• DOI:10.1088/1361-6455/aaddd1
• Corpus ID: 53711074

30-Fold Increase in Atom-Cavity Coupling Using a Parabolic Ring Cavity

@inproceedings{Cox201830FoldII,
  title={30-Fold Increase in Atom-Cavity Coupling Using a Parabolic Ring Cavity},
  author={Kevin C. Cox and David H. Meyer and Nathan Schine and Fredrik K. Fatemi and Paul D. Kunz},
  year={2018}
}

• K. Cox, D. H. Meyer, +2 authors P. Kunz
• Published 15 February 2018
• Physics

Optical cavities are one of the best ways to increase atom-light coupling and will be a key ingredient for future quantum technologies that rely on light-matter interfaces. We demonstrate that traveling-wave “ring” cavities can achieve a greatly reduced mode waist w, leading to larger atom-cavity coupling strength, relative to conventional standing-wave cavities for given mirror separation and stability. Additionally, ring cavities can achieve arbitrary transverse-mode spacing simultaneously with the large mode-waist reductions. Following these principles, we build a parabolic atom-ring cavity system that achieves strong collective coupling NC = 15(1) between N = 10 Rb atoms and a ring cavity with a single-atom cooperativity C that is a factor of 35(5) times greater than what could be achieved with a near-confocal standing-wave cavity with the same mirror separation and finesse. By using parabolic mirrors, we eliminate astigmatism–which can otherwise preclude stable operation–and increase optical access to the atoms. Cavities based on these principles, with enhanced coupling and large mirror separation, will be particularly useful for achieving strong coupling with ions, Rydberg atoms, or other strongly interacting particles, which often have undesirable interactions with nearby surfaces. 30-Fold Increase in Atom-Cavity Coupling Using a Parabolic Ring Cavity 2 Optical cavities increase the coupling between light and matter by an enormous factor (as much as 10) given by the average number of times light bounces between the mirrors before being transmitted or absorbed, approximately the cavity finesse F . For this reason, optical cavities will continue to be a key ingredient for emerging quantum technologies across a wide range of qubit platforms including neutral atoms [1], ions [2], and solid-state qubits [3]. Optical cavity technology continues to advance in a variety of useful directions, for example: extreme stability and finesse [4], integrated photonic waveguides [5, 6], and record atom-cavity coupling achieved with fiber cavities [7]. Within this broad spectrum of cavity technologies, one of the most useful optical cavity designs is still the two-mirror standing-wave (i.e. Fabry-Perot) cavity. While these cavities have recently enabled exquisite quantum control [8, 9, 10, 11], they also have several serious potential drawbacks: 1) The standing-wave results in spatially inhomogeneous light-matter coupling. 2) One cannot simultaneously make the cavity tightly focusing, stable, and single mode. 3) Small-mode cavities limit optical access and readily develop mirror surface charges that perturb strongly interacting qubits, such as ions and Rydberg atoms [12, 13, 2]. Ring cavities, made with three or more mirrors, make a directional path for light and naturally provide a solution to the first challenge [14, 15, 16, 17]. In this work we solve challenges 2) and 3) by presenting a ring cavity with increased coupling relative to a comparable standing-wave cavity while maintaining stability, large mirror separation, and tunability of transverse modes. The large coupling enhancements we discuss can be achieved using either parabolic mirrors, as done here, or a four-mirror “bowtie” configuration. While bowtie enhancement cavities have been used in lasers [18], nonlinear optics, and high power laser cavities [19], only one recent experiment has explicitly placed atomic qubits inside a ring cavity using a geometry where large coupling enhancements are possible [20, 21]. Here we emphasize the attractive properties of ring cavities, particularly the ability to engineer small mode waists, with the hope that they can be utilized to a greater degree to advance cavity-based quantum technology. In this paper, we demonstrate strong collective coupling between an ensemble of Rb atoms and a parabolic ring cavity, diagrammed in Fig. 1 a). The cavity mirrors reside outside the vacuum chamber in a geomtry with one short focusing arm (length d1), containing a cloud of laser-cooled and trapped atoms, and one long nearly-collimated arm (length d2). This ring cavity allows a significantly smaller waist w1 = 12(1) μm compared to a single-mode standing-wave cavity of the same length, resulting in a factor of 35(5) increase in atom-photon coupling. Additionally, we demonstrate that the frequency splitting between transverse cavity modes can be tuned arbitrarily while maintaining a small waist, a level of control that is impossible with standing-wave cavities. Parabolic mirrors eliminate astigmatism and increase optical access, opening the possibility for addressing the atoms at small angles [22]. The outside-vacuum design has allowed rapid prototype iteration and implementation in our quantum experiments. Despite the moderate finesse F = 80(3) due to traversing the vacuum windows, we achieve a collective cooperativity of NC = 15(1) using only 10 Rb atoms. 30-Fold Increase in Atom-Cavity Coupling Using a Parabolic Ring Cavity 3 C o u p lin g E n h a n ce m e n t