Quantum-enhanced parameter estimation in continuously monitored boundary time crystals

Abstract: We investigate quantum-enhanced parameter estimation in boundary time crystals (BTCs) via continuous monitoring. By analytically deriving the global quantum Fisher information rate, we show that in the time-crystal phase the ultimate precision exhibits a cubic scaling with the system size, $f_{\mathrm{global}}\sim N^3$, surpassing both the sensitivity at the critical point and the standard quantum limit (SQL). We then numerically demonstrate that this bound can be attained already at finite $N$ using experimentally accessible strategies: continuous photodetection and, in particular, continuous homodyne detection. Moving towards realistic implementations, we derive the fundamental precision limits for inefficient detection ($\eta<1$). While inefficiencies asymptotically restore SQL scaling, a constant-factor quantum advantage remains possible, diverging as $\eta!\to!1$. Numerical simulations show that homodyne detection outperforms photodetection in approaching the ultimate bound and consistently provides a collective advantage over independent single-qubit protocols, which grows with $N$.