### Beam splitter - Wikipedia

Keywords: beams-splitter, interferometer, Mach-Zehnder, photon, correlation, Quantum physics theory, or more precisely quantum optics as is dealt with in . case of half-silvered mirrors, as will be discussed in sub-sections and Quantum mechanics allows “entangled states” of two distant systems. Entanglement or classical correlations? Quantum interference splitter (half- silvered mirror):?? . How is this different from a classical correlation? initial system. Buried in and intended for , the capsule was a glass Their scheme exploits quantum entanglement, a phenomenon in which particles or points in a The correlation appears to violate “locality,” the rule that states that . at a partially silvered mirror, so that half the photons took one path and.

But we can probe deeper into this mystery with an experiment. This is precisely what the quantum eraser experiment does. The Quantum Eraser Like several concepts in quantum theory, originally thought experiments were developed to explore an idea or approach, but technology has advanced to the point where we can actually carry out some of them.

The quantum eraser experiment is one such experiment, and was carried out at the University of Maryland in The experiment starts with visible light photons traveling through a double slit. The exiting light immediately hits a prism which splits a single photon into an entangled pair. A lens then directs one of the photons to detector D0. The other photon goes to another prism. What happens next depends on which slit the original photon came through.

If it came from the top slit path pictured in redit will go to a half-silvered mirror BSb.

If it came from the bottom slit path pictured in bluethe prism will direct it to half-silvered mirror BSa. The photons from Mb head to another half-silvered mirror BSc. A similar action occurs with photons coming through the bottom slit. They will hit BSa, which sends photons to detector D3 and mirror Ma. Note that no photons from the top slit can reach detector D3. No photons from the bottom slit can reach detector D4. Note that it is not possible to determine which slit the photons that hit D1 and D2 originated from.

So this is what we have: The Coincidence Counter allows us to assign a photon that strikes D0 to its entangled partner, which strikes D1 — D4. So we put 12v on the Arduino Uno and let the photons loose. D1 and D2 show an interference pattern. And this makes sense.

We cannot know which slit the photons detected at D1 and D2 came through. So they act as a wave. And we know which slit the photons detected at D3 and D4 came through, so they act like particles. But this is not the point of the experiment. Note how the bands are opposite of each other in R01 and R This corresponds to the R01 and R02 waveforms in the top image. See the Wiki for more information on why this occurs. The neat stuff is going on at detector D0.

For every photon that hits D1 — D4, it has an entangled partner that hits D0 8ns earlier. Like the other detectors, D0 can resolve a particle or wave pattern. And this, my fellow hackers, should be impossible. The photons hit D0 8 nanoseconds before D1 — D4. We are forced to consider an impossible scenario: The photons that end up at D1 and D2 must be sending information 8ns into the past to tell its entangled partner at D0 to become a wave.

The photons that end up at D3 and D4 must be sending information 8ns into the past to tell its entangled partner at D0 to become a particle. This, my friends, is a simplified explanation of what the quantum eraser is all about. However, our problem lies not with what appears to be undeniable time travel. Our problem is that how we view the natural world is not compatible in the quantum realm.

## Beam splitter

To ask if it is a wave or particle is nonsense. Encourage your sons, daughters, nieces and nephews to take the helm and study quantum theory.

Stir their curiosity… there are stories yet to be told, and discoveries that remain to be made. The short answer is: But the longer answer is, under certain conditions: Let's examine the question of what basis to best describe this quantum experiment so as to better understand the quantum facts.

EQ 1 is expressed in the path basis and shows that path A is correlated and entangled with path B. The experiment illustrated in Fig. I will also introduce a third representation called the "eraser basis".

If you expected that this note was going to be a bit of poetic fluff, you will be seriously disappointed. Raw, uncensored quantum physics ahead! About the path basis photon B for instance in Fig.

Photon B takes either path B1 or path B2. And this fact can be verified using photon detectors B1 and B2. Remember, Mother Nature on the quantum level is responsive to the questions that you ask. If you go looking for photon paths, Nature will give you photon paths. Screen pattern of A photons What about photon A, that no longer travels in two beams but has been merged by a converging lens onto a photon sensitive screen? Well, the rules of quantum theory state that if you know which path a photon took, it cannot form an interference pattern.

We can determine which path A took by looking at the B counters. If B1 clicks then photon A took path A1, giving rise to pattern 1 of Fig. If B2 clicks then photon A took path A2, giving rise to pattern 2 of Fig.

Both these patterns are featureless blurs. Adding them together gives a featureless blur with twice the intensity. No matter how hard you stare at these pixels, you will never see an interference pattern.

If we possess path info about photon A, no interference is possible -- if photon A took one path, it simply could not have gone thru both slits. This argument depends on the fact that the paths of the B photons carry info about the paths of the A photons.

But suppose we alter our experiment by destroying B's path information with a "quantum eraser"? If the eraser leaves us unable to tell which path A took, will we then be able to observe the A photon interfering at the screen? Let's take a look at how a quantum eraser works. Beam splitter operating as Quantum Eraser The quantum eraser consists of a half-silvered mirror.

