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- W3100559158 abstract "Is a quantum algorithm capable of implementing an if-clause? Given a black-box subroutine, a $d$-dimensional unitary $Uin U(d)$, a quantum if-clause would correspond to applying it to an input qudit if and only if the value of a control qubit is $1$. It was previously shown by Thompson et al. and Araujo et al. that implementations using single access to the oracle $U$ are impossible. Our main result is a strong generalization of this impossibility result: we prove that there is no unitary oracle algorithm implementing $control_phi(U)=left|0right>!!left !!left<1right|otimes U$, for some $U$-dependent phase $phi(U)$, even if allowed any finite number of calls to $U$ and $U^dagger$, and even if required to only approximate the desired operator $control_phi(U)$. Even further, there is no such emph{postselection} oracle algorithm, i.e. a unitary oracle algorithm followed by a binary success/fail measurement, that reports success and implements $control_phi(U)$ with a nonzero probability for each $Uin U(d)$. Our proof relies on topological arguments which can be viewed as a modification of the Borsuk-Ulam theorem. Combining the topological arguments with the algorithm of Dong et al. in fact leads to an interesting dichotomy: implementing $control_phi(U^m)$ in our model is possible if and only if the integer $m$ is a multiple of the oracle's dimension $d$. Our impossibility no longer holds if the model is relaxed, either by dropping the worst-case requirement that the algorithm works for all $Uin U(d)$, or, remarkably, by allowing measurements more general than a binary postselection measurement. We observe that for both relaxations, inefficient algorithms exist, and it remains open whether efficient ones do." @default.
- W3100559158 created "2020-11-23" @default.
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- W3100559158 date "2020-11-19" @default.
- W3100559158 modified "2023-09-27" @default.
- W3100559158 title "Topological obstructions to implementing controlled unknown unitaries" @default.
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