What is a qubit?

If you have never studied quantum computing, this page is your starting point. You do not need any physics: we will go slowly.

Bits, but quantum

A classical computer stores information in bits. A bit can be in one of two states: 0 or 1, like a switch that is off or on.

A qubit (quantum bit) is the quantum version of that idea, with two important differences:

1. Superposition. Until it is measured, a qubit can be both 0 and 1 at once, with weights (amplitudes) for each option.
2. Entanglement. Several qubits can become so tightly linked that the state of one depends on another, even when they are far apart.

It sounds like magic, but it is simply how matter behaves at very small scales. The mathematics behind it is linear algebra: a qubit is a vector of two complex numbers.

What it looks like in vqubit

You do not have to handle those vectors by hand. In vqubit the first thing is to decide where each qubit lives. That "where" is called a quantum identifier, or Qid:

import vqubit as vq

q0 = vq.LineQubit(0)      # qubit 0 of a line
q1 = vq.LineQubit(1)      # qubit 1 of the same line

A line (LineQubit) is just a simple model: qubits are numbered 0, 1, 2, .... There are also grids (GridQubit), which imitate the physical layout of many real processors, and named qubits (NamedQubit).

Superposition with a single gate

The Hadamard gate (H) puts a qubit into superposition: an equal split between 0 and 1.

circuit = vq.Circuit(vq.H(q0))
state = vq.Simulator().simulate(circuit, qubit_order=[q0])
print(state)

If you measure that qubit many times, you will see 0 roughly half the time and 1 the other half. Superposition does not mean it is "both at once" in the everyday sense: it means that, until you measure it, its probabilities are spread out.

Measurement

Measuring a qubit forces it to "decide" between 0 and 1. The outcome is random, with probabilities that depend on the state. In vqubit:

result = vq.Simulator().run(vq.Circuit(vq.H(q0), vq.measure(q0, key="result")), repetitions=1000)
print(result.histogram(key="result"))

Next steps

Now that you know what a qubit is, learn what you can do to it: the quantum gates.