Understanding the Measurement and Interaction of Electrons: A Beginner's Guide

[durant35]

If you say, something is "small" you've to say, compared to what. The uncertainties of position and momentum (or the position in phase space), which obey the Heisenberg uncertainty relation $\Delta x \Delta p_x \geq \hbar/2$, are usually very small compared to the necessary resolution of the phase-space position on a macroscopic scale. This means that very many different quantum states cannot be distinguished on a macroscopic scale. Also usually it is hard to isolate a macroscopic system sufficiently from the environment, so that you have always a mixture of many quantum states due to this perturbance of the system by interactions with the environment, which leads to decoherence and thus classical behavior.

On the other hand there are astonishing examples for the quantum behavior of macroscopic objects. E.g.,

http://physicsworld.com/cws/article/news/2011/dec/02/diamonds-entangled-at-room-temperature

Believe it or not, I saw this article while googleing some stuff. How is this possible, I mean how didn't the decoherence kick in?

 

[vanhees71]

That's a good question. I'm not 100% sure, but the reason must be that the energy gap between the used phonon mode to the next excited state is very large and thus that even at room temperature the probability for transitions is very low. Perhaps you find the detailed answer in the Science article:

https://people.phys.ethz.ch/~reimk/Media/Science-2011-Lee-1253-6.pdf

 

[durant35]

That's a good question. I'm not 100% sure, but the reason must be that the energy gap between the used phonon mode to the next excited state is very large and thus that even at room temperature the probability for transitions is very low. Perhaps you find the detailed answer in the Science article:

https://people.phys.ethz.ch/~reimk/Media/Science-2011-Lee-1253-6.pdf

I'm sorry, but I don't understand it. I know only a little bit about entaglement and as far as I know it occurs when the wavefunctions overlap so that the system acts as one. How could the wavefunctions overlap spatially at that kind of temperature, what did actually happen? It is mentioned that each of the diamons was simultaneously in the state of 'vibrating and non vibrating' which is even more confusing, could you please clarify it a bit?

 

[vanhees71]

Ok, I'll try. I only have to find the time to read the paper! Please be patient with me ;-)).

 

[durant35]

Ok, I'll try. I only have to find the time to read the paper! Please be patient with me ;-)).

No problem, thank you.
:)

 

[durant35]

Do superpositions of macroscopic objects occur naturally in the world or do the experimenters have to induce them like with the diamonds in question?

 

[bhobba]

Do superpositions of macroscopic objects occur naturally in the world or do the experimenters have to induce them like with the diamonds in question?

The technical meaning of superposition is rather mathematical and makes questions like the above not well posed so can't be answered.

A better query would be do quantum effects occur in the everyday world around us. Yes - but they usually are not obvious. For example transistors work because of things called holes which are in fact quasi particles and depend entirely on QM. There are others like the strange behaviour of liquid helium. How common is this strange stuff - maybe more common than we generally think - but its seems the exception rather than the rule - mostly classical physics is good enough

That said even the simple phenomena of light traveling though glass is rather complex and dependant on advanced QM. Do a post about it and and someone into solid state physics may explain what's really going on - its quite interesting.

Thanks
Bill

 

[durant35]

Thanks Bill. What I meant was quantum effects on the macro scale like the 'vibrating-non vibrating' state from the diamonds experiment. I know that many things like transistors work on the miscroscopic qm background. But can a state like the one mentioned occur without experiments?

 

[bhobba]

But can a state like the one mentioned occur without experiments?

To the best of my knowledge - no.

Thanks
Bill

 

[durant35]

To the best of my knowledge - no.

Thanks
Bill

Ok. Do you know how did they manage to achieve the entaglement in conditions that aren't cold and isolated?

 

[zonde]

That's a good question. I'm not 100% sure, but the reason must be that the energy gap between the used phonon mode to the next excited state is very large and thus that even at room temperature the probability for transitions is very low. Perhaps you find the detailed answer in the Science article:

https://people.phys.ethz.ch/~reimk/Media/Science-2011-Lee-1253-6.pdf

It seems that phonons used in the experiment are of much higher frequency than that of thermal vibrations. And these phonons are relatively stable because of specific structure of crystal ("bulk vibration consisting of two counter-oscillating sublattices within the diamond structure.")

On the first glance the experiment seems to show rather bizarre effect - phonon shared by two distant diamonds. But on the second glance it seems that experiment is consistent with explanation that there are two polarization entangled phonons in crystals.

 

[bhobba]

Ok. Do you know how did they manage to achieve the entaglement in conditions that aren't cold and isolated?

No.

Thanks
Bill

 

[durant35]

It seems that phonons used in the experiment are of much higher frequency than that of thermal vibrations. And these phonons are relatively stable because of specific structure of crystal ("bulk vibration consisting of two counter-oscillating sublattices within the diamond structure.")

