Ball and Biercuk EPJ Quantum Technology (2015)2:11 DOI 10.1140/epjqt/s40507-015-0022-4 R E S E A R C H Open Access Walsh-synthesized noise filters for quantum logic Harrison Ball1,2 and Michael J Biercuk1,2* *Correspondence:
[email protected] Abstract 1ARC Centre for Engineered Quantum Systems, School of We study a novel class of open-loop control protocols constructed to perform Physics, The University of Sydney, arbitrary nontrivial single-qubit logic operations robust against time-dependent Sydney, NSW 2006, Australia non-Markovian noise. Amplitude and phase modulation protocols are crafted 2National Measurement Institute, West Lindfield, Sydney, NSW 2070, leveraging insights from functional synthesis and the basis set of Walsh functions. We Australia employ the experimentally validated generalized filter-transfer function formalism in order to find optimized control protocols for target operations in SU(2) by defining a cost function for the filter-transfer function to be minimized through the applied modulation. Our work details the various techniques by which we define and then optimize the filter-synthesis process in the Walsh basis, including the definition of specific analytic design rules which serve to efficiently constrain the available synthesis space. This approach yields modulated-gate constructions consisting of chains of discrete pulse-segments of arbitrary form, whose modulation envelopes possess intrinsic compatibility with digital logic and clocking. We derive novel families of Walsh-modulated noise filters designed to suppress dephasing and coherent amplitude-damping noise, and describe how well-known sequences derived in NMR also fall within the Walsh-synthesis framework. Finally, our work considers the effects of realistic experimental constraints such as limited modulation bandwidth on achievable filter performance.