Quantum metrology makes precise measurements that cannot be achieved classically. Typically, entanglement of N particles is used to measure a phase with precision ∆φ = 1/N, called the Heisenberg limit. This exceeds the ∆φ = 1/ √ N precision limit possible with N non-entangled particles, called the standard quantum limit (SQL). The signal-to-noise ratio (SNR) for a given light intensity is limited by SQL, which is critical for measurements where the probe light intensity is limited in order to avoid damaging the sample. The SQL can be tackled using entangled particles.