Bad Qubits — Learn Quantum by Doing

Multiple choice

Under independent (uncorrelated Markovian) dephasing, the Huelga result shows the best entangled probe strategy achieves what scaling of the phase uncertainty with the number of probes N?

1/N scaling (the full Heisenberg limit), preserved despite the noise
1/N^2 scaling, an improvement beyond the Heisenberg limit
No dependence on N at all
1/sqrt(N) scaling — the same as independent probes, so entanglement gives only a constant-factor gain