Experimental detection of nonclassical correlations in mixed-state quantum computation

Abstract

We report on an experiment to detect non-classical correlations in a highly mixed state. The correlations are characterized by the quantum discord and are observed using four qubits in a liquid state nuclear magnetic resonance quantum information processor. The state analyzed is the output of a DQC1 computation, whose input is a single quantum bit accompanied by n maximally mixed qubits. This model of computation outperforms the best known classical algorithms, and although it contains vanishing entanglement it is known to have quantum correlations characterized by the quantum discord. This experiment detects non-vanishing quantum discord, ensuring the existence of non-classical correlations as measured by the quantum discord.

Unified view of correlations using the square-norm distance

“…On the other hand, in the case of mixedstate computation, in certain computational tasks quantum speed-up can be achieved using separable (unentangled) states, like in the so-called deterministic quantum computation with one qubit (DQC1) protocol [5]. This speed-up has been linked [6] to the presence of quantum discord [7,8], considered as a quantifier of the quantum part of correlations present in a bipartite system and defined as the difference between two quantum analogues of the classical mutual information [9,10].…”

Quantum discord as resource for remote state preparation

“…Obviously, such states have the benefit of being easier to prepare and more robust against losses and experimental imperfections. In fact, there are quantum computational models based on mixed, separable states, most notably the so-called deterministic quantum computation with one qubit (DQC1) 5 , which has recently been demonstrated experimentally [20][21][22] . In this context, quantum discord has been proposed as the resource that can provide the enhancement for the computation 23,24 , but its relation to the computational speed-up remains ambiguous 15,25 .…”

Anomalous decay of quantum correlations of quantum-dot qubits

“…Although entanglement itself is a very important resource for a number of applications [5], including quantum computation, quantum cryptography, or teleportation, separability (the lack of entanglement) does not automatically exclude the presence of quantum correlations [6]. This is in particular the reason why quantum computation models relying on mixed, separable (not entangled) states [7][8][9] are possible.The quantum discord [10, 11] is a measure of quantum correlations (see, however Refs. [11,12] for Holevo-type and thermodynamic based measures) which captures correlations beyond entanglement; it is defined as the difference of two classically equivalent formulas for mutual information and is non-negative.…”

“…Although entanglement itself is a very important resource for a number of applications [5], including quantum computation, quantum cryptography, or teleportation, separability (the lack of entanglement) does not automatically exclude the presence of quantum correlations [6]. This is in particular the reason why quantum computation models relying on mixed, separable (not entangled) states [7][8][9] are possible.…”