Subhertz interferometry at the quantum noise limit

Fundamental scientific research and applications such as metrology require high precision phase measurements, which require the use of highly sensitive optical interferometers such as the laser interferometer gravitational-wave observatory. Despite achieving remarkable shot-noise-limited sensitivity levels, interferometric measurements of low signals and gravitational waves have remained a great challenge. This can be attributed to the dominant classical noises i.e. seismic, optical and thermal at the low-frequency region. In recent literature, multifrequency interferometry has been employed to obtain uncontaminated interferometric signals and sub-shot-noise phase measurements with high-frequency broadband squeezing. Unfortunately, practical reduction of the subhertz classical noises below the shot-noise level which is regarded as a necessity for accurate phase measurements has not been fully explored.

To this effect, scientists at Hubei University of Technology Dr. Sheng Feng and Dr. Boya Xie together with Dr. Peng Yang from Hubei Polytechnic University investigated the subhertz phase measurements based on a two-frequency Mach-Zehnder interferometer. The interferometer was operated at a dark fringe in which the signal-to-noise ratio in the phase measurements was kept optimal. Essentially, the subhertz interferometer was experimentally demonstrated at standard quantum noise limit using dual-frequency coherent probe light at the input while the output was measured by phase-sensitive heterodyne detection. This was because the dark-fringe light intensity is quadratic in phase and has weak power required for the production of observable photoelectric signals during direct detection. The work is currently published in the journal, Optics Letters.

It was important to first calculate the quantum noise limited sensitivity of the interferometer. Secondly, the feasibility of the noise floor of the interferometric signal was experimentally demonstrated to achieve the shot-noise level. Meanwhile, the interferometer at the dark fringe was locked to control the relative phase between the oscillator of the heterodyne detector and the signal light.

The authors successfully managed to suppress the classical noise to achieve 1 Hz that was at the standard quantum noise level. However, it was worth noting that it was relatively hard to realize completely dark fringes. This was due to the imperfect interference visibility at the interferometer output. It resulted in the dominance of the relative intensity noise and phase noise of the laser light over the shot noise especially in the frequency band. On the other hand, improving the interference visibility closer to unity resulted in darker fringes which may reach the shot-noise-limited sensitivity.

When broadband squeezed light is injected in the unused port of the interferometer, sub-shot-noise subhertz interferometry may be implemented. The developed interferometry has several advantages. For instance, no control is required for the relative phase between the squeezed light and the probe beam because they share the same propagation path inside the interferometer. Additionally, the efficiency of the presented interferometer can be improved by increasing the power ratio of the interfering light over the oscillator to prevent loss of the squeezed light. Altogether, the study provides essential information that will pave way for the realization of subhertz interferometry beyond the shot-noise limit.