Coherence Time

The duration over which a quantum processor's qubits can maintain their quantum state to perform calculations.

Coherence time is the duration over which a quantum processor's qubits can maintain their quantum state (such as superposition or entanglement) to perform calculations. In quantum computing, qubits are highly sensitive to their surrounding environments. Any external disturbance, such as temperature fluctuations, electromagnetic interference, or physical vibrations, causes the qubits to lose their quantum properties and decay into classical states—a destructive process known as decoherence. Because quantum computations must be completed before decoherence occurs, coherence time dictates the maximum number of logic gates (operations) a quantum computer can execute in a single run. If a system's coherence time is too short, errors accumulate rapidly, rendering the calculation useless. Consequently, extending coherence times is one of the most critical engineering hurdles in quantum hardware development. For technology investors, tracking coherence times across competing hardware platforms (such as superconducting qubits, trapped ions, or neutral atoms) is a key way to assess technological maturity. However, investors must evaluate coherence times alongside gate speeds and gate fidelity. A system with a long coherence time but very slow gate operations may perform fewer calculations than a system with a shorter coherence time but ultra-fast gate speeds.

A quantum processor has a qubit coherence time of 100 microseconds (0.0001 seconds). If a single quantum gate operation takes 50 nanoseconds (0.00000005 seconds) to execute, the maximum number of operations that can be performed before decoherence is 100,000 nanoseconds / 50 nanoseconds = 2,000 gates.