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---
title: "Simon's algorithm"
sort_title: "Simon's algorithm"
date: 2021-05-01
categories:
- Quantum information
- Algorithms
layout: "concept"
---
**Simon's algorithm** was the first proof that quantum computers
are able to solve some problems *exponentially* faster
than classical computers.
In the same spirit as
the [Deutsch-Jozsa algorithm](/know/concept/deutsch-jozsa-algorithm/)
and the [Bernstein-Vazirani algorithm](/know/concept/bernstein-vazirani-algorithm/),
the problem it solves, known as **Simon's problem**,
is of no practical use,
but nevertheless Simon's algorithm is an important landmark.
Simon's problem is this:
we are given a "black box" function $$f(x)$$
that takes an $$n$$-bit input $$x$$
and returns an $$n$$-bit output.
We are promised that there exists an $$s$$ such that for all $$x_1$$ and $$x_2$$:
$$\begin{aligned}
f(x_1)
= f(x_2)
\quad \Leftrightarrow \quad
x_2 = s \oplus x_1
\end{aligned}$$
In other words, regardless of what $$f(x)$$ does behind the scenes,
its output is the same for inputs $$x_1$$ and $$x_2$$
if and only if $$x_2 = s \oplus x_1$$,
or, equivalently, $$x_1 = s \oplus x_2$$.
The goal is to find the $$n$$-bit number $$s$$, using as few calls to $$f$$ as possible.
There are two cases:
if $$s = 0$$, then $$f$$ is one-to-one, since $$x_2 = 0 \oplus x_1 = x_1$$.
Otherwise, if $$s \neq 0$$, then $$f$$ is two-to-one by definition:
for every $$x_1$$ there exists exactly one $$x_2$$ such that $$x_2 = s \oplus x_1$$.
A classical computer solves this by randomly guessing inputs,
until it finds two that give the same output,
and then $$s = x_1 \oplus x_2$$.
For $$n$$-bit numbers, this takes $$\mathcal{O}(\sqrt{2^n})$$ guesses
(the square root is due to the birthday paradox).
A quantum computer needs to query $$f$$ only $$\mathcal{O}(n)$$ times,
although the exact number varies due to the algorithm's probabilistic nature.
It uses the following circuit:
{% include image.html file="simons-circuit.png" width="52%" alt="Simon's circuit" %}
The XOR oracle $$U_f$$ implements $$f$$,
and has the following action for $$n$$-bit $$a$$ and $$b$$:
$$\begin{aligned}
\Ket{a} \Ket{b}
\quad \to \boxed{U_f} \to \quad
\Ket{a} \Ket{b \oplus f(a)}
\end{aligned}$$
Starting from the state $$\Ket{0}^{\otimes 2 n}$$,
we apply the [Hadamard gate](/know/concept/quantum-gate/) $$H$$
to each of the first $$n$$ qubits:
$$\begin{aligned}
\Ket{0}^{\otimes n} \Ket{0}^{\otimes n}
\quad \to \boxed{H^{\otimes n}} \to \quad
\Ket{+}^{\otimes n} \Ket{0}^{\otimes n}
= \frac{1}{\sqrt{2^n}} \sum_{x = 0}^{2^n - 1} \Ket{x} \Ket{0}^{\otimes n}
\end{aligned}$$
Where $$\Ket{x}$$ is shorthand for $$\Ket{x}_1 \cdots \Ket{x}_n$$.
In other words, we now have an equal superposition of all possible inputs $$x$$,
with a constant $$\Ket{0}^{\otimes n}$$ beside it.
We give this to the oracle $$U_f$$:
$$\begin{aligned}
\frac{1}{\sqrt{2^n}} \sum_{x = 0}^{2^n - 1} \Ket{x} \Ket{0}^{\otimes n}
\quad \to \boxed{U_f} \to \quad
\frac{1}{\sqrt{2^n}} \sum_{x = 0}^{2^n - 1} \Ket{x} \Ket{f(x)}
\end{aligned}$$
Then we apply $$H^{\otimes n}$$ to the first $$n$$ qubits again,
which, thanks to the definition of the Hadamard transform,
yields the following,
where $$x \cdot y$$ is the bitwise dot product:
$$\begin{aligned}
\frac{1}{\sqrt{2^n}} \sum_{x = 0}^{2^n - 1} \Ket{x} \Ket{f(x)}
\quad \to \boxed{H^{\otimes n}} \to \quad
&\frac{1}{2^n} \sum_{x = 0}^{2^n - 1} \bigg( \sum_{y = 0}^{2^n - 1} (-1)^{x \cdot y} \Ket{y} \bigg) \Ket{f(x)}
\end{aligned}$$
Next, we measure all qubits.
