Four copies enable nonzero certification.

Yu-Xin Wu and colleagues from Zhejiang University of Science and Technology report that six copies of a three-qubit state are sufficient to achieve a near-optimal measurement, reaching a certification probability of (39 + 2S)τ^2/296, where S represents the sum of the traces of squared density matrices. The work characterizes the trade-off between entanglement certification, reading out the three-tangle and retaining the original quantum state. Increasing from four to five copies of the state not only boosts the optimal success probability from τ2/48 to 5τ2/72, but also allows the original unknown state to be retained for further processing. The researchers demonstrate these gains using local Schur measurements and conditional local operations with classical communication. The spectral limit for binary readout of the three-tangle is defined by a polynomial multiplication operator, according to new work characterizing optimal finite-copy entanglement readout. Researchers at Zhejiang University of Science and Technology detail how this operator impacts…