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Glossary

Quantum glossary

Coherence time (T1, T2)

Also: t1, t2, relaxation time, dephasing time

How long a physical qubit keeps its quantum information: T1 for energy relaxation, T2 for loss of phase.

T1 is the time for |1⟩ to decay to |0⟩; T2 is the time for a superposition to lose its phase relationship, and T2 ≤ 2T1. Divide the coherence time by the gate time and you get a rough budget for how many operations fit before the state degrades.

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Related terms

  • Decoherence — The loss of quantum behaviour as a system becomes entangled with its uncontrolled environment.
  • Noise — Any unwanted process that disturbs a quantum computation: imperfect gates, decoherence, crosstalk, readout errors.