There are measuring instruments that measure distance by measuring the time a lightbeam takes to come back. An example is a LICA scanner, that uses this tecnology to make 3D models of objects using laser beams.
Question is: How is it possible to measure how long light takes to get from point A to point B and back if light travels so fast? Isn't electronics too slow to measure such tiny time differences? What about the time electronic components need to change its state?
In general such systems do not use time of flight measurements, at least not directly. Light is just too fast for current systems, if you want better repeatability than a few meters for a cost effective system. There are a few things you can do: geometric measurement, interferometry, modulation.
The simplest thing you can do is to use triangulation. This is how the cheap distance measurement equipment used in construction works. You shine a beam, it reflects, and you pick it up from a different sensor near the receiver. Measure the angle, and you can get a relatively coarse repeatability in the 10's of mm range.
If you need better repeatability, you can use interferometry. You simply measure the difference in phase shift. This can get you to um repeatability. The problem here is that the wavelength changes with atmospheric conditions e.g. 0.1 K rise in temperature generates a 1ppm change. You need to compensate for temperature, pressure, humidity etc. Furthermore, this will give you incremental readings. You need to combine this with the former method.
A relatively new method is to send a modulated waveform. When you receive it back you can use the fact that you know its shape. This then reduces to an optimization problem. A convex optimization problem to be precise. The main disadvantage here is that there is a relationship between bandwidth, and time: the uncertainty principle. But you can get very accurate results.
Light covers 1 metre in 3.33 ns. That 3.33 ns (if it were a waveform) would have a frequency of 300 MHz so, if you had an oscillator running at 10 GHz you'd have a distance resolution of 3 cm. If you had a higher frequency you'd get a better distance resolution.
I'm not saying that these sorts of circuits are trivial but they do exist.
