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Double Slit Experiment
in Quantum Physics


Introduction

The double slit experiment is based on the wave/particle duality of subatomic particles. As simple a concept as it is, it produces some of  the most astounding experimental results of quantum mechanics. This experiment stands the heart of quantum weirdness, because it is one of the strangest results in science. The original double silt experiment used photons of light, but later versions used electrons but the results are the same The double slit experiment is actually responsible for starting Quantum Physics


Single slit

Single slit with Classical Particles

Shooting classical particles that is small balls of mater at a screen with an single slit in it produces a simple spray of particles on second screen behind it.

A good example of this would be bullets shot from a gun at a sheet of steel that they can not penetrate with one slit in it, you would get just this type of pattern.

Single slit  with a wave

When a wave passes through a single slit it produces a similar patter, that of a bright spot in the center that dims as one move away from the center.

A wave of water through a single slit is a good example of this affect in action.


Double slit

Double slit with classical particles

When you add a second slit to the first screen classical particles would tend to form two spays of particles on the second screen. This just the single slit time two.

This is the pattern that bullets shot from a gun at a sheet of steel that they can not penetrate with two slits in it. This makes sense and it is  ine one would expect to see.
When a wave passes through a double slit it produces the bright and dark striped pattern seen here This patter results from the interference of the two waves produce by the slits.

This affect can be seen in a wave of water that passes through a double slit.


Quantum Particles

Single slit with quantum particles

Shooting quantum particles that is subatomic particles at a screen with an single slit in it produces the simple spray of particles on second screen behind it.

So far so good. This result is actually consistent with both waves and particles.

Double slit quantum particles

This is where things start getting strange. When shooting quantum particles at a screen with two slits in it. One would expect to get two spays of particles on the second screen like what you get with classical particles such as bullets .

However when one actually shoots quantum particles at a screen with two slits in it. Like a wave quantum particles form a series of several light and dark strips as shown below.

This shows the wave nature of quantum particles.

As strange as this is the quantum world gets even stranger.


Double slit quantum particles shot one at a time

One possibility that was considered is that the quantum particles are somehow interacting to form the series of l light and dark strips. The solution was to shoot the particles one at a time so that they can not interact.

Double-slit-experiment by Dr. Tonomura showing a build-up of an interference pattern of 100 single electrons.
GNU Free Documentation License

Double-slit-experiment by Dr. Tonomura showing a build-up of an interference pattern of 3,000 single electrons.
GNU Free Documentation License

Double-slit-experiment by Dr. Tonomura showing a build-up of an interference pattern of 20,000 single electrons.
GNU Free Documentation License

Double-slit-experiment by Dr. Tonomura showing a build-up of an interference pattern of 700,000 single electrons
GNU Free Documentation License

Note that the electrons still form an interference pattern even when shot one at time. This shows that the wave nature of subatomic particles is inherent in each particle. These results actually seem to require each electron to go through both slits at the same time.

However  this is not the end of quantum mechanical strangeness because it gets even stranger.


Double slit quantum particles shot one at a time being watched

The notion that one particle could actually go through both slits at the same time is so ant-intuitive that physicists had to look to see which slit each particle actually went through. To look experimenters put detectors by the slits so as to look to see which slit each particle actually goes through. What they saw was a real mind blower.

The result was that the subatomic particles formed a spray single line behind each slit. Just like classical particles would suggesting that the subatomic particle some how knew when they were being watched and behaved differently because of it.

Even this is not the end of the strangeness because thing are about to get even stranger.

 Suspecting that the detectors were causing the different results experimenters turned off the recorder, while leaving the detectors on. This process also included the recorder on but not actually recording anything.

To every ones surprise they got the interference pattern.

Experimenters even had the recorder on but the tape was removed while leaving the detectors on.

They got the interference pattern

The subatomic particles only produce the double lines when the information about them was actually being recorded. This was really strange in deed because it could only be concluded that it was the conscious act of observing that affects the experiment and not the measuring devices them selves. 


Quantum Eraser Experiments

These are a new set of double slit experiments that closes the gaps. These experiments eliminate many possible explanations for what is going on.

What follows is based on the paper:

Double-slit quantum eraser
Published in Physical Review
February 20, 2002

Warning what follows may blow your mind.

