Linear optical quantum metrology with single photons --- Experimental errors, resource counting, and quantum Cram\'er-Rao bounds

Understand

Quantum number-path entanglement is a resource for super-sensitive quantum metrology and in particular provides for sub-shotnoise or even Heisenberg-limited sensitivity.

• However, such number-path entanglement has thought to have been resource intensive to create in the first place --- typically requiring either very strong nonlinearities, or nondeterministic preparation schemes with feed-forward, which are difficult to implement.
• Recently in [Phys.
• Rev.