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arXiv:2511.04987 · Physical Review Research

Single and Double-click High-Rate Entanglement Generation Between Distant Ions Using Multiplexed Atomic Ensembles

Benedikt Tissot, Soubhadra Maiti, Emil R. Hellebek, Anders Søndberg Sørensen

quant-phSubmitted Nov 7, 2025• Announced Nov 10, 2025• 1 citation• Revised May 18, 2026 (v2)
v1 · Nov 7, 2025v2 · May 18, 2026
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Abstract

In an accompanying paper [arxiv:2511.04488], we introduced an approach to interface trapped-ion quantum processors with ensemble-based quantum memories by matching a spontaneous parametric down conversion source to both the ions and the memories. This enables rapid entanglement generation between single trapped ions separated by distances of hundreds of kilometers. In this article, we extend the protocol and provide additional details of the analysis. Particularly, we compare a double-click and single-click approaches for the ion edge nodes. The double-click approach relaxes the phase stability requirement but is strongly affected by finite efficiencies. Choosing the optimal protocol thus depends on the access to the phase stabilization as well as the efficiencies of the interfaces of the ions and ensemble-based memories.

Comments
21 pages, 6 figures, code supporting the findings of this manuscript is provided as ancillary files

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In an accompanying paper [arxiv:2511.04488], we introduced an approach to interface trapped-ion quantum processors with ensemble-based quantum memories by matching a spontaneous parametric down conversion source to both the ions and the memories. This enables rapid entanglement generation between single trapped ions separated by distances of hundreds of kilometers. In this article, we extend the protocol and provide additional details of the analysis. Particularly, we compare a double-click and single-click approaches for the ion edge nodes. The double-click approach relaxes the phase stability requirement but is strongly affected by finite efficiencies. Choosing the optimal protocol thus depends on the access to the phase stabilization as well as the efficiencies of the interfaces of the ions and ensemble-based memories.
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.
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.
Experiments in quantum optics are motivated by fundamental questions about the quantum mechanical description of matter and light as well as by applications in quantum technologies such as quantum information processing. Exploiting the quantum character of both matter and light has turned out to be extremely rewarding for novel technological applications in the emerging fields of quantum communication and computation. Here we consider the implementation of quantum information technology with trapped ions and single photons, focusing on two key issues: the entanglement of distant atoms and the controlled interaction of a single photon with a single atom. This thesis describes the construction of a double ion trap apparatus including the necessary frequency-stabilized laser systems, and first experiments to investigate the generation of distant atomic entanglement, the implementation of a simple quantum network and controlled single-atom single-photon interaction. The experimental setup comprises two ion traps at a distance of one meter, which are independent but identical in construction. We introduce novel realizations of a photoionization method for calcium atoms, a laser frequency stabilization scheme and a radio frequency pulse sequencer. Linear Paul traps are used to load individual or strings of calcium ions. High-numerical aperture objectives mounted on sub-micrometer positioning systems allow us to address and manipulate a single ion with strongly focused light fields and collect the emitted fluorescence photons from a large solid angle. With these bidirectional single-atom single-photon interfaces the essential building blocks of a quantum network with atoms and photons are available. One of the main objectives we pursue with the double-trap apparatus is the generation of entanglement between ions in different traps, which is one of the key tasks in a quantum network. We compare single- and two-photon schemes for probabilistic entanglement creation and analyze how the performance of the schemes depends on photon detection efficiency. As a first step towards the experimental realization of such schemes we measure the second order correlation function of fluorescence photons emitted from two distant trapped ions. We observe high-contrast quantum (Hong-Ou-Mandel) interference, proving the indistinguishability of photons emitted from different ions, which is a prerequisite for the considered entanglement schemes. To investigate the interaction of single photons and a single atom in free space we combine the ion trap apparatus with a tunable, narrowband spontaneous parametric down-conversion source. The high-numerical aperture objectives allow us to efficiently couple bandwidth-tunable photons to a single trapped ion. Such atom-photon interaction at the single-particle level is the most fundamental process in quantum optics and lies literally at the focus of many experimental implementations of quantum information