All articles starting with Q
2,252 articles
Quantum channel
A quantum channel, or quantum communication channel, is a completely positive, trace-preserving linear map on density matrices that describes any physical change of a quantum state, including gates, measurements, noise, and communication.
Quantum chaos
Quantum chaos is the field of physics that studies quantum fingerprints of classical chaos, identified indirectly through energy-level statistics, eigenfunctions, and dynamics, since quantum evolution is linear.
Quantum Cheshire cat
The quantum Cheshire cat is a proposed quantum effect in which a particle's property, such as spin, appears in a different place from the particle, proposed in 2013.
Quantum chromodynamics
Quantum chromodynamics (QCD) is the quantum field theory of the strong interaction that binds quarks and gluons into protons and neutrons; developed in the early 1970s, it generates most ordinary mass.
Quantum circuit
A quantum circuit, also called the circuit model of quantum computation, is a sequence of unitary gates, measurements, and classical control acting on qubits; ideal circuits are reversible.
Quantum circuit complexity
Quantum circuit complexity measures how large a quantum circuit must be to implement a given unitary or Boolean function, counting gates (size) and parallel layers (depth).
Quantum circuit optimization
Quantum circuit optimization is a family of methods in quantum computing that rewrite circuits into equivalent ones with fewer gates or fewer qubits, vital on noisy hardware.
Quantum circuit simulation
Quantum circuit simulation is the classical computation of quantum circuit outputs; an N-qubit state vector needs 2^N amplitudes, so a 16 GB machine caps out near 30 qubits.
Quantum circuit transpilation
Quantum circuit transpilation is the compiler step that rewrites an abstract quantum circuit into one a specific device can execute, decomposing gates into native sets, assigning qubits, and inserting SWAPs.
Quantum Circuits
S. Harvey Moseley is an American astronomer and instrumentation physicist elected to the National Academy of Sciences in 2023, known for NASA's COBE, cryogenic X-ray detectors, and JWST microshutters.
Quantum clock synchronization
Quantum clock synchronization (QCS) is a set of protocols that establish a shared time reference between distant clocks using entanglement, single-photon interference, and other quantum correlations.
Quantum cloning
Quantum cloning is the approximate copying of unknown quantum states by a quantum cloning machine, since the no-cloning theorem forbids perfect replication; the best copies are measured by fidelity.
Quantum cohomology
Quantum cohomology is a deformation of the ordinary cohomology ring of a manifold in which the product counts holomorphic curves, introduced in the early 1990s from quantum string theory.
Quantum communication complexity
Quantum communication complexity studies how many qubits and how much shared entanglement two parties must exchange to compute a function, with quantum protocols sometimes exponentially reducing communication.
Quantum communication network
A quantum communication network is a system of quantum nodes that distributes entanglement between any two nodes, enabling quantum key distribution, teleportation, and distributed quantum computing.
Quantum communication primitives
Quantum communication primitives are minimal tasks that use quantum resources like entanglement, chiefly quantum teleportation, superdense coding, and quantum key distribution, bounded by no-cloning and no-signaling.
Quantum computing
Quantum computing is computing with qubits, which exploit superposition and entanglement to solve problems classical computers handle poorly; today's error-prone machines aim at molecular simulation and cryptography.
Quantum computing for chemistry and electronic structure
Quantum computing for chemistry and electronic structure applies quantum computers to molecular simulation; variational algorithms suit near-term hardware, while quantum phase estimation with error correction targets fault-tolerant machines.
Quantum conditional entropy
Quantum conditional entropy, H(A|B) = H(AB) − H(B), measures the entropy of one part of a quantum system given the other, and can be negative when the parts are entangled.
Quantum confinement effect
The quantum confinement effect is the change in a material's electronic and optical properties when its dimensions shrink to nanoscale, making its energy levels discrete and size-dependent.
Quantum contextuality
Quantum contextuality is the failure of quantum mechanics to admit a classical model where a measurement result is independent of other jointly performed compatible measurements.
Quantum convolutional neural network
A quantum convolutional neural network (QCNN) is a variational quantum machine learning model introduced by Cong, Choi, and Lukin in Nature Physics in 2019, using O(log N) parameters.
Quantum critical point
A quantum critical point is where a quantum system undergoes a continuous phase transition at zero temperature, driven by a non-thermal parameter such as magnetic field, pressure, or chemical doping.
Quantum cryptography
Quantum cryptography, also known as quantum cryptology, is a set of methods using quantum mechanics to secure information, best known for quantum key distribution, whose BB84 protocol dates to 1984.
Quantum cutting
Quantum cutting is a photoluminescence process that converts one high-energy photon into two lower-energy photons, first observed in 1974 and studied for boosting silicon solar cells.
Quantum Darwinism
Quantum Darwinism is a framework proposed by Wojciech Zurek in 2009 that explains how objective classical reality emerges from quantum mechanics through redundant records in the environment.
Quantum data locking
Quantum data locking (QDL) is a quantum communication phenomenon in which a short secret key can encrypt an arbitrarily long quantum-encoded message, defying classical key-length requirements.
Quantum decoherence
Quantum decoherence is the loss of quantum interference that occurs when a quantum system entangles with its environment, making its behavior effectively classical and posing the main obstacle to quantum computing.
Quantum defect theory
Quantum defect theory (QDT) is an atomic and molecular physics method that describes photoabsorption and electron-ion scattering using a few short-range quantum-defect parameters with analytic Coulomb wave functions.
Quantum depolarizing channel
The quantum depolarizing channel is a noise model in quantum information theory where a state is kept with some probability and otherwise replaced by the maximally mixed state.