# Experimental protection of arbitrary states in a two-qubit subspace by nested Uhrig dynamical decoupling

## TL;DR

This paper demonstrates experimentally that a three-layer nested Uhrig dynamical decoupling sequence effectively preserves arbitrary two-qubit states in an NMR quantum processor, showing good average fidelity improvements over unprotected states.

## Contribution

It is the first experimental demonstration of nested Uhrig dynamical decoupling protecting arbitrary two-qubit states in a subspace.

## Key findings

- NUDD preserves states with high fidelity
- Performance varies among states but is generally good
- Fidelity with NUDD exceeds unprotected states

## Abstract

We experimentally demonstrate the efficacy of a three-layer nested Uhrig dynamical decoupling (NUDD) sequence to preserve arbitrary quantum states in a two-dimensional subspace of the four-dimensional two-qubit Hilbert space, on an NMR quantum information processor. The effect of the state preservation is studied first on four known states, including two product states and two maximally entangled Bell states. Next, to evaluate the preservation capacity of the NUDD scheme, we apply it to eight randomly generated states in the subspace. Although, the preservation of different states varies, the scheme on the average performs very well. The complete tomographs of the states at different time points are used to compute fidelity. State fidelities using NUDD protection are compared with those obtained without using any protection. The nested pulse schemes are complex in nature and require careful experimental implementation.