Improved Decoy-state and Flag-state Squashing Methods
arxiv(2024)
Abstract
In this work, we present an improved analysis for decoy-state methods,
enhancing both achievable key rates and recovering analytical results for the
single intensity scenario. Our primary focus is improving the shortcomings
observed in current decoy-state methods, particularly recovering results when
employing no decoy intensities. Our methods enable the continuous interpolation
across varying numbers of intensity settings. Additionally, we extend
decoy-state techniques to encompass scenarios where intensities vary depending
on the signal state, thereby relaxing the constraints on experimental
implementations. Our findings demonstrate that a minimum of two intensities are
sufficient for high asymptotic secret key rates, thereby further softening
experimental requirements. Additionally, we address inherent imperfections
within detection setups like imperfect beamsplitters. We derive provable secure
lower bounds on the subspace population estimation, which is required for
certain squashing methods such as the flag-state squasher. These analytical
bounds allow us to encompass arbitrary passive linear optical setups, and
together with intensities varying with each signal state, lets us include a
broad class of experimental setups.
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