Fundamental quantum limits of optomechanical sensors
Dominic Branford and Animesh Datta, working in collaboration with Haixing Miao (University of Birmingham), have published a paper on the fundamental quantum limits of optomechanical sensors in Physical Review Letters (DOI: 10.1103/PhysRevLett.121.110505).
Being able to measure very weak forces is central to many applications, such as the direct detection of gravitational waves and monitoring subterranean movement of magma in volancially-active areas. The strength of a force can be inferred through its effect of displacing a mass: the displacement can be sensed by illuminating it with a laser and observing the reflected light, a case of optomechanical sensing. While using higher intensity light increases the reflected signal, allowing finer measurement, the laser itself causes additional motion of the mass which appears as a "ponderomotive squeezing" of the reflected light.
The ponderomotive squeezing is a quantum effect that changes the photon number statistics, sometimes reducing and sometimes increasing noise. However, since it is uncontrollable, the overall effect tends to degrade the sensor’s precision.
In this work, Dominic, Haixing and Animesh study the best precision attainable by optomechanical sensors when multi-coloured light is used. Such a technique requires elaborate optics: several lasers are now needed. The ponderomotive effect now leads to even more complicated states of light, with quantum correlations between the different colours. Once again, the uncontrollable nature of the correlations leads to additional noise and uncertainty, limiting the resolution of the sensor. We provide the limits quantum mechanics imposes on the precision of such sensors, encompassing practical and optimal measurement schemes. Moreover, in spite of the more elaborate optics, the use of multiple frequencies does not improve the fundamental limits. By showing how multicoloured light fails to provide an advantage, the work suggests that new ideas for improving the precision of optomechanical sensors ought to be sought out and explored.