probe approach.
These results establish that entanglement with ancillae is a valuable approach
for delivering quantum-enhanced metrology. Our new approach to
entanglement-assisted quantum metrology via a simple linear-optical
interferometric network with easy-to-prepare photonic inputs provides a path
towards practical quantum metrology.
