Time and the Quantum

[1over137]

Hi all.

I have found a very interesting article[1] and would like to share it with you. It's about time and the quantum eraser. Let's do the experiment as depicted on the Figure 1 at the end of this post.

Figure 1A:
Light impinges from the left on the two-level atom located at sites 1 and 2. An atom excited to level a emits a photon gamma. We look at the interference of these photons at the screen. Because both atoms are finally in the state b after the emission of photons, it is not possible to determine which atom contributed the the photon gamma. A large number of such experiments are carried out; i.e., any one photon will yield one count on the screen, and it takes many such photon events to build up an interference pattern. This is an anlogue og the usual Young's double-slit experiment. Instead of the usual light beams through two pin holes, we have considered scattered light from two atoms. The key to the appearance of the interference is the lack of which-path information for the photons.

Figure 1B:
In the case where the atoms have three levels, the drive field excites the atoms from the ground state c to the excited state a. The atom in a state a can then emit photon gamma and end up in state b. Here, the photon detected on the screen leaves behind which-path information;that is, the atom responsible for contributing the photon gamma is in level b, whereas the other atom remains in level c. The precise mathematical description of photons gamma1 and gamma2 is the same in cases a and b. It is only the presence of the passive observer state that kills the interference. (NOT the presence of conscious mind!)

Figure 1C:
Here we introduce another field that takes the atom from level b to bprime and, after the emission of a photon phi at the bprime-c transition, ends up in level c. Now the final state of both the atoms is c, and a measurement of internal states cannot provide us the which-path information. It would therefore seem that the interference fringes will be restored, but a careful analysis indicates that the which path information is still available through the photon phi. A measurement on the photon phi can tell us which atom contributed to the photon gamma. The big question is whether we can erase the which-path information contained in the photon phi and recover the interference fringes?

Let us now turn to the Figure 2 at the end of this post.

We again consider two atoms of the type shown in Figure C located at sites 1 and 2. A pair of photons gamma and phi are emitted either by the atom 1 or by the atom 2. The gamma photon as before proceeds to the screen on the right and is detected by a detector on screen D at location x0. A repeat of this experiment yields essentially random distribution of photons on the screen.

What about the appearance and disapearence of interference fringes discussed above? For this purpose, we look at the photon phi. We consider only those instances where the phi photon scattered from atom 1 proceeds to the beam splitter B1 and the phi photon scatterd from the atom 2 proceeds to B2. At either of these 50/50 beam splitters, the photon phi has a 50% probability of proceeding to detectors D3 (from photon scattered from 1) and to D4 (for photon scattered from 2). On the other hand, there is also a 50% probability that the photon will be reflected and proceed to another 50/50 beam splitter B. For these photons there is an equal probability of being detected at detectors D1 and D2.

If the photon phi is detected at the detector D3, it has necessarily come from the atom 1 and could not have come from the atom 2. Similarly, detection at D4 means that the photon phi came from the atom 2. For such events, we can also conclude the the corresponding photon gamma was also scattered from the same atom.
That is, we have which-way information if detectors D3 and D4 register a count.
If the photon phi is detected at D1, there is an equal probability that it may have come from the atom 1 or it may have come from the atom 2. Thus we have erased the information about which atom scattered the photon phi, and there is no which-path information available for the corresponding photon gamma. The same can be said about the photon phi detected at D2.
After this experiment is done a large number of times, we shall have roughly 25% of phi photons detected each at D1, D2, D3 and D4. The corresponding spatial distribution of gamma photons will be as mentioned above completely random. Next we do a sorting process. We separate out all the events where the phi photons are detected at D1, D2, D3 and D4. For these four groups of events we locate the position of the detected gamma photons on the screen D.

The key result is that, for the events corresponding to the detection of phi photons at detectors D3 and D4 there are no interference fringes but we obtain conjugate (pi phase shifted) interference fringes for those events where the phi photons are detected at D1 and D2. For this set of data, there is no which-path information available for the corresponding gamma photons.

Suppose we place the phi photon detectors far away. Then the future measurements on these photons influence the way we think about the gamma photons today (or yesterday!). For example, we can conclude that gamma photons whose phi partners were succesfully used to ascertain which-path information can be described as having (in the past) originated from site 1 or site 2. We can also conclude that the gamma photons whose phi partners had their which-path informations erased cannot be described as having (in the past) originated from site 1 or site 2 but must be described, in the same sense, as having come from both sites. The future helps shape the story we tell of the past.

