Decoherence is the primary enemy of quantum computation. A qubit in superposition is extraordinarily sensitive to its environment — thermal fluctuations, electromagnetic interference, vibrations, and even stray photons can cause the qubit to lose its quantum state and behave like an ordinary classical bit. The time a qubit can maintain its quantum properties is called its coherence time, and extending this window is one of the central engineering challenges in quantum computing.
Coherence times vary dramatically across qubit technologies. Superconducting qubits (IBM, Google) typically maintain coherence for 100-300 microseconds. Trapped-ion qubits (IonQ, Quantinuum) can remain coherent for seconds or even minutes, thanks to the natural isolation of individual atoms in vacuum. Nitrogen-vacancy centers in diamond and certain nuclear spin qubits have demonstrated coherence times of hours under special conditions. Each qubit technology applies different strategies to fight decoherence, from dilution refrigerators that cool superconducting chips to near absolute zero, to electromagnetic shielding and vibration isolation.
The relationship between coherence time and gate speed determines how many operations a quantum computer can perform before errors overwhelm the computation. This ratio — sometimes expressed as the T2/gate-time ratio — is a key figure of merit for any quantum platform. Even with the best coherence times available, practical quantum algorithms require quantum error correction to extend effective computation far beyond what raw hardware coherence allows. For deeper coverage, see DeepTechIntel's quantum computing section.