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Frontiers in Optics

A Cold-Atom Superradiant Laser with <1 Intracavity Photon

Justin Bohnet, Zilong Chen, Joshua M. Weiner, Kevin C. Cox, Dominic Meiser, Murray Holland, James K. Thompson

Published Jan 1, 2012• 0 citations
Publisher

Abstract

We demonstrate a laser that operates with <1 intracavity photon, such that the laser-cooled gain medium act as the primary flywheel for the oscillator’s phase. Prospects for ultranarrow and novel passive/active oscillators are also considered.
DOI
10.1364/fio.2012.fw5f.2

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We demonstrate a cold-atom Raman laser that operates quasi-continuously with as few as 0.2 photons on average inside the cavity and with a single-atom Raman decay rate below 1 Hz. We demonstrate that the laser’s coherence is primarily stored within the atoms, and consider the relationship between the Schawlow-Townes linewidth and the standard quantum limit on phase estimation using unentangled atoms. This proof-of-principle experiment may lead to novel hybrid sensors and guide the development of future ultra-stable lasers with intrinsic insensitivity to currently-limiting thermal and technical sources of cavity mirror vibration.
Abstract : We demonstrated that an optical laser can operate with less than 1 photons on average inside the cavity while also obtaining significant spectral narrowing of the emitted light relative to the single atom transition linewidth. We further explored the power output properties and threshold conditions of this laser. We made direct measurements that showed that the lasing frequency can be made greater than 10000 times less sensitive to the resonance frequency of the optical cavity than is the case for normal lasers or for passive optical reference cavities. Such a large reduction in sensitivity would essentially remove both thermal and technical mirror vibrations as a limitation on laser linewidth. Other studies included how phase information can be stored and manipulated inside of our laser, such that it may be possible to have both an active laser and a passive Ramsey-like sensor in the same setup. We considered the fundamental limits on the atomic quantum information that can be extracted from the laser light and found that it corresponded closely to the fundamental atomic standard quantum limit on phase estimation.
We experimentally study the relaxation oscillations and amplitude stability properties of an optical laser operating deep into the bad-cavity regime using a laser-cooled ^{87}Rb Raman laser. By combining measurements of the laser light field with nondemolition measurements of the atomic populations, we infer the response of the gain medium represented by a collective atomic Bloch vector. The results are qualitatively explained with a simple model. Measurements and theory are extended to include the effect of intermediate repumping states on the closed-loop stability of the oscillator and the role of cavity feedback on stabilizing or enhancing relaxation oscillations. This experimental study of the stability of an optical laser operating deep into the bad-cavity regime will guide future development of superradiant lasers with ultranarrow linewidths.
A cold atomic gas with an inverted population on a transition coupled to a field mode of an optical resonator constitutes a generic model of a laser. For quasi-continuous operation, external pumping, trapping and cooling of the atoms is required to confine them in order to achieve enough gain inside the resonator. As inverted atoms are high-field seekers in blue detuned light fields, tuning the cavity mode to the blue side of the atomic gain transition allows for combining lasing with stimulated cavity cooling and dipole trapping of the atoms at the antinodes of the laser field. We study such a configuration using a semiclassical description of particle motion along the cavity axis. In extension of earlier work we include free space atomic and cavity decay as well as atomic dipole-dipole interactions and their corresponding forces. We show that for a proper choice of parameters even in the bad cavity limit the atoms can create a sufficiently strong field inside the resonator such that they are trapped and cooled via the superradiant lasing action with less than one photon on average inside the cavity.