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arXiv:2101.04236 · Physical Review A

Improving entanglement generation rates in trapped-ion quantum networks using nondestructive photon measurement and storage

John Hannegan, James Siverns, Jake Cassell, Qudsia Sara Quraishi

Published May 27, 2021• 10 citations
arXivHTMLPDF

Abstract

Long-range quantum information processing will require the integration of different technologies to form hybrid architectures combining the strengths of multiple quantum systems. In this work, we propose a hybrid networking architecture designed to improve entanglement rates in quantum networks based on trapped ions. Trapped ions are excellent candidates as network nodes, but photon losses make long-distance networking difficult. To overcome some losses and extend the range of trapped-ion-based networks, we propose including neutral-atom-based nondestructive single-photon detection and single-photon storage in between networking nodes, forming a hybrid network. This work builds on recently demonstrated optical frequency conversion of single photons emitted by trapped ions. We derive the average two-node entanglement rate for this proposed network architecture as a function of distance. Using reasonable experimental parameters, we show that this proposed quantum network can generate remote entanglement rates up to a factor of 100 larger than that of an equivalent homogeneous network at both near-IR and C-band wavelengths for distances up to 50 km.

DOI
10.1103/physreva.103.052433

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Long-range quantum information processing will require the integration of different technologies to form hybrid architectures combining the strengths of multiple quantum systems. In this work, we propose a hybrid networking architecture designed to improve entanglement rates in quantum networks based on trapped ions. Trapped ions are excellent candidates as network nodes, but photon losses make long-distance networking difficult. To overcome some losses and extend the range of trapped-ion-based networks, we propose including neutral-atom-based nondestructive single-photon detection and single-photon storage in between networking nodes, forming a hybrid network. This work builds on recently demonstrated optical frequency conversion of single photons emitted by trapped ions. We derive the average two-node entanglement rate for this proposed network architecture as a function of distance. Using reasonable experimental parameters, we show that this proposed quantum network can generate remote entanglement rates up to a factor of 100 larger than that of an equivalent homogeneous network at both near-IR and C-band wavelengths for distances up to 50 km.
Different quantum systems possess different favorable qualities. On the one hand, ensemble-based quantum memories are suited for fast multiplexed long-range entanglement generation. On the other hand, single-atomic systems provide access to gates for processing of information. Both of those can provide advantages for high-rate entanglement generation within quantum networks. We develop a hybrid architecture that takes advantage of these properties by combining trapped-ion nodes and nodes comprised of spontaneous parametric down conversion photon pair sources and absorptive memories based on rare-earth ion ensembles. To this end, we solve the central challenge of matching the different bandwidths of photons emitted by those systems in an initial entanglement-generation step. This enables the parallel execution of multiple probabilistic tasks in the initial stage. As a particular example, we show that our approach can lead to a significant speed-up for the fundamental task of creating ion-ion entanglement over hundreds of kilometers in a quantum network.
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.
Future quantum networks will require the ability to produce matter-photon entanglement at photon frequencies not naturally emitted from the matter qubit. This allows for a hybrid network architecture, where these photons can couple to other tools and quantum technologies useful for tasks such as multiplexing, routing, and storage, but which operate at wavelengths different from that of the matter qubit source, while also reducing network losses. Here, we demonstrate entanglement between a trapped ion and a 780-nm photon, a wavelength that can interact with neutral-Rb-based quantum networking devices. A single barium ion is used to produce 493-nm photons, entangled with the ion, which are then frequency converted to 780 nm while preserving the entanglement. We generate ion-photon entanglement with fidelities $\ensuremath{\ge}0.93$(2) and $\ensuremath{\ge}0.84$(2) for 493-nm and 780-nm photons respectively with the fidelity drop arising predominantly from a reduction in the signal-noise of our detectors at 780 nm compared with at 493 nm.
Quantum networking is an established and growing field of research, with applications in many other fields of quantum technology, as well as fundamental research in quantum mechanics. A quantum network consists of multiple quantum devices that can interchange quantum information or be entangled with each other. Trapped ions coupled to optical cavities are among the most promising platforms in quantum technology broadly and networking specifically, with the ability to initialise and store quantum states in the ions and, via the cavity, transfer those states onto photons, which can interact with distant systems. This thesis describes two experiments using an ion trap with integrated high finesse optical cavity as a source of single photons. In the first, the photons were converted to a standard telecommunications wavelength through quantum frequency conversion and transmitted over 10 km of optical fibre. The attenuation experienced by the photons in optical fibre without frequency conversion would make transmission over such a long distance impractical. The converted photons were shown to maintain their nonclassical statistics, meaning such a system could be used to create quantum networks over long distances. A novel scheme for producing single photons from the ion-cavity system was demonstrated. This scheme significantly reduces the impact of decoherence in the system compared to commonly used schemes through selection of the initial state of the ion. This scheme may be directly applied to ion-photon entanglement and would lead to improved fidelity in ion-ion entanglement or state transfer. Both experiments represent significant steps towards the building of quantum networks based on ions coupled to optical cavities. They lay the groundwork for future experiments using the same system to entangle ions with photons and eventually with ions in other traps.
We analyze the performance of a quantum repeater protocol based on single trapped ions. At each node, single trapped ions embedded into high-finesse cavities emit single photons whose polarization is entangled with the ion state. A specific detection of two photons at a central station located half-way between two nodes heralds the entanglement of two remote ions. Entanglement can be extended to long distances by applying successive entanglement swapping operations based on two-ion gate operations that have already been demonstrated experimentally with high precision. Our calculation shows that the distribution rate of entanglement achievable with such an ion-based quantum repeater protocol is higher by orders of magnitude than the rates that are achievable with the best known schemes based on atomic-ensemble memories and linear optics. The main reason is that for trapped ions the entanglement swapping operations are performed deterministically, in contrast to success probabilities below 50% per swapping with linear optics. The scheme requires efficient collection of the emitted photons, which can be achieved with cavities, and efficient conversion of their wavelength, which can be done via stimulated parametric down-conversion. We also suggest how to realize temporal multiplexing, which offers additional significant speed ups in entanglement distribution, with trapped ions.