On the first glance the experiment seems to show rather bizarre effect - phonon shared by two distant diamonds. But on the second glance it seems that experiment is consistent with explanation that there are two polarization entangled phonons in crystals.

But how did they manage to cross paths of the two diamonds, I've red that electron paths had something to do about it.

 

[zonde]

But how did they manage to cross paths of the two diamonds, I've red that electron paths had something to do about it.

They didn't of course.
First they shine a pump laser pulse on both diamonds. A photon splits in phonon and redder photon than the rest of pump photons. They collect these redder photons from both crystals and analyze them together using two polarization beam splitters and half wave plate. After short time they shine second pulse on diamonds and phonon combines with one pump photon and creates bluer photon. These are collected and analyzed together in separate channel using two PBSes and wave plates.

 

[durant35]

But is that a vibration of a diamond per se or a state where the phonon is spread as a wave in both diamonds?

 

[zonde]

But is that a vibration of a diamond per se or a state where the phonon is spread as a wave in both diamonds?

For me it seems that this experiment is consistent with explanation that there is phonon in each diamond but they are polarization entangled.
Idea that there is single phonon spread over both diamonds seems bizarre. Say where is the energy of phonon then?

 

[durant35]

For me it seems that this experiment is consistent with explanation that there is phonon in each diamond but they are polarization entangled.
Idea that there is single phonon spread over both diamonds seems bizarre. Say where is the energy of phonon then?

What is polarization entaglement and how is it obtainable in two objects spread by distance which wavefunctions don't cross paths? I have much less understanding than you about this topic so I hope you can help me with it and with my lack of knowledge. Thank you.

 

[zonde]

What is polarization entaglement and how is it obtainable in two objects spread by distance which wavefunctions don't cross paths? I have much less understanding than you about this topic so I hope you can help me with it and with my lack of knowledge. Thank you.

Let's first find out how much do you know about polarization of light. Do you know how to get polarized light, say as described here: https://en.wikipedia.org/wiki/Polarizer ?

 

[durant35]

Okay, I've red it. I think I understand the basics. Can you continue please?

 

[zonde]

Generally we speak about photon polarization entanglement. Usually polarization entangled photons are produce using parametric down-conversion in specific arrangements.
Polarization entangled photons have a property that when you measure polarization of one photon from the pair the other one is certain to have the same polarization (or opposite depending on entanglement type) even when two measurements are performed at distant places.

 

[durant35]

Generally we speak about photon polarization entanglement. Usually polarization entangled photons are produce using parametric down-conversion in specific arrangements.
Polarization entangled photons have a property that when you measure polarization of one photon from the pair the other one is certain to have the same polarization (or opposite depending on entanglement type) even when two measurements are performed at distant places.

Okay, thanks for the explanation. But one thing are photons and other are relatively big diamonds which are quite localized unlike photons. How is entaglement by polarization obtainable for this kind of an object?

 

[zonde]

Okay, thanks for the explanation. But one thing are photons and other are relatively big diamonds which are quite localized unlike photons. How is entaglement by polarization obtainable for this kind of an object?

Diamonds or rather phonons are not measured directly but rather first converted into photons and then photon polarization is measured, simply stated.

 

[durant35]

Okay, now that makes much more sense. Could this experiment be obtaniable in some other circumstances or diamonds are specific because of their specific structure?

 

[durant35]

I reworked the experiment in my mind and I think I understand a bit of it. Do photons exhibit similar behavior while passing through other bodies that are already vibrating (like our bodies) so that they put them in a state of vibrating and non vibrating?

 

[vanhees71]

Ok. Do you know how did they manage to achieve the entaglement in conditions that aren't cold and isolated?

A free source of the paper is

https://www.researchgate.net/profile/Xian-Min_Jin/publication/51855622_Entangling_Macroscopic_Diamonds_at_Room_Temperature/links/00463519f66babe7f6000000.pdf

 

[durant35]

A free source of the paper is

https://www.researchgate.net/profile/Xian-Min_Jin/publication/51855622_Entangling_Macroscopic_Diamonds_at_Room_Temperature/links/00463519f66babe7f6000000.pdf

You've just made my day harder :P Because there are many mentioned examples of macro entaglement and I don't understand how are they achieved.
Why the diamond experiment is/isn't obtainable in other objects like our bodies?

 

[mfb]

All the excited states have to be well isolated from the environment. This is possible (for fractions of a second) in nice clean crystals like diamond, it is not possible in human bodies.
There is some evidence that very brief quantum effects are relevant in photosynthesis (e.g. this news), but those states are (a) on the level of a few molecules and (b) extremely short-living.

 

[durant35]

What about macroscopic light and entaglement?

 

[durant35]

Why doesn't the excited state spread to the environment that observes it so it also becomes excited?

 

[mfb]

Why doesn't the excited state spread to the environment that observes it so it also becomes excited?

That question does not make sense.

And what is "macroscopic light"? Light does not have a size.