80 articles
Accessible information
Accessible information is a quantity in quantum information theory measuring how much classical information can actually be extracted from quantum states by the best possible measurement.
Amplitude damping channel
The amplitude damping channel is a quantum channel modeling energy loss from a qubit to its environment, with the excited state decaying to the ground state with probability γ.
Biased-noise surface code
The biased-noise surface code is a surface code variant for quantum error correction that exploits noise bias, where dephasing dominates; the best-known example is the XZZX surface code.
Bloch sphere
The Bloch sphere is a unit sphere representing a single qubit's states as points, with pure states on the surface, mixed states inside, and unitary gates acting as rotations.
Bosonic and Gaussian channel capacity
Bosonic and Gaussian channel capacity is the study of how much classical, quantum, or secret-key information can be reliably transmitted per use of lossy light channels.
Bound entanglement and the distillability problem
Bound entanglement is mixed-state quantum entanglement that cannot be distilled into singlets by LOCC, established by the Horodecki 1998 theorem; whether NPT bound entanglement exists remains open.
Cat qubit
A cat qubit is a superconducting qubit, named for Schrödinger's cat, that encodes one logical qubit in a microwave cavity with exponentially suppressed bit-flip errors.
Classical capacity
The classical capacity of a quantum channel is the maximum rate, in bits per use, at which classical information can be transmitted reliably, the quantum analog of Shannon capacity.
Coherent information
Coherent information is a measure in quantum information theory, defined as a channel's output entropy minus its environment's entropy, introduced by Schumacher and Nielsen in 1998.
Concatenated codes
Concatenated codes are quantum error-correcting codes in which each qubit of one code is encoded by another, introduced by Knill and Laflamme in 1996 and used in the first threshold-theorem proofs.
Decoherence-free subspace
A decoherence-free subspace (DFS) is a subspace of a quantum system's Hilbert space in which states evolve unitarily, providing passive error avoidance when noise is symmetric.
Depolarizing channel
The depolarizing channel is the standard symmetric noise model in quantum information theory, applying bit flips, phase flips, and combined flips with equal probability, analogous to the binary symmetric channel.
Diamond norm
The diamond norm, also called the completely bounded trace norm, is a norm on linear maps introduced by Alexei Kitaev to measure the worst-case distinguishability of two quantum channels.
Eigenstate thermalization hypothesis
The eigenstate thermalization hypothesis (ETH) is a quantum statistical mechanics conjecture that individual eigenstates of nonintegrable systems are locally thermal, explaining how closed quantum systems equilibrate.
Entanglement catalysis
Entanglement catalysis (quantum catalysis) is a quantum information phenomenon where an entangled catalyst state enables otherwise impossible transformations, returned unchanged, introduced by Jonathan and Plenio in 1999.
Entanglement distillation
Entanglement distillation, also called entanglement purification, is a quantum information protocol that turns many noisy entangled pairs into fewer near-perfect ones, enabling faithful quantum teleportation through noisy channels.
Entanglement in continuous-variable systems
Entanglement in continuous-variable systems is the nonseparability of bosonic modes such as optical field modes, whose entanglement is carried by continuous quadratures like position and momentum.
Entanglement in many-body quantum systems
Entanglement in many-body quantum systems studies how entanglement is distributed and structured across many interacting particles, using tools like entanglement entropy, the entanglement spectrum, and topological entanglement entropy.
Entanglement measure
An entanglement measure is a function assigning a nonnegative number to a quantum state to quantify its entanglement, vanishing on separable states and normalized on maximally entangled pairs.
Entanglement of formation
The entanglement of formation (EoF) is a measure of quantum entanglement for a bipartite quantum state, defined as the minimum average entanglement over all pure-state ensembles representing the mixed state.
Entanglement witness
An entanglement witness is a Hermitian observable used to certify quantum entanglement. It is a practical alternative to full state tomography, using negative expectation values to prove entanglement.
Entanglement-assisted classical capacity
The entanglement-assisted classical capacity, C_E, of a quantum channel is the maximum reliable classical bit rate with unlimited shared entanglement, characterized by Bennett, Shor, Smolin, and Thapliyal in 1999.
Entropic uncertainty relations
An entropic uncertainty relation is a lower bound on the Shannon entropies of outcomes from incompatible quantum measurements, constraining the full spread rather than variances.
Erasure correction
Erasure correction is the branch of quantum error correction that protects quantum information against erasure errors, faults whose location is known even though the error's effect is not.
Fannes–Audenaert inequality
The Fannes–Audenaert inequality is a sharp upper bound on the difference between the von Neumann entropies of two quantum states in terms of their trace distance, refining Mark Fannes' 1973 inequality.
Fidelity (quantum mechanics)
Fidelity in quantum mechanics measures how similar two quantum states are, from 0 to 1, and is used to benchmark teleportation, quantum gates, and prepared states.
Five-qubit code
The five-qubit code, also known as the Laflamme code, is a quantum error-correcting code denoted [[5,1,3]] that encodes one logical qubit into five, the minimum for correcting any single-qubit error.
Fracton
A fracton is a quasiparticle excitation of a quantum many-body system that cannot move freely, giving rise to fracton phases that defy standard topological order.
Frequency-bin entanglement
Frequency-bin entanglement is a form of quantum entanglement where photon pairs share correlations across discrete frequency bins rather than polarization or paths, used for fiber-based quantum key distribution.
Generalized relative entropy
Generalized relative entropy is any function of two quantum states extending the classical relative entropy, a family including Petz's quantum f-divergences, Rényi relative entropies, and min- and max-relative entropies.