In today’s most sensitive position sensors — gravitational wave antennae — an optical interferometer is used to measure variations in the length of two arms with an extreme precision (the expected relative signal is of 10^-21 for Advanced LIGO). At these sensitivities, noise reduction is of paramount importance to be able to detect any signal. Classical noise (thermal, seismic etc.) can be overcome by careful engineering. However, quantum noise — that which remains when all classical noise is absent — is much harder to suppress.

When an optical interferometer is used to measure the position of an object, laser light of a definite frequency is reflected off of the object. Classical physics tells us that there is no light at frequencies far from that of the laser. Quantum mechanics however tells us that there are intrinsic fluctuations of photons at these frequencies — i.e. a photon “vacuum” is not completely “dark”. These, so-called “vacuum” fluctuations — a form of quantum noise — can exert a force on the object whose position is being measured, leading to an unavoidable disturbance in its position. 

In recently performed experiment, Prof. Kippenberg’s group has shown how these quantum mechanical fluctuations of light can be modified through their interaction with the same mechanical object whose position it disturbs.

The experimental setup consists of a glass nanostring (red in the opening picture) placed in the extreme proximity of a round optical micro-cavity (blue). The coupling of the string with light stored near the edge of the cavity (in a whispering gallery mode, analogous to some sound modes in a big, circular building) is exploited to partially suppress quantum noise in the cavity.

The system is carefully engineered and operated in an environment cooled to just a few degrees above absolute zero so as to ensure that the mechanical oscillator is appreciably driven by the quantum fluctuations of the light. The experiment demonstrates how correlations developed due to this disturbance can be used to suppress the fluctuations in the light below the level set by the vacuum. The yellow region in the above figure, plotting the light signal at various frequencies, depicts this via the red trace (observed level of fluctuations) dipping below the blue trace (observed level of the vacuum). Such states of light, called “squeezed” states, feature a level of noise smaller than the uncoupled light at some frequencies. The two prominent peaks in the red trace are due to the motion of the mechanical oscillator. 

Despite the fact that quantum noise is suppressed at some frequencies, it is important to note that at other frequencies excess quantum noise is added. Nature does not offer any “free lunch”.

In several state-of-the-art sensitive applications which employ light for metrology, the vacuum fluctuations of light set a limit to the measurement precision. Thus, the ability to generate “squeezed” states with reduced noise is expected to be a major boon for future applications, including, but not limited to, gravitational wave detection. Although the frequencies of interest for gravitational wave detection are much lower than what has been demonstrated in this experiment, it is in principle possible to generate squeezed states inside a gravitational wave antenna, just as shown here, to improve its performance.

Reference:
“Appearance and Disappearance of Quantum Correlations in Measurement-Based Feedback Control of a Mechanical Oscillator”, V. Sudhir, D. J. Wilson, R. Schilling, H. Schütz, S. A. Fedorov, A. H. Ghadimi, A. Nunnenkamp, and T. J. Kippenberg, Phys. Rev. X 7, 011001 
DOI: https://doi.org/10.1103/PhysRevX.7.011001