  Quantum entanglement is a resource in quantum metrology that can be
distributed to two orthogonal physical quantities for the enhancement of their
joint measurement sensitivity, as demonstrated in quantum dense metrology. On
the other hand, we can also devote all the quantum resource to phase
measurement only for optimum measurement sensitivity. Here, we experimentally
implement a dual-beam scheme in an SU(1,1) interferometer for the optimum phase
measurement sensitivity. We demonstrate a 3.9-dB improvement in signal-to-noise
ratio over the optimum classical method and this is 3-dB better than the
traditional single-beam scheme. Furthermore, such a scheme also realizes a
quantum optical tap of quantum entangled fields and has the full advantages of
an SU(1,1) interferometer for practical applications in quantum metrology and
quantum information.
