Decoherence is the central enemy of quantum computing. It describes the process by which a qubit's fragile quantum state — its superposition and entanglement — degrades through interaction with the surrounding environment. Thermal fluctuations, electromagnetic noise, vibrations, cosmic rays, and even stray photons can cause a qubit to lose its quantum information and collapse into a classical state.
Decoherence manifests in two primary ways: relaxation (T1 decay), where an excited qubit spontaneously decays to its ground state, and dephasing (T2 decay), where the relative phase between superposition components randomizes. T2 is always less than or equal to 2×T1. Modern superconducting qubits achieve T1 and T2 times of 100-500 microseconds; trapped-ion qubits can reach seconds to minutes; neutral atom qubits typically achieve hundreds of milliseconds to seconds.
The practical impact of decoherence is that all quantum operations must complete within the coherence time window, and errors accumulate with every gate operation. This is why quantum error correction is essential for large-scale quantum computing — without it, decoherence makes long computations impossible. Engineering efforts to combat decoherence include extreme cooling (dilution refrigerators at 10-15 millikelvin), electromagnetic shielding, vibration isolation, and materials science improvements to reduce defects in qubit fabrication.