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Glossary

Quantum glossary

Decoherence

The loss of quantum behaviour as a system becomes entangled with its uncontrolled environment.

Nothing mysterious “collapses”: information about the qubit leaks into its surroundings, and from the qubit’s point of view a superposition turns into an ordinary coin toss. It is the central engineering obstacle to building quantum computers.

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Related terms

  • Coherence time (T1, T2) — How long a physical qubit keeps its quantum information: T1 for energy relaxation, T2 for loss of phase.
  • Density matrix — A matrix description of a quantum state that can also represent uncertainty about which state you have.
  • Mixed state — A state that is a statistical mixture of pure states, describing classical uncertainty on top of quantum uncertainty.