I read this article today and was somewhat puzzled by this quote:

[...] according to quantum mechanics, the process of measuring the motion also influences it. In the experiment, this 'quantum measurement backaction' is caused by the inevitable quantum fluctuations of the laser light. In the framework of quantum optics, these are caused by quantum fluctuations of the electromagnetic field in empty space (vacuum). Odd as it sounds, this effect left clear signatures in the Niels Bohr Institute experiments' data, namely strong correlations between the quantum fluctuations of the light, and the mechanical motion as measured by light.

In particular, how, experimentally, does one independently measure the quantum fluctuations of the laser so that its value can be compared to the measured mechanical motion of the membrane?

If I correctly understand the research paper, it is done via ponderomotive squeezing in which the amplitude fluctuations of the laser are correlated with phase variations due to the change in the physical length of the optical cavity, which I assume is due to the fluctuation of the experimental membrane. Is that correct? If so, how can we separate the quantum fluctuations of the laser from the mechanical motion of the membrane?


For the context described in the paper, the laser noise due to quantum fluctuations is simply the shot noise. The reason this form of noise is sometimes described this way comes from the fact that a coherent laser beam can be described as a "displaced vacuum state" (equivalent to the vacuum but with an average intensity not equal to zero). Due to considerations related to the Heisenberg Uncertainty principle, such a state will have noise in a measurement (which is the same noise you'd see from a vacuum measurement, i.e. which is the same form of noise you'd see if you tried to make a measurement with the laser turned off).

  • $\begingroup$ Thanks for the answer. What I don't understand is how one can separate the quantum noise of the laser from the random fluctuations of the membrane if the only way the latter is measured is via the phase of the returned laser light. What am I missing? $\endgroup$ – Edward Dec 23 '16 at 0:00

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