Abstract
In this paper, we consider the quantum switch, which is one of the indefinite causal structures, in the study of quantum channel estimation. We show that such an indefinite causal order cannot provide any advantage for estimating quantum channels when the Kraus operators of the channel are commutative. We investigate the effects of the quantum switch when studying unitary qubit channels and amplitude damping qubit channels with noncommutative Kraus operators.
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Z. Yin was partially supported by NSFC No. 12031004.
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Appendices
Appendix A: Generators of Lie algebras su(2) and su(4)
The generators of Lie algebra su(2) are given by
And the generators of Lie algebra su(4) are given by
Appendix B: Coefficients of \(\rho _\theta \) in (29)
Recall that
The generalized Bloch expression of \(\rho _\theta \) is given by
with coefficients
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Gu, J., Yin, Z. & Li, L. Quantum channel estimations via indefinite causal order. Quantum Inf Process 22, 369 (2023). https://doi.org/10.1007/s11128-023-04118-7
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DOI: https://doi.org/10.1007/s11128-023-04118-7