Quantum discord and the power of one qubit

TL;DR

This paper investigates the quantum correlations in the DQC1 model using quantum discord, revealing nonclassical correlations despite the absence of entanglement, and proposes discord as a resource measure.

Contribution

It demonstrates that quantum discord is nonzero in DQC1, providing a new perspective on quantum resources beyond entanglement in mixed-state quantum computation.

Findings

Quantum discord is nonzero in DQC1 for typical instances.

Nonzero discord indicates presence of nonclassical correlations without entanglement.

Quantum discord can serve as a resource measure in mixed-state quantum computation.

Abstract

We use quantum discord to characterize the correlations present in the quantum computational model DQC1, introduced by Knill and Laflamme [Phys. Rev. Lett. 81, 5672 (1998)]. The model involves a collection of qubits in the completely mixed state coupled to a single control qubit that has nonzero purity. The initial state, operations, and measurements in the model all point to a natural bipartite split between the control qubit and the mixed ones. Although there is no entanglement between these two parts, we show that the quantum discord across this split is nonzero for typical instances of the DQC1 ciruit. Nonzero values of discord indicate the presence of nonclassical correlations. We propose quantum discord as figure of merit for characterizing the resources present in this computational model.