Does adding quantum 'helpers' improve precision measurements? No.

Quantum sensors sometimes use extra 'ancilla' modes — retained locally while a signal probes a system — hoping entanglement buys extra precision. This work proves that for lossless passive optical systems, no amount of ancilla energy improves the quantum Fisher information beyond what a straightforward signal probe achieves. Entanglement turns out to be unnecessary too, simplifying practical quantum sensing designs considerably.