Squeezing out noise

Researchers at AEI Hannover develop new method to stabilize laser power, enabling more precise measurements in low-power experiments

May 21, 2025

Laser light is a versatile tool for scientists: Many precision experiments in fields ranging from life sciences to gravitational-wave detection are only possible with lasers. While the use of high laser power allows for more precise measurements, this may not be possible in certain experiments. At low laser power, precision is fundamentally limited by shot noise: fluctuations in the laser power associated with fundamental variations of the number of light quanta in the laser beam. Now, researchers at the Max Planck Institute for Gravitational Physics and at Leibniz University Hannover have developed a novel way to reduce shot noise using a taylored version of darkness, namely a squeezed quantum mechanical vacuum state. This is the first realization of a bright squeezed laser beam via power stabilization with squeezed vacuum. It reduces laser power noise over a broad frequency band by up to a factor of 3.7 below the previous fundamental shot noise limit. The results could improve experiments limited to low laser power levels. In their study, the researchers also discuss how their experiment could be extended to use the same method to reduce the power fluctuations of high-power lasers, such as those used in current and future gravitational-wave detectors, e.g., the Einstein Telescope.

Paper abstract

Low-power-noise lasers are essential for many high-precision experiments. Shot noise imposes a fundamental limit on the power stability achievable with classical light sources. In this letter, we report the generation of bright squeezed light in the audio band, which can improve the sensitivity of otherwise shot-noise-limited measurements. It is (to our knowledge) the first experimental realization of a bright squeezed light source via the use of squeezed vacuum in a power stabilization scheme. We demonstrate a laser beam at a power of 9.9 mW with a broadband power noise suppression below the classical stabilization limit by up to 8.7+0.3−0.4 dB in the audio band, resulting in a bright squeezed beam with a power noise of 5.7+0.3−0.4 dB below shot noise for frequencies from 6 kHz to 20 kHz. We further conceptually outline a method to surpass this result in the future.

Double logarithmic plot of the output relative power noise. The green curve shows the classically stabilized relative power noise. The teal and orange curve show the squeezed and anti-squeezed measurements, respectively. The out-of-loop photo detector dark noise was subtracted (not shown; 13 dB below the classical limit at 10 kHz). The peaks at 19.6 kHz and its harmonics stem from the power supply of the used network analyzer, and the corresponding dips in the teal curve are an artifact of the dark noise subtraction. The dashed lines give the calculated quantum noise levels of the classical stabilization (black), the shot noise (gray), and squeezed (light blue) and anti-squeezed (light orange) operation. The in-loop electronic noise (blue) and residual technical noise (red) in uncorrelated sum with the squeezed quantum noise yield the expected squeezed relative power noise (purple).

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