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Quantum computing glossary

Qubits, superposition, entanglement, quantum gates, Grover’s and Shor’s algorithms, error correction: quantum computing terms explained in plain English.

29 terms · new to the topic? Start with what is a quantum computer?

• Qubit · A qubit is the basic unit of information in a quantum computer: measured, it gives 0 or 1, but before measurement it can be in a superposition of both.
• Superposition · Superposition is the quantum state in which a qubit, before it is measured, holds the states 0 and 1 together with definite amplitudes.
• Quantum measurement · Quantum measurement ends a qubit’s superposition and returns 0 or 1, with the result chosen at random according to the amplitudes.
• Quantum entanglement · Entanglement means several qubits share a joint quantum state, so their measurement results are correlated even when the qubits are far apart.
• Quantum interference · Quantum interference is the way amplitudes add up like waves, raising the probability of some results and cancelling others to zero.
• Quantum gate · A quantum gate is a basic operation that reversibly changes the state of one or more qubits; quantum programs are sequences of gates.
• Hadamard gate (H) · The Hadamard gate is a single-qubit quantum gate that turns a qubit in |0⟩ into an equal superposition of 0 and 1.
• CNOT gate · The CNOT (controlled-NOT) gate is a two-qubit gate that flips the target qubit when the control qubit is 1; it is the basic tool for creating entanglement.
• Quantum circuit · A quantum circuit is a quantum program made of gates applied to qubits in order, followed by measurements.
• Bell state · A Bell state is one of four special states in which two qubits are entangled in the simplest and strongest way; the best known gives only 00 or 11 when measured.
• Grover’s algorithm · Grover’s algorithm finds a marked item among N unsorted items in about √N steps, a quadratic speed-up over classical search.
• Shor’s algorithm · Shor’s algorithm factors large numbers exponentially faster than known classical methods, which is why it threatens RSA encryption.
• Quantum error correction · Quantum error correction spreads one logical qubit over many physical qubits so that errors caused by noise can be found and fixed without measuring the qubits directly.
• Surface code · The surface code arranges qubits on a two-dimensional grid and uses only operations between neighbouring qubits; it is the most widely tested quantum error-correcting code today.
• Decoherence · Decoherence is a qubit gradually losing its superposition and entanglement, its quantum properties, through unwanted interaction with its environment.
• NISQ (noisy intermediate-scale quantum) · NISQ is the current era of quantum computing, named by John Preskill in 2018: devices with tens to a few hundred noisy qubits and no error correction.
• VQE (variational quantum eigensolver) · VQE is a hybrid quantum algorithm that approximates the ground-state energy of a molecule by tuning a parameterised quantum circuit in a classical optimisation loop.
• QAOA (quantum approximate optimisation algorithm) · QAOA combines parameterised quantum circuits with classical optimisation to search for approximate solutions to combinatorial optimisation problems such as MaxCut.
• Quantum teleportation · Quantum teleportation is a protocol that transfers the unknown state of a qubit to another qubit using a shared entangled pair and two classical bits.
• Quantum key distribution (QKD, BB84) · Quantum key distribution lets two parties share a secret encryption key while detecting eavesdropping attempts thanks to the nature of quantum measurement.
• Quantum supremacy · Quantum supremacy is the demonstration that a quantum computer performs a specific task that the most powerful classical computers cannot complete in reasonable time.
• Qiskit · Qiskit is IBM’s open-source Python library for quantum programming; it lets you build circuits, run them on simulators and send them to IBM quantum computers.
• Bloch sphere · The Bloch sphere is a geometric picture that shows every state of a single qubit as a point on the surface of a sphere: the north pole is |0⟩, the south pole |1⟩.
• Deutsch–Jozsa algorithm · The Deutsch–Jozsa algorithm is one of the first quantum algorithms: it decides with certainty, in a single query, whether a function is constant or balanced.
• Bernstein–Vazirani algorithm · The Bernstein–Vazirani algorithm finds the hidden bit string s inside a function f(x) = s·x in a single query, instead of the n queries needed classically.
• Quantum Fourier transform (QFT) · The quantum Fourier transform (QFT) computes the discrete Fourier transform of the amplitudes with H and controlled-phase gates; it underlies period finding and phase estimation.
• Quantum phase estimation (QPE) · Quantum phase estimation (QPE) reads the phase a gate adds to one of its eigenstates as a binary number in counting qubits, using phase kickback and the inverse Fourier transform.
• No-cloning theorem · The no-cloning theorem states that it is impossible to make an exact copy of an unknown quantum state; it is a fundamental constraint for quantum cryptography and error correction.
• Post-quantum cryptography (PQC) · Post-quantum cryptography means encryption algorithms that run on today’s classical computers and are believed to stay secure against large quantum computers.