Electron Spin and the Double-Slit Experiment

[Wick]

Does anyone out there know if the double-slit has been accomplished with electrons that are all spinning in the same "direction"? If so could you please tell me if the observations were interesting in any way? If it hasn't been accomplished, could you either accomplish it or tell me what you would expect to see using a thought experiment.

Regards!

Wick

[Graybeard]

Yes. Its called the Delayed-Choice Experiment, and so far as I know it is a thought experiment. A different variation is called the Quantum Eraser Experiment which has been carried out.

If you think it helps then post back and I will load a good schematic diagram I have of it where both experiments are combined.

cool bananas ... greg

[Wick]

Yes. Its called the Delayed-Choice Experiment, and so far as I know it is a thought experiment. A different variation is called the Quantum Eraser Experiment which has been carried out.

If you think it helps then post back and I will load a good schematic diagram I have of it where both experiments are combined.

cool bananas ... greg

Thanks, Greg! My specific question has to do with how or if the double slit experiment looks different when electrons with aligned angular momentum are used in the experiment. If you think you schematic would help me to understand this, I would greatly appreciate it. The quantum Eraser experiment may be too complicated for my needs (to much potential macroscopic interaction. What I'm looking for is an experiment in which electrons with aligned angular momentum are sent through a slit or biprism and the interference pattern recorded and compared with the experiment with electrons of randomly aligned angular momentum.

Regards!

Wick

[Graybeard]

I don't know of any that do exactly that. Once you have detected spin the particle has coherence. Prior to the detection nothing was known.

It seems to me that once you know spin, then you have changed the outcome of the experiment.

From the WIKI. (my bold)
"Nothing is observable regarding it between the time a photon is emitted (which experimenters can at least locate in time by determining the time at which energy was supplied to the electron emitter) and the time it appears as the delivery of energy to some detector screen (e.g., as a measurement on a "charge-coupled device" such as the substitute for photographic emulsion in a digital camera)

The delayed choice and q-eraser experiments attempt to circumvent this indirectly. I don't think that aligning particles before transmission will produce an interference pattern. Experiments on spin-alignment were performed using Bell's Theorem with equal counter-intuitive results, but this is not the same as the double-slit.

Also spin is subject to different probabilities according to the angle of detection. As the axis always appears towards the detector, regardless of which angle you detect from (another counter intuitive ?) This would mean that it would be impossible to know spin up until the moment it was detected on the screen. This would seem to make the experiment you are looking for impossible. (I think ... wiser heads might know better)

You may have to sift thru all of the experiments to find what suits your needs. Perhaps if you post your purpose some of the others on here will know.

cool bananas ... greg

[Wick]

Thanks, Graybeard,

I also suspect that it would not produce an interference pattern. But I wonder why. If all of the electrons have aligned spin, we are not tracking which electron is passing through which slit. I had always thought that detection at the slit was what "collapsed the wave function". Why is it that electrons with aligned spin function would not interfere with one another.

I'm assuming the same would be true of photons and electrons in this regard.

While I'm showing my ignorance (I have lots of it!!!), I was recently reading that the spin axis can be oriented in any direction for quantum particles. Some even postulate that quantum particles spin along every possible axis but when we measure it we are only able to measure one of the axes around which a given particle spins. This strange property become important in entanglement experiments.

Can you tell me, Graybeard, what it means when we say that each quantum particle is spinning around every possible axis?

The reason I'm asking these questions is because I have a feeling (there he goes again) that spin has a bearing on where quantum particles strike the screen, but I have been told that when the quantum particles pass through the slit or biprism, they are actually realigned "on the way through" which randomizes the ordering of each particles angular momentum.

This makes the experiment slightly suspect to me.

I know this experiment has been preformed a billion times and that I'm rehashing territory that has already been paved and settled, but I'm trying to come to terms with the counter-intuitive nature of the quanta and its taking some time for me.

Sorry to keep bothering you. And thanks for you patience.

Wick

[Graybeard]

I also suspect that it would not produce an interference pattern. But I wonder why. If all of the electrons have aligned spin, we are not tracking which electron is passing through which slit. I had always thought that detection at the slit was what "collapsed the wave function". Why is it that electrons with aligned spin function would not interfere with one another.

First of all, I'm not 100% sure your experiment can't be done, or hasn't been done.

Detection of a particle anywhere, not just at the slit, 'collapses the wave function', tho not everybody sees it in these terms. I think a good general term is that it 'coheres'. By aligning the spin you have interacted with the particle, in fact you have 'detected' it.

As well you can't really say that the electrons or particles interfere with each other and thereby produce the interference pattern. Because when the experiment is slowed down to say a single electron released once every 10 seconds the wave pattern is still produced. So in effect, a single particle 'waves' or interferes with itself.

But once you have 'flagged' a particle such as aligning its spin, it can no longer interfere with itself. But you can unflag it during transit, and the interference pattern will re-appear again. Because you no longer hold information on it.

The first attempt to cause the particle to 'own up' to which slit it had passed thru was the Delayed Choice. The reasoning went that if knowing which slit it had passed thru was preventing the interference pattern, then lets leave the detection until after it has passed thru the slits, interfered with itself, and has now formed into its interference pattern trajectory. (picture it on its flight path as it approaches to impact and at that moment, detect it. Now you know which slit it has come thru, and as well it is forced to form an interference pattern on the screen. Doesn't work.

