Categories: Algorithms, Quantum information.

# Bernstein-Vazirani algorithm

In quantum information, the Bernstein-Vazirani algorithm proves the supremacy of quantum computers over classical deterministic or probabilistic computers. It is extremely similar to the Deutsch-Jozsa algorithm, and even uses the same circuit.

It solves a very artificial problem: we are given a “black box” function $f(x)$ that takes an $N$-bit $x$ and returns a single bit, which we are promised is the lowest bit of the bitwise dot product of $x$ with an unknown $N$-bit string $s$:

\begin{aligned} f(x) = s \cdot x \:\:(\bmod \: 2) = (s_1 x_1 + s_2 x_2 + \:...\: + s_N x_N) \:\:(\bmod \: 2) \end{aligned}

The goal is to find $s$. To solve this problem, a classical computer would need to call $f(x)$ exactly $N$ times with $x = 2^n$ for $n \in \{ 0, ..., N \!-\! 1\}$. However, the Bernstein-Vazirani algorithm allows a quantum computer to do it with only a single query. It uses the following circuit:

Where $U_f$ is a phase oracle, whose action is defined as follows, where $\Ket{x} = \Ket{x_1} \cdots \Ket{x_N}$:

\begin{aligned} \Ket{x} \quad \to \boxed{U_f} \to \quad (-1)^{f(x)} \Ket{x} = (-1)^{s \cdot x} \Ket{x} \end{aligned}

That is, it introduces a phase flip based on the value of $f(x)$. For an example implementation of such an oracle, see the Deutsch-Jozsa algorithm: its circuit is identical to this one, but describes $U_f$ in a different (but equivalent) way.

Starting from the state $\Ket{0}^{\otimes N}$, applying the Hadamard gate $H$ to all qubits yields:

\begin{aligned} \Ket{0}^{\otimes N} \quad \to \boxed{H^{\otimes N}} \to \quad \Ket{+}^{\otimes N} = \frac{1}{\sqrt{2^N}} \sum_{x = 0}^{2^N - 1} \Ket{x} \end{aligned}

This is an equal superposition of all candidates $\Ket{x}$, which we feed to the oracle:

\begin{aligned} \frac{1}{\sqrt{2^N}} \sum_{x = 0}^{2^N - 1} \Ket{x} \quad \to \boxed{U_f} \to \quad \frac{1}{\sqrt{2^N}} \sum_{x = 0}^{2^N - 1} (-1)^{s \cdot x} \Ket{x} \end{aligned}

Then, using the definition of the Hadamard transform and the fact that it is its own inverse, one final set of $H$-gates leads us to:

\begin{aligned} \frac{1}{\sqrt{2^N}} \sum_{x = 0}^{2^N - 1} (-1)^{s \cdot x} \Ket{x} \quad \to \boxed{H^{\otimes N}} \to \quad \Ket{s} = \Ket{s_1} \cdots \Ket{s_N} \end{aligned}

Which, upon measurement, gives us the desired binary representation of $s$. For comparison, the Deutsch-Jozsa algorithm only cares whether $s = 0$ or $s \neq 0$, whereas this algorithm is interested in the exact value of $s$.

1. J.S. Neergaard-Nielsen, Quantum information: lectures notes, 2021, unpublished.
2. S. Aaronson, Introduction to quantum information science: lecture notes, 2018, unpublished.