Bang–bang refocusing of a qubit exposed to telegraph noise, Quantum Information Processing 3

Solid state qubits promise the great advantage of being naturally scalable to large quantum computer architectures, but they also possess the significant disadvan-tage of being intrinsically exposed to many sources of noise in the macroscopic solid-state environment. With suitably chosen systems such as superconductors, many of sources of noise can be suppressed. However, imprecision in nanofabri-cation will inevitably induce defects and disorder, such as charged impurities in the device material or substrate. Such defects generically produce telegraph noise and can hence be modelled as bistable fluctuators. We demonstrate the possibil-ity of the active suppression of such telegraph noise by bang–bang control through an exhaustive study of a qubit coupled to a single bistable fluctuator. We use a stochastic Schrödinger equation, which is solved both numerically and analyt-ically. The resulting dynamics can be visualized as diffusion of a spin vector on the Bloch sphere. We find that bang–bang control suppresses the effect of a bista-ble fluctuator by a factor roughly equalling the ratio of the bang–bang period and the typical fluctuator period. Therefore, we show the bang–bang protocol work