Extremely Squeezed Light

Artist's impression of a squeezed light source. A non-linear crystal and a partially reflective mirror are the components of an optical parametric amplifier. (Source: A. Franzen and T. Steinhaus)

Artist’s impression of a squeezed light source. A non-linear crystal and a partially reflective mirror are the components of an optical parametric amplifier. (Source: A. Franzen & T. Steinhaus, AEI)

Laser light is one of the tools of the trade for gravitational-wave astronomy, which began one year ago with the first direct observation of merging black holes. Interferometric detectors such as Advanced LIGO, Virgo and GEO600 measure kilometer-long distances to the precision of a fraction of an atomic nucleus using laser light.

The precision of these detectors is – among other things – limited by the quantum mechanical “noise” of the light. AEI researchers have been working for many years on the development of ever better light sources with quantum noise better than nature usually allows – so-called squeezed light These light sources have been improving the sensitivity of the gravitational-wave detector GEO600 near Hanover, Germany, since 2010. In a laboratory setting, AEI scientists have in its own turns now broken their own world record from 2010 for the generation of squeezed light. They suppressed the quantum noise by a factor of 32. To achieve this, the physicists improved upon existing experiments with low-loss optical components and specially optimized photodetectors.

“At the Albert Einstein Institute, we have in the past years created a unique wealth of experience in optimizing and applying squeezed light,” says Henning Vahlbruch, postdoc at the AEI. “Based on our previous experiments, we could now break our own world record and at the same time experimentally show an entirely new application of squeezed light.”

The extremely strong squeezed light enables an entirely new method to determine the quantum efficiency of photodetectors. These devices play an important role in many modern experiments, including gravitational-wave detectors. There they measure the light at the detector output, whose fluctuations reveal gravitational wave signals. A crucial factor is how efficiently the photodetector converts the impinging light into electrical current.

While previous methods to determine this quantum efficiency require specially calibrated light sources, this is not necessary for the new technique developed in Hanover. The trick lies in the simplicity of the measurement. The squeezing strength of the laser light registered by the photodetectors could in principle be infinitely large. In reality, however, it is limited by optical losses. In their experiment, the Hanover physicists measured a total loss of 2.5 % ± 0.1 % of the laser light. Now, they only had to determine all losses caused by lenses, mirrors, and other optical elements. The found that these could account for 2.0 % of the total loss. The remaining 0.5 % must be the loss at the photodetector. The researchers thereby determined a quantum efficiency of 99.5 % ± 0.5 % for their specially optimized photodetector.

“Our new technique is an elegant solution to determine the quantum efficiency, because it is in a sense a very direct measurement,” says Moritz Mehmet, postdoc at the AEI. “With the squeezed light we neatly circumvent the laborious calibration of reference light sources. Another plus is that our method can be used quite easily at different light powers.”

In the future the new world record in squeezed light will be applied in gravitational-wave detectors such as Advanced LIGO, Virgo, and GEO600. Their sensitivity can be improved further by the use of similar squeezed light sources and by minimizing optical losses. Planned third-generation detectors such as the Einstein Telescope will also depend on squeezed light. Laboratory experiments with even more strongly squeezed light will enable even more precise measurements of the quantum efficiency of photodetectors. In the long term, they could become an independent and precise alternative to established methods and enable other applications in quantum metrology. (Source: AEI)

Reference: H. Vahlbruch et al.: Detection of 15 dB Squeezed States of Light and their Application for the Absolute Calibration of Photoelectric Quantum Efficiency; Phys. Rev. Lett. 117, 110801 (2016); DOI: 10.1103/PhysRevLett.117.110801

Link: Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institute, AEI), Hanover, Germany

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