Qubits

What are qubits?

A qubit is a physical system, such as an atom or a photon, that can be put into a superposition of two states. Each state represents a binary value (i.e., a zero or a one). For example, when an atom is in its lowest energy state, it could represent a zero and when it is in an excited state, it could represent a one. For the photon, horizontal polarization could correspond to a zero and vertical polarization could correspond to a one.

Qubits, short for quantum bits, are distinguished from classical bits by their ability to be put into superpositions of their two states. This means that they can be in the zero state and the one state at the same time. This is a fundamental quantum phenomenon. That is, classical bits cannot do this. For further details, see the knowledge article on superposition.

Qubits are the building blocks of quantum computers and they can display a plethora of quantum phenomena, most notably entanglement. For further details, see the knowledge article on entanglement.

Why are qubits useful and important?

Qubits are just as important for quantum computers as classical bits are for ordinary computers. That is to say, they are at the heart of everything. More concretely, quantum computers rely on having a large number of high quality qubits. Let us take the number and the quality in turn.

Due to the phenomenon of superposition: a single qubit can represent two values at the same time. This is difficult to imagine because in the familiar world of classical computers, a bit can only be a zero or a one - it can't be both at once.

Superposition also means that two qubits can represent four values at the same time; three qubits can represent eight; four qubits, sixteen; and so on. With each additional qubit, the number of values that can be represented in a single instance doubles. This is exponential growth and it gives a quantum computer a dramatic ability to operate in a way that classical computers cannot.

Higher quality qubits can maintain such superpositions for longer periods of time. This means that the quantum computer has more time to manipulate all the values and complete more intricate calculations before noisy interactions cause errors to build up.