Phase sensitivity enhancement in an SU(1,1) interferometer via tunable bell states

We investigate phase estimation in an SU(1,1) interferometer using a tunable Bell-state input and single-port intensity detection. Under an equal mean-photon-number constraint, we compare the Bell-state input with a two-mode squeezed vacuum (TMSV) input. The scheme employs a photon-number superposition of the vacuum state and the two-mode single-photon state, which can be prepared via quantum-scissors truncation of a TMSV state. We derive closed-form expressions for the internal mean photon number, quantum Fisher information (QFI), intensity-detection phase sensitivity, and loss-dependent sensitivity. The results show that, in the considered parameter regime, the optimized Bell-state input can provide lower phase uncertainty than the corresponding TMSV benchmark. Under external photon loss, the optimized Bell state remains below the standard quantum limit even at 𝑇 = 0 . 5 . QFI analysis further reveals that this advantage originates from the phase information encoded in the tunable photon-number superposition, rather than solely from the detection scheme.