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The theory of entanglement-assisted quantum metrology

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The quantum Fisher information (QFI) measures the amount of information that a quantum state carries about an unknown parameter. Given a quantum channel, its entanglement-assisted QFI is the maximum QFI of the output state assuming an input state over the system and an arbitrarily large ancilla. Consider N identical copies of a quantum channel, the channel QFI grows either linearly or quadratically with N asymptotically. Here we obtain a simple criterion that determines whether the scaling is linear or quadratic. In both cases, we found a quantum error correction protocol achieving the channel QFI asymptotically and an SDP to solve the optimal code.