Optimized dynamical protection of nonclassical correlation in a quantum algorithm

TL;DR

This paper demonstrates an integrated approach combining dynamical decoupling with quantum gates via optimal control to protect nonclassical correlations, like quantum discord, during quantum algorithms, using NMR experiments.

Contribution

It introduces a method to enhance quantum gate robustness against decoherence by integrating dynamical decoupling with optimal control, specifically protecting nonclassical correlations in quantum algorithms.

Findings

Phase alternating DD sequences improve protection.

π/2 pulse sequences perform as well or better than π pulses.

Experimental validation with NMR shows significant decoherence suppression.

Abstract

A quantum memory interacts with its environment and loses information via decoherence as well as incoherence. A robust quantum control that prepares, preserves, and manipulates nonclassical correlations even in the presence of environmental influence is of paramount importance in quantum information processing. A well-known technique to suppress decoherence, namely Dynamical Decoupling (DD), consists of a sequence of rapid flips applied to the system in order to refocus the system-environment interactions. In this work, we integrate DD with quantum gates using optimal control techniques to realize robust quantum gates which offer protection against decoherence. To investigate the protection of non-classical correlation, we study the evolution of quantum discord in Grover's search algorithm implemented with dynamically protected gates. Using a two-qubit NMR system, we experimentally demonstrate a significant protection against decoherence and incoherence. We find better performances by phase alternating DD sequences with suitable spacings between the DD pulses. Interestingly, we also find that DD sequences based on $\pi/2$ pulses perform as well as or even better than those with $\pi$ pulses in protecting the non-classical correlation. We also support the experimental results by analyzing the robustness of various DD schemes.