Decoherence Budget

The decoherence budget is an informal but useful concept describing how much total error a quantum circuit can accumulate before the output becomes dominated by noise rather than meaningful computation. It depends on qubit T1/T2 times, gate error rates, and circuit depth. The total error per qubit scales roughly as (circuit depth × gate time / T2). Circuits that exceed the decoherence budget produce results that are essentially random. Managing the decoherence budget requires minimizing circuit depth through transpiler optimization, using native gates, and applying error mitigation techniques. As NISQ hardware improves, the decoherence budget grows — allowing deeper, more complex algorithms to run reliably. Fault-tolerant quantum computing eliminates the decoherence budget entirely through quantum error correction.