The elusive Heisenberg limit in quantum-enhanced metrology

TLDR

Quantum precision enhancement is crucial for advanced metrological experiments such as gravitational wave detection and atomic clock calibration, yet precision is limited by shot noise scaling as 1/√N, which quantum entanglement can overcome to achieve the Heisenberg scaling of 1/N. The study demonstrates that decoherence limits quantum enhancement to a constant factor in the infinite‑probe limit. The authors develop efficient, geometry‑based tools using semi‑definite programming to derive bounds for decoherence models relevant to metrology, including depolarization, dephasing, spontaneous emission, and photon loss. The maximal achievable quantum precision under decoherence is a constant factor, not the ideal quadratic improvement.

Abstract

Quantum precision enhancement is of fundamental importance for the development of advanced metrological optical experiments, such as gravitational wave detection and frequency calibration with atomic clocks. Precision in these experiments is strongly limited by the 1/√N shot noise factor with N being the number of probes (photons, atoms) employed in the experiment. Quantum theory provides tools to overcome the bound by using entangled probes. In an idealized scenario this gives rise to the Heisenberg scaling of precision 1/N. Here we show that when decoherence is taken into account, the maximal possible quantum enhancement in the asymptotic limit of infinite N amounts generically to a constant factor rather than quadratic improvement. We provide efficient and intuitive tools for deriving the bounds based on the geometry of quantum channels and semi-definite programming. We apply these tools to derive bounds for models of decoherence relevant for metrological applications including: depolarization, dephasing, spontaneous emission and photon loss.