Synopsis: Bad Cavities for Precise Lasers

The frequency of a laser based on trapped ultracold atoms can be made insensitive to fluctuations in the laser cavity’s length.
Synopsis figure
M. Norcia and J. K. Tompson, Phys. Rev. X (2016)

Lasers with narrow frequency spectra are the basis of today’s most accurate clocks and of fundamental studies of atomic physics or general relativity. Their accuracy is currently limited by fluctuations in the length of reference optical cavities that stabilize the laser spectrum. These fluctuations cause the cavity’s resonant frequency to jump about. Matthew Norcia and James Thompson at JILA in Colorado have now developed a laser whose sensitivity to fluctuations in its cavity length can be suppressed. The scheme could potentially lead to lasers with a frequency precision surpassing that of any existing laser.

The authors’ laser can operate in a superradiant, or “bad cavity” regime. In this regime, atoms in the laser gain medium synchronize to emit light at a frequency dependent only on atomic properties. As a consequence, length changes of a relatively lossy laser cavity do not affect the output frequency. Previous theoretical studies suggested that a superradiant laser using cold alkaline-earth metals could attain extraordinarily narrow linewidths. Norcia and Thompson demonstrated a step towards such a laser by trapping up to 60,000 ultracold strontium atoms in an optical lattice confined within a cavity.

Their laser operates on electronic transitions that are—to a first approximation—forbidden. The lifetime of these forbidden transitions is 1000 times longer than that of allowed transitions of the same wavelength. In the frequency domain, this longer decay time leads to a narrower emission spectrum. While the demonstrated laser linewidth is still much larger than that of today’s most precise lasers, the researchers suggest that the same techniques may be applied to forbidden transitions with even longer lifetimes, yielding orders-of-magnitude improvements.

This research is published in Physical Review X.

–Matteo Rini


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