Beam B1 comes in, is half reflected and half transmitted and sent into outputs B3 and B4. Likewise beam B2 comes in, is half reflected and half transmitted and is also sent into outputs B3 and B4. The eraser adds together these two input beams in such a manner as to entirely destroy information about which path the input photon took before entering the beam splitter.

The equations for this path info erasure operation are: If that minus sign were not there, more energy would come out of the eraser than went in -- humans could create endless energy out of mirrors. However the law of conservation of energy is a very strong prejudice in the physics community and seems to be obeyed to the letter by Nature as well. This minus sign is a special case of the Stokes relations for light traveling across interfaces.

Sir George Stokes was an Irish physicist from County Sligo, who also made important discoveries in fluid mechanics.

The names for the bases are my own inventions but the physics equations are entirely conventional. I chose the symbols M and W to label these two wave functions because the letters M and W are flipped versions of one another, just as the two wave functions are in the interference sense, precisely one another's opposites, as we shall see.

These two wave functions by themselves contain no information about "which path" the B photon took from the source, but taken together the two wave functions could in principle still be combined in an "anti-eraser" which would resurrect the "which path" info of photon B.

However if photon B is actually detected after the eraser either by "Bobski" at detector B3 or by "Boris" at detector B4, then B's path information is definitively erased and cannot ever be recovered. A signal from either Boris or Bobski means: Quantum Eraser produces 2 sets of fringes.

### QUANTUM TANTRA: Some Notes on Quantum Entanglement

Does erasing B photon's which-path information and via their perfect entanglement also erasing A photon's path info as well now produce interference when both of A's paths converge on the same screen. The answer is Yes. Erasing B's path info produces interference fringes on A's screen. But B's eraser and A's screen could be light years apart. In the physics literature, Bob and Alice are iconic figures constantly obsessed with exploiting quantum entanglement to exchange superluminal messages between space-like separated locations.

And on the surface, it looks as though path-entangled photons can do the job, because erasing Bob's path will produce Alice's fringes. And not erasing Bob's path will make Alice's fringes disappear. Of course this entangled eraser scheme can't possibly work. Such a scheme would violate Einstein's laws of relativity. But one can't simply invoke relativity to explain away the eraser scheme.

## The Quantum Eraser

Any such alleged FTL signaling proposal must be refuted on its own terms, by using only the laws of quantum mechanics. When Bobski announces that he has erased photon B's path, all of photon A's screen pixels that correlate with Bobski's message form a fringe pattern Z.

When Boris announces that he has erased photon B's path, all of photon A's screen pixels that correlate with Boris's message form a second fringe pattern "Anti-Z". As shown in Fig. So Alice's fringes appear when triggered by messages either from Bobski or Boris, but no fringes appear when there is no way to tell whether the dot on Alice's screen was correlated with a click at counter B3 or a click at counter B4.

Thus, absent a trigger signal which must be sent at light speed or slowerwhat happens at Bob's location remains at Bob's location. FTL signaling using this entangled eraser scheme is impossible.

Computing the observed pattern of photons on Alice's screen. To see how this fringe and anti-fringe business works, we calculate the intensity of the patterns at a single location on Alice's screen.

These two beams have been slightly shifted for clarity, In reality they would be exactly superposed. Squaring the wave function we obtain the probability for a photon to hit the screen.

Here you can explicitly see that the interferences terms at location x cancel when both terms are summed. Since x is an arbitrary position on the screen, if the interference term cancels at x, then the interference terms cancel everywhere. Each of these wave functions produces interference fringes on Alice's screen. But these two sets of interference fringes exactly cancel one another. Path-entangled systems possess a variety of "magical" qualities which I will only mention in passing.

**Your Relationship is a Mirror**

There is, for instance, the so-called "delayed-choice" quantum eraser which involves invoking Bob's quantum eraser long after Alice's photon pattern has been indelibly recorded. Hence apparently after-the-fact producing matched sets of mutually canceling Alice fringes that correlate with Bobski's and Boris's detector events. But wait, there's more.

The path eraser pictured in Fig.

### Photon and half-silvered mirror | Physics Forums

And Bob can carry out this invisible Alice fringe pattern shifting in the "delayed-choice" mode, that is, long after Alice has already recorded every pixel of her pattern "in stone". Given the peculiar behavior of this simplest of all quantum entanglements, it is hard not to imagine that "something" must be being transmitted faster-than-light or even backwards-in-time from Bob to Alice. Something must be really being sent and really fast too in these experiments. But physicists can't really say what that "something" might be.

We have introduced here three different photon representations, the path basis P, the screen basis S and the eraser basis E. Applying these three bases separately to photon A or photon B, we could obtain nine different experiments on photon entanglement. Both SS and EE have such similar behaviors that I will just consider the SS case for which the wave function in the screen - screen basis looks like this: Likewise Alice's anti-interference pattern anti-Z is perfectly correlated with Bob's anti-Z pattern.