Imagine that the beam splitters and the four photon phi detectors are on the other site of the universe!
And imagine that you choose to place the beamsplitter B after the photon gamma reached the detector D.
(I can next time share with you the delayed choice quantm eraser experiment if there is an interest among people.)

[1]: Y. Aharonov, M. S. Zubairy: Time and the Quantum, Science, Vol.307, 2005
http://www.columbia.edu/cu/physics/pdf-files/Scully.pdf

[RickD]

你讲中国话，给我。

[Reactionary]

Uistinu dojmljiv eksperiment. Šteta je, međutim, što ja ne razumijem gotovo ništa. Možeš li nam pojasniti svakodnevnim jezikom? Koje su implikacije tog eksperimenta?

[RickD]

Uistinu dojmljiv eksperiment. Šteta je, međutim, što ja ne razumijem gotovo ništa. Možeš li nam pojasniti svakodnevnim jezikom? Koje su implikacije tog eksperimenta?

Truly impressive experiment. It is a pity, however, as I understand almost nothing. Can you explain in everyday language? What are the implications of this experiment

I hear ya, Reactionary.

Hana, can you dumb it down for us simple minded folk?

[Furstentum Liechtenstein]

http://www.columbia.edu/cu/physics/pdf-files/Scully.pdf

It may be easier to understand 1/137's post if you read the link she provided, above. Even so, for me, it was like reading Scientific American...that is, difficult but not impossible to understand. Hana's summary is quite good:

The future helps shape the story we tell of the past.

...which is a twist of the ''Chicken or the Egg? conundrum.*

FL

*''Which came first, the chicken or the egg?''

[1over137]

I'd like to add the following [1]:

"Again, let me emphasize that the future measurements do not change anything at all about things that took place in your experiment today; the future measurements do not in any way change the data you collected today. But the future measurements do influence the kinds of details you can invoke when you subsequently describe what happened today. Before you have the results of the idler photon measurements, you really can't say anything at all about the which-path history of any given signal photon. Hovewer, once you have the results, you conclude that signal photons whose idler partners were succesfully used to ascertain which-path information can be described as having -years earlier- traveled either left or right. You also conclude that signal photons whose idler partners had their which-path information erased cannot" be described as having -years earlier- definitely gone one way or the other. We thus see that the future helps shape the story you tell of the past.
These experiments are a magnificient affront to our conventional notions of space and time. Something that takes place long after and far away from something else nevertheless is vital to our description of that something else. By any classical -commonsense- reckoning, that's, well, crazy. Of course, that's the point: classical reckoning is the wrong kind of reckoning to use in quantum universe. We have learned from the Einstein-Podolsky-Rosen discussion that quantum physics is not local in space. If you have fully absorbed that lesson - a tough one to accept in its onw right - these experiments, which involve a kind of entaglement across space and through time, may not seem thouroughly outlandish. But by the standards of daily experience, they certainly are."

[1]: Briane Greene: The Fabric of the Cosmos.

[1over137]

There is a remarkable experiment done only recently (published in April 2012).
news article: http://www.sciencedaily.com/releases/20 ... 131902.htm
scientific article: http://arxiv.org/ftp/arxiv/papers/1203/1203.4834.pdf (you may understand page 1 and end of page 6)

quotes from there:

"if we attempt to attribute an objective meaning to the quantum state of a single system, curious paradoxes appear: quantum effects mimic not only instantaneous action-at-a-distance but also, as seen here, influence of future actions on past events, even after there events have been irrevocably recorded."

"However, there is never a paradox if the quantum state is viewed as to be no more than a "catalogue of our knowledge". Then the state is a probability list for all possible measurement outcomes, the relative temporal order of the three observer's events is irrelevant and no physical interactions whatsoever between these events, epsecially in the past, are necessary to explain the delayed-choice-entanglement swapping."

