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Quantum channel estimations via indefinite causal order

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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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Acknowledgements

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

$$\begin{aligned} \eta _{1}=\left( \begin{array}{cc} 0&{}\quad 1\\ 1&{}\quad 0\nonumber \\ \end{array}\right) , \quad \eta _{2}=\left( \begin{array}{cc} 1&{}\quad 0\\ 0&{}\quad 1\nonumber \\ \end{array} \right) , \quad \eta _{3}=\left( \begin{array}{cc} 0&{}\quad -i\\ i&{}\quad 0 \end{array} \right) . \end{aligned}$$

And the generators of Lie algebra su(4) are given by

$$\begin{aligned} \eta _{1}= & {} \left( \begin{array}{cccc} 0&{}\quad 1&{}\quad 0&{}\quad 0\\ 1&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array} \right) , \quad \eta _{2}=\left( \begin{array}{cccc} 0&{}\quad 0&{}\quad 1&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 1&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array}\right) , \quad \eta _{3}=\left( \begin{array}{cccc} 0&{}\quad 0&{}\quad 0&{}\quad 1\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 1&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array}\right) ,\\ \eta _{4}= & {} \left( \begin{array}{cccc} 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 1&{}\quad 0\\ 0&{}\quad 1&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array}\right) , \eta _{5}= \left( \begin{array}{cccc} 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 1\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 1&{}\quad 0&{}\quad 0\\ \end{array} \right) , \quad \eta _{6}= \left( \begin{array}{cccc} 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 1\\ 0&{}\quad 0&{}\quad 1&{}\quad 0\\ \end{array} \right) , \\ \eta _{7}= & {} \left( \begin{array}{cccc} 0&{}\quad -i&{}\quad 0&{}\quad 0\\ i&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array} \right) , \eta _{8}= \left( \begin{array}{cccc} 0&{}\quad 0&{}\quad -i&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ i&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array} \right) , \eta _{9}= \left( \begin{array}{cccc} 0&{}\quad 0&{}\quad 0&{}\quad -i\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ i&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array} \right) , \\ \eta _{10}= & {} \left( \begin{array}{cccc} 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad -i&{}\quad 0\\ 0&{}\quad i&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array} \right) , \eta _{11}= \left( \begin{array}{cccc} 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad -i\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad i&{}\quad 0&{}\quad 0\\ \end{array} \right) , \quad \eta _{12}= \left( \begin{array}{cccc} 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad -i\\ 0&{}\quad 0&{}\quad i&{}\quad 0\\ \end{array} \right) ,\\ \eta _{13}= & {} \left( \begin{array}{cccc} 1&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad -1&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array} \right) , \quad \eta _{14}=\frac{1}{\sqrt{3}} \left( \begin{array}{cccc} 1&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 1&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad -2&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad 0\\ \end{array} \right) , \\ \eta _{15}= & {} \frac{1}{\sqrt{6}} \left( \begin{array}{cccc} 1&{}\quad 0&{}\quad 0&{}\quad 0\\ 0&{}\quad 1&{}\quad 0&{}\quad 0\\ 0&{}\quad 0&{}\quad 1&{}\quad 0\\ 0&{}\quad 0&{}\quad 0&{}\quad -3\\ \end{array} \right) . \end{aligned}$$

Appendix B: Coefficients of \(\rho _\theta \) in (29)

