Strong coupling between an atom and an electromagnetic resonator is an important condition in cavity quantum electrodynamics (QED). While strong coupling in various physical systems has been achieved so far, it remained elusive for single atomic ions. In this paper we demonstrate for the first time the coupling of a single ion to an optical cavity with a coupling strength exceeding both atomic and cavity decay rates. 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 QED utilizing single trapped ions in the strong coupling regime for quantum optics and quantum technologies.
We study experimentally and theoretically a dense ensemble of negatively charged nitrogen-vacancy centers in diamond coupled to a high $Q$ superconducting coplanar waveguide cavity mode at low temperature. The nitrogen-vacancy centers are modeled as effective spin one defects with inhomogeneous frequency distribution. For a large enough ensemble the effective magnetic coupling of the collective spin dominates the mode losses and inhomogeneous broadening of the ensemble and the system exhibits well resolved normal mode splitting in probe transmission spectra. We use several theoretical approaches to model the probe spectra and the number and frequency distribution of the spins. This analysis reveals an only slowly temperature dependent q-Gaussian energy distribution of the defects with a yet unexplained decrease of effectively coupled spins at very low temperatures below $unit{100}{millikelvin}$. Based on the system parameters we predict the possibility to implement an extremely stable maser by adding an external pump to the system.
We demonstrate an all-fiber cavity QED system with a trapped single atom in the strong coupling regime. We use a nanofiber Fabry-Perot cavity, that is, an optical nanofiber sandwiched by two fiber-Bragg-grating mirrors. Measurements of the cavity transmission spectrum with a single atom in a state-insensitive nanofiber trap clearly reveal the vacuum Rabi splitting.
The quadrupole S$_{1/2}$ -- D$_{5/2}$ optical transition of a single trapped Ca$^+$ ion, well suited for encoding a quantum bit of information, is coherently coupled to the standing wave field of a high finesse cavity. The coupling is verified by observing the ions response to both spatial and temporal variations of the intracavity field. We also achieve deterministic coupling of the cavity mode to the ions vibrational state by selectively exciting vibrational state-changing transitions and by controlling the position of the ion in the standing wave field with nanometer-precision.
We present a novel hybrid system where an optical cavity is integrated with a microfabricated planar-electrode ion trap. The trap electrodes produce a tunable periodic potential allowing the trapping of up to 50 separate ion chains spaced by 160 $mu$m along the cavity axis. Each chain can contain up to 20 individually addressable Ybtextsuperscript{+} ions coupled to the cavity mode. We demonstrate deterministic distribution of ions between the sites of the electrostatic periodic potential and control of the ion-cavity coupling. The measured strength of this coupling should allow access to the strong collective coupling regime with $lesssim$10 ions. The optical cavity could serve as a quantum information bus between ions or be used to generate a strong wavelength-scale periodic optical potential.
We experimentally demonstrate a ring geometry all-fiber cavity system for cavity quantum electrodynamics with an ensemble of cold atoms. The fiber cavity contains a nanofiber section which mediates atom-light interactions through an evanescent field. We observe well-resolved, vacuum Rabi splitting of the cavity transmission spectrum in the weak driving limit due to a collective enhancement of the coupling rate by the ensemble of atoms within the evanescent field, and we present a simple theoretical model to describe this. In addition, we demonstrate a method to control and stabilize the resonant frequency of the cavity by utilizing the thermal properties of the nanofiber.