So then the Quantum Eraser experiment was designed to split the particle into entangled 'singlet pairs' or 'sisters' with a splitter at the exit of both slits. Now one of the sisters could be steered off and observed, while the other is left undetected to hit the screen and form the interference pattern. But as long as the sister was observed no pattern was formed by the undetected particle. However if the sister is unobserved then the partner forms the interference pattern.

The Delayed Choice Quantum Eraser captures the sister and puts this particle into a sort of holding pattern. Now the possibility exists, that by observing the sister sometime in the future, after the undetected particle has hit the screen we can have both interference pattern and 'which slit' information. But the undetected particle does not form an interference pattern while the sister is held. But if the sisters are released in the future and their partners are remapped on the screen,(the records checked, and separated out from those whose sisters were observed) an interference pattern emerges. This experiment involves many beam splitters and detectors .... and I have given a very brief description of its true nature and effects.

I don't think I have given a very good explanation here, but I'm doing it from memory ... and I'm a bit drunk. You would do better to look it up. In the end, its the act of observation that determines a particles 'coherence'. The coherence is caused by any interaction and not just by human observation.

Can you tell me, Graybeard, what it means when we say that each quantum particle is spinning around every possible axis?

This means to me, that the particle's axis of spin cannot be known until it is observed, and it is always observed with its axis towards the detector. But perhaps you are asking for a concept, ? Einstein argued against this by using entangled particles. If you know the spin of one, you by default, know the spin of the other, ergo: a particles spin can be known. Bell's Theorem proved him wrong by detecting singlet particles along different spin axis. Since then there has been no serious challenge to QM's counter intuitive behaviour.

The reason I'm asking these questions is because I have a feeling (there he goes again) that spin has a bearing on where quantum particles strike the screen, but I have been told that when the quantum particles pass through the slit or biprism, they are actually realigned "on the way through" which randomizes the ordering of each particles angular momentum.This makes the experiment slightly suspect to me.

I think your referring to one of the experiments above. Beam splitters are considered trustworthy in experiments. This was Dave's field I believe. It was his job to build the measuring equipment to measure the theorists predictions. I will PM him a link to here and see if he can help

Sorry to keep bothering you. And thanks for you patience.

Wick

Its no bother, I'm just an amateur like most.

cool bananas ... greg

[Graybeard]

Hi Wick .... I PM'd Dave .... here is his reply.

Hi Greg;
Excellent question; had to think on it for a bit! Since the magnetic moment of the electron does not determine it’s wave function, I doubt if it would influence the results of the experiment if they were all aligned. As far as I know, there has never been a specific experiment of this type. Electrons tend to align their axis parallel to the direction of travel and thus they would naturally align with the same spin orientation if they are within proximity. Current through a wire is a good example of this effect and produces magnetic fields that are all oriented in the same direction around the wire. You would need to do the experiment one electron at a time to eliminate this influence.

If you like, you can quote me to Wick.

What would be more interesting is if an AC potential of high frequency were to be placed on the double slit during the experiment.
Dave

PS: the delayed observation experiment has nothing to do with his question.

Hope this helps.

cool bananas ... greg

[analog]

Electrons tend to align their axis parallel to the direction of travel and thus they would naturally align with the same spin orientation if they are within proximity.

Correct me if I'm wrong Dave, but to reference such to the macro scale, I think I recall you suggesting this to me a while back about the polar orientation of galaxies, as this being suggestive that the entire universe still maintains a degree of linear motion.

What would be more interesting is if an AC potential of high frequency were to be placed on the double slit during the experiment.

Just trying to think this through and perhaps risk showing my true ignorance , but if the slits where contained within a metal structure, would it not create a radiating electromagnetic field within the openings (i.e. similar to acting as an antenna?), which created a wall of resonating interference which interacted with the wave function of the electron as it attempted to propagate through the slits?

Perhaps a clash of varying frequencies of the wave state of matter; projected as the electron, and projecting as the EM waves? Depending on the frequency used, would the information of the electron be unrecognisably altered as it emerged on the other side? Would it dissipate back to the Aethereal state of matter causing further EM radiation? Would it only cause a new and third (more random) type of observed pattern at the detector?

Perhaps, I'm way off, but I'm picturing some QED (Feynman diagrams) type interactions going on in there, but I just exceeded my intellectual boundaries about two thoughts ago, and probably shouldn't have even opened my mouth.....rotflmao.

later,

Tim

[Graybeard]

Just trying to think this through and perhaps risk showing my true ignorance, but if the slits where contained within a metal structure, would it not create a radiating electromagnetic field within the openings (i.e. similar to acting as an antenna?), which created a wall of resonating interference which interacted with the wave function of the electron as it attempted to propagate through the slits?

Yes .. I'm having difficulty with it to. A hi frequency AC potential would have an effect. It would produce a fluctuating magnetic field around the two slits, especially if the metal was laminated. However, I'm still left with the problem that we are interfering with the wave function of the particle and therefore the results would be no interference pattern.

What we really need is to get that old bugger Dave away from the TV and get his arse over here with some further explanations. Explanations tailored around our limited understandings ... lol. I've already tried to get him here Tim, perhaps you have a go....

cool bananas ... greg.

[dleviwing]

The physical orientation and dimensions of the slits are highly dependent on the wavelength of the EM waves you wish to experiment with. In the case of an electron, those wavelengths are extremely short and thus we can only do a double slit experiment with electrons within the confines of a crystal structures; the slits are actually gaps between the molecules of the crystal. I might mention that semiconductors are crystals; you may wish to read about quantum effects in the manufacturing of semiconductor devices; think about the effects of several billion slits on the wave functions of electrons.