Recall that

$$\begin{aligned} \rho _{\theta }= & {} \mathcal {S}(\mathcal {U}_{1\theta },\mathcal {U}_{2\theta })(\rho ;|c\rangle ) = \begin{pmatrix} pE&{}\sqrt{p(1-p)}G\\ \sqrt{p(1-p)}H&{}(1-p)M\\ \end{pmatrix},\\ E= & {} \frac{1}{4} \{ 2+2(2q-1)\cos 2\theta +\sqrt{q(1-q)}(2-2\cos 4\theta ) \} |0\rangle \langle 0|\\{} & {} + \frac{1}{4} \{ 4\sqrt{q(1-q)}\cos 2\theta -2i[(1-2q)\sin 2\theta +\sqrt{q(1-q)}\sin 4\theta ] \} |0\rangle \langle 1|\\{} & {} + \frac{1}{4} \{ 4\sqrt{q(1-q)}\cos 2\theta +2i[(1-2q) \sin 2\theta +\sqrt{q(1-q)}\sin 4\theta ] \} |1\rangle \langle 0|\\{} & {} + \frac{1}{4} \{ 2+2(1-2q)\cos 2\theta +\sqrt{q(1-q)}(2\cos 4\theta -2)\}|1\rangle \langle 1|,\\ G= & {} \frac{1}{4} \{[3q-1+2\cos 2\theta +(q-1)\cos 4\theta +\sqrt{q(1-q)}(1-\cos 4\theta )]\\{} & {} -i[(2q-2)\sin 2\theta +(1-q)\sin 4\theta +\sqrt{q(1-q)}(\sin 4\theta -2\sin 2\theta )]\}|0\rangle \langle 0|\\{} & {} +\frac{1}{4} \{[\sqrt{q(1-q)}(3+\cos 4\theta )+q(1-\cos 4\theta )]\\{} & {} -i[-q\sin 4\theta +2(1-q)\sin 2\theta +\sqrt{q(1-q)}(\sin 4\theta +2\sin 2\theta )]\}|0\rangle \langle 1|\nonumber \\{} & {} +\frac{1}{4} \{[(1-q)(\cos 4\theta -1)+\sqrt{q(1-q)}(3+\cos 4\theta )]\\{} & {} -i[2q\sin 2\theta -(1-q)\sin 4\theta -\sqrt{q(1-q)}(\sin 4\theta +2\sin 2\theta )]\}|1\rangle \langle 0|\\{} & {} +\frac{1}{4} \{[2\cos 2\theta +2-3q+(\sqrt{q(1-q)}-q)\cos 4\theta -\sqrt{q(1-q)}]\\{} & {} -i[(2q-2\sqrt{q(1-q)})\sin 2\theta +(\sqrt{q(1-q)}-q)\sin 4\theta ]\} |1\rangle \langle 1|,\\ H= & {} \frac{1}{4}\{[3q-1+2\cos 2\theta +(q-1)\cos 4\theta +\sqrt{q(1-q)}(1-\cos 4\theta )]\\{} & {} +i[(2q-2)\sin 2\theta +(1-q)\sin 4\theta +\sqrt{q(1-q)}(\sin 4\theta -2\sin 2\theta )]\}|0\rangle \langle 0|\\{} & {} +\frac{1}{4} \{[(1-q)(\cos 4\theta -1)+\sqrt{q(1-q)}(3+\cos 4\theta )]\\{} & {} +i[2q\sin 2\theta -(1-q)\sin 4\theta -\sqrt{q(1-q)}(\sin 4\theta +2\sin 2\theta )]\}|0\rangle \langle 1|\\{} & {} +\frac{1}{4}\{[q(1-\cos 4\theta )+\sqrt{q(1-q)}(3+\cos 4\theta )]\\{} & {} +i[2(1-q)\sin 2\theta +\sqrt{q(1-q)}(2\sin 2\theta +\sin 4\theta )-q\sin 4\theta ]\}|1\rangle \langle 0|\\{} & {} +\frac{1}{4} \{[2\cos 2\theta +2-3q-\sqrt{q(1-q)}+(\sqrt{q(1-q)}-q)\cos 4\theta ]\\{} & {} +i[(2q-2\sqrt{q(1-q)})\sin 2\theta +(\sqrt{q(1-q)}-q)\sin 4\theta ] \} |1\rangle \langle 1|,\\ M= & {} \frac{1}{4} \{2+2(2q-1)\cos 2\theta \} |0\rangle \langle 0|\\{} & {} +\frac{1}{4} \{[(2q-1)(1-\cos 4\theta )+4\sqrt{q(1-q)}\cos 2\theta ]-i[(1-2q)\sin 4\theta \\{} & {} +4\sqrt{q(1-q)}\sin 2\theta ]\}|0\rangle \langle 1|+\frac{1}{4} \{[(2q-1)(1-\cos 4\theta )+4\sqrt{q(1-q)}\cos 2\theta ]\\{} & {} +i[(1-2q)\sin 4\theta +4\sqrt{q(1-q)}\sin 2\theta ]\}|1\rangle \langle 0|\\{} & {} +\frac{1}{4} \{2+2(1-2q)\cos 2\theta \} |1\rangle \langle 1|. \end{aligned}$$

