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arXiv:1512.09346 · Physical Review Letters

Optimized Multi-Ion Cavity Coupling

Stephen Patrick Begley, Markus O. Vogt, Gurpreet Kaur Gulati, Hiroki Takahashi, Matthias Keller

Published May 31, 2016• 36 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevlett.116.223001

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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.
Entanglement is an established resource in quantum information processing, and there is a clear imperative to study many-body systems both in quantum technology applications and for probing fundamental physical laws. Ion trap cavity quantum electrodynamics is a highly promising platform for research. Prerequisites are the controlled coupling of many ions to the cavity field along with the ability to initialize the quantum states and drive coherent transitions between them. A coaxial ion trap and high finesse cavity system has been shown to couple strings of up to five ions to the cavity mode with nearly optimal coupling strength thanks to precise control over their positions in the standing wave and their mutual Coulomb interaction. The predictive power of the theoretical model demonstrates that the scheme can be extended to more ions or to higher coupling regimes. In a separate experiment it has been demonstrated that a quantum state can be initialized, before coherently transferring the population to a qubit state through the cavity interaction. The emission of polarized photons in the cavity mode has been measured, taking the system closer to the generation of cluster states for quantum information research and fundamental studies in many-body entanglement. Building on the aforementioned work, an infrastructure has been put in place for the direct observation of vacuum Rabi oscillations between a single ion and the cavity. In this scheme, the contribution to the dynamics from the dominant incoherent channel is minimized through post-selection of the data. As a quantum system coupled to a reservoir with memory, it will provide experimental constraints on theoretical work in the field of non-Markovian dynamics.
In this work, I detail the reconstruction and upgrades performed on the axial cavity ion trap in the ITCM group at the university of Sussex, and the measurement of the coupling of multiple ions to the cavity mode. This enables the optimal coupling between the ions and the cavity by adjusting the ions position in the radial and axial positions. This covers new ground in extending the optimal coupling beyond two ions which is of great importance for experiments with several ions in an optical cavity. The thesis outlines the background theory of light-matter interaction and cavity QED, before describing the physical ion trap hardware and its assembly. A description of the laser and cavity systems is provided, including techniques for locking both to stable references. A number of novel measurement techniques for measuring and maximising the stability of the ions and cavities are presented, including micromotion minimisation, spectroscopy, magnetic field compensation using the ground state Hanle effect, and Raman spectroscopy. These techniques enable the measurement of crucial parameters of the atomic transitions and the cavity. The work culminates in a description of the optimisation of the coupling between ion strings and the cavity first by adjusting the radial trap position by means of variable capacitors attached to RF electrodes, and then axially by means of adjusting the endcap potentials and therefore the spacing between ions to obtain the greatest localisation while still positioning the ions close to the antinodes of the cavity field.
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.
Precise control over internal states of atomic ions in ion traps has been possible for many years enabling high precision spectroscopy of atomic transitions. Trapped ions are suitable candidates for quantum information processing due to their long storage times, low decoherence rates and high readout efficiency. Recent advances in micro-machining fibre optic mirrors and integrated fibre optic-ion trap systems for cavity QED allows one to realise strong coupling between an ion and a cavity mode by reducing the mode volume. For strong ion-cavity coupling, deterministic transfer of quantum states between ions and photons is possible. We have designed and constructed a miniaturised linear Paul trap with an integrated optical cavity, intended for coupling multiple ions to the cavity mode. The RF-electrode separation is 50μm, while the endcap separation is 350μm. The endcap electrodes are narrow tubes of inner diameter 300μm allowing optical fibers through the tubes to form the cavity. The trap depth we calculate to be 650meV for a 10V applied potential at 20MHz. The 4P1⁄2→ 3D3⁄2 transition of the atomic ion 40Ca+ with Γ = 24π × 22MHz can be coupled to an optical fibre Fabry-Perot cavity with length of 380µm, finesse of 33,000 and cavity linewidth k = 2π ×11MHz.The ion-cavity coherent coupling g = 2π × 19:5MHz of this system will put it in single-ion strong coupling regime with cooperativity, C=1.5.
Strong coupling between an atom and an electromagnetic resonator is an important condition in cavity quantum electrodynamics. While strong coupling in various physical systems has been achieved so far, it remained elusive for single atomic ions. Here, we achieve a coupling strength of 2π×(12.3±0.1) MHz between a single ^{40}Ca^{+} ion and an optical cavity, exceeding both atomic and cavity decay rates which are 2π×11.5 and 2π×(4.1±0.1) MHz, respectively. We use cavity assisted Raman spectroscopy to precisely characterize the ion-cavity coupling strength and observe a spectrum featuring the normal mode splitting in the cavity transmission due to the ion-cavity interaction. Our work paves the way towards new applications of cavity quantum electrodynamics utilizing single trapped ions in the strong coupling regime for quantum optics and quantum technologies.
The coupling of a single ion to an optical cavity is a promising route towards scalable quantum technologies. This study presents the design, initial construction and simulations of two different ion traps for quantum computation with novel capabilities. The first system is an end cap system that utilises Fabry Perot cavities formed by the facets of fibres. The small mode volumes (cavity length ~ 300 µm) of the fibre cavities are ideal to achieve high cooperativity and coupling strength. This trap features the novel ability to mechanically adjust the position of the ion by perturbing/distorting the trapping potential with grounded side electrodes. A capability crucial to maximising the ion-cavity coupling. The design incorporates a number of improvements over the previous iteration by Takahashi et al. such as increased optical access, increased mode matching and improved mechanical stability of the cavity. To verify and test the design modifications; Pound-Drever-Hall cavity locking was set up. The test set up failed to lock to an error signal due to a mechanically unstable translation stage. The other design presented is a linear micro trap intended to trap strings of ions and does not use fibre based cavities. This design features multiple different regions reserved for storage, computation and communication and aims to implement a shuttling scheme that will allow the selective loading of single ions from a reservoir of ions into the desired regions. The fabrication process would utilise methods in microfabrication. Simulations of the trapping dynamics were used to optimise the electrode geometries and splitting protocol.