Fig. 1. General process of quantum metrology

Fig. 2. Four different parallel strategies for estimation of unknown parameter. (a) Classical-classical strategy; (b) classical-quantum strategy; (c) quantum-classical strategy; (d) quantum-quantum strategy

Fig. 3. Sequential strategy

Fig. 4. General construction of Mach-Zehnder interferometer

Fig. 5. Fundamental principle of Hong-Ou-Mandel interference

Fig. 6. Structures of different types of interferometers. (a) Michelson interferometer; (b) Sagnac interferometer

Fig. 7. Lossy optical interferometer with transmissivity η of encoding phase

Fig. 8. Schematic of multiple phases estimation

Fig. 9. Rayleigh criterion in classical imaging systems