Sub–shot-noise sensitivities without entanglement

We critically analyze the commonly maintained statement that entanglement is necessary to beat the shot-noise limit or standard quantum limit (SQL) in the sensitivity with which certain parameters can be measured in interferometric experiments. We consider in detail three different physical realizations of a beam splitter and a Mach-Zehnder interferometer using photons, massive bosons, and distinguishable spins or qubits. We study several input states and the corresponding definitions of entanglement. We show that mode entanglement is not required for photons or massive bosons for beating the SQL. In particular, with a fluctuating number of two-mode bosons, the shot-noise limit can be beaten by nonentangled bosonic states with all bosons in one mode. As a consequence, the trivially present entanglement due to symmetrization that appears when writing bosonic states in terms of individual particles is a useful resource for precision measurements.