Decoherence
Decoherence is the loss of quantum coherence as a system interacts with its environment, turning superpositions into classical mixtures.
The core problem
No qubit is perfectly isolated. Interaction with the surrounding environment — stray fields, vibrations, thermal photons — entangles the qubit with degrees of freedom you cannot track. Averaging over that inaccessible environment converts a coherent superposition into a classical mixture. This is decoherence, and it is the dominant obstacle to building quantum computers.
What is lost
In the density matrix, decoherence suppresses the off-diagonal coherence terms while leaving the diagonal populations. A pure superposition (|0>+|1>)/sqrt(2), whose density matrix has equal off-diagonal entries, decays toward the mixed diagonal state — statistically 0 or 1, with the quantum interference capacity gone.
Two timescales
- T1 — energy relaxation: an excited |1> decays toward |0>
- T2 — dephasing: the relative phase between |0> and |1> randomises
T2 is at most 2*T1 and is usually shorter, because phase is more fragile than energy. These times are the headline coherence specifications of any qubit technology.
Why it looks classical
Decoherence explains the appearance of a classical world: macroscopic objects couple so strongly to their surroundings that superpositions vanish almost instantly, which is why we never see a everyday object in two places at once. For a computer, though, this same process is a leak that corrupts the calculation.
Fighting it
Three strategies are combined: build better-isolated hardware to lengthen coherence times, run gates faster than the decoherence timescale, and use quantum error correction to detect and reverse errors faster than they accumulate. Fault tolerance succeeds only when the physical error rate per gate is pushed below a threshold set by the code.