Entangled quantum probes for dynamical environmental noise

Dipartimento di Fisica, Universita degli Studi di Milano, 20133 Milano, Italy`(Dated: March 12, 2015)We address the use of entangled qubits as quantum probes to characterize the dynamical noise induced bycomplex environments. In particular, we show that entangled probes improve estimation of the correlation timefor a broad class of environmental noises compared to any sequential strategy involving single qubit preparation.The effect is present when the noise is faster than a threshold value, a regime which may always be achievedby tuning the coupling between the quantum probe and the environment inducing the noise. Our scheme ex-ploits time-dependent sensitivity of quantum systems to decoherence and does not require dynamical controlon the probes. We derive the optimal interaction time and the optimal probe preparation, showing that it corre-sponds to multiqubit GHZ states when entanglement is useful. We also show robustness of the scheme againstdepolarization or dephasing of the probe, and discuss simple measurements approaching optimal precision.