The generalized Bloch expression of \(\rho _\theta \) is given by

$$\begin{aligned} \rho _{\theta }=\frac{1}{4}1\hspace{-2.22214pt}{\textrm{l}}_{4}+\frac{1}{2} \sum _{i=1}^{15} \omega _i \eta _i, \end{aligned}$$

with coefficients

$$\begin{aligned} \omega _{1}&=2p\sqrt{q(1-q)}\cos 2\theta , \\ \omega _{2}&=\frac{\sqrt{p(1-p)}}{2}[3q-1+\sqrt{q(1-q)}+2\cos 2\theta +(q-1-\sqrt{q(1-q)})\cos 4\theta ],\\ \omega _{3}&=\frac{\sqrt{p(1-p)}}{2}[q+3\sqrt{q(1-q)}-(q-\sqrt{q(1-q)})\cos 4\theta ],\\ \omega _{4}&=\frac{\sqrt{p(1-p)}}{2}[q-1+3\sqrt{q(1-q)}+(1-q+\sqrt{q(1-q)})\cos 4\theta ],\\ \omega _{5}&=\frac{\sqrt{p(1-p)}}{2}[2-3q-\sqrt{q(1-q)}+2\cos 2\theta +(\sqrt{q(1-q)}-q)\cos 4\theta ],\\ \omega _{6}&=\frac{1-p}{2}[2q-1-(2q-1)\cos 4\theta +4\sqrt{q(1-q)}\cos 2\theta ],\\ \omega _{7}&=p[(1-2q)\sin 2\theta +\sqrt{q(1-q)}\sin 4\theta ],\\ \omega _{8}&=\frac{\sqrt{p(1-p)}}{2}[(2q-2-2\sqrt{q(1-q)})\sin 2\theta +(1-q+\sqrt{q(1-q)})\sin 4\theta ],\\ \omega _{9}&=\frac{\sqrt{p(1-p)}}{2}[(2-2q+2\sqrt{q(1-q)})\sin 2\theta +(\sqrt{q(1-q)}-q)\sin 4\theta ],\\ \omega _{10}&=\frac{\sqrt{p(1-p)}}{2}[(2q-2\sqrt{q(1-q)})\sin 2\theta -(1-q+\sqrt{q(1-q)})\sin 4\theta ],\\ \omega _{11}&=\frac{\sqrt{p(1-p)}}{2}[(2q-2\sqrt{q(1-q)})\sin 2\theta +(\sqrt{q(1-q)}-q)\sin 4\theta ],\\ \omega _{12}&=\frac{1-p}{2}[(1-2q)\sin 4\theta +4\sqrt{q(1-q)}\sin 2\theta ],\\ \omega _{13}&=p[(2q-1)\cos 2\theta +\sqrt{q(1-q)}(1-\cos 4\theta )],\\ \omega _{14}&=\frac{1}{\sqrt{3}}-\frac{1}{\sqrt{3}}(1-p)[2+(2q-1)\cos 2\theta ],\\ \omega _{15}&=\frac{1}{\sqrt{6}}-\frac{\sqrt{6}}{3}(1-p)[1+(1-2q)\cos 2\theta ]. \end{aligned}$$

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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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