# A Is measurement a yes/no process?

1. Mar 2, 2016

### naima

the wave/particle duality is no more seen as the opposite sides of a coin. We now see it as a point in a segment [0 , 1].
It is the same with pure states. The visibility of the fringes varies between 0 and 1.
In regards to measurement, We are often in the old yes/no habit:
At a some point the system ceases to evolve unitarily, measurement forces it to jump. This jump is irreversible and everybody can see the same output.
Cannot we go further?
What happens when the macroscopic device becomes mesoscopic or has the size of few atoms?
Is there a place between measurement and no measurement?

2. Mar 2, 2016

### A. Neumaier

In principle one can resolve the jump by fine time probing. But if you read a meter, you consider it to be a meaurement only after the pointer oscillations died out so that there is a well-defined answer.

It is just as when you count the number of people in a room while someone enters the room.
Does it make sense to say 3.25 people are in the room at a particular time and 3.45 a fraction of a seconds later? It is better to wait an then say that the number jumped from 3 to 4.

3. Mar 2, 2016

### Staff: Mentor

No.

Measurement in modern times is well defined - it either happens or not.

Thanks
Bill

4. Mar 2, 2016

### naima

can a neutron be a measurement device? can an atom?
At which level amplification is enough to have an output result?

5. Mar 2, 2016

### Staff: Mentor

Suppse we say that measure, observe, interact-with are all synonyms in this context. How would you phrase your question then?

6. Mar 2, 2016

### Demystifier

It is possible to observe a mesoscopic system by a macroscopic apparatus. By watching the density matrix of such a mesoscopic system at different times, one can see how non-diagonal matrix elements continually diminish with time.

7. Mar 2, 2016

### Staff: Mentor

Its when decoherence has occurred. That is determined by a specific set-up and model. So make your query more exact and you may get an answer.

Thanks
Bill

8. Mar 2, 2016

### naima

So there is a place between measurement and no measurement for partial or weak or unsharp measurement.
the problem was not if a mesoscopic system could be measured by a macroscopic apparatus but if a particle could ne measured by a mesoscopic appararus. And when there is not a sufficient amplification one can merge the possible outputs so that there is no measurement.
We are far from the yes/no starting point.

9. Mar 2, 2016

### stevendaryl

Staff Emeritus
Really? It would seem to me that a measurement is just an interaction between the system being measured and the device performing the measurement. You have to study in detail the nature of that interaction to determine what (if anything) is being measured (or from the other direction, you have to carefully design the device so that it measures whatever it is you are interested in measuring).

What is your definition of measurement that makes it a definite yes/no thing?

10. Mar 2, 2016

### stevendaryl

Staff Emeritus
I would think that perhaps every measurement necessarily involves decoherence, but not that every instance of decoherence implies a measurement. To relate the word "measurement" to what it meant pre-quantum mechanics (and what it still means in other branches of science), I would think that measurement would require a correspondence between various values of the thing being measured and various macroscopically distinguishable states of the measuring device. I wouldn't think that every instance of decoherence would set up such correspondence.

11. Mar 2, 2016

### Staff: Mentor

Its when a mixed state diagonal in a specific basis results from some interaction.

Thanks
Bill

12. Mar 2, 2016

### naima

A measurement is what gives an output.
I would like to add that when you observe the density matrix of the particle, you really get an output: it is the level of decoherence. if it is diag(0.5 0.5) you have a complete information about the decoherence level but you have no information about the pointer state of the measurement device.

Last edited: Mar 2, 2016
13. Mar 2, 2016

### naima

Accordind to you, measurement occurs when decoherence occured.
Could you elaborate the role of decoherence in the Stern Gerlach device?
the electron can be described by a spin two level variable and by the vertical component of its position. We also have a magnetic field. Is there decoherence?
in which basis? Do we need a distant screen for this decoherence?
Instead of a screen we can merge the paths with another field to undone the measurement.
Suppose now that we have a screen. We will have a macroscopic spot and so a measurement with an output. Is there at this place that decoherence occured?
What happens if the screen is not macroscopic but you have only a dozen of atoms here and there? Decoherence being incomplete, have we a weak measurement? Maybe the beams can be merged after the atoms.

14. Mar 2, 2016

### DrChinese

I think we are talking about several different things. Certainly there are such things as weak measurements. The HUP even allows you to measure partial information about non-commuting attributes. You simply don't have high statistical confidence with those results. Decoherence itself is not all or nothing for a particle, as you can cause collapse on one basis (say spin) without affecting another (momentum).

So when we talk about yes/no for measurements: are we talking generally? Because some of these points yield different answers as you dive into the question.

15. Mar 2, 2016

### Staff: Mentor

I am not familiar with the decoherence account of that experiment.

But there is plenty of literature on it eg:
http://arxiv.org/abs/1507.08541

Thanks
Bill