Fidelity enhancement by logical qubit encoding

TL;DR

This paper demonstrates high-fidelity control of logical qubits encoded in a decoherence-free subspace using NMR, showing improved fidelity over full Hilbert space control by leveraging dipolar couplings.

Contribution

It introduces a method for logical qubit encoding in a DFS with high control fidelity in an NMR system, despite unknown Hamiltonian details.

Findings

Logical Bell state creation in DFS

Higher fidelity control in DFS than full space

Effective use of dipolar couplings for control

Abstract

We demonstrate coherent control of two logical qubits encoded in a decoherence free subspace (DFS) of four dipolar-coupled protons in an NMR quantum information processor. A pseudo-pure fiducial state is created in the DFS, and a unitary logical qubit entangling operator evolves the system to a logical Bell state. The four-spin molecule is partially aligned by a liquid crystal solvent, which introduces strong dipolar couplings among the spins. Although the system Hamiltonian is never fully specified, we demonstrate high fidelity control over the logical degrees of freedom. In fact, the DFS encoding leads to higher fidelity control than is available in the full four-spin Hilbert space.