All articles starting with Q
2,252 articles
Quantum radio-frequency sensing with spin ensembles
Quantum radio-frequency sensing with spin ensembles uses collective quantum states of electron or nuclear spins, often NV centers in diamond, to detect radio-frequency and microwave magnetic fields.
Quantum random number generation
Quantum random number generation produces random numbers from quantum-mechanical processes, whose outcomes are intrinsically probabilistic rather than merely hard to predict, supplying entropy for cryptography and simulation.
Quantum random number generator
A quantum random number generator (QRNG) is a device that produces random numbers from quantum processes, such as photon measurements, with unpredictability guaranteed by quantum mechanics rather than algorithms.
Quantum recommendation systems
A quantum recommendation algorithm introduced by Iordanis Kerenidis and Anupam Prakash in 2016 that samples a product a user is likely to value in polylogarithmic time, later matched classically by Tang's 2019 algorithm.
Quantum reference frames
A quantum reference frame (QRF) is a quantum system used as the frame of reference for describing other systems, consolidated in a 2019 paper by Giacomini, Castro-Ruiz, and Brukner.
Quantum reinforcement learning
Quantum reinforcement learning (QRL) is a machine learning approach combining reinforcement learning with quantum computing, first framed in a 2008 paper by Daoyi Dong and colleagues.
Quantum relative entropy
The quantum relative entropy is a directed measure of distinguishability between two quantum states, defined as S(ρ||σ) = Tr ρ(log ρ − log σ), and is always non-negative.
Quantum Rényi entropy
The quantum Rényi entropy is a one-parameter family of entropy measures on quantum states that generalizes the von Neumann entropy, recovering it as α approaches 1.
Quantum repeater
A quantum repeater is a device placed along a quantum channel to extend entanglement distribution beyond direct transmission, since quantum states cannot be copied and amplified.
Quantum reservoir computing
Quantum reservoir computing (QRC) is a machine learning method in which a fixed quantum system processes temporal inputs while training is confined to a classical readout layer.
Quantum reservoir engineering
Quantum reservoir engineering, also called engineered dissipation or quantum bath engineering, couples quantum systems to engineered environments so that dissipation drives them into desired steady states, aiding error correction and sensing.
Quantum resource theory
Quantum resource theory is a quantum information framework that treats properties like entanglement and coherence as quantifiable resources, defined by free states and free operations.
Quantum scarring
Quantum scarring is the anomalous concentration of quantum eigenstate probability density along unstable periodic orbits of a chaotic system, first shown by Eric Heller in 1984.
Quantum scrambling
Quantum scrambling is the process by which information stored in a few degrees of freedom of a quantum many-body system spreads into nonlocal, entangled correlations, measured by the OTOC.
Quantum secret sharing
Quantum secret sharing is a quantum cryptographic protocol in which a dealer splits a secret among parties so only authorized subsets can reconstruct it, introduced in 1999.
Quantum secure direct communication
Quantum secure direct communication (QSDC) is a quantum communication protocol that transmits secret messages directly over a quantum channel, without prior key distribution and provably secure against quantum computing.
Quantum sensing
Quantum sensing is the use of quantum systems or phenomena to measure physical quantities such as magnetic fields, time, and gravity, with precision classical instruments cannot reach.
Quantum sensor
A quantum sensor is a measurement device using quantum properties, such as atomic energy levels and spin, to achieve sensitivity beyond what classical physics allows.
Quantum sieving of hydrogen isotopes in porous frameworks
Quantum sieving of hydrogen isotopes is a separation effect in which nanoporous materials like zeolites and MOFs separate H₂ from D₂ through quantum differences in diffusion and adsorption, proposed in 1995.
Quantum signal processing
Quantum signal processing (QSP) is a quantum algorithmic technique applying polynomial transformations to a block-encoded operator, introduced by Low and Chuang in 2017; with QSVT it underlies Hamiltonian simulation and other major quantum algorithms.
Quantum simulation
Quantum simulation is the approximate computational solution of the Schrödinger equation for molecules and materials, used in quantum chemistry and condensed matter physics; Feynman proposed quantum-computer simulation in 1982.
Quantum simulation of lattice gauge theories
Quantum simulation of lattice gauge theories uses cold atoms, trapped ions, and quantum computers to emulate lattice gauge theories such as QCD, avoiding the classical sign problem.
Quantum simulation of lattice gauge theories with dynamical matter
Quantum simulation of lattice gauge theories with dynamical matter uses controllable quantum systems, such as cold atoms and trapped ions, to emulate gauge fields coupled to mobile matter fields.
Quantum simulation of many-body and lattice models
Quantum simulation of many-body and lattice models uses engineered quantum systems, analog or digital, to emulate Hamiltonians like the Hubbard model that are intractable classically.
Quantum simulation of the Schwinger model
Quantum simulation of the Schwinger model uses trapped ions or superconducting qubits to reproduce the Schwinger model, quantum electrodynamics in one dimension, first demonstrated at Innsbruck in 2016.
Quantum simulator
A quantum simulator is a quantum device that uses entanglement to solve scientific and computational problems intractable for classical computers, an idea proposed by Richard Feynman in 1982.
Quantum singular value transformation
Quantum singular value transformation (QSVT) is a quantum algorithmic framework introduced by Gilyén, Su, Low, and Chuang in 2019 that applies polynomial functions to a matrix's singular values.
Quantum speed limit
A quantum speed limit, or quantum speed limit theorem, is a fundamental lower bound on the minimal time a quantum system needs to evolve between two distinguishable states.
Quantum spin Hall effect
The quantum spin Hall effect is a topological state of matter with an insulating bulk and spin-polarized conducting edges, predicted by Kane and Mele in 2005.
Quantum spin Hall effect of light
The quantum spin Hall effect of light, or photonic QSHE, is the photonic counterpart of the electronic quantum spin Hall effect, locking light's spin to its propagation direction.