Resolving frequencies in a time-dependent field is classically limited by the measurement bandwidth. Using tools from quantum metrology and quantum control may overcome this limit, yet the full advantage afforded by entanglement so far remains elusive. Here we map the problem of frequency measurement to that of estimating a global dephasing quantum channel. In this way, we determine the ultimate quantum limits of frequency estimation in stochastic ac sensing. We find exact quantum Fisher information bounds for estimating frequency and frequency differences of stochastic fields. In particular, given two close signals with frequency separation ${\omega }_r$, we find that the quantum Fisher information for the separation estimation is approximately $2/{\omega }_r^2$, i.e., inversely proportional to the separation parameter. The bounds are achievable in certain regimes by superpositions of Dicke states. GHZ states are suboptimal but improve precision over unentangled states, achieving Heisenberg scaling in the low-bandwidth limit. This Letter establishes a robust framework for stochastic ac signal sensing that can be extended to arbitrary time-dependent and stochastic fields.

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• Quantum control
• Quantum information theory
• Quantum metrology
• Quantum parameter estimation