The experiment uses single photons from an argon ion pump laser with a wave length 351.1 nm. A special nonlinear crystal called beta-barium borate (BBO) is used produce entangled photon pairs each it twice the original wavelength. (702.2 nm) Each of these photons go in a different direction as shown by the labels p and s on the above illustration.

The photons go to two different detectors the s photons goes to Ds and the p photons goes to Dp. Detector Ds provides both position and polarization of s photons while detector Dp provides polarization of p photons. This information is then recorded by a recording device.

The gray line labeled s –Ds is the point along the p photon path that is the same distance as is traveled by the s photon to Ds. Note that at this point Dp is before Ds such that p photons are detected before s photons.

Placing a double slit in the path of the s photons produces the interference pattern of the classical double slit experiment.

However as before this is where things start getting strange.

QWP1

QWP2

Quarter Wave Plates (QWP) are special crystals that change linearly polarized light into circularly polarized light. In this case there are two quarter wave plates. For a photon of a given linear polarization one plate changes it to right circular polarization while the other changes it to left circular polarization.

Polarization
of p photon

Polarization of
s photon
before QWP

Polarization of
s photon after
QWP1 and slit 1

Polarization of
s photon after
QWP2 and slit 2

x

y

R

L

y

x

L

R

Since the s and p photons are entangled knowing the polarization of the p photon allows one to know the original polarization of the s photon. Placing these in front of each slit makes it possible to figure out which slit the s photon went through, without disturbing it.

As a result we get the double peak pattern instead of the interference pattern. Since the QWP’s don’t disturb the s photon it proves that the detectors are not what changes the result of the experiment.

Placing a polarizer in the path of the p photon destroys our information that tells us the s photon’s original polarization so it is no longer possible to know which slit the s photon went through. The result is that the interference pattern comes back.

Delayed Quantum Erasure

If you still have any working neurons left what comes next will probably fry what’s left.

Delayed erasure erases the p photon polarization after the s photon is detected.

Extending the p photons distance to Dp to beyond s photons distance to Ds produces the interference pattern without the QWP’s.
When the QWP’s are put back in place the interference pattern is lost again and the experiment produces the double peak pattern again.
Now placing a polarizer in the path of the p photon after distance the s photon travels to Ds( s-Ds) What we would expect to see and what common sense suggests that since the s photon is detected before the p photon’s polarization information is destroyed is that we should get the double peak pattern. However the interference pattern comes back again.

So what do we have here?

QWP’s form a measuring system that tags the s photons without disturbing them. This system needs additional information from p photon to determine which slit the s photon came through. When that information is measured the tagged photons form a double peak pattern. When this information is destroyed the tagged photons form an interference pattern.

In each case the tagging QWP’s are in place, the Dp and Ds detectors are detecting the p and s photons respectively, the information from both sets of detectors is sent to a recorder and is actually recorded.
The same results occur whether or not the p photon information is destroyed before or after the s photon is detected. This prevents the s photon from being affected by the change in p photon do to entanglement.

When the results of this experiment are analyzed the only real change is in the content of the information being measured and recorded. In one case the information allows one to find out which slit the s photon went through. In the other the information dose not allows one to find out which slit the s photon went through. This shows that the deterring factor is the information. It is the information about an event that causes the wave function collapse or decoherence if you prefer.


So what is happening?

In general the wave function represents multiple possibilities. When specific information is measured about an event those possibilities are reduced to one producing the needed information. It then produces a new wave function of multiple possibilities consistent with that one possibility.

All of this put together indicates that what we call reality is actually immaterial at its most fundamental level. At its most fundamental level the universe seems to be composed entirely of information. Making it more like a sophisticated computer program than a world of material objects.

It is not just the existence of the information but its ability of it to eventually be accessed by the experimenter. That experimenter is ultimately a human being that is a conscious entity. The information that makes up reality seems to be calculated as need for a conscience entity so as to produce a consistent reality.

One thing this does show is that consciousness is a fundamental part of reality. This is not new age thought but what the observation of subatomic particles is actually telling us. It is consistent with the Biblical view of God as creator since the universe would be a result of His consciousness. It is also consistent with the Biblical view of God creating the Universe out of nothing.
 


Reference

Reference

Explained ! The Double Slit Experiment

Explaining the Double Slit Experiment -Physicist Thomas Campbell

A Double-Slit Quantum Eraser Experiment

 


 

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