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The Quantum Eraser

September 7, 2016 | Posted in News

Richard Feynmann noted more than once that complementarity is the central mystery that lies at the heart of quantum theory. Complementarity rules the world of the very small… the quantum world, and surmises that particles and waves are indistinguishable from one other. That they are one and the same. That it is nonsensical to think of something, or even try to visualize that something as an individual “particle” or a “wave.” That the particle/wave/whatever-you-want-to-call-it is in this sort of superposition, where it is neither particle nor wave. It is only the act of trying to measure what it is that disengages the cloaking device and the particle or wave nature is revealed. Look for a particle, and you’ll find a particle. Look for a wave instead, and instead you’ll find a wave.

Complementarity arises from the limits placed on measuring things in the quantum world with classical measuring devices. It turns out that when you try to measure things that are really really really small, some issues come up… some fundamental issues.  For instance, you can’t really know exactly where a sub-atomic particle is located in space. You can only know where it is within a certain probability, and this probability is distributed through space in the form of a wave. Understanding uncertainty in measurement is key to avoiding the disbelief that hits you when thinking about complementarity.

This article is a continuation of the one linked above. I shall pick up where I left off, in that everyone agrees that measurement on the quantum scale presents some big problems. However, not everyone agrees what these problems mean. Some, such as Albert Einstein, say that just because something cannot be measured doesn’t mean it’s not there. Others, including most mainstream physicists, say the opposite — that if something cannot be measured, it for all practical purposes is not there. We shall continue on our journey by using modern technology to peer into the murky world of complementarity. But first, a quick review.

The Double Slit Experiment — Where It All Began

Firstly, there are purists out there that will disagree with my approach to explaining these concepts. I must plead with you that it is not my goal to submit this article to The Scientific Journal for review. My goal is simply to rip away the complexities that naturally follow this advanced topic, and present it in an easy-to-understand format that anyone can enjoy and learn from. But by all means feel free to expand on anything in the comments!

Complementarity was developed to help understand the results of laboratory experiments. Today, the idea of complementarity resides at the heart of what is known as the Copenhagen interpretation of quantum mechanics. There are other interpretations out there, but the Copenhagen model is the most widely accepted.

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Source: Rollins.edu

The laboratory experiments I speak of revolve around the double slit experiment, which can differentiate between a particle and a wave. Imagine you’re at a gun range and you put up a large target. In between you and the target is erected a large steel wall with two narrow slits…maybe six inches wide and two feet apart. You fire a few hundred rounds with your machine gun, and then observe the pattern on the target. You will find an obvious pattern – two narrow lines where the bullets went through the slits.

Now let us take our large steel wall with the two slits and stick it in a lake, so that the slits are just above the surface. Behind the wall, we’ll place some type of detector that can detect waves. We toss a large rock into the lake and watch the resulting wave emanate from the point of impact and strike the wall. On the other side of the wall, two other waves appear from the slits. The slits will each act like a wave source. The waves from each source will interfere with each other and produce a distinct pattern on our detector wall. It’s known as an interference pattern, and consists of several lines of different intensities.

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Source: maths.org

Now, you should see where we’re going with this. If we have an unknown substance, and we want to know if it is made of particles or waves, we can perform this experiment. Light, for instance, will produce an interference pattern. And that makes perfect sense – it’s an electromagnetic wave. One would think that sub-atomic particles would produce a pattern like our machine gun bullets did – two distinct lines. It turns out that this is not the case. They will produce an interference pattern as well. And that most certainly does not make sense.

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Source: maths.org

But physicists are clever, and decided to try firing one particle at a time at the double slit. About one particle per hour. But it yields the same result — an interference pattern! The particle is acting like a wave, as if it went through both slits at the same time! That’s impossible! We must take a closer look. We will observe the single particle to see which slit it goes through. Turns out that when you do this, you will get the double line pattern like you expected. If we look at it, we will see a particle. If we don’t look at it, we will see a wave. And thus was born the concept of complementarity.

The idea that “observation determines reality” gets into a philosophical quagmire that I’m not touching with a 39 foot HF antenna. But we can probe deeper into this mystery with an experiment. What if we could observe the particle/wave/whatever AFTER it goes through the slit and BEFORE it hits the detector wall? 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 1999.

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 red), it will go to a half-silvered mirror BSb. If it came from the bottom slit (path pictured in blue), the prism will direct it to half-silvered mirror BSa. Note that “BS” stands for “Beam Splitter”: a half-silvered mirror will allow 50% of the photons to pass, and will reflect the other 50%.

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Source: wikipedia.org

The BSb mirror will send 50% of the photons from the top slit to detector D4 and the other 50% to the mirror Mb. The photons from Mb head to another half-silvered mirror BSc. This mirror will send 50% of the photons to detectors D1 and Drespectively.

A similar action occurs with photons coming through the bottom slit. They will hit BSa, which sends photons to detector D3 and mirror Ma. From Ma, they will go to mirror BSc, which takes half of the photons to D1 and the other half to D2.

In the end, photons from the top slit will go to detectors’ D1, D2 and D4. Note that no photons from the top slit can reach detector D3. Photons from the bottom slit will go to detectors’ D1, D2 and 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:

  • Top slit = D4
  • Bottom slit = D3
  • Unknowable = D1 and D2

er_06Detector D0 lies on the shortest path, so a photon will strike it approximately 8 nanoseconds before its entangled partner reaches another detector. 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. This is what we find — D3 and D4 (labeled “R0n” in the Wiki images) show a particle pattern. 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.

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Note how the bands are opposite of each other in R01 and R02. 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. This is doing exactly what we wanted to do — we’re looking at the particle AFTER it goes through the slit (via D0), but BEFORE it hits the detector wall, which in this case is made of detectors’ D1 – D4.

What they found was that the photons that hit D0 always — as in 100% of them — correlated to their partner photons. And this, my fellow hackers, should be impossible. Why? Because:

  • The photons hit D0 8 nanoseconds before D1 – D4.
  • The photons have a 50/50 chance of hitting D1/D2 or D3/D4.

How then can the photon that hits D0 know if its entangled partner went to D1/D2 or D3/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. It “erases” the past, preventing us from ever knowing which slit the photon came through. Bohr was right: complementarity is real, impossible as it might seem. 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. To ask if it is a wave or particle is nonsense. To ask if it’s even there is nonsense. There is no such thing as “there” in the quantum realm. Concepts like time and space, cause and effect …have different meanings there… meanings that we’re still not sure of to this very day.

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. Many of which are surely greater than the greatest fiction, but whose fantastic implications are rooted in a very real reality — the next frontier of modern science.


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Source: The Quantum Eraser

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