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[bell_bohm] Re: double slit experiment

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  • manyworlds
    Discussion of thought experiment with separate electrons and a nice Java applet may be find at :
    Message 1 of 8 , Nov 12, 2002
      Discussion of thought experiment with separate electrons and a nice Java
      applet may be find at :
      *Maybe we could turn down the electron gun until the electrons were coming
      out slowly enough for us to be sure it was one at a time. Lucky for us it
      does just that. Use the minus and plus keys on your keyboard to control the
      speed of the gun, and slow it down a lot. Then press your backspace key to
      clear the screen.
      Hey! The interference lines are building up anyway! How can it do that if
      the electrons are really like little bullets? What are the electrons
      interfering with? This is so strange...
      (...) Well, it's not really possible to set up the experiment just the way
      we've shown it here, with electrons being shot at a screen through a pair of
      slits. The two slit experiment with light has been done many
      times--originally by Thomas Young in 1801--but it's just not practical to do
      exactly the same experiment with electrons. The equipment would have to be
      made on an impossibly small scale to show the effects we've been discussing.
      So the applet you saw is what's known as a thought experiment. It shows the
      results that would be obtained, according to quantum theory, if a
      hypothetical experiment like this could be performed.*

      First single-photon measurements were reported in 1999:
      16 July 1999:
      *The ability of physicists to control single quantum particles, such as
      individual atoms and photons, has increased greatly in recent years and has
      allowed many "thought" experiments to be actually performed in the
      laboratory. Experimental techniques have now advanced to the stage where it
      is possible to repeatedly observe a single photon without destroying it. In
      this latest breakthrough physicists at the Ecole Normale Supérieure in Paris
      used rubidium atoms to observe the photon in a superconducting niobium
      cavity (Nature 400 239).*
      The Paris team used lasers to first select rubidium atoms with a very
      well-defined velocity, and then prepare them in a highly-excited so-called
      Rydberg state. The atoms were then passed through the niobium cavity, which
      can store a single microwave photon for up to 1 millisecond. The experiment
      is designed such that the energy of the microwave photon is the same as the
      energy difference between two Rydberg in the atom.
      If there is no photon in the cavity, nothing happens to the atom. If there
      is one photon, however, the phase of the wave function describing the atom
      is changed and this can be measured using interference techniques. These
      techniques can also detect if the photon is in a quantum superposition of
      zero-photon and single-photon states. If a second atom is sent through the
      device, it yields the same result as the first one, showing that a QND
      measurement has been made. It is not possible to extend the technique to
      higher numbers of photons but it could be used to make a quantum logic

      *1999 was a good year for precision experiments in quantum mechanics. Two of
      the highlights were the repeated measurement of a single photon in a
      superconducting cavity and the observation of quantum interference effects
      in a beam of carbon-60 molecules. The single-photon experiment at the Ecole
      Normale Supérieure in Paris was an example of a quantum non-demolition
      measurement: the team was able to repeatedly observe the photon without
      destroying it. The carbon-60 experiment at the University of Vienna in
      Austria observed wave-like behaviour in a beam of carbon-60 molecules - the
      largest particle for which quantum interference effects have been observed.*

      In 1999 Prof. Anton Zeilinger's Quantum Optics Group (U. Vienna)
      had developed an interferometer for large molecules:
      Diffraction and Interference with Fullerenes: Wave-particle duality of C60
      *We have observed de Broglie wave interference of the buckminsterfullerene
      C60 with a wavelength of about 3 pm through diffraction at a SiNx absorption
      grating with 100 nm period. This molecule is the by far most complex object
      revealing wave behaviour so
      far. The buckyball is the most stable fullerene with a mass of 720 atomic
      units, composed of 60 tightly bound carbon atoms.
      (...)Quantum interference experiments with large molecules, of the kind
      first reported here, open up many novel possibilities among them decoherence
      studies and nanolithography experiments.*
      (...)Note that the ratio between the diameter of a buckyball (1 nm) and the
      width of our diffraction grating slits (50 nm) compares favorably with the
      ratio between the diameter of a football (22 cm) and the width of a goal
      (732 cm) according to FIFA standards.
      The distance between the source and the detector corresponds in this scaling
      to the distance between the Earth and the moon.
      - C60 powder is heated to ~ 600 ... 700°C in a resistively heated oven.
      - The velocity distribution is very broad and faster than purely thermal.
      - The molecular de Broglie wavelength is centered at ~ 2.5 pm.
      - The de Broglie wave length is thus ~ 400 times smaller than the size of
      the particle
      (1nm diameter of the electron shell)
      - The hot and divergent beam is collimated to a pencil of ~ 10µrad
      Buckyballs that pass the second slit are transversely cold
      - The SiN diffraction grating has a nominal gap width of 50nm and a grating
      constant of 100nm
      - After free evolution over 1m the molecules are detected via thermionic
      ionization by a tightly
      focused Argon ion laser beam at 24 W.
      - The positive ions are counted by a secondary electron counting system.
      Interpretation of the diffraction curve
      - Interference fringes can clearly be seen*

