What is a qubit?

How it works

A classical bit holds a 0 or a 1. A qubit can be 0, 1 or a blend of both, a condition called superposition, according to NIST's quantum computing explainer. Qubits can also be entangled, meaning their quantum states are linked instead of independent. The number of combinations a machine can represent at once doubles with every qubit added. Two qubits cover four and three cover eight. The catch, NIST notes, is that measuring the qubits when a calculation ends yields only a little information about those combinations.

The word was in print by April 1995, when Benjamin Schumacher published a Physical Review A paper. It counted quantum information in spin-1/2 systems, which it called "quantum bits" or "qubits."

Types of qubits

A qubit is a role, not a single device. NIST lists several ways to build one:

• trapped ions (electrically charged atoms held in a vacuum by electric or magnetic fields)
• tiny circuits of superconductors (materials with no electrical resistance at very cold temperatures)
• neutral atoms held by laser light
• single photons
• electrons confined in semiconductors such as silicon

NIST says ion qubits keep a superposition for a long time but compute comparatively slowly. It says superconducting qubits are quick and can be produced with existing chip-making methods, though their quantum states are more fragile and do not last as long.

Where things stand in 2026

Qubits remain error-prone. The NIST page, last updated on May 28, 2026, says the best quantum computers have hundreds of linked qubits and err about once per thousand operations. A classical computer makes about one error per quintillion calculations, it says. It adds that running Shor's code-breaking algorithm may take millions of qubits.

Larger arrays exist in the lab. Caltech said in September 2025 that its physicists had trapped 6,100 neutral-atom qubits in one grid, which it called the largest qubit array yet assembled. Entangling them was listed as a next step.

Because physical qubits make errors, hardware is also measured in logical qubits. Each is encoded redundantly in many physical qubits. IEEE Spectrum reported in June 2026 that the record holders were then QuEra, which claimed 96 logical qubits, and Quantinuum, which claimed 94. The U.S. Department of Energy said that month that a competition it announced aims to demonstrate fault-tolerant systems with logical qubits in the low hundreds in 2028.