Two-qubit binegativity can increase under local amplitude damping

Abstract

It has been conjectured that the binegativity of two-qubit states is an entanglement monotone. We present a full-rank state as a counterexample: applying amplitude damping to only one of the qubits can increase the binegativity of the state. The output state remains full rank, and the amount of increase can be calculated exactly. In addition, we construct a family of counterexamples that vary continuously with a parameter. In the corresponding one-way local operations and classical communication protocol, the probability-weighted average of the binegativity over the branches after the local measurement is also higher than that of the initial state.