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I am reading/studying the famous Nielsen and Chuang's book and ran into this interesting question and I don't quite understand the $f(x)$. It says it simply maps from $m$ to $n$ bits. But don't we have to know what $f(x)$ is to build a circuit using Ts and CNOTs? I think I miss something here.

Thanks.

Problem 4.1: (Computable phase shifts) Let $m$ and $n$ be positive integers. Suppose $f:\left\{0, \ldots, 2^{m}-1\right\} \rightarrow\left\{0, \ldots, 2^{n}-1\right\}$ is a classical function from $m$ to $n$ bits which may be computed reversibly using $T$ Toffoli gates, as described in Section 3.2.5. That is, the function $(x, y) \rightarrow(x, y \oplus f(x))$ may be implemented using $T$ Toffoli gates. Give a quantum circuit using $2 T+n$ (or fewer) one, two, and three qubit gates to implement the unitary operation defined by $$ |x\rangle \rightarrow \exp \left(\frac{-2 i \pi f(x)}{2^{n}}\right)|x\rangle . $$

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    $\begingroup$ Have you thought about treating $f$ as a black box? For example can you treat $f$ abstractly and construct a circuit of $2T+n$ qubit gates to convert the values of $f$ into phases and use those to conditionally rotate the amplitude of $\vert x\rangle$? $\endgroup$ Feb 1, 2022 at 22:16
  • $\begingroup$ @MarkS I will think about it. Thanks! $\endgroup$ Feb 2, 2022 at 2:21

1 Answer 1

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One possible way would go as follows:

  1. Compute $\hat P|x,y\rangle \equiv |x,y \oplus f(x)\rangle$ using $T$ Toffoli gates.
  2. Expand $q=\sum\limits_{j=0}^{n-1} q_j 2^j$ with $q_j$ being the digits of a binary representation of $0\le q\le 2^{n-1}$.
  3. Now observe that \begin{align*} \hat O|x,q\rangle&\equiv\mathrm e^{2\pi\mathrm iyn/2^n}\prod\limits_{j=0}^{n-1}\exp\left(-\pi\mathrm i q_j2^{j-n+1}\right) \Bigg|x,q\bigg\rangle \\ &= \exp\left(-2\pi\mathrm i/2^n\left(\sum\limits_{j=0}^{n-1}q_j2^j-y\right)\right) \Bigg|x,q\bigg\rangle \\ &= \exp\left(-2\pi\mathrm i/2^n\left(q-y\right)\right) |x,q\rangle, \end{align*} so that for $q = y\oplus f(x)$, we obtain $\hat O|x,y\oplus f(x)\rangle=\exp\left(-2\pi\mathrm i/2^nf(x)\right)|x,y\oplus f(x)\rangle.$ The operation $\prod\limits_{j=0}^{n-1}\exp\left(-\pi\mathrm i q_j2^{j-n+1}\right)$ can be implemented using $n$ single-qubit phase gates $\hat U_{n-1}\cdots\hat U_0$ each acting as $\hat U_j=\begin{pmatrix}1&0\\0&\mathrm e^{-\pi\mathrm i 2^{j-n+1}}\end{pmatrix}$ while the global $y$-dependent phase can be neglected.
  4. Uncompute $\hat P^\dagger|x,y \oplus f(x)\rangle = |x,y\rangle$, using $T$ Toffoli gates.

The total cost of the operation $\hat P^\dagger\hat O\hat P$ therefore is $2T+n$ gates, as required.

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