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arXiv:2107.13230 · Nanophotonics

Long‐range qubit entanglement via rolled‐up zero‐index waveguide

Ibrahim Issah, Mohsin Habib, Hümeyra Çağlayan

Published Nov 17, 2021• 9 citations
arXivHTMLPDF

Abstract

Abstract Preservation of an entangled state in a quantum system is one of the major goals in quantum technological applications. However, entanglement can be quickly lost into dissipation when the effective interaction among the qubits becomes smaller compared to the noise‐injection from the environment. Thus, a medium that can sustain the entanglement of distantly spaced qubits is essential for practical implementations. This work introduces the fabrication of a rolled‐up zero‐index waveguide which can serve as a unique reservoir for the long‐range qubit–qubit entanglement. We also present the numerical evaluation of the concurrence (entanglement measure) via Ansys Lumerical FDTD simulations using the parameters determined experimentally. The calculations demonstrate the feasibility and supremacy of the experimental method. We develop and fabricate this novel structure using cost‐effective self‐rolling techniques. The results of this study redefine the range of light‐matter interactions and show the potential of the rolled‐up zero‐index waveguides for various classical and quantum applications such as quantum communication, quantum information processing, and superradiance.

DOI
10.1515/nanoph-2021-0453

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Abstract Entanglement is paramount in quantum information processing. Many quantum systems suffer from spatial decoherence in distances over a wavelength and cannot be sustained over short time periods due to dissipation. However, long range solutions are required for the development of quantum information processing on chip. Photonic reservoirs mediating the interactions between qubits and their environment are suggested. Recent research takes advantage of extended wavelength inside near-zero refractive index media to solve the long-range problem along with less sensitivity on the position of quantum emitters. However, those recent proposals use plasmonic epsilon near-zero waveguides that are intrinsically lossy. Here, we propose a fully dielectric platform, compatible with the Nitrogen Vacancy (NV) diamond centers on-chip technology, to drastically improve the range of entanglement over 17 free-space wavelengths, or approximatively 12.5 µm, using mu near-zero metamaterials. We evaluate transient and steady state concurrence demonstrating an order of magnitude enhancement compared to previous works. This is, to the best of our knowledge, the first time that such a long distance is reported using this strategy. Moreover, value of the zero time delay second order correlation function g12(2)(0){g}_{12}^{(2)}(0)g12(2)​(0) g 12 ( 2 ) ( 0 ) are provided, showing antibunching signature correlated with a high degree of concurrence.
Preparation and measurement of an entangled state are essential in quantum information applications. The recent theoretical works on preparing and measuring entangled states via coupled quantum dot–waveguide platforms are promising. However, an alternative strategy is still needed for more accessible implementations. In this Article, we propose a simple but powerful medium for quantum information processing and communication consisting of a zero-index photonic crystal (PC) waveguide (WG). By exploiting the zero-index behavior of the PC WG, we overcome the difficulty of sensitivity in quantum dots’ location, which is one of the main challenges for preparing and measuring a two-qubit entangled state. Moreover, our platform possesses the feature of tunability, which allows it to operate in different wavelength regimes. The suggested medium is also promising for realizing long-range spatial coherence as well as long-range entanglement.
Abstract Multi‐level (qudit) entangled photon states are a key resource for both fundamental physics and advanced applied science, as they can significantly boost the capabilities of novel technologies such as quantum communications, cryptography, sensing, metrology, and computing. The benefits of using photons for advanced applications draw on their unique properties: photons can propagate over long distances while preserving state coherence, and they possess multiple degrees of freedom (such as time and frequency) that allow scalable access to higher dimensional state encoding, all while maintaining low platform footprint and complexity. In the context of out‐of‐lab use, photon generation and processing through integrated devices and off‐the‐shelf components are in high demand. Similarly, multi‐level entanglement detection must be experimentally practical, i.e., ideally requiring feasible single‐qudit projections and high noise tolerance. Here, we focus on multi‐level optical Bell and cluster states as a critical resource for quantum technologies, as well as on universal witness operators for their feasible detection and entanglement characterization. Time‐ and frequency‐entangled states are the main platform considered in this context. We review a promising approach for the scalable, cost‐effective generation and processing of these states by using integrated quantum frequency combs and fiber‐based devices, respectively. We finally report an experimentally practical entanglement identification and characterization technique based on witness operators that is valid for any complex photon state and provides a good compromise between experimental feasibility and noise robustness. The results reported here can pave the way toward boosting the implementation of quantum technologies in integrated and widely accessible photonic platforms.
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Abstract Shared entanglement can significantly amplify classical correlations between systems interacting over a limited quantum channel. A natural avenue is to use entanglement of the same dimension as the channel because this allows for unitary encodings, which preserve global coherence until a measurement is performed. Contrasting this, a distributed task based on a qubit channel is demonstrated, for which irreversible encoding operations can outperform any possible coherence‐preserving protocol. This corresponds to using high‐dimensional entanglement and encoding information by compressing one of the subsystems into a qubit. Demonstrating this phenomenon requires the preparation of a 4D maximally entangled state, the compression of two qubits into one and joint qubit‐ququart entangled measurements, with all modules executed at near‐optimal fidelity. A proof‐of‐principle experiment is reported that achieves the advantage by realizing separate systems in distinct and independently controlled paths of a single photon. This result demonstrates the relevance of high‐dimensional entanglement and non‐unitary operations for enhancing the communication capabilities of standard qubit transmissions.