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The question aims to this issue : if there is some technological arrangement (or action) to take over the particle/system in order to keep it in a coherent state, then the field, (force or whatever) keeping it away from interacting with "an external system" isn't it itself an interaction?

I mean, supposing you reach enough isolation to avoid decoherence.

How do you know the particle is still there?

Thanks

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I took the liberty of editing your syntax, and I hope I have not obscured your question. Maybe you should clarify what type of coherence you are talking about. For example, laser light is the result of coherence among zillions of atoms and photons and obviously there are lots of particles involved so their existence is indubitable. –  anna v Jun 29 '11 at 7:08

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If you're talking about building a quantum computer, then there are some modes of the system which you need to keep isolated so that you can make sure that any coherence of these modes is preserved, but there are other modes of the system that you use to control the system, and these aren't isolated. This idea is also used in quantum error correction. This process uses active control on certain modes of a system to suppress the decoherence of other modes of the system. You can be sure that the system is still there by observing the modes that don't need to be isolated. A similar idea is used in building a quantum logic clock, which is the most accurate clock ever built.

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Thanks for the answer, I have commented in the @Peter Morgan answer that my question aim to the measure problem. Here you talk about modes, I've follow the links but I see no references to modes, perhaps that's the answer I am looking for, How can a mode be manipulated/measured and keep other isolated, it seems very complex. Could you explain it further? thanks –  HDE Jul 5 '11 at 14:47
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Modes might not be quite the right word. For quantum error correction, the quantum state space generated by the qubits in a quantum error correcting code is expressed as the tensor product of two subspaces, one of which is used for measuring the errors, and the other is used for encoding the information. In general, the idea is to use control over some degrees of freedom of the system to make other degrees of freedom behave as if they were more isolated –  Peter Shor Jul 6 '11 at 2:59
    
Shor, Thanks, that's roughly what I am asking. Do you have any examples (or a name to do web searching) for these kind of processes, I mean, how control over some degrees of freedom is implemented? –  HDE Jul 6 '11 at 12:33
    
I believe there are some good not-too-technical introductions to quantum error correction around. I don't know whether there are for ion-trap quantum logic clocks. –  Peter Shor Jul 6 '11 at 12:58
    
Thanks, the idea of controlling only some degrees of freedom and the "ion-trap", are good enough info to start with –  HDE Jul 6 '11 at 13:44

There's a lot of creativity in constructing an experiment, exploiting many different interactions in many different configurations, so there isn't a one size fits all Answer to your Question.

One source of decoherence, however, are thermal fluctuations of the EM field (the immediate environment), which are driven by the thermal fluctuations of whatever surrounds the system of experimental interest. To reduce the effects of decoherence, we can surround the system by something cold, so that the EM field is more driven by the cold surrounding, and much less driven by the hotter surroundings that are further away. How we keep the cold surroundings from heating up, and how we create the cold surroundings in the first place, are relatively modern miracles of invention, which in Physics experiments are likely to be multi-stage exploits. To some considerable extent, as refrigeration technology improves, so improves Physics.

We can't totally eliminate thermal fluctuations of the EM field, insofar as the 3rd law of thermodynamics is empirically supported, so yes, the surroundings still affect the system of experimental interest, but less.

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Interesting, I was thinking how inevitable is radiation, gravity, electromagnetism, so that source, thermal fluctuations, is a key, my question is related with the measure problem, in the sense of an usual idea that "before" making the measurement, we already know that there is a particle there, for example in double slit, sending electrons one at a time How could we know that a single electron is sent if we haven measured or counted it?, or How can it have been counted or measured if it wasn't measured?, that sound a bit weird for me –  HDE Jul 5 '11 at 14:42
    
@HDE You know you have a single electron from the synchrotron radiation in some experiments, as the example in my answer.Or from a coincidence circuit that gives a signal that an electron has passed and is going towards the target, in other simple experiments. en.wikipedia.org/wiki/Particle_detector for complicated ones. –  anna v Jul 5 '11 at 15:01
    
@anna v you said "..circuit that gives a signal that an electron has passed" so Is the electron isolated?, as I understand what comes out is not "an electron" anymore but a "counted electron",How could an electron be isolated if it need to interact to inform to "be there"? Perhaps answer is measuring independent properties, in classical physics when you put a mass in a weighing scale no matter that original position were modified because you want to know its weight, but in QM is a big problem because choose could be mandatory and perhaps there are not independient properties.. –  HDE Jul 5 '11 at 16:12
    
@HDE If one person throws a ball to a second one and the second one throws the ball in the basket, is it a different ball that went through the basket? You can make a simple setup with two counters and you know an electron passed and went for the slits where you have a third counter that counts an electron with the appropriate coincidence and area coverage. What is so hard to understand? –  anna v Jul 5 '11 at 17:55
    
@anna v You know it, balls are Classic objects, macroscopic properties could be measured one at a time without affecting them much but because of Uncertainty principle we can't do that kind of measures with electrons, after measuring its position we gain uncertain in momentum, every measure changes the wave function irreversibly. –  HDE Jul 5 '11 at 18:53

As an experimentalist, I will first address this last summary of your question:

How do you know the particle is still there?

Let us define the terms of the question:

Particle.

a) In particle physics we know a particle was there by the tracks it leaves in a bubble chamber.

b)By the signals it sets off as it passes and ionizes

The measurements have shown us that we deal with very small dimensions in all quantities, mass, size etc.

We also have found that particles follow quantum dynamics and the solutions of the appropriate equations of motion.

Generally: can I trap one particle and "know" it is there? I have not done it, but it is being done billions of times a second at the accelerators. If I went to the trouble to design an experiment that has trapped a single proton in a magnetic configuration, I would know it was there from the radiation it would emmit as it oscillated in the magnetic trap.

Usually though, because of the very small values accompanying the existence of a particle one deals with a flux of them at a time.

Now coherence. Coherence is the term describing the quantum mechanical solution of the equations of more than one particle, and refer to the phase differences between those particles : i.e. coherence means that those phase differences remain constant . Described as quantum mechanical waves, the particles are "in step". If you only have one particle, as in my gedanken experiment above, the quantum mechanical solution is known and phases can only be defined with respect to the field. As long as energy is supplied to my proton this description will hold.

The "know the particle" phrase should become "know the particles" phrase.

Coherence is observed macroscopically:

in laser light

in superconducting magnets, over kilometers of wire length.

in superfluidity.

All these require zillions of particles and no question should arise if they are there or not. The answer by Peter Morgan addresses the question of stability of such systems.

Now I suspect you are asking the question from statements of coherence and the density matrix formulation. This has to do with the quantum mechanical statistical behaviour of many particles, so again, your one particle question does not compute. You should maybe clarify in your head what you really want to learn about coherence. Maybe the density matrix formalism confuses you?

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I've added some comments on others answers clarifying the concept of "how do you know the particle is there" question, thanks –  HDE Jul 5 '11 at 14:50

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