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Physical Review Letters

Continuous Collective Strong Coupling of Strontium Atoms to a High Finesse Ring Cavity

Julia Cline, Vera M. Schäfer, Zhijing Niu, Dylan J. Young, Tai Hyun Yoon, James K. Thompson

Published Jan 10, 2025• 13 citations
Publisher

Abstract

We demonstrate continuous loading and strong collective coupling of atoms to a high finesse ring cavity. A series of spatially separated laser cooling stages delivers a continuous flux of 2.1(3)×10^{7} ^{88}Sr atoms/s into an intracavity magic wavelength lattice where the atoms are deterministically transported inside the ring cavity. We observe a continuous atom-cavity vacuum Rabi splitting, demonstrating the first steady-state strong collective coupling on a narrow linewidth atomic transition. This Letter opens the path to future continuous cold atom devices, such as a continuous wave superradiant laser with millihertz linewidth and low vibration sensitivity, enabling searches for new physics and technologies.

DOI
10.1103/physrevlett.134.013403

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We demonstrate continuous loading and strong collective coupling of atoms to a high finesse ring cavity. A series of spatially separated laser cooling stages delivers a continuous flux of 2.1(3)×10^{7} ^{88}Sr atoms/s into an intracavity magic wavelength lattice where the atoms are deterministically transported inside the ring cavity. We observe a continuous atom-cavity vacuum Rabi splitting, demonstrating the first steady-state strong collective coupling on a narrow linewidth atomic transition. This Letter opens the path to future continuous cold atom devices, such as a continuous wave superradiant laser with millihertz linewidth and low vibration sensitivity, enabling searches for new physics and technologies.
We demonstrate continuous loading of strontium atoms into a high finesse ring cavity and observe continuous strong collective coupling in the form of a vacuum Rabi splitting between the atoms and the cavity on the 7.5 kHz transition 1S0^1{\rm S}_01S0​ to 3P1^3{\rm P}_13P1​. The atoms are loaded into the cavity from a 3D narrow linewidth molasses, thus avoiding large magnetic field gradients and associated broadening of transition frequencies. The ring cavity allows us to realize a deterministic conveyor belt to transport atoms away from the loading region where the laser cooling beams lead to broadening of the strontium clock transition. We trap up to 10610^6106 atoms in an intracavity 813 nm lattice in the Lamb-Dicke regime, and transport the atoms along the cavity axis. This work opens the path to the creation of a continuous wave superradiant laser with millihertz linewidth enabling searches for new physics and the use of high-precision optical frequency references outside of low vibration laboratory environments.
Laser-cooled gases of atoms interacting with the field of an optical cavity are a powerful tool for quantum sensing and the simulation of open and closed quantum systems. They can display spontaneous self-organisation phase transitions, time crystals, new lasing mechanisms, squeezed states for quantum sensing, protection of quantum coherence, and dynamical phase transitions. However, all of these phenomena are explored in a discontinuous manner due to the need to stop and reload a new ensemble of atoms. Here we report the observation of hours-long continuous lasing from laser-cooled 88^{88}88Sr atoms continuously loaded into a ring cavity. The required inversion to produce lasing arises from inversion in the atomic momentum degree of freedom, a mechanism related directly to self-organization phase transitions and collective atomic recoil lasing, both of which were previously only observed in a cyclic fashion compared to the truly continuous behavior here. Further, the sensitivity of the lasing frequency to cavity frequency changes is 120 fold suppressed due to an atomic loss mechanism, opening an interesting new path to compensate cavity frequency noise for realizing narrow frequency references. This work opens the way for continuous cavity QED quantum simulation experiments as well as continuous superradiant lasers.
Steady-state collective emission from ensembles of laser cooled atoms has been proposed as a method for generating sub-millihertz linewidth optical lasers, with potential for broad impacts across science and technology. We have built a model system that tests key predictions for such active oscillators using a Raman laser with laser cooled atoms as the gain medium. The laser operates deep in the bad-cavity, or superradiant, regime of laser physics, where the cavity decay rate is much greater than the atomic coherence decay rate. Specifically, we demonstrate that a system of 106 87Rb atoms trapped in a 1D standing wave optical lattice can spontaneously synchronize and collectively emit a quasi-continuous coherent optical output, even when the intracavity field contains on average < 1 photon.
Abstract By considering linear scattering of laser-driven cold atoms inside an undriven high-finesse optical resonator, we experimentally demonstrate effects unique to a strongly coupled vacuum field. Arranging the atoms in an incommensurate lattice with respect to the radiation wavelength, the Bragg scattering into the cavity can be suppressed by destructive interference: the atomic array is subradiant to the cavity mode under transverse illumination. We show however, that strong collective coupling leads to a drastic modification of the excitation spectrum, as evidenced by well-resolved vacuum Rabi splitting in the intensity of the fluctuations. Furthermore, we demonstrate a significant polarization rotation in the linear scattering off the subradiant array via Raman scattering induced by the strongly coupled vacuum field.
We present experiments on ensemble cavity quantum electrodynamics with cold potassium atoms in a high-finesse ring cavity.Potassium-39 atoms are cooled in a two-dimensional magneto-optical trap and transferred to a three-dimensional trap which intersects the cavity mode.The apparatus is described in detail and the first observations of strong coupling with potassium atoms are presented.Collective strong coupling of atoms and light is demonstrated via the splitting of the cavity transmission spectrum and the avoided crossing of the normal modes.