Bounds for Apparent Second-Law Violations in Quantum Trajectories

Abstract

Negative stochastic entropy production is commonly called an apparent violation of the second law. In general quantum-trajectory dynamics, however, the physical entropy production $σ$ need not obey a forward detailed fluctuation theorem. A general arbitrary-coupling formulation identifies a dynamical-asymmetry term $σ^\ast$ that completes it into $Ω=σ+σ^\ast$, whose mean is $\langleΩ\rangle=Σ+Σ^\ast$. We prove that the likelihood-ratio sign is optimal among reversal-odd trajectory observables and use this fact to transfer an established sharp fluctuation-theorem floor to the tie-corrected physical sign statistic $Π_σ=\Pr(σ<0)+\Pr(σ=0)/2$. When $\Pr(σ=0)=0$, the result reads $\Pr(σ<0)\ge[1-\langleΩ\rangle/g(\langleΩ\rangle)]/2$, where $g$ is the inverse of $a\mapsto a\tanh(a/2)$. The physical integral fluctuation theorem simultaneously suppresses large negative events, producing a quantitative ``frequent but mild&#39;&#39; law, while the sign imbalance lower-bounds the hidden mean $Σ^\ast$. We formulate the measured-record protocol explicitly and illustrate and numerically audit the tie-corrected theorem in random finite-coupling collision models and a coherently driven qubit interacting with thermal ancillas.