Reducing the signal-to-noise ratio to get a lock on quantum signals

Chris Lee

Reducing the signal-to-noise ratio to get a lock on quantum signals

There is an art to making good measurements. In the ideal case, you want a really sensitive measuring device so that you can measure the smallest of changes to the signal you are interested in. But what you find is that most of the sensitivity is wasted, because in addition to signal, there is always noise. And for most "on the bleeding edge" experiments, the noise is huge compared to the signal.

In classical physics, there is a seemingly magical machine, called a lock-in amplifier, that extracts signal from the deepest and darkest pits of noise. But it seemed that the way quantum mechanics worked would make a quantum lock-in relatively useless. It turns out that, with a bit of clever thinking, you can make a quantum lock-in, and the resulting measurements have even more exquisite sensitivity.

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