Spectral investigation of the noise influencing multi-qubit states

TL;DR

This paper experimentally characterizes noise in multi-qubit and decoherence-free states using NMR, analyzing noise spectra and scaling, and predicts the effectiveness of dynamical decoupling sequences for quantum coherence preservation.

Contribution

It introduces a comprehensive experimental approach to characterize noise in multi-qubit states and decoherence-free subspaces, and evaluates dynamical decoupling performance in complex quantum systems.

Findings

Noise spectra of multi-qubit states were successfully measured.

Scaling of noise with coherence order was observed.

Uhrig DD sequence improves coherence based on noise predictions.

Abstract

Characterizing and understanding noise affecting quantum states has immense benefits in spectroscopy as well as in realizing quantum devices. Transverse relaxation times under a set of dynamical decoupling (DD) sequences with varying interpulse delays were earlier used for obtaining the noise spectral densities of single-qubit coherences. In this work, using a pair of homonuclear spins and NMR techniques, we experimentally characterize noise in certain decoherence-free subspaces. We also explore the noise of similar states in a heteronuclear spin pair. Further, using a 10-qubit system, we investigate noise profiles of various multiqubit coherences and study the scaling of noise with respect to the coherence order. Finally, using the experimentally obtained noise spectrum of the 10-qubit NOON state, we predict the performance of a Uhrig DD sequence and verify it experimentally.