Researchers demonstrate superradiant emission from the 150-s lifetime strontium clock transition.
In this work we realize a narrow spectroscopic feature using a technique that we refer to as magnetically-induced optical transparency. A cold ensemble of 88 Sr atoms interacts with a single mode of a high-finesse optical cavity via the 7.5 kHz linewidth, spin forbidden 1 S0 to 3 P1 transition. By applying a magnetic field that shifts two excited state Zeeman levels, we open a transmission window through the cavity where the collective vacuum Rabi splitting due to a single level would create destructive interference for probe transmission. The spectroscopic feature approaches the atomic transition linewidth, which is much narrower than the cavity linewidth, and is highly immune to the reference cavity length fluctuations that limit current state-of-the-art laser frequency stability.There has been a dedicated effort in recent years to improve the frequency stability of lasers [1] used to probe optical atomic clocks [2][3][4]. Improvements in these precision measurement technologies are essential for advancing a broad range of scientific pursuits such as searching for variations in fundamental constants [5] , gravitational wave detection [6,7], and physics beyond the standard model [8,9]. Associated improvements in atomic clocks would also advance recent work on relativistic geodesy [10].The frequency stability of current state-of-the-art lasers is limited by thermal fluctuations in the reference cavity mirror coatings, substrates, and spacer [11]. This problem can be alleviated by creating systems that rely on an ensemble of atoms, rather than the reference cavity, to achieve stable optical coherence. Recent approaches include cavity-assisted non-linear spectroscopy [12][13][14] and superradiant lasers [15][16][17][18]. Both approaches use narrow forbidden transitions with linewidths ranging from 7.5 kHz to 1 mHz. These novel systems are absolute frequency references and are intrinsically less sensitive to both fundamental thermal and technical vibrations that create noise on the optical cavity's resonance frequency.Here we demonstrate a new linear spectroscopy approach in which a static magnetic field can induce optical transparency in the transmission spectrum of an optical cavity. The center frequency of the transparency window is shown to be insensitive to changes in the cavityresonance frequency and to first-order Zeeman shifts. The observed linewidth of the feature approaches the natural linewidth of the 7.5 kHz optical transition and can be insensitive to inhomogeneous broadening of the atomic transition frequency. The linewidth of the feature is an important attribute for laser stabilization, as a laser stabilized to a narrow spectroscopic feature is less sensitive to technical offsets than a laser stabilized to a broader feature. In the future, it might be possible to extend this technique to even narrower optical transitions for enhanced spectroscopic sensitivity in atoms such as calcium and magnesium.In partial analogy to electromagnetically induced transparency (EIT) [19][20][21], we refer to this eff...
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