Why are other systems so sensitive to decoherence?

[craigi]

It seems that many systems currently studied, are very fragile to dechorence. Great care is taken to isolate them from even the smallest amount of interatction with the outside environment.

Yet, the most well known quantum mechanics experiment, the double slit experiment, seems incredibly rebust to decoherence. It seems that you can put pretty much anything you like in there aside from actually detecting which slit the particle went through, and the quantum effect remains.

Why is this setup so robust and why are other systems so sensitive to decoherence?

 

[UltrafastPED]

Because the light that is used has a very long coherence length ... this is classical coherence, but it carries over.

This is true for light from very distant objects (sun, stars), for lasers, etc.

 

[craigi]

Because the light that is used has a very long coherence length ... this is classical coherence, but it carries over.

This is true for light from very distant objects (sun, stars), for lasers, etc.

Presumably the double slit experiments performed with heavy particles were performed under very careful isolation conditions.

The experiment is also performed with white light, I don't recall any issues with having to take special care with isolation, in this case.

 

[UltrafastPED]

The white light will come from the sun (as it was in the earliest experiments), passed through a small hole prior to the slits. I don't think it works so well (or at all) with an incoherent source like an incandescent bulb.

I have also done the two slit experiment with electrons: electron diffraction through crystalline materials. The electron source was very coherent.For an introduction see: http://users.wfu.edu/ucerkb/WFU Home/Phy114/L15-Interference.pdf

 

[craigi]

Because the light that is used has a very long coherence length ... this is classical coherence, but it carries over.

This is true for light from very distant objects (sun, stars), for lasers, etc.

The white light will come from the sun (as it was in the earliest experiments), passed through a small hole prior to the slits. I don't think it works so well (or at all) with an incoherent source like an incandescent bulb.

I have also done the two slit experiment with electrons: electron diffraction through crystalline materials. The electron source was very coherent.For an introduction see: http://users.wfu.edu/ucerkb/WFU Home/Phy114/L15-Interference.pdf

Why would the light from the sun be coherent? I'd expect it to be incoherent like a bulb. Those photons are just being generated through spontaneous electron subshell transitions and I can't think of any process along the way that would bring them into phase.

If I'm correct, the experiment can be performed using light from a filament bulb passed through a grating consisting of very thin vertical lines.

Something else that has just crossed my mind, that I'd not considered before is that as an incohrerent light source is made dimmer there comes a point where it must behave like a coherent light source when the individual photon wave packets are no longer likely to overlap.

Can anyone confirm this or have I got something wrong?

 

[UltrafastPED]

If you are so sure of your position you should attempt the experiment. It doesn't require expensive equipment ... an opaque card, a razor blade to make the slits, something for the light to fall on ... and a few calculations to determine the slit separation and the distance to the detector screen ... which can be a piece of white paper.

As for why sunlight is coherent ... I will let you think about this a bit. It does require some analysis, but with some effort on your part you will understand. In any case try your setup with sunlight through a pinhole first, then with your light bulb through the same pinhole.

Let us know how it turns out!

 

[craigi]

If you are so sure of your position you should attempt the experiment. It doesn't require expensive equipment ... an opaque card, a razor blade to make the slits, something for the light to fall on ... and a few calculations to determine the slit separation and the distance to the detector screen ... which can be a piece of white paper.

As for why sunlight is coherent ... I will let you think about this a bit. It does require some analysis, but with some effort on your part you will understand. In any case try your setup with sunlight through a pinhole first, then with your light bulb through the same pinhole.

Let us know how it turns out!

Nope. I have no idea what you mean.

My best guess is that you're confusing the spatial coherence of starlight which appears as a single point, more or less, in the sky, with the spatial coherence of the sun which represents a distributed light source to us here on earth. Passing either sunlight or a light from a filament bulb through a pinhole would give us a similar degree of spatial coherence, presuming that they present the same solid angle to the pinhole.

Am I wrong?

 

[UltrafastPED]

Yes, you are wrong. How much optics have you studied? Where are you in school?

 

[craigi]

Yes, you are wrong. How much optics have you studied? Where are you in school?

I left school a long time ago. My optics study was just at undergrad level. I seem to remember scoring about 90% in the examination of the module, but it was a long time ago, so I've almost certainly forgotten more than I remember. I'm really not going to work out what you mean, so it'd be easier if you just tell me.

edit...

Are you talking about the fact that the sun is often considered a parallel light source to us here on Earth?

 

[Cthugha]

My best guess is that you're confusing the spatial coherence of starlight which appears as a single point, more or less, in the sky, with the spatial coherence of the sun which represents a distributed light source to us here on earth. Passing either sunlight or a light from a filament bulb through a pinhole would give us a similar degree of spatial coherence, presuming that they present the same solid angle to the pinhole.

Am I wrong?

No, you are exactly right. A double slit measures the spatial coherence of a light source in a given experimental geometry. Spatial coherence is more or less given by the angular size of the source as seen by the double slit. This means you can increase the spatial coherence of a light beam by just placing it further away from the double slit or putting a small pinhole in the light path which increases spatial coherence, too, as the pinhole acts like a point-like light source. A light bulb will give exactly the same result when using the same pinhole (and of course the same spectral range - broadband white light will create a superposition of many different interference patterns for the different parts of the spectrum).

This is also the reason why the famous Hanbury Brown-Twiss experiment worked with light from Sirius (long distance - high spatial coherence), but not with sunlight (too close - low spatial coherence). Besides that, both sunlight and a light bulb share the same characteristics in terms of quantum optics: both are thermal light sources with the photon statistics being Bose-Einstein statistics.

