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Tunneling time from spin fluctuations in Larmor clock

Tunneling time, time needed for a quantum particle to tunnel through a potential energy barrier, can be measured by a duration marker. One such marker is spin reorientation due to Larmor precession. With a weak magnetic field in $z$ direction, the Larmor clock reads two times, $τ_y$ and $τ_z$, for a potential energy barrier along the $y$ axis. The problem is to determine the actual tunneling time (ATT). B{ü}ttiker defines $\sqrt{τ_y^2 + τ_z^2}$ to be the ATT. Steinberg and others, on the other hand, identify $τ_y$ with the ATT. The B{ü}ttiker and Steinberg times are based on average spin components but in non-commuting spin system average of one component requires the other two to fluctuate. In the present work, we study the effects of spin fluctuations and show that the ATT can well be $τ_y + \frac{τ_z^2}{τ_y}$. We analyze the ATT candidates and reveal that the fluctuation-based ATT acts as a transmission time in all of the low-barrier, high-barrier, thick-barrier and classical dynamics limits. We extract this new ATT using the most recent experimental data by the Steinberg group. The new ATT qualifies as a viable tunneling time formula.