  We address the use of entangled qubits as quantum probes to characterize the
noise induced by complex environments. In particular, we show that a joint
measurement on entangled probes can improve estimation of the correlation time
for a broad class of environmental noises compared to any sequential strategy
involving single qubit preparation. The enhancement appears when the noise is
faster than a threshold value, a regime which may always be achieved by tuning
the coupling between the quantum probe and the environment inducing the noise.
Our scheme exploits time-dependent sensitivity of quantum systems to
decoherence and does not require dynamical control on the probes. We derive the
optimal interaction time and the optimal probe preparation, showing that it
corresponds to multiqubit GHZ states when entanglement is useful. We also show
robustness of the scheme against depolarization or dephasing of the probe, and
discuss simple measurements approaching optimal precision.