 

[craigi]

No, you are exactly right. A double slit measures the spatial coherence of a light source in a given experimental geometry. Spatial coherence is more or less given by the angular size of the source as seen by the double slit. This means you can increase the spatial coherence of a light beam by just placing it further away from the double slit or putting a small pinhole in the light path which increases spatial coherence, too, as the pinhole acts like a point-like light source. A light bulb will give exactly the same result when using the same pinhole (and of course the same spectral range - broadband white light will create a superposition of many different interference patterns for the different parts of the spectrum).

This is also the reason why the famous Hanbury Brown-Twiss experiment worked with light from Sirius (long distance - high spatial coherence), but not with sunlight (too close - low spatial coherence). Besides that, both sunlight and a light bulb share the same characteristics in terms of quantum optics: both are thermal light sources with the photon statistics being Bose-Einstein statistics.

Thanks for clearing that up.

So back to my original question. What is it about the double slit experiment, including it's implementation with photons, heavy elementary particles and molecules, that makes it so robust to decoherence whereas many other quantum systems that are currently being studied are extremely fragile to it?

Is it that the superposition is only required to remain stable, for each individual particle for a very small amount of time? That the particles used are weakly interacting with the medium that they're passing through? Is it that each particle only really interacts with itself rather than existing in a more complex preparation?

 

[Cthugha]

So back to my original question. What is it about the double slit experiment, including it's implementation with photons, heavy elementary particles and molecules, that makes it so robust to decoherence whereas many other quantum systems that are currently being studied are extremely fragile to it?

There are several points to consider. First: the double slit works on the single particle level. So you can send single particles in repeatedly and in the ensemble average, you will get the interference pattern. Second: The double slit relies on fixed phase difference based on the geometry of the slit, so interactions which randomize phase or generally speaking change the state of the particle going through the double slit will be the cause for messing up the patterns.

The probability of a scattering event taking place in the double slit setup is pretty small for light (unless you create fog or steam or something like that) in the atmosphere. Electrons and other massive particles can scatter from atoms or molecules in the atmosphere. So the double slit experiment will not work for those particles unless you create some vacuum.

Going back to the first point, it is important to note that decoherence scales exponentially with system size. It is moderately easy to shield a single particle from its surroundings, but it is more complicated to make sure each of n particles does not interact with its surroundings or one of the other particles. I am not quite sure which systems you are specifically aiming at, but for example qubits for quantum computing are usually very delicate. You may be able to isolate a single qubit and keep it from decohering, but that is not especially useful. Instead you want many qubits to create a sensible quantum computer. Even worse, you do not just want them to sit there, but you want to initialize them, manipulate them and read them out. Doing this without introducing decoherence is challenging. Other systems may have a more complicated environment. Shielding an electron in vacuum is easy. Shielding an electron spin in the middle of a semiconductor quantum dot is a completely different issue.

 

[craigi]

There are several points to consider. First: the double slit works on the single particle level. So you can send single particles in repeatedly and in the ensemble average, you will get the interference pattern. Second: The double slit relies on fixed phase difference based on the geometry of the slit, so interactions which randomize phase or generally speaking change the state of the particle going through the double slit will be the cause for messing up the patterns.

The probability of a scattering event taking place in the double slit setup is pretty small for light (unless you create fog or steam or something like that) in the atmosphere. Electrons and other massive particles can scatter from atoms or molecules in the atmosphere. So the double slit experiment will not work for those particles unless you create some vacuum.

Going back to the first point, it is important to note that decoherence scales exponentially with system size. It is moderately easy to shield a single particle from its surroundings, but it is more complicated to make sure each of n particles does not interact with its surroundings or one of the other particles. I am not quite sure which systems you are specifically aiming at, but for example qubits for quantum computing are usually very delicate. You may be able to isolate a single qubit and keep it from decohering, but that is not especially useful. Instead you want many qubits to create a sensible quantum computer. Even worse, you do not just want them to sit there, but you want to initialize them, manipulate them and read them out. Doing this without introducing decoherence is challenging. Other systems may have a more complicated environment. Shielding an electron in vacuum is easy. Shielding an electron spin in the middle of a semiconductor quantum dot is a completely different issue.

Well we know that the scattering process in itself can't always result in decoherence otherwise interference could never be obtained when mirrors are incorporated into the experiment.

Though it's easy to see that scattering is going to be a precursor to possible decoherence. Do we know if entanglement as a result of scattering is a prerequisite for decoherence? I think we already know that even under entanglement the interference could be preserved, until the entangled particle is observed. If this is the case, what constitutes observing the entangled particle? Also, if the entangled particle remains unobserved in time until the source particle has scattered with the target to contribute to the inference pattern then is the pattern still observed? To what extent can we rely on time ordered causality here?

Am I on the right lines here or am I missing something?

 

[Cthugha]

Well we know that the scattering process in itself can't always result in decoherence otherwise interference could never be obtained when mirrors are incorporated into the experiment.

Scattering and reflection are two very different processes. Reflection is a coherent process that introduces some phase shift to a light beam, but does not change the state of the light field irreversibly. That means that - unless you detect the photon somewhere - there is no way of knowing that a photon indeed hit a mirror. It did not leave any trace. The mirror does not end up in an orthogonal state to its initial one. So the process is reversible. Other processes like rotating the polarization of a light beam using a wave plate belong into the same category.

Incoherent scattering on the other hand is a very different process. The photon in question and the thing it scatters from both end up in a different state and the phase gets randomized. Now there is a trace from which you might be able to conclude that the photon was there and the scattering process took place. The interaction is irreversible. The latter processes cause decoherence. The former do not. To answer your other question: Any irreversible interaction qualifies as an observation in your sense.