Quantum coherence

Quantum coherence is the preservation of definite phase relationships among components of a quantum system. It enables quantum interference and is lost when interactions with the environment make those relationships inaccessible.

Quantum superposition: A quantum state formed from a linear combination of possible states. Coherent superpositions retain phase relationships that allow their components to interfere.
Quantum state: The mathematical description of a quantum system's physical properties and measurement probabilities. Coherence is a property of a quantum state relative to a chosen basis.
Quantum computing: Computation that uses quantum states and operations to process information. Quantum algorithms use coherent evolution to combine amplitudes and produce useful interference.
Quantum coherence time: The characteristic duration over which a quantum system preserves a specified form of coherence. Different coherence times quantify how long a device can maintain phase-sensitive behavior.
Quantum interference: The enhancement or suppression of quantum probabilities when alternative amplitudes combine. Interference is the observable consequence of phase relationships among alternatives.
Quantum amplitude: A complex number whose squared magnitude contributes to a quantum measurement probability. Coherent amplitudes combine with their relative phases before probabilities are calculated.
Quantum error correction: Methods for protecting quantum information from errors using encoded states and recovery operations. Error-correcting codes help preserve logical coherence despite noise in physical qubits.
Quantum noise: Uncertainty and fluctuations that arise in quantum systems and measurements. Noise can randomize relative phases and limit coherence in experiments.
Density matrix: A mathematical representation of a quantum state, including statistical mixtures and coherent superpositions. Its off-diagonal elements encode coherence between basis states.
Pure state: A quantum state describable by a single state vector, with maximal information about the system. Pure states can contain coherence, though purity alone does not guarantee it in every basis.

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