The order in which we do this does not matter,
but, for clarity, let us measure the last $$n$$ qubits first,
yielding $$\Ket{f(x_1)}$$ for some $$x_1$$.
Doing this leaves the $$2n$$ qubits in the following state,
where $$f(x_1) = f(x_2)$$ and $$x_2 = s \oplus x_1$$:
$$\begin{alignedat}{2}
&\mathrm{if} \: s = 0: \qquad
&&\frac{1}{\sqrt{2^{n}}} \sum_{y = 0}^{2^n - 1} (-1)^{x_1 \cdot y} \Ket{y} \Ket{f(x_1)}
\\
&\mathrm{if} \: s \neq 0: \qquad
&&\frac{1}{\sqrt{2^{n+1}}} \sum_{y = 0}^{2^n - 1} \Big( (-1)^{x_1 \cdot y} + (-1)^{x_2 \cdot y} \Big) \Ket{y} \Ket{f(x_1)}
\end{alignedat}$$
If $$s = 0$$, we get an equiprobable superposition of all $$y$$.
So, when we measure the first $$n$$ qubits, the result is a uniformly random number,
regardless of the phase $$(-1)^{x_1 \cdot y}$$.
If $$s \neq 0$$, the situation is more interesting,
because we can only measure $$y$$-values where:
$$\begin{aligned}
(-1)^{x_1 \cdot y} + (-1)^{x_2 \cdot y} \neq 0
\end{aligned}$$
Since $$x_2 = s \oplus x_1$$ by definition,
we can rewrite this as follows:
$$\begin{aligned}
(-1)^{x_1 \cdot y} + (-1)^{x_1 \cdot y \oplus s \cdot y}
= (-1)^{x_1 \cdot y} + (-1)^{x_1 \cdot y} (-1)^{s \cdot y}
\neq 0
\end{aligned}$$
Clearly, the expression can only be nonzero if $$s \cdot y$$ is even.
In other words, when we measure the first $$n$$ qubits,
we get a random $$y$$-value,
for which $$s \cdot y$$ is guaranteed to be even.
In both cases $$s = 0$$ and $$s \neq 0$$,
we measure a $$y$$-value that satisfies the equation:
$$\begin{aligned}
s \cdot y = 0 \:\:(\bmod 2)
\end{aligned}$$
This tells us something about $$s$$, albeit not much.
But if we run Simon's algorithm $$N$$ times,
we get various $$y$$-values $$y_1, ..., y_N$$,
from which we can build a system of linear equations:
$$\begin{aligned}
s \cdot y_1 &= 0 \:\:(\bmod 2)
\\
s \cdot y_2 &= 0 \:\:(\bmod 2)
\\
&\:\:\vdots
\\
s \cdot y_N &= 0 \:\:(\bmod 2)
\end{aligned}$$
This can be solved efficiently by a classical computer.
In the best-case scenario, all those $$y$$-values would be linearly independent
(when regarded as vectors of bits),
in which case only $$N = n - 1$$ equations would be necessary.
Simon's algorithm is therefore $$\mathcal{O}(n)$$.
It may feel like "cheating" to use a classical computer at the end.
Remember that the point of this algorithm is to limit the number of oracle queries,
which we did successfully.
Querying an oracle might be a very expensive operation,
so that is a big improvement!
That said, Simon's algorithm currently has no known practical uses.
## References
1. J.S. Neergaard-Nielsen,
*Quantum information: lectures notes*,
2021, unpublished.
2. S. Aaronson,
*Introduction to quantum information science: lecture notes*,
2018, unpublished.
|
