What is Threshold Probability

Introduction to Quantum Computing

Threshold Theorem

The quantum threshold theorem states that a quantum computer with a physical error rate below a certain threshold can, through application of quantum error correction schemes, suppress the logical error rate to arbitrarily low levels. 

The threshold theorem resolves whether quantum computers, in practice, can perform long computations without succumbing to noise.  As small, constant error is inevitable, this could mean that quantum computers with imperfect gates can only apply a constant number of gates before the computation is destroyed by noise.

Begin with a family of circuits and error free gates.