[Byblos]

There is a remarkable experiment done only recently (published in April 2012).
news article: http://www.sciencedaily.com/releases/20 ... 131902.htm
scientific article: http://arxiv.org/ftp/arxiv/papers/1203/1203.4834.pdf (you may understand page 1 and end of page 6)

quotes from there:

"if we attempt to attribute an objective meaning to the quantum state of a single system, curious paradoxes appear: quantum effects mimic not only instantaneous action-at-a-distance but also, as seen here, influence of future actions on past events, even after there events have been irrevocably recorded."

"However, there is never a paradox if the quantum state is viewed as to be no more than a "catalogue of our knowledge". Then the state is a probability list for all possible measurement outcomes, the relative temporal order of the three observer's events is irrelevant and no physical interactions whatsoever between these events, epsecially in the past, are necessary to explain the delayed-choice-entanglement swapping."

Is there a way you can give us an example or an analogy to describe in laymen's terms what this all means? I feel like we're all dancing around the implications but we're not close enough to appreciate (comprehend) them.

[1over137]

I have to do that step by step. The first step is to comprehend quantum entanglement. Brian Greene is master in explaining physics to public audience. I will take an example from his book "The fabric of the cosmos." (He got a Pulitzer prize for that.)

Imagine that agent Mulder is in America and agent Scully is in her family's estate in Provence. Both received a box in which are little lightproof titanium boxes numbered from 1 to 1000, and a letter saying that both would be recieving an identical package. Letter says that each little titanium box contains an alien sphere that will flesh red or blue the moment the little door on its side is opened. The letter says that before any given box is opened, the sphere has the capacity to flash either red or blue, and it RANDOMLY decides between the two colors at the moment the door is opened. The letter further says that although the boxes work exactly the same for both agents - even though the spheres inside each one of the little boxes RANDOMLY choose between flashing red or blue - the agent's boxes somehow work in tandem. The letter claims that there is a mysterious connection, so that if there is a blue flash when Mulder opens his box number 1, Scully will also find a blue flash when she opens her box number 1; if Mulder sees a red flash when he opens box 2, Scully will also see a red flash in box 2, and so on.

Now, both agents open their little boxes and on comparing colors that flash inside each little box by phone, Scully and Mulder do indeed find the agreement predicted in the letter. Scully now thinks that the sphere inside each of their little boxes has been programmed to flash red or it has been programmed to flash blue when the door to its little box is opened. But Mulder says to Scully that the letter says each alien sphere RANDOMLY chooses between flashing blue and flashing red when the door is opened, not that the sphere has been preprogrammed to choose one color or the other. Scully is still suspicious. The letter also informs them that not only will opening the door to a little box cause the sphere inside to flash, but any other tampering with the little box to figure out how it works - for example if one tries to examine the sphere's color composition or chemical make up before the door is opened - will also cause it to flash. In other words, one cannot analyze the supposed random selection of red or blue because any such attempt will contaminate the very experiment one is trying to carry out. So Scully says to Mulder that his tiny green men are pretty clever - they've set things up so their ruse can't be unmasked.

Brian Greene writes:
"It would seem that Scully has this one soundly wrapped up on the side of science. Yet, here's the thing. Quantum mechanicians - scientists, not aliens - have for nearly eighty years been making claims about how the universe works that closely parallel those described in the letter. The rub is that there is now strong scientific evidence that a viewpoint along the lines of Mulder's - not Scully's - is supported by the data. For instance, according to quantum mechanics, a particle can hang in a state of limbo between having one or another particular property - like an 'alien' sphere hovering between flashing red and flashing blue before the door to its box is opened - and only when the particle is looked at (measured) does it randomly commit to one definite property or another. As if this weren't strange enough, quantum mechanics also predicts that there can be connections between particles, similar to those claimed to exist between the aliens spheres. Two particles canbe so entwined by quantum effects that their random selection of one property or another is correlated: just as each of the alien spheres chooses randomly between red and blue and yet, somehow, the colors chosen by spheres in boxes with the same number are correlated (both flashing red or both flashing blue), the properties chosen randomly by two particles, even if they are far apart in space, can similarly be aligned perfectly. Roughly speaking, even though the two particles are widely separated, quantum mechanics shows that whatever one particle does, the other will do too."

(Sorry for the delay.)

[Edit: Sorry for misinforming you but Brian Greene got his Pulitzer prize for his another book: "Elegant Universe". Anyway, The Fabric of the Cosmos is a great book worth deserving another Pulitzer prize : ). Highly recommended.]