Quantum gate

A quantum gate is not a switch. A classical gate flips a bit from 0 to 1. A quantum gate rotates a qubit.

What this Short covers:

1. A qubit is like an arrow on a globe (the Bloch sphere): 0 at the north pole, 1 at the south pole, and it can point anywhere on the surface.
2. A single-qubit gate turns that arrow. Even the X gate, the quantum NOT, is a half-turn.
3. Turn it halfway, onto the equator, and you get an equal superposition of 0 and 1 (what the Hadamard gate does from 0).
4. Two-qubit gates like CNOT let one qubit (the control) decide what happens to another (the target). With the control in superposition, this creates entanglement.
5. A quantum program (a circuit) is a sequence of gates, with measurement at the end.

Note: once two qubits are entangled they no longer have separate arrows, and measuring one does not send a signal to the other.

Fine print for the physics crowd: the "arrow on a globe" is the Bloch sphere, and every single-qubit gate is a rotation of it. Once two qubits are entangled they no longer have separate arrows, and measuring one does not send a signal to the other. "Only look at the end" is true for standard circuits; IBM's dynamic circuits can measure mid-way.

1. IBM Quantum, Basics of Quantum Informationquantum.cloud.ibm.com
2. Bloch sphereen.wikipedia.org
3. Quantum logic gateen.wikipedia.org
4. Controlled NOT gateen.wikipedia.org
5. quantum.cloud.ibm.com