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arXiv:2109.01978 · Physical Review A

Doubly ionized lanthanum as a qubit candidate for quantum networks

Steven Olmschenk

Published Apr 25, 2022• 1 citation
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Abstract

We propose doubly ionized lanthanum (${\mathrm{La}}^{2+}$) as a possible qubit candidate for quantum networks. Transitions between the lowest levels in the atom are in the infrared, enabling a direct matter-light interface amenable to long-distance quantum communication. These transitions could also be used to directly laser-cool trapped ${\mathrm{La}}^{2+}$ ions. The rich hyperfine structure of the ion may allow for a qubit stored in magnetic-field insensitive states as well as protocols for atom-photon entanglement.

DOI
10.1103/physreva.105.042617

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We propose doubly ionized lanthanum (La2+{\mathrm{La}}^{2+}La2+) as a possible qubit candidate for quantum networks. Transitions between the lowest levels in the atom are in the infrared, enabling a direct matter-light interface amenable to long-distance quantum communication. These transitions could also be used to directly laser-cool trapped La2+{\mathrm{La}}^{2+}La2+ ions. The rich hyperfine structure of the ion may allow for a qubit stored in magnetic-field insensitive states as well as protocols for atom-photon entanglement.
Despite the tremendous advances in laser cooling of neutral atoms and positive ions, no negatively charged ion has been directly laser cooled. The negative ion of lanthanum, La(-), has been proposed as the best candidate for laser cooling of any atomic anion [ and , Phys. Rev. A 81, 032503 (2010)]. Tunable infrared laser photodetachment spectroscopy is used to measure the bound-state structure of La(-), revealing a spectrum of unprecedented richness with multiple bound-bound electric dipole transitions. The potential laser-cooling transition ((3)F(2)(e)→(3)D(1)(o)) is identified and its excitation energy is measured. The results confirm that La^{-} is a very promising negative ion for laser-cooling applications.
Laser-cooled atomic ions are standards for quantum information science, acting as qubit memories with unsurpassed levels of quantum coherence while also allowing near-perfect measurement. When qubit state-dependent optical dipole forces are applied to a collection of trapped ions, their Coulomb interaction is modulated in a way that allows the entanglement of the qubits through quantum gates that form the basis of a quantum computer. Similar optical forces allow the simulation of quantum many-body physics, where recent experiments are approaching a level of complexity that cannot be modelled with conventional computers. Scaling to much larger numbers of qubits can be accomplished by coupling trapped ion qubits through optical photons, where entanglement over remote distances can be used for quantum communication and large-scale distributed quantum computers. Laser sources and quantum optical techniques are the workhorse for such quantum networks, and will continue to lead the way as future quantum hardware is developed.
Quantum computing promises solutions to some of the world's most important problems that classical computers have failed to address. The trapped-ion-based quantum computing platform has a lot of advantages for doing so: ions are perfectly identical and near-perfectly isolated, feature long coherent times, and allow high-fidelity individual laser-controlled operations. One of the greatest remaining obstacles in trapped-ion-based quantum computing is the issue of scalability. The approach that we take to address this issue is a modular architecture: separate ion traps, each with a manageable number of ions, are interconnected via photonic links. To avoid photon-generated crosstalk between qubits and utilize advantages of different kinds of ions for each role, we use two distinct species - ¹⁷¹Yb⁺ as memory qubits and ¹³⁸Ba⁺ as communication qubits. The qubits based on ¹⁷¹Yb⁺ are defined within the hyperfine "clock" states characterized by a very long coherence time while ¹³⁸Ba⁺ ions feature visible-range wavelength emission lines. Current optical and fiber technologies are more efficient in this range than at shorter wavelengths. We present a theoretical description and experimental demonstration of the key elements of a quantum network based on the mixed-species paradigm. The first one is entanglement between an atomic qubit and the polarization degree of freedom of a pure single photon. We observe a value of the second-order correlation function g⁽²⁾(0) = (8.1 ± 2.3)⨉10⁻⁵ without background subtraction, which is consistent with the lowest reported value in any system. Next, we show mixed-species entangling gates with two ions using the Mølmer-Sørensen and Cirac-Zoller protocols. Finally, we theoretically generalize mixed-species entangling gates to long ion chains and characterize the roles of normal modes there. In addition, we explore sympathetic cooling efficiency in such mixed-species crystals. Besides these developments, we demonstrate new techniques for manipulating states within the D₃⸝₂-manifold of zero-nuclear-spin ions - a part of a protected qubit scheme promising seconds-long coherence times proposed by Aharon et al. in 2013. As a next step, we provide a detailed description of the protocols for three- and four-node networks with mixed species, along with a novel design for the third trap with in-vacuum optics to optimize light collection.
Trapped atomic ions are standards for quantum information processing, as all of the fundamental quantum operations have been demonstrated in small collections of atoms. Current work is concentrated on scaling ion traps to larger numbers of interacting qubits and the generation of massive entangled states. We discuss progress in the quantum networking of trapped atomic ions, using the Coulomb interaction for demonstrations of simple quantum simulations of magnetism, ultrafast laser pulses for entanglement, and finally probabilistic photonic interactions to bridge entanglement over long distances. © Published under licence by IOP Publishing Ltd.
Trapped atomic ions are standards for quantum information processing, as all of the fundamental quantum operations have been demonstrated in small collections of atoms. Current work is concentrated on scaling ion traps to larger numbers of interacting qubits and the generation of massive entangled states. We discuss progress in the quantum networking of trapped atomic ions, using the Coulomb interaction for demonstrations of simple quantum simulations of magnetism, ultrafast laser pulses for entanglement, and finally probabilistic photonic interactions to bridge entanglement over long distances. © Published under licence by IOP Publishing Ltd.
Trapped ions are a promising platform for local quantum information processing. In order to distribute this quantum information over long distances, we can take advantage of optical cavities, which ofier a coherent interface between matter and light, enabling the transfer of quantum information from stationary qubits such as ions onto photons. We demonstrate such an interface by coupling trapped ions to a cavity and have recently shown that a quantum state can be faithfully transferred from a single ion onto a single photon. In particular, this transfer can be improved by taking advantage of a collective effect between multiple ions, namely, superradiant emission into the cavity. In this proof-of-principle experiment, we tune the phase of a two-ion entangled state between sub- and superradiance. The superradiant coupling is then used to enhance the transfer of quantum information onto a photon from a logical qubit encoded in the two ions. Finally, prospects for linking together distant ions in cavities via a quantum network are discussed. Toward this goal, we outline a fiber-based ion-cavity experiment which allows access to the single-ion strong-coupling regime.