Skip to content
QuantumO

Quantum Gates and Circuits

IntermediateQuantum computing7 min read

By QuantumO Editorial · Scientific reviewer: not yet assigned · Published · Updated

In short

A quantum gate is a reversible operation that transforms qubit states; a quantum circuit is a time-ordered sequence of such gates ending in measurement. A small universal set — for example Hadamard, phase, and CNOT gates — is enough to approximate any quantum computation.

Watch the lesson

Video coming soon

This entry is a CMS placeholder. Add the YouTube video ID, thumbnail, duration and publication date to activate the player.

Visit the channel
Video details and transcript

Single-qubit gates rotate; two-qubit gates entangle

A Hadamard gate turns a definite 0 into an even combination of 0 and 1. Phase gates adjust the relative sign or phase between amplitudes — the ingredient interference needs.

Two-qubit gates such as CNOT make one qubit's behaviour depend on another, which is how entanglement is created inside a circuit.

Reversibility and universality

Apart from measurement, quantum gates are reversible: no information is discarded. A universal gate set can approximate any operation to any desired accuracy, much as NAND alone suffices classically.

Real hardware supports only a few native gates; compilers rewrite circuits into those, which is why the same algorithm can be much deeper on one machine than another.

Analogy: rotations of a globe

Single-qubit gates act like rotations of a globe, moving a point on the Bloch sphere. The analogy is precise for one qubit and fails entirely for two, where entanglement has no such picture.

Common misconceptions

Claim: Quantum gates are just faster logic gates.
More accurately: They are reversible and act on amplitudes rather than definite values, which makes them a different kind of operation, not a faster one.

Important terminology

Quantum gate
A quantum gate is a reversible operation that transforms the state of one or more qubits.
Quantum circuit
A quantum circuit is a sequence of quantum gates applied to qubits, usually ending in measurement.
Entanglement
Entanglement is a link between quantum systems such that the whole system has a definite description while its individual parts do not.
Superposition
Superposition is a quantum state formed by combining other states, with an amplitude attached to each.

Frequently asked questions

What is a quantum gate?
A reversible operation applied to one or more qubits that transforms their quantum state, such as the Hadamard or CNOT gate.
Video transcript

Transcript not available yet. Transcripts are stored as an editable CMS field on each video record.

Sources and further reading

Keep going