Multiple-spin coherence transfer in linear Ising spin chains and beyond: numerically-optimized pulses and experiments

TL;DR

This paper investigates optimal control pulses for multiple-spin coherence transfer in linear Ising spin chains, combining numerical optimization and experiments to improve quantum information transfer and spectral analysis.

Contribution

It introduces a systematic numerical approach supporting the sufficiency of restricted controls for time-optimal coherence transfer, and demonstrates experimental robustness in realistic spin systems.

Findings

Restricted control families are sufficient for time-optimal transfers.

Pulse sequences are effective even with long-range couplings.

Experimental implementation shows robustness under relaxation and imperfections.

Abstract

We study multiple-spin coherence transfers in linear Ising spin chains with nearest neighbor couplings. These constitute a model for efficient information transfers in future quantum computing devices and for many multi-dimensional experiments for the assignment of complex spectra in nuclear magnetic resonance spectroscopy. We complement prior analytic techniques for multiple-spin coherence transfers with a systematic numerical study where we obtain strong evidence that a certain analytically-motivated family of restricted controls is sufficient for time-optimality. In the case of a linear three-spin system, additional evidence suggests that prior analytic pulse sequences using this family of restricted controls are time-optimal even for arbitrary local controls. In addition, we compare the pulse sequences for linear Ising spin chains to pulse sequences for more realistic spin systems with additional long-range couplings between non-adjacent spins. We experimentally implement the derived pulse sequences in three and four spin systems and demonstrate that they are applicable in realistic settings under relaxation and experimental imperfections-in particular-by deriving broadband pulse sequences which are robust with respect to frequency offsets.