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What is a Qubit?

A qubit, also known as a quantum bit, is the basic unit of information used in quantum computing. Much like how today’s computers fundamentally work by manipulating bits that exist in two different states, quantum computers manipulate qubits that can exist in either of two basis states or the superposition of both states at the same time. Qubits represent the states of atoms or subatomic particles while bits represent the states of a transistor. Because particles are generally thought of as existing across all possible states at the same time, qubits can store much more information than bits and provide an inherent opportunity to be used in parallel computing.

Examples of Qubits

There are many different examples of qubits being used to test and demonstrate the capabilities of quantum computers. Below are some of the main qubit archetypes for which other hybrid qubit designs are built:

Flux Qubits - These are also known as Persistent Current Qubits and are micrometer-sized circuits where an external magnetic flux is applied to create a persistent current flow. The basis states in this case are the magnetic flux quanta trapped in a superconducting ring.

Charge Qubits - These are created in a tiny circuit that contains a superconducting island, a Josephson junction, and a superconducting reservoir. In this case, the basis states are the charge states of the superconducting island.

Phase Qubits - These are created in a circuit with a Josephson junction that is operated in a zero-voltage state. The basis states are the different quantum charge oscillation amplitudes across a Josephson junction, where the charge and the phase are analogous to momentum and position respectively of a quantum harmonic oscillator.