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65

EDIT updated (improved) description of phase detection circuit There are two principles used in these systems. The first is the time-of-flight principle. As you noted, if you wanted to get down to 3 mm accuracy, you need timing resolution of 20 ps (20, not 10, because you would be timing the round trip of the light). That's challenging - certainly not ...


24

The timing circuit doesn't have to run that fast. It just needs a time-to-digital converter which has a high enough resolution (0.1ns is nearly trivial with off the shelf CMOS technology) and then it can average many pulses (hundreds or thousands) to get the resolution improved by another order of magnitude. These are all fairly standard engineering ...


20

You don't have to run a clock that fast, and you don't need any new physical principles either, just some clever electronic design, mixing analog and digital components and making a few critical parts (switches, in essence) very fast. One simple technique, as described here on wikipedia, is a two-slope ramp. At the start of the time to be measured, you ...


10

Instead of attempting to time the round-trip of individual pulses (which depends on a good way to separate reflected pulses from ambient noise), you can also build a phase-locked loop. Control the sending of outgoing pulses by a voltage-controlled oscillator, sending one pulse at each rising zero crossing. Whenever you see an incoming pulse just before the ...


6

Laser is coherent light, so with a technique called interferometry you can actually measure distance with a resolution of less than a micro-meter, regardless of your timing resolution. It should be noted that the measurement produced by interferometry has half-wavelength periodicity (e.g. 200-350nm for visible light). This means that in order to absolutely ...


3

Physics is not a bureaucracy, so there are no "official" documents like that. There are people called "metrologists" who think all week long about how to make physical standards more precise and they do have fairly well thought out ideas with what physical effects one should start to make precise measurements. These are not necessarily the standards that one ...


3

Do you have access to a precision balance? Then you could weigh the plate, and using the known dimensions of the plate and the density of copper, compute the thickness. For $5\,{\rm cm} \times 5\,{\rm cm} \times 30\,\mu{\rm m}$ the weight would be $0.672\,{\rm g}$ for example. The precision of that measurement depends on how accurately you can measure the ...


2

You are right that the magnetic field, or rather the apparatus used to generate it, absorbs some angular momentum from the quantum spin. This apparatus includes, for example, some coils of wire containing more than $10^{23}$ electrons, as well as many other macroscopic components. The upshot is that increasing the angular momentum of this apparatus by an ...


2

Here's how I understand your question: A and B are space-like separated and make a measurement on a single particle that has equal (or just non-vanishing) probabilities of being in A's or B's region. You now ponder how the measurement process works on a deeper level. Could the collapse be a dynamical (i.e. time dependent) process? I think it can not. If ...


1

The method with measuring the instantaneous weight while swinging the arm with a fixed angular velocity sounds rather impractical. You might try to combine data from multiple sources. I assumed that you are talking about the bit of limb from elbow up to and including the hand, with the hand stretched, but it turns out that you wish to exclude the hand. The ...


1

Wave function collapse is not global, it is fictional. Let's suppose that the state is $\alpha|X=0\rangle+\beta|X=1\rangle$, where $|\alpha|^2+|\beta|^2=1$. When Alice measures the state, an operation is applied that correlates both Alice and the environment with the value of $X$, like so ...


1

The surrounding system is in a very large very mixed state, and that washes out the distinction in angular momentum. Trivial Example Consider the trace distances between the following nearly-maximally-mixed density matrices: $$\begin{align} D \left( \frac{1}{2} \begin{bmatrix} 1& & \\ &1& \\ & &0 \end{bmatrix}, \frac{1}{2} ...



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