coherence time (T1, T2)

A qubit is fragile. The moment you put it into a delicate quantum state, the outside world starts nudging it back toward something ordinary, and the quantum information you carefully wrote in begins to leak away. Coherence time is the answer to a simple question: how long does a qubit stay quantum before that leakage scrambles it? Think of it like a spinning coin balanced on its edge, holding a precise tilt. Coherence time measures how long it keeps that tilt before stray vibrations make it wobble and fall flat.

There are two clocks. T1 is the energy-relaxation time: a qubit excited into |1> tends to dump that energy into its environment and slide back down to |0>, the way a hot object cools to room temperature. T2 is the dephasing time, and it tracks something subtler. A qubit's superposition carries a relative phase between |0> and |1> (the alpha and beta amplitudes), and that phase is where interference, the thing that actually makes quantum algorithms work, lives. Random environmental noise jitters the phase until it becomes meaningless. T2 is usually shorter than T1, because anything that flips the energy also disturbs the phase, but extra noise can spoil the phase on its own.

In practice these times are the budget every computation spends. Each gate takes some time to run, so the ratio of coherence time to gate time roughly sets how many operations you can chain together before the answer drowns in noise. On today's machines that budget is small, which is the core reason we are still in the NISQ era and not yet running large fault-tolerant algorithms. Longer coherence is not the whole story (gate accuracy and qubit count matter too), but it is one of the hard physical limits everyone is fighting against.