Coherence protection and decay mechanism in qubit ensembles under concatenated continuous driving

TL;DR

This paper demonstrates how concatenated continuous driving (CCD) can significantly enhance the coherence times of dense spin qubit ensembles, with experimental validation on NV centers showing up to 15-fold improvements.

Contribution

The study provides a detailed Floquet theory analysis of CCD dynamics, optimizing driving parameters for maximal coherence protection in dense qubit ensembles.

Findings

15-fold increase in coherence time for unknown states

500-fold increase for known states via optimized driving

Identification of dominant decay mechanisms in NV ensembles

Abstract

Dense ensembles of spin qubits are valuable for quantum applications, even though their coherence protection remains challenging. Continuous dynamical decoupling can protect ensemble qubits from noise while allowing gate operations, but it is hindered by the additional noise introduced by the driving. Concatenated continuous driving (CCD) techniques can, in principle, mitigate this problem. Here we provide deeper insights into the dynamics under CCD, based on Floquet theory, that lead to optimized state protection by adjusting driving parameters in the CCD scheme to induce mode evolution control. We experimentally demonstrate the improved control by simultaneously addressing a dense Nitrogen-vacancy (NV) ensemble with $10^{10}$ spins. We achieve an experimental 15-fold improvement in coherence time for an arbitrary, unknown state, and a 500-fold improvement for an arbitrary, known state, corresponding to driving the sidebands and the center band of the resulting Mollow triplet, respectively. We can achieve such coherence time gains by optimizing the driving parameters to take into account the noise affecting our system. By extending the generalized Bloch equation approach to the CCD scenario, we identify the noise sources that dominate the decay mechanisms in NV ensembles, confirm our model by experimental results, and identify the driving strengths yielding optimal coherence. Our results can be directly used to optimize qubit coherence protection under continuous driving and bath driving, and enable applications in robust pulse design and quantum sensing.