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SPIE Quantum Information Science, Sensing, and Computation

Towards using trapped ions as memory nodes in a photon-mediated quantum network

Boyan Tabakov, Jackson Bell, Benjamin Bonenfant, Paul R. Cook, Lester Disney, Justin Phillips, Kaitlin Poole, Laura Wessing, Jameson P. O'Reilly, Tyler Dolezal, Kathy-Anne Soderberg, Daniela F. Bogorin

Published May 16, 2018• 3 citations
Publisher

Abstract

Quantum networking exploits features of quantum mechanics to provide ultrasecure networks that are both tamper-proof and tamper-evident. Such networks can be implemented as distant memory nodes connected via photon-based interfaces. Trapped ions are nearly ideal quantum network nodes due to the precise control possible over both their internal and external degrees of freedom as well as for their superior performance as long-term quantum memories. Photon-based qubits are the natural choice to transfer information within the network due to their ability to transmit quantum information over long distances and the capability to process information ”on-the-fly” between the memory nodes. We present the quantum research being done at the Air Force Research Laboratory (AFRL) with a focus on trapped ion qubits, the short- and long-term goals of the lab, and some of the unique resources we have access to at AFRL.

DOI
10.1117/12.2305058

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Science•Feb 11, 2021•1 citation
Quantum networking exploits features of quantum mechanics to provide ultrasecure networks that are both tamper-proof and tamper-evident. Such networks can be implemented as distant memory nodes connected via photon-based interfaces. Trapped ions are nearly ideal quantum network nodes due to the precise control possible over both their internal and external degrees of freedom as well as for their superior performance as long-term quantum memories. Photon-based qubits are the natural choice to transfer information within the network due to their ability to transmit quantum information over long distances and the capability to process information ”on-the-fly” between the memory nodes. We present the quantum research being done at the Air Force Research Laboratory (AFRL) with a focus on trapped ion qubits, the short- and long-term goals of the lab, and some of the unique resources we have access to at AFRL.
Trapped ions are one of the leading platforms for quantum computation and networking, with long coherence times and high fidelity qubit operation. We exploit these attributes to build a quantum network with the use of three trapped ion modules connected via photonic links. In each of these systems, we have ions of different species for use as memory or communication qubits. Our newest module contains two high NA=0.8 objectives, each covering 20% of the solid angle of photons emitted from the ion, making it suitable for use as the middle node of the network. Photons are collected from each chamber, and we generate remote entanglement between each pair of nodes by interfering the photons on a beamsplitter. This heralds entanglement of the communication ions in the pair of network nodes, which we then swap onto the memory qubits. I will discuss our progress towards creating a Greenberger–Horne–Zeilinger (GHZ) state between the memory ions in the three nodes.
Increasing the number of qubits that can be controlled in a quantum system represents an essential challenge to the field of quantum computing. Quantum networks consisting of nodes for local information processing and photonic channels to distribute entanglement between different nodes represent a promising modular approach to achieve this scaling. Trapped atomic ions are an ideal candidate for quantum network nodes, with long-lived identical qubit memories that can be locally entangled through their Coulomb interaction and remotely entangled through photonic channels. In this work I will first discuss our established toolkit for using 171Yb+ and 138Ba+ ions individually or together within a quantum node. Next I will show how the 138Ba+ toolkit has been extended to allow for quantum operations in the 52D3/2 manifold. I will then demonstrate how we can generate ion-photon entanglement as a resource to connect separate nodes with a focus on some important improvements which will allow us to implement it as part of a larger network. These improvements include first the use of separate memory (171Yb+ ) and photon generating (138Ba+ ) ions. Additionally, the use of separate atomic lines within 138Ba+ for excitation and collection allows us to preserve integrity of this photonic interface by ensuring the purity of the single photons that are produced. To this end I demonstrate a single-photon source for quantum networking based on a trapped 138Ba+ ion with a single photon purity of g2(0) = (8.1 ± 2.3) × 10−5 without background subtraction. Trade-offs between the photonic generation rate and the memory-photon entanglement fidelity for the case of polarization photonic qubits are also examined and optimized by tailoring the spatial mode of the collected light. These techniques should be useful in constructing larger ion-photon networks.
Trapped atomic ions are a leading platform for quantum information networks, with long-lived identical qubit memories that can be locally entangled through their Coulomb interaction and remotely entangled through photonic channels. However, performing both local and remote operations in a single node of a quantum network requires extreme isolation between spectator qubit memories and qubits associated with the photonic interface. We achieve this isolation by cotrapping ^{171}Yb^{+} and ^{138}Ba^{+} qubits. We further demonstrate the ingredients of a scalable ion trap network node with two distinct experiments that consist of entangling the mixed species qubit pair through their collective motion and entangling a ^{138}Ba^{+} qubit with an emitted visible photon.
Trapped ions are excellent candidates for quantum nodes, as they possess many desirable features of a network node including long lifetimes, on-site processing capability, and production of photonic flying qubits. However, unlike classical networks in which data may be transmitted in optical fibers and where the range of communication is readily extended with amplifiers, quantum systems often emit photons that have a limited propagation range in optical fibers and, by virtue of the nature of a quantum state, cannot be noiselessly amplified. Here, we first describe a method to extract flying qubits from a Ba+ trapped ion via shelving to a long-lived, low-lying D-state with higher entanglement probabilities compared with current strong and weak excitation methods. We show a projected fidelity of ≈89% of the ion–photon entanglement. We compare several methods of ion–photon entanglement generation, and we show how the fidelity and entanglement probability varies as a function of the photon collection optic’s numerical aperture. We then outline an approach for quantum frequency conversion of the photons emitted by the Ba+ ion to the telecommunication range for long-distance networking and to 780 nm for potential entanglement with rubidium-based quantum memories. Our approach is significant for extending the range of quantum networks and for the development of hybrid quantum networks compromised of different types of quantum memories.
Quantum Networks Photons, being robust and fast, are ideal carriers of quantum information in long-distance quantum networks. For a practical communication platform, the photons also need to be manipulated at the local level at nodes or exchange points across the network, and this requires information to be reliably encoded, stored, and retrieved from the photonic qubits. Craiciu et al. demonstrate such functionality using an ensemble of erbium ions embedded within a solid-state matrix of yttrium orthosilicate. With the ions operating as a dynamic quantum memory for photons at communication wavelengths and the approach being compatible with integrated photonics, the results offer promise for possible integration into large-scale optical fiber networks. Optica 8 , 114 (2021).