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arXiv:1301.7110 · Physical Review A

Entanglement-free certification of entangling gates

Marcelo Pereira de Almeida, Mile Gu, Alessandro Fedrizzi, Matthew A. Broome, Timothy C. Ralph, Andrew G. White

Published Apr 23, 2014• 20 citations
arXivHTMLPDF

Abstract

Not all quantum protocols require entanglement to outperform their classical alternatives. The nonclassical correlations that lead to a quantum advantage are conjectured to be captured by quantum discord. Here we demonstrate that discord has an immediate practical application: it allows a client who lacks the ability to generate entanglement or conduct quantum measurements to certify whether an untrusted party has entangling gates. We implement our protocol in the discrete-variable regime with photonic qubits and show its success in the presence of high levels of noise and imperfect gate operations. Our technique offers a practical method to test claims of quantum processing and to benchmark entangling operations for physical architectures in which only highly mixed states are available.

DOI
10.1103/physreva.89.042323

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Not all quantum protocols require entanglement to outperform their classical alternatives. The nonclassical correlations that lead to a quantum advantage are conjectured to be captured by quantum discord. Here we demonstrate that discord has an immediate practical application: it allows a client who lacks the ability to generate entanglement or conduct quantum measurements to certify whether an untrusted party has entangling gates. We implement our protocol in the discrete-variable regime with photonic qubits and show its success in the presence of high levels of noise and imperfect gate operations. Our technique offers a practical method to test claims of quantum processing and to benchmark entangling operations for physical architectures in which only highly mixed states are available.
The threshold between classical and nonclassical two-qubit states is drawn at the place when these states can no longer be described by classical correlations, i.e., quantum discord or entanglement appear. However, to check if the correlations are classical (in terms of quantum discord and entanglement) it is sufficient to witness the lack of quantum discord because its zero value implies the lack of entanglement. We explain how the indicator of quantum discord introduced by Girolami and Adesso [Phys. Rev. Lett. 108, 150403 (2012)] can be practically measured in linear-optical systems using standard beam splitters and photon detectors. We study the efficiency of the setup assuming both ideal and real components and show that the efficiency of the proposed implementation is better than the full two-photon quantum tomography. Thus, we demonstrate that a class of experiments previously available on NMR platform can be implemented in optical systems.
Quantum correlations and nonprojective measurements underlie a plethora of information-theoretic tasks, otherwise impossible in the classical world. Existing schemes to certify such nonclassical resources in a device-independent manner require seed randomness-which is often costly and vulnerable to loopholes-for choosing the local measurements performed on different parts of a multipartite quantum system. In this Letter, we propose and experimentally implement a semi-device-independent certification technique for both quantum correlations and nonprojective measurements without seed randomness. Our test is semi-device independent in the sense that it requires only prior knowledge of the dimension of the parts. We experimentally show a novel quantum advantage in correlated coin tossing by producing specific correlated coins from pairs of photons entangled in their transverse spatial modes. We establish the advantage by showing that the correlated coin obtained from the entangled photons cannot be obtained from two two-level classical correlated coins. The quantum advantage requires performing qubit trine positive operator-valued measures (POVMs) on each part of the entangled pair, thus also certifying such POVMs in a semi-device-independent manner. This proof of concept firmly establishes a new cost-effective certification technique for both generating nonclassical shared randomness and implementing nonclassical measurements, which will be important for future multiparty quantum communications.
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 systems in mixed states can be unentangled and yet still nonclassically correlated. These correlations can be quantified by the quantum discord and might provide a resource for quantum information processing tasks. By precisely controlling the interaction of two ionic qubits with their environment, we investigate the capability of noise to generate discord. Firstly, we show that noise acting on only one quantum system can generate discord between two. States generated in this way are restricted in terms of the rank of their correlation matrix. Secondly, we show that classically correlated noise processes are capable of generating a much broader range of discordant states with correlation matrices of any rank. Our results show that noise processes prevalent in many physical systems can automatically generate nonclassical correlations and highlight fundamental differences between discord and entanglement.
Deterministic quantum computation with one pure qubit (DQC1) is an efficient model of computation that uses highly mixed states. Unlike pure-state models, its power is not derived from the generation of a large amount of entanglement. Instead it has been proposed that other nonclassical correlations are responsible for the computational speedup, and that these can be captured by the quantum discord. In this Letter we implement DQC1 in an all-optical architecture, and experimentally observe the generated correlations. We find no entanglement, but large amounts of quantum discord-except in three cases where an efficient classical simulation is always possible. Our results show that even fully separable, highly mixed, states can contain intrinsically quantum mechanical correlations and that these could offer a valuable resource for quantum information technologies.
Dissimilar notions of quantum correlations have been established, each being motivated through particular applications in quantum information science and each competing for being recognized as the most relevant measure of quantumness. In this contribution, we experimentally realize a form of quantum correlation that exists even in the absence of entanglement and discord. We certify the presence of such quantum correlations via negativities in the regularized two-mode Glauber-Sudarshan function. Our data show compatibility with an incoherent mixture of orthonormal photon-number states, ruling out quantum coherence and other kinds of quantum resources. By construction, the quantumness of our state is robust against dephasing, thus requiring fewer experimental resources to ensure stability. In addition, we theoretically show how multimode entanglement can be activated based on the generated, nonentangled state. Therefore, we implement a robust kind of nonclassical photon-photon correlated state with useful applications in quantum information processing.