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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

Discord as a consumable resource

Mile Gu, Helen M. Chrzanowski, Syed M. Assad, Thomas Symul, Kavan Kishore Modi, Timothy C. Ralph, Vlatko Vedral, Ping Koy Lam

Published Jun 1, 2013• 1 citation
Publisher

Abstract

Quantum discord is conjectured to be a more general quantum resource than entanglement. We support this conjecture by showing, via experimental Gaussian optics, that quantum processors can harness discord to perform tasks classical counterparts cannot.
DOI
10.1109/cleopr.2013.6600227

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For many years, the notion of quantum correlations was equated with the notion of quantum entanglement. The wide range of quantum protocols whose extra-powers over classical protocol were shown to originate from quantum entanglement mostly motivated this. For example, it was shown that for certain class of problems, a quantum computer could provide exponential speed-up over its classical counterpart, given that it relied on quantum entangled resources. Recently, however, the requirement of possessing entanglement to perform efficient quantum computation has been questioned both theoretically [1] and experimentally [2]. It appears that a non-classical quantity called quantum discord [3] is all that is required.
Coherent interactions that generate negligible entanglement can still exhibit unique quantum behaviour. This observation has motivated a search beyond entanglement for a complete description of all quantum correlations. Quantum discord is a promising candidate. Here, we experimentally demonstrate that under certain measurement constraints, discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interactions. The inability to access this information by any other means allows us to use discord to directly quantify this ‘quantum advantage’.
Abstract The study of quantum information processing seeks to characterize the resources that enable quantum information processing to perform tasks that are unfeasible or inefficient for classical information processing. Quantum cryptography is one such task, and researchers have identified entanglement as a sufficient resource for secure key generation. However, quantum discord, another type of quantum correlation beyond entanglement, has been found to be necessary for guaranteeing secure communication. Despite this, it remains a long-standing problem to clearly understand the role that discord plays and how to effectively utilize discord for analyzing security in specific quantum cryptography protocols. Here, we strictly prove that discord determines the secrecy and show its practical interest by considering a three-state quantum key distribution protocol. Its robustness against imperfections in qubit sources and measurements as well as basis misalignment, results in better key rates than BB84 protocol. Those advantages are experimentally demonstrated via photonic phase encoding systems.
Quantum discord characterizes "nonclassicality" of correlations in quantum mechanics. It has been proposed as the key resource present in certain quantum communication tasks and quantum computational models without containing much entanglement. We obtain a necessary and sufficient condition for the existence of nonzero quantum discord for any dimensional bipartite states. This condition is easily experimentally implementable. Based on this, we propose a geometrical way of quantifying quantum discord. For two qubits this results in a closed form of expression for discord. We apply our results to the model of deterministic quantum computation with one qubit, showing that quantum discord is unlikely to be the reason behind its speedup.
Discordant states appear in a large number of quantum phenomena and seem to be a good indicator of divergence from classicality. While there is evidence that they are essential for a quantum algorithm to have an advantage over a classical one, their precise role is unclear. We examine the role of discord in quantum algorithms using the paradigmatic framework of restricted distributed quantum gates and show that manipulating discordant states using local operations has an associated cost in terms of entanglement and communication resources. Changing discord reduces the total correlations and reversible operations on discordant states usually require nonlocal resources. Discord alone is, however, not enough to determine the need for entanglement. A more general type of similar quantities, which we call KKK discord, is introduced as a further constraint on the kinds of operations that can be performed without entanglement resources.
Quantum illumination employs entanglement to detect reflecting objects in environments so noisy that all entanglement is destroyed. This appears paradoxical: the benefit of entanglement outlasts entanglement itself. Here we demonstrate that the resilience of quantum illumination is due to quantum discord - a more resilient form of quantum correlations. We prove a direct equality between the performance gain in quantum illumination and the amount of discord which is expended to resolve the target. Discord outlasts entanglement; and in harnessing this discord, quantum illumination outperforms all conventional techniques. This simultaneously explains why quantum illumination thrives in entanglement breaking noise, and confirms discord is a resource of immediate value.