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arXiv:1610.02755 · Europhysics Letters (EPL)

Experimentally freezing quantum discord in a dephasing environment using dynamical decoupling

Harpreet Singh, Arvind, Kavita Dorai

Published Jun 1, 2017• 16 citations
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Abstract

The discovery of the intriguing phenomenon that certain kinds of quantum correlations remain impervious to noise up to a specific point in time and then suddenly decay, has generated immense recent interest. We exploit dynamical decoupling sequences to prolong the persistence of time-invariant quantum correlations in a system of two NMR qubits decohering in independent dephasing environments. We experimentally prepare two-qubit Bell-diagonal quantum states that interact with individual noise channels and demonstrate that we are able to freeze quantum correlations over long time scales via dynamical decoupling. Our results have important implications for experimental quantum control and for quantum information processing protocols.

DOI
10.1209/0295-5075/118/50001

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The discovery of the intriguing phenomenon that certain kinds of quantum correlations remain impervious to noise up to a specific point in time and then suddenly decay, has generated immense recent interest. We exploit dynamical decoupling sequences to prolong the persistence of time-invariant quantum correlations in a system of two NMR qubits decohering in independent dephasing environments. We experimentally prepare two-qubit Bell-diagonal quantum states that interact with individual noise channels and demonstrate that we are able to freeze quantum correlations over long time scales via dynamical decoupling. Our results have important implications for experimental quantum control and for quantum information processing protocols.
Dynamical decoupling protocols are one of the most used tools for efficient quantum error corrections and for reservoir engineering. In this paper we study the effect of dynamical decoupling pulses on the preservation of both quantum and classical correlations, and their influence on the intriguing phenomenon of time-invariant discord. We study two qubits experiencing local dephasing with an Ohmic class spectrum and subject to dynamical decoupling protocols. We investigate the connection between the dynamics of both classical and quantum correlations and the behavior of information flow between system and environment. Finally, we establish a set of necessary conditions for which time-invariant discord can be created using dynamically decoupling techniques.
Quantum information processing (QIP) based on high-dimensional quantum systems provides unique advantages and new potentials where high-dimensional quantum correlations (QCs) play vital roles. Exploring the resistance of QCs against noises is crucial as QCs are fragile due to complex and unavoidable system-environment interactions. In this study, we investigate the performance of high-dimensional QCs under local dephasing noise using a single nitrogen-vacancy center in diamond. A freezing phenomenon in the high-dimensional quantum discord dynamics was observed, showing that discord is robust against local dephasing noise. Utilizing a robustness metric known as the freezing index, we find that the discord of qutrits outperforms their qubit counterparts when confronted with dephasing noise. Furthermore, we develop a geometric picture to explain this intriguing freezing phenomenon. Our findings highlight the potential of utilizing discord as a physical resource for advancing QIP in high-dimensional quantum settings.
Quantum information processing (QIP) based on high-dimensional quantum systems provides unique advantages and new potentials where high-dimensional quantum correlations (QCs) play vital roles. Exploring the resistance of QCs against noises is crucial as QCs are fragile due to complex and unavoidable system-environment interactions. In this study, we investigate the performance of high-dimensional QCs under local dephasing noise using a single nitrogen-vacancy center in diamond. A freezing phenomenon in the high-dimensional quantum discord dynamics was observed, showing discord is robust against local dephasing noise. Utilizing a robustness metric known as freezing index, we found that the discord of qutrits outperforms their qubits counterpart when confronted with dephasing noise. Furthermore, we developed a geometric picture to explain this intriguing freezing phenomenon phenomenon. Our findings highlight the potential of utilizing discord as a physical resource for advancing QIP in high-dimensional quantum settings.
We investigate evolution of quantum correlations in ensembles of two-qubit nuclear spin systems via nuclear magnetic resonance techniques. We use discord as a measure of quantum correlations and the Werner state as an explicit example. We, first, introduce different ways of measuring discord and geometric discord in two-qubit systems and then describe the following experimental studies: (a) We quantitatively measure discord for Werner-like states prepared using an entangling pulse sequence. An initial thermal state with zero discord is gradually and periodically transformed into a mixed state with maximum discord. The experimental and simulated behavior of rise and fall of discord agree fairly well. (b) We examine the efficiency of dynamical decoupling sequences in preserving quantum correlations. In our experimental setup, the dynamical decoupling sequences preserved the traceless parts of the density matrices at high fidelity. But they could not maintain the purity of the quantum states and so were unable to keep the discord from decaying. (c) We observe the evolution of discord for a singlet-triplet mixed state during a radio-frequency spin-lock. A simple relaxation model describes the evolution of discord, and the accompanying evolution of fidelity of the long-lived singlet state, reasonably well.
Quantum correlations quantified by quantum discord have been experimentally demonstrated as important physical resources in quantum information processing. Because of the inevitable interactions between the quantum system and the environment, it is essential to characterize the quantum correlation's dynamics in open systems and to protect the quantum correlation from being lost in the environment. Herein it is experimentally demonstrated in the Si:P system that there is a sudden change (at a critical time ${t}_{c}\ensuremath{\approx}166$ ns) in the decay rates of classical and quantum correlations. To protect the quantum correlation, we achieve an extension of the critical time by 50 times to $8\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}$s by applying a two-flip dynamical decoupling pulse sequence. Moreover, we observe the phenomenon of the revival of both quantum and classical correlations. The experimental observation of a nondecay interval for quantum correlation and its great extension in a solid-state system with genuine noise makes the use of quantum discord as a physical resource in quantum information processing more practicable.
Sudden transitions of one-norm geometric quantum discord and quantum discord of quantum systems have been observed in experiments recently. We propose a scheme to control sudden transitions of quantum correlations of two qubits within two spatially separated thermal baths or one common thermal bath via bang-bang pulses with a finite period. For the two-bath case, correlations decrease with time, and the sudden transition behavior can be efficiently adjusted by bang-bang pulses. For one common bath case, the thermal bath may play a constructive or destructive role in the generation of steady-state correlations, depending on the initial state of qubits.