Arbitrary quantum control of qubits in the presence of universal noise

Abstract

In this manuscript we address the problem of deriving \emph{analytic} expressions for calculating universal decoherence-induced errors in qubits undergoing arbitrary, unitary, time-dependent quantum-control protocols. For a qubit undergoing unitary decoherence the evolution of a qubit state in the presence of time-varying semiclassical fields may be treated geometrically. We show that the fidelity of an arbitrary control operation may then be expressed to arbitrary order in terms of experimentally relevant spectral characteristics of the noise and the control over all Cartesian directions and accounting for noise cross-correlations. We formulate \emph{control matrices} in the time domain to capture the effects of piecewise-constant control, and convert them to generalized Fourier-domain filter functions. Such generalized filter functions may therefore be derived for complex temporally modulated control protocols, accounting for susceptibility to rotations of the qubit state vector in three dimensions. Taken together, this framework provides a computationally efficient means to calculate the effects of universal noise on arbitrary quantum control protocols without the need for time-consuming simulations of Bloch vector evolution. As a concrete example, we apply our method to treating the problem of dynamical decoupling incorporating realistic control pulses of arbitrary duration or form, including the replacement of simple $\pi$-pulses with complex dynamically corrected gates.