path: root/source/know/concept/quantum-gate/index.md

---
title: "Quantum gate"
sort_title: "Quantum gate"
date: 2021-03-29
categories:
- Quantum information
layout: "concept"
---

In quantum computing, **quantum gates** are the equivalent
of classical binary logic gates such as $$\mathrm{NOT}$$, $$\mathrm{AND}$$, etc.
Because of the continuous nature of qubits,
the number of possible quantum gates is uncountably infinite,
so we only consider the most important examples here.



## One-qubit gates

As an example, consider the following most general single-qubit state $$\ket{\psi}$$:

$$\begin{aligned}
    \ket{\psi}
    = \alpha \ket{0} + \beta \ket{1}
    = \begin{bmatrix} \alpha \\ \beta \end{bmatrix}
\end{aligned}$$

Arguably the most famous and most fundamental quantum gates are the **Pauli matrices**:

$$\begin{aligned}
    \boxed{
        X =
        \begin{bmatrix}
            0 & 1 \\
            1 & 0
        \end{bmatrix}
    }
    \qquad
    \boxed{
        Y =
        \begin{bmatrix}
            0 & -i \\
            i & 0
        \end{bmatrix}
    }
    \qquad
    \boxed{
        Z =
        \begin{bmatrix}
            1 & 0 \\
            0 & -1
        \end{bmatrix}
    }
\end{aligned}$$

They have the following effect on $$\ket{\psi}$$.
Note that $$X$$ is equivalent to the classical $$\mathrm{NOT}$$ gate
(and is often given that name),
and $$Z$$ is sometimes called the **phase-flip gate**:

$$\begin{aligned}
    X \ket{\psi}
    = \begin{bmatrix} \beta \\ \alpha \end{bmatrix}
    \qquad
    Y \ket{\psi}
    = \begin{bmatrix} -i \beta \\ i \alpha \end{bmatrix}
    \qquad
    Z \ket{\psi}
    = \begin{bmatrix} \alpha \\ -\beta \end{bmatrix}
\end{aligned}$$

In fact, $$Z$$ is a specific case of the **phase shift gate** $$R_\phi$$,
which modifies the qubit's phase without changing its amplitudes.
For an angle $$\phi$$, it is given by:

$$\begin{aligned}
    \boxed{
        R_\phi =
        \begin{bmatrix}
            1 & 0 \\
            0 & e^{i \phi}
        \end{bmatrix}
    }
\end{aligned}$$

For $$\phi = \pi$$, we recover the Pauli-$$Z$$ gate.
In general, the action of $$R_\phi$$ is as follows:

$$\begin{aligned}
    R_\phi \ket{\psi}
    = \begin{bmatrix} \alpha \\ e^{i \phi} \beta \end{bmatrix}
\end{aligned}$$

Two common special cases of $$R_\phi$$
are $$\phi = \pi/2$$ and $$\phi = \pi/4$$,
respectively called $$S$$ and $$T$$:

$$\begin{aligned}
    \boxed{
        S = R_{\pi/2} =
        \begin{bmatrix}
            1 & 0 \\
            0 & i
        \end{bmatrix}
    }
    \qquad \quad
    \boxed{
        T = R_{\pi/4} =
        \frac{1}{\sqrt{2}}
        \begin{bmatrix}
            \sqrt{2} & 0 \\
            0 & 1 + i
        \end{bmatrix}
    }
\end{aligned}$$

Finally, we have the **Hadamard gate** $$H$$,
which is defined as follows:

$$\begin{aligned}
    \boxed{
        H = \frac{1}{\sqrt{2}}
        \begin{bmatrix}
            1 & 1 \\
            1 & -1
        \end{bmatrix}
    }
\end{aligned}$$

Its action consists of rotating the qubit
by $$\pi$$ around the axis $$(X + Z) / \sqrt{2}$$ of
the [Bloch sphere](/know/concept/bloch-sphere/):

$$\begin{aligned}
    H \ket{\psi}
    = \frac{1}{\sqrt{2}} \begin{bmatrix} \alpha + \beta \\ \alpha - \beta \end{bmatrix}
\end{aligned}$$

Notably, it maps the eigenstates of $$X$$ and $$Z$$ to each other,
and is its own inverse (i.e. unitary):

$$\begin{aligned}
    H \ket{0} = \ket{+}
    \qquad
    H \ket{1} = \ket{-}
    \qquad
    H \ket{+} = \ket{0}
    \qquad
    H \ket{-} = \ket{1}
\end{aligned}$$

The **Clifford gates** are a set including $$X$$, $$Y$$, $$Z$$, $$H$$ and $$S$$,
or more generally any gates that rotate
by multiples of $$\pi/2$$ around the Bloch sphere.
This set is **not universal**, meaning that if we start from $$\ket{0}$$,
we can only reach $$\ket{0}$$, $$\ket{1}$$, $$\ket{+}$$, $$\ket{-}$$, $$\ket{+i}$$ $$\ket{-i}$$ using these gates.

If we add *any* non-Clifford gate, for example $$T$$,
then we can reach any point on the Bloch sphere,
which means that the set is **universal**.

However, there is a problem: a qubit has an uncountable infinity of states,
but a quantum circuit consists of a countably infinite sequence of gates, at most.
Therefore, technically, we can never reach the whole Bloch sphere,
but we *can* come up with circuits that approximate a target state to some degree $$\varepsilon$$.
This is the definition of universality:
any state can be approximated.



