Qubit

A qubit is a two-level quantum system whose state can be written as α|0⟩ + β|1⟩, where α and β are complex probability amplitudes satisfying |α|² + |β|² = 1. The symbols |0⟩ and |1⟩ are orthogonal computational-basis states, not necessarily physical objects called zero and one. A measurement in that basis produces 0 with probability |α|² or 1 with probability |β|².

Unlike a classical bit, a qubit is not simply an unknown choice between two values. Relative phase—the relationship between the complex amplitudes—affects later interference and can be observed through suitable measurements. The complete pure-state space is represented geometrically by the Bloch sphere; antipodal points describe orthogonal states, while mixed states occupy its interior.¹

Quantum gates transform qubits reversibly through unitary operations, changing amplitudes and phases without directly revealing the encoded state. Common single-qubit gates include the Pauli gates, the Hadamard gate, and phase-rotation gates; controlled two-qubit gates can create entanglement.

Measurement converts quantum information into classical information and generally disturbs the measured state. Entangled qubits have joint states that cannot be factored into independent states, so their measurement statistics exhibit correlations stronger than those permitted by classical local models. Entanglement does not permit faster-than-light communication, because an individual measurement result remains intrinsically random.

Quantum algorithms exploit interference to increase the probability of useful outcomes. This advantage is problem-dependent: quantum computers are not universally faster than classical computers, and many tasks have no known meaningful quantum improvement.

A qubit is an abstract information unit that can be implemented by many physical systems. Leading platforms include superconducting electrical circuits, trapped ions, semiconductor spin qubits, neutral atoms, and photons; each offers different compromises among coherence, control speed, connectivity, fabrication, and measurement.²

Superconducting qubits are engineered circuits containing Josephson junctions and are controlled with microwave pulses. Trapped-ion qubits use internal states of ions held by electromagnetic fields, while semiconductor devices encode states in electron or hole spins. Photonic qubits may use polarization, path, or time-bin degrees of freedom, making them attractive for communication but often requiring specialized sources and detectors.

Physical qubits are vulnerable to noise from unwanted interactions with their surroundings. Coherence time measures how long selected quantum properties persist, but useful performance also depends on gate and readout errors, calibration, crosstalk, and the architecture connecting qubits.

Reliable quantum computation requires quantum error correction because an unknown quantum state cannot be copied for ordinary redundancy. Error-correcting codes distribute one logical qubit across many physical qubits, allowing certain errors to be detected indirectly through syndrome measurements without measuring the encoded information itself.³

One important family, the surface code, uses a two-dimensional array of qubits and local checks; its appeal comes from relatively high tolerance to certain noise models, although the overhead can be substantial.⁴ A logical qubit may therefore require many physical qubits, especially when operations must be fault tolerant.

Qubits also appear outside general-purpose processors. Quantum sensors use carefully controlled states to measure fields, time, acceleration, or gravity, while quantum communication protocols use properties such as measurement disturbance and entanglement. The term can thus describe an information abstraction, a laboratory degree of freedom, or a protected logical unit—not only a chip component.

Superposition

A quantum state formed from a linear combination of basis states, with complex amplitudes that determine measurement probabilities.

Entanglement

A nonseparable quantum correlation between systems whose joint state cannot be expressed as independent states.

Coherence

The preservation of well-defined quantum phase relationships needed for interference.

Logical qubit

A qubit encoded across multiple physical qubits to protect information from errors.