2013/07/01 by Sergey Knysh, Knysh, Sergey I., Gabriel A. Durkin +1 · 2 citations
Computer Science · Physics and Astronomy · #Quantum Information and Cryptography #Advanced Thermodynamics and Statistical Mechanics #Quantum many-body systems
paper · pdf · doi:10.48550/arxiv.1307.0470
Coherent ensembles of N qubits present an advantage in quantum phase\nestimation over separable mixtures, but coherence decay due to classical phase\ndiffusion reduces overall precision. In some contexts, the strength of\ndiffusion may be the parameter of interest. We examine estimation of both phase\nand diffusion in large spin systems using a novel mathematical formulation. For\nthe first time, we show a closed form expression for the quantum Fisher\ninformation for estimation of a unitary parameter in a noisy environment. The\noptimal probe state has a non-Gaussian profile and differs also from the\ncanonical phase state; it saturates a new tight precision bound. For noise\nbelow a critical threshold, entanglement always leads to enhanced precision,\nbut the shot-noise limit is beaten only by a constant factor, independent of\nN. We provide upper and lower bounds to this factor, valid in low and high\nnoise regimes. Unlike other noise types, it is shown for N \≫ 1 that phase\nand diffusion can be measured simultaneously and optimally.\n