## Two-qubit gates

As an example, let us consider
the following two pure one-qubit states $$\ket{\psi_1}$$ and $$\ket{\psi_2}$$:

$$\begin{aligned}
    \ket{\psi_1}
    = \alpha_1 \ket{0} + \beta_1 \ket{1}
    = \begin{bmatrix} \alpha_1 \\ \beta_1 \end{bmatrix}
    \qquad \quad
    \ket{\psi_2}
    = \alpha_2 \ket{0} + \beta_2 \ket{1}
    = \begin{bmatrix} \alpha_2 \\ \beta_2 \end{bmatrix}
\end{aligned}$$

The composite state of both qubits, assuming they are pure,
is then their tensor product $$\otimes$$:

$$\begin{aligned}
    \ket{\psi_1 \psi_2}
    = \ket{\psi_1} \otimes \ket{\psi_2}
    &= \alpha_1 \alpha_2 \ket{00} + \alpha_1 \beta_2 \ket{01} + \beta_1 \alpha_2 \ket{10} + \beta_1 \beta_2 \ket{11}
    \\
    &= c_{00} \ket{00} + c_{01} \ket{01} + c_{10} \ket{10} + c_{11} \ket{11}
\end{aligned}$$

Note that a two-qubit system may be [entangled](/know/concept/quantum-entanglement/),
in which case the coefficients $$c_{00}$$ etc. cannot be written as products,
i.e. $$\ket{\psi_2}$$ cannot be expressed separately from $$\ket{\psi_1}$$, and vice versa.
In other words, the action of a two-qubit gate
can be expressed in the basis of $$\ket{00}$$, $$\ket{01}$$, $$\ket{10}$$ and $$\ket{11}$$,
but not always in the basis of $$\ket{0}_1$$, $$\ket{1}_1$$, $$\ket{0}_2$$ and $$\ket{1}_2$$.

With this noted, the first two-qubit gate is $$\mathrm{SWAP}$$,
which simply swaps $$\ket{\psi_1}$$ and $$\ket{\psi_2}$$:

{% include image.html file="swap.png" width="22%"
    alt="SWAP gate diagram" %}

$$\begin{aligned}
    \boxed{
        \mathrm{SWAP} =
        \begin{bmatrix}
            1 & 0 & 0 & 0 \\
            0 & 0 & 1 & 0 \\
            0 & 1 & 0 & 0 \\
            0 & 0 & 0 & 1
        \end{bmatrix}
    }
\end{aligned}$$

This matrix is given in the basis of $$\ket{00}$$, $$\ket{01}$$, $$\ket{10}$$ and $$\ket{11}$$.
Note that $$\mathrm{SWAP}$$ cannot generate entanglement,
so if its input is separable, its output is too.
In any case, its effect is clear:

$$\begin{aligned}
    \mathrm{SWAP} \ket{\psi_1 \psi_2}
    &= c_{00} \ket{00} + c_{10} \ket{01} + c_{01} \ket{10} + c_{11} \ket{11}
\end{aligned}$$

Next, there is the **controlled NOT gate** $$\mathrm{CNOT}$$,
which "flips" (applies $$X$$ to) $$\ket{\psi_2}$$ if $$\ket{\psi_1}$$ is true:

{% include image.html file="cnot.png" width="22%"
    alt="CNOT gate diagram" %}

$$\begin{aligned}
    \boxed{
        \mathrm{CNOT} =
        \begin{bmatrix}
            1 & 0 & 0 & 0 \\
            0 & 1 & 0 & 0 \\
            0 & 0 & 0 & 1 \\
            0 & 0 & 1 & 0
        \end{bmatrix}
    }
\end{aligned}$$

That is, it swaps the last two coefficients $$c_{10}$$ and $$c_{11}$$ in the composite state vector:

$$\begin{aligned}
    \mathrm{CNOT} \ket{\psi_1 \psi_2}
    &= c_{00} \ket{00} + c_{01} \ket{01} + c_{11} \ket{10} + c_{10} \ket{11}
\end{aligned}$$

More generally, from every one-qubit gate $$U$$,
we can define a two-qubit **controlled U gate** $$\mathrm{CU}$$,
which applies $$U$$ to $$\ket{\psi_2}$$ if $$\ket{\psi_1}$$ is true:

{% include image.html file="cu.png" width="22%"
    alt="CU gate diagram" %}

$$\begin{aligned}
    \boxed{
        \mathrm{CU} =
        \begin{bmatrix}
            1 & 0 & 0 & 0 \\
            0 & 1 & 0 & 0 \\
            0 & 0 & u_{00} & u_{01} \\
            0 & 0 & u_{10} & u_{11}
        \end{bmatrix}
    }
\end{aligned}$$

Where the lower-right 2x2 block is simply $$U$$.
The general action of this gate is given by:

$$\begin{aligned}
    \mathrm{CU} \ket{\psi_1 \psi_2}
    &= c_{00} \ket{00} + c_{01} \ket{01} + (c_{10} u_{00} + c_{11} u_{01}) \ket{10} + (c_{10} u_{10} + c_{11} u_{11}) \ket{11}
\end{aligned}$$

A set of gates is **universal** if all possible mappings
from $$n$$ to $$n$$ qubits can be approximated using only these gates.
A minimal universal set is $$\{\mathrm{CNOT}, T, S\}$$,
and there exist many others.



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