Using quantum coherence, entanglement, or squeezing to measure physical quantities below the classical shot-noise (standard-quantum) limit. Platforms: NV-centre diamond magnetometry (spin coherence times T₂ up to ms at room temperature;…
quantum-sensing
NV-centre diamond magnetometry: optically-detected magnetic resonance
NV-centre (nitrogen-vacancy) defect in diamond: a substitutional N adjacent to a C-atom vacancy, in the negative charge state, has a…
Squeezed-vacuum injection: 3-dB shot-noise reduction below SQL in LIGO
Caves 1981 proposal: inject a squeezed-vacuum state at the dark-port of a Michelson-interferometric gravity-wave detector such that the…
QND measurement: [A_obs, H_sys+H_int] = 0 → back-action-evasion
Braginsky 1974 / Caves-Thorne-Drever-Sandberg-Zimmermann 1980 quantum non-demolition (QND) measurement: an observable  whose measurement…
Heisenberg limit: δφ ≥ 1/N vs SQL δφ ≥ 1/√N — ratio √N enhancement
For N-probe interferometric phase estimation the Cramér-Rao bound from the quantum Fisher information F_Q gives δφ ≥ 1/√F_Q. Uncorrelated…
NV DC-magnetometry sensitivity: η_B = ℏ/(g_e μ_B · √(N) · T₂* · √t)
Taylor-Cappellaro-Lukin 2008: the NV-centre shot-noise-limited DC-magnetometry sensitivity per unit measurement-time bandwidth is η_B =…
Squeezed phase noise: δφ_sqz = e^{−r}/√N (Caves 1981)
Caves 1981 squeezed-state Mach-Zehnder phase-estimation: injecting a squeezed-vacuum state with squeezing parameter r (amplitude e^{−r} < 1…