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The Mach Zehnder Interferometer

Exerd

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This light experiment never ceases to amaze me. I thought others would find it interesting.

The experiment is an example of how light in the quantum particle state seems to sometimes travel in two different paths when only a single photon is sent. It is as if a mirror, non-existent particle travels through the air and somehow communicates with the other particle!

Hopefully this small experiment will cause some of you to learn about the fascinations of quantum mechanics and further down the line, string theory.

Here is a photo used from Wikipedia showing the setup of the experiment:

Mach-zender-interferometer.png


Here is the way it works. Light enters as a beam from the lower left hand corner. Its first object of incidence is a 50/50 beam splitter.

There is a 50% chance the impeding particles will travel through the splitter, and a 50% chance the particles will deflect off of the splitter and take a turn upwards.

There is a mirror which reflects 100% of the particles in the top left corner, and the lower right corner of the box.

No matter which path the light takes after the first beam splitter, it is guaranteed to reflect off of a 100% mirror as the second object of incidence.

When a solid beam of photons enters the interferometer, the light traveling through the splitter forward to the lower right mirror retains its original polarization, and reverses polarization when it reflects off of that lower right mirror.

The light which is reflected off of the first beam splitter immediately switches its polarization, but then hits the upper left mirror where it reflects again, therefore switching polarization again.

Now, the beams both arrive at the upper right beam splitter with opposite polarization, and cancel each other out. In this circumstance, the beams land on detector #2, 100% of the time. They always cancel each other out and go up to that detector at the end, and never land on detector #1.

So, while you imagine this, think about the fact that this is happening because there are two beams of photons, each traveling different paths, and those paths dictate the polarization, which then cancel each other out.

What would happen if you had the precision of an instrument which could only send one single photon down the path, instead of a stream of photons?

Since there is only one single photon making the journey, when it arrived at the final splitter, it would have no other particle meeting it there to cause polarization to cancel out, right?

Wrong!

When detector #2 is monitored, every time a single photon is sent, that photon always chooses detector #2. This can be very confusing to grasp, but is what really happens.

Well, maybe this is not the way it works, you think to yourself. You decide to modify the experiment. Instead of having a mirror in the lower right hand corner, you remove that mirror, and replace it with a photon sensor. Now, you can detect when a photon hits that new sensor. When it does not hit that sensor, you are monitoring the other two sensors, #1 and #2 to see if it went to one of those.

Something very strange now happens.

50% of the time, the single particle lands on your new sensor, documenting the fact that the particle can take two paths.

The other 50% of the time, the particle does not make it to that new sensor. It happens to take the other path. What is very strange is that now when the particle does not land on the new sensor, 25% of the time it lands on detector #2, and 25% of the time it now lands on detector #1!

The particles which travel the other path, somehow "know" that you are checking the other path with a sensor. They are no longer governed by the rule of opposite polarization, where they before always went to sensor #2. Somehow presumably, there is an invisible particle communicating back to the measurable particle. Weird stuff!

I tried my best to explain that, I hope that you can understand it. If not, maybe someone better with explanations can word it for those who cannot.

;)
 
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pullbangdead

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Schrodinger's Cat (multiple "dimensions"/wavefunction collapse with observation) and single photon interference together, I like it!

Thought experiment: Now what about IF you had a magical detector/mirror? Ie a way to simultaneously detect the photon at the lower right hand corner AND allow said photon to continue on its journey undisturbed. What would you see at the original detectors? You've now observed the photon before the original detectors, collapsing its wavefunction, and allowed it to continue on. What fraction of single photons do you see on each detector?
 
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Exerd

Active member
Joined
Jan 2, 2010
Messages
448
Points
28
Schrodinger's Cat (multiple "dimensions"/wavefunction collapse with observation) and single photon interference together, I like it!

Thought experiment: Now what about IF you had a magical detector/mirror? Ie a way to simultaneously detect the photon at the lower right hand corner AND allow said photon to continue on its journey undisturbed. What would you see at the original detectors? You've now observed the photon before the original detectors, collapsing its wavefunction, and allowed it to continue on. What fraction of single photons do you see on each detector?

Or, what if you had that photon traveling millions of miles, and after the photon already deflected from the first beam splitter, you then switched the lower right mirror with a detector? Would it still obey?

It is some crazy stuff. It makes you open up and realize that there is some "magic" out there which we cannot yet describe, and that the closest language we have to interpret it is with advanced mathematics.

As they say, observation dictates the path. Without observation, it is presumed that the particle travels in every possible direction. It is in an infinite amount of states at the same time.

I think the utmost fascinating study is superstring theory. It essentially states that everything exists within a vast foam of particles. As you move, all of those string-like particles stay in the same relative location, but you are just a giant vibratory wave moving through that foam, composed of trillions upon trillions of vibrating strings. Even when there is no mass in a vacuum, that foam still exists there, but its vibratory state is low enough so that the particles create no mass within our dimension and in that area. When you begin to think about it this way, it seems easier to grasp the concept of 11 dimensional hyperspace, because those vibrations can play in many ways like a guitar string. When they vibrate in one type of node, it is within our dimension. When they vibrate in another node, it is in a different dimension. Other dimensions could presumably exist right on top of each other because of the ability for the particle strings to vibrate in many nodes at once.

I'm not so sure if any others are understanding this. :yabbem:
 




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