Hardware & Errors
Coherence Time
The duration a qubit can maintain its quantum state before decoherence causes it to lose quantum information.
Definition
Coherence time describes how long a qubit can preserve its quantum properties — superposition and phase relationships — before environmental noise causes it to decay into a classical-like state. Longer coherence times allow more operations to be performed before errors accumulate.
Technical Definition
Two specific coherence times are commonly measured: T₁ (relaxation time, how long before a qubit in |1⟩ decays toward |0⟩) and T₂ (dephasing time, how long phase information is preserved). T₂ is always less than or equal to twice T₁ in standard models.
Visual Explanation: An Analogy
Think of coherence time like how long a spinning top keeps spinning before friction and air resistance slow it down and it falls over — the longer it spins, the more 'useful time' you have before it loses its special spinning state.
Real-World Use Cases
- A key benchmark for comparing different quantum hardware platforms
- Determines how many gate operations can realistically be performed in a circuit
- Trapped-ion and neutral atom systems generally report longer coherence times than superconducting qubits