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Contemporary Physics

Cavity-QED with single trapped ions

Matthias Keller

Published Jan 2, 2022• 7 citations
Publisher

Abstract

Combining optical cavities with trapped atomic ions is a powerful tool to investigate the interaction of light with matter on the most fundamental scale: a single atom with a single particle of light. Recent advances have not only demonstrated the emission of single photons, ion-photon entanglement and ion-ion entanglement but paved the way to use these systems in modern quantum technologies. Ion-cavity systems are now one of the most promising technologies for realising a scalable quantum computer and quantum networks. While the integration of optical cavities and ion traps is challenging, new techniques and technologies made it possible to demonstrate important building blocks for such systems.

DOI
10.1080/00107514.2022.2118385

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Combining optical cavities with trapped atomic ions is a powerful tool to investigate the interaction of light with matter on the most fundamental scale: a single atom with a single particle of light. Recent advances have not only demonstrated the emission of single photons, ion-photon entanglement and ion-ion entanglement but paved the way to use these systems in modern quantum technologies. Ion-cavity systems are now one of the most promising technologies for realising a scalable quantum computer and quantum networks. While the integration of optical cavities and ion traps is challenging, new techniques and technologies made it possible to demonstrate important building blocks for such systems.
We review the recent developments towards strong coupling of a single ion in cavity QED, in particular by using optical fiber cavities. We also summarize our efforts along the same line being pursued at Sussex University. The main issue in cavity QED with trapped ions has been the incompatibility of dielectric mirrors in trapping potentials. As consequence the mirrors of a cavity have to be further retracted from the ion, rendering the cavity volume rather big to sacrifice the ion-cavity coupling. However, the emerging fiber cavity technology is expected to largely mitigate this difficulty and yet achieve excellent resonator-quality. Achieving strong coupling with single trapped ions enables one to build a highly reliable and controllable quantum interface between matter and light to be used in quantum network. It could also open up new possibilities such as manipulating photonic quantum states via the quantum computing toolbox of ion trap.
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.
Recent technological advances in cavity quantum electrodynamics (CQED) are paving the way to utilize multiple quantum emitters confined in a single optical cavity. In such systems, it is crucially important to control the quantum mechanical coupling of individual emitters to the cavity mode. In this regard, combining ion trap technologies with CQED provides a particularly promising approach due to the well-established motional control over trapped ions. Here, we experimentally demonstrate coupling of up to five trapped ions in a string to a high-finesse optical cavity. By changing the axial position and spacing of the ions in a fully deterministic manner, we systematically characterize their coupling to the cavity mode through visibility measurements of the cavity emission. In good agreement with the theoretical model, the results demonstrate that the geometrical configuration of multiple trapped ions can be manipulated to obtain optimal cavity coupling. Our system presents a new ground for exploring CQED with multiple quantum emitters, enabled by the highly controllable collective light-matter interaction.
Long-range transport of quantum information across a network is most readily achieved through the use of photons as the information carriers. The nodes of such a quantum network are most naturally a quantum memory or information processor, which typically do not rely upon photonic qubits. Realizing a large-scale distributed quantum network therefore requires an efficient interface between the two physical manifestations of the quantum information. The most promising platform for realizing a quantum computer is the manipulation of trapped atomic ions, as this system has demonstrated all the fundamental requirements to realize a quantum computer. Additionally, trapped ions posses long coherence times necessary for quantum memories. The interface between atomic and photonic qubits relies on a high photon emission rate into the collected solid angle. Most methods of photon collection for quantum networks use a high numerical aperture microscope objective, which only collects on the order of 1% of all the emitted photons. This results in a small probability of successfully linking two quantum nodes. In this work, I present the design, simulation, and implementation of a trapped ion cavity QED system capable of improving the photon collection efficiency. A single ytterbium ion is coupled to a high-finesse optical cavity. The photon scatter rate into the cavity mode is enhanced by a factor of 130 over the free-space scatter rate into the cavity solid angle. This represents the first step towards realizing a protocol to entangle the polarization of an emitted photon to the Zeeman sublevels of the ion with high photon collection efficiency. Simulations of the experimental system indicate that with an optical cavity up to 4% of the light emitted by the ion can be collected.
The realisation of efficient ion-photon quantum interfaces is an essential step towards developing operational quantum information networks. Individual trapped ions have been shown as promising candidates for quantum information processing nodes, while effective ion manipulation and photonic links between nodes can be realised by deterministic coupling of ions to optical cavities. However, integrating optical cavities into ion traps remains a challenge as the trapping potential is affected by the presence of any dielectric or conductor.
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.