processing protocols and of studies of the (quantum) limits of matter -light interaction, which were so far restricted to weak coherent light fields. Here we observe individual atomic absorption events of one photon of an entangled pair by employing a quantum jump detection scheme. To characterize and control the interaction process we modify the spectral properties of the down conversion photons and perform single-photon spectroscopy of a single ion. To go beyond experiments with cohe rent fields we study temporal correlations between the absorption of photons from the spontaneous parametric down conversion source by a single ion and the detection of the corresponding partner photon. For the applied scheme we observe promising indications that the quantum correlations of photons produced in spontaneous parametric down conversion are preserved in the resonant interaction with a single atom. The implementation of more efficient atom-light coupling and entanglement transfer schemes with our apparatus is outlined.
Entanglement distribution between distant nodes is a critical requirement for the realization of quantum networks. Recently, entanglement between distant atomic qubits has been demonstrated experimentally [1]. This scheme uses the projective measurement of the photons emitted by the two distant atoms to entangle them. Another strategy is to generate entangled photon pairs by a parametric down-conversion process, and make them interact with the distant atoms, transferring the entanglement from photon to atomic qubits [2, 3]. Here we describe our activities towards studying this second kind of entanglement distribution scheme.
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.
Irhe field of quantum information processing (QIP) has evolved rapidly in the last decade. Strings of single rapped ions have proven to be one of the most promising candidates for the implementation of quantum information processing tasks such as universal gate operations or eleportation. Besides the challenge of scaling up these quantum processors to a larger number of ions, another important problem is to interconnect different processors on a macroscopic scale and integrate them into a quantum network. Irhis thesis describes the construction of a new experiment dedicated to the study of quantum communication and quantum networking with laser cooled single ions and first experiments carried out with it. Two linear Paul traps for 4DCa+ have been set up at a distance of one meter. Two-photon interference of the fluorescence light of two remotely trapped single ions could be observed with a contrast of 80%. This proves the suitability of the setup for the creation of remote entanglement, an important building block for quantum networking. Irhe first part of the thesis describes the experimental setup that ~as constructed from scratch. It consists of a newly designed trap apparatus and the laser setup. The apparatus was designed to ulfill the requirement of interferometric stability, which is a pre requisite for one remote entanglement protocol. Another important feature are two high numerical aperture in-vacuum lenses hat optimize the collection efficiency of photons scattered by he ions. In order to frequency-stabilize the laser systems, a ransfer stabilization scheme referenced to a transition in atomic ~s has been developed. The stabilization chain comprises low-cost open transfer cavities and offers very good long term stability. Irhe second part of the thesis presents the experimental results starting with the calibration of the two traps through excitation spectra. After that, the realization of two-photon interference rom single ions under continuous excitation is presented. The model analysis of the data usinq eiqht level Bloch equations reveals the individual coherence properties of the interfering ~ingle photons. The coherence time of the fluorescence light does not exceed 20- ns, indicating that incoherent scattering is ~ominant. fA,n analysis of the efficiency of methods for remote entanglement relying on two-photon and single-photon interference, focussing on he role of the detection efficiency is discussed. It is revealed hat Single-photon schemes are more efficient if the detection ~fficiency is low (like in present experiments). The created setup is suitable for the realization of both types of protocols, based pn single- and two-photon interference. Jhe conditioned dynamics of photon emission of a single ion are studied using correlation functions. The engineering of second ~rder correlation functions used as a tool which relies on conditional dynamics, might have interesting applications in probabilistic quantum information processing. pingle-atom single-photon interfaces are an important application in quantum networks. As opposed to the emission of single photons by single atoms, the absorption of Single photons by single atoms cannot be controlled in a similar way. With the future goal of realizing a heralded absorption of a single photon by a single atom, one of the ion traps was linked to a Spontaneous Parametric Down Conversion source. Quantum jump spectroscopy on a single ion using single photons from this source is shown. Finally the reported two-photon interference could be extended also to pulsed excitation. This marks an important technological step towards remote entanglement.