The Double Slit Experiment with matter waves Repeating the classic double slit e

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The Double Slit Experiment with matter waves

Repeating the classic double slit experiment with matter particles has been accomplished several times in various ways. For a good history of the experiments with electrons, read

www.physics.rutgers.edu/~steves/501/links/double_slit_experiment.pdf. That article mentions Feynman's statement about the matter double slit experiment:

“Most discussions of double-slit experiments with particles refer to Feynman's quote in his lectures: "We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery." Feynman went on to add: "We should say right away that you should not try to set up this experiment. This experiment has never been done in just this way. The trouble is that the apparatus would have to be made on an impossibly small scale to show the effects we are interested in."

The double-slit experiment, and variations, is often used as a tool to explore the particle-wave duality of quantum objects. All experiments performed used light until 1961 when Claus Johnson at Tübingen performed it with electrons.

Later, in 1974, Merli at Milan performed the experiment with one electron at a time moving through the system. This experiment was repeated in 1989 at Hitachi by Tonomura with more sensitive detectors and with the same result. Watch the linked video clip to see the result: http://youtube.com/watch?v=DfPeprQ7oGc

With multiple particles in the system, arguments could be made that the particles were colliding in such a way as to produce the interference pattern. With a single particle, no such argument was possible.

[1] With a single particle moving through the system, how can you explain the development of the interference pattern?

[2] Between the source and the detector, where must the electron have traveled?

[3] When a detector is placed near one slit to detect the electron if it passes through, the double slit pattern collapses and a single slit pattern is seen instead. How does this refine your account of what the electron “does as it passes through the system”?

[4] So, with no detectors at the slits, which slit does the electron pass through on its way to the detector?

How the VW Quantum was conceived

The experiment has now been carried out on the largest molecule yet, a phthallocyanine derivative with 114 atoms.

The results of the research, led by Thomas Juffmann, also of the University of Vienna, were published online March 25 in the journal Nature Nanotechnology.”

http://www.livescience.com/19268-quantum-double-slit-experiment-largest-molecules.html

Real-time single-molecule imaging of quantum interference

Thomas Juffmann,   Adriana Milic, Michael Müllneritsch, Peter Asenbaum,   Alexander Tsukernik, Jens Tüxen, Marcel Mayor, Ori Cheshnovsky & Markus Arndt  

Nature Nanotechnology 7, 297–300 (2012)

doi:10.1038/nnano.2012.34

Received 30 December 2011 Published online 25 March 2012

Figure 2: Single-molecule imaging of PcH2 with subwavelength accuracy.

Plots show photon numbers at various position at six different time points (starting at the back), as extracted from the frames of a movie recorded with an EMCCD camera.

Figure 3: Build-up of quantum interference.

a–e, Selected frames from a false-colour movie recorded with an EMCCD camera showing the build-up of the quantum interference pattern for PcH2 molecules. Images were recorded before deposition of the molecules (a) and 2 min (b), 20 min

http://physicsworld.com/cws/article/news/2012/mar/29/quantum-interference-the-movie

The team also created a silicon-nitride diffraction grating with a separation of 100 nm between slits. This ensured that the diffraction angle was large enough to be resolved after the molecules passed through the slits. Furthermore, the grating was just 10 nm thick – around 16 times thinner than previous gratings – in order to reduce interactions between the molecules and the grating material.

Now, physicists at institutes in Austria, Israel, Switzerland and Germany have watched in real time as interference patterns were created by 58-atom phthalocyanine molecules (C32H18N8) and 114-atom phthalocyanine derivatives (C48H26F24N8O8) – the latter being the largest ever molecule to be studied in this way. The molecules were produced using micro-evaporation, in which a laser was focused on a thin layer of the compound. This reduced the heat load to the sample, preventing the molecules from decomposing and providing the researchers with an intense and coherent beam of large organic molecules.

Another important innovation was the use of fluorescence microscopy to detect the molecules. This involved exciting the molecules with a laser, and their emitted light was imaged onto an electron-multiplying charge-coupled device (EMCCD) camera. This technique, which allowed each molecule's position to be determined with an accuracy of 10 nm, was around 10,000 times more sensitive than previous detection methods.

Phthallocyanine:

Digging even further: Choosing how to detect the result after the particles pass through the slits.

More elaborate versions of the experiment, proposed by John Wheeler in 1978, in which the choice of how the particle will be detected is made AFTER it passes through the slits have now been carried out, with bizarre results. Watch the following video clips: https://www.youtube.com/watch?v=KnpCH9VRvPg., https://www.youtube.com/watch?v=8ORLN_KwAgs.

And answer the questions on the next page.

[1] What is the Copenhagen interpretation of QM?

[2] In the Copenhagen interpretation, what happens when a measurement of a quantum system is made?

[3] How does the "physical interpretation" of the wavefunction differ from the Copenhagen interpretation?

The experiment involves the production of an entangled pair of particles.

This effect has been extensively investigated:

To begin understanding these, watch the following clips: https://www.youtube.com/watch?v=tafGL02EUOA&t=29s and

https://www.youtube.com/watch?v=LcqHFo8s3Ts&t=7s or https://www.youtube.com/watch?v=WFi9QYnqiYw&t=287s (full hour lecture)

and answer the following questions:

[4] What does it mean that two particles are entangled?

[5] When one of the pair is detected and its properties measured, what happens to the other one of the pair?

[6] Before either particle is detected, what is known about the two particles which are entangled?

[7] What was Einstein's objection to this prediction of the quantum theory?

In another series of lectures, Susskind gives a "classical" version of entanglement. Al places two coins in two boxes: a dime in one and a nickel in the other. A courier comes, chooses one at random and delivers it to Nathan 10,000 km away. Nathan calls Al and says the box has arrived. Al opens the box left behind and knows instantly what is in the box Nathan received, even before he opens it. Nathan still doesn't know what is in his box unless Al tells him or he opens his box.

[8] How does the quantum version differ from this example?

[9] Revisiting question 6, what do the results of the delayed choice double slit experiment seem to say about the states of the entangled particles before they are detected? Are they already set or are we just ignorant of what state they are already in?

If you want more on these issues, the following link is a 75 minute lecture by one of the European investigators at the forefront of theresearch into such questions, Serge Haroche https://www.youtube.com/watch?v=pM_GN2zJPgQ.

Explanation / Answer

1) When an electron is passing through the double slit, we cannot measure its position at which it is going. We do not know the path of the electron and of course we know the probability of finding electron in a certain path.

If we consider the probability amplitude for electron which is complex in nature and have ability to interfere with other probability amplitudes at the detection plane. This is similar to the doubleslit interference pattern with light. So we observe the fringes at the detection plane.

2) We do not have any information about the path followed by the electron between the source and detector. This is the heart of the interference experiment with matter waves.

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