      They observed 200 counts/s for single peak (without grating)
      1250 counts/50s central peak + 2 additional peaks 650 counts/50s (with
      (25 counts/s + 2 x 13 counts/s)
      Is this enough to speak about C60 balls parade through the slits of the
      I believe it is. There is very little probability that two different C60
      balls met themselves at the same point of the screen. So - in most cases
      this is just a single ball at every count with its matter wave being
      actually passed through many of slits of the grating simultaneously.
      And how it is possible that it went through at least two gates 50 nm apart
      being 1 nm particle and having associated de Broglie wavelength 0.0025 nm?
      It just does mean that the associated wave packet has the transverse space
      range of 2000 wavelengths, at least.

      They reported new version of this experiment lately... Now they use a
      standing light wave as the diffraction grating for the Fullerenes C60 and
      *The standing light wave constitutes a periodic structure with a periodicity
      of half of the laser wavelength, i.e. 257 nm. The most probable velocity of
      the fullerenes amounts to 120 m/s, which corresponds to a de Broglie
      wavelength of 4,6 pm (4,6*10-12 m) for C60 and 4,0 pm for C70. So we expect
      diffraction angles for these fullerenes of 18 µrad and 15 µrad,
      respectively. In a photon picture the observed deflection amounts to twice
      the photon recoil of the green laser photons.
      By varying the power of the standing light wave the induced phase shift and
      so also the relative height of the individual diffraction orders can be
      varied, as shown in figure 2. For comparison also the undiffracted
      beamprofile is shown on top. In contrast to atoms the absorption of the
      'grating' photons doesn't lead to spontaneous emission but to an internal
      heating of the molecule, so the absorption of n photons deflects the
      fullerene by n photon recoils. Twice the mean number of absorbed photons is
      given by the imaginary part of the mean phase shift , quoted in fig. 2. The
      resolution of our detector is good enough to resolve the individual
      diffraction peaks but the absorption of an odd number of photons fills up
      the minima in between and decreases the contrast.
      (...) In contrast to the extremely fragile material structures used in our
      previous interference experiments standing light waves proved to be a
      promising alternative - especially for the coherent manipulation of even
      larger molecules: they have perfect periodicity, high transmission and
      cannot be blocked or destroyed by the molecules.*

      In 2000 the group of Prof. David E. Pritchard of Massachusetts Institute of
      operated atom interferometer of interest here:
      *We are pioneering new measurement techniques using coherent atom optics
      (such as beam-splitters, mirrors and lenses) to manipulate matter waves. We
      operate an atom interferometer, similar to a Mach-Zhender optical
      interferometer, which splits deBrogile waves of matter into two physically
      separated paths. After an interaction region where each atom can pass
      simultaneously on both sides of a metal foil, the matter waves recombine,
      forming interference fringes. We monitor the phase and contrast of these
      fringes, which are extremely sensitive to any interactions experienced by
      the atoms.
      In the year 2000 we completed three experiments on decoherence. Presently,
      in Spring 2001, we are midway through a measurement of the matter wave index
      of refraction, and we are developing a novel atom optic for velocity
      multiplexing. Each project described in this report refines atom
      interferometry as a tool for making measurements of atomic properties and
      probing fundamental issues in quantum physics.

      And photons once more... The most promising seems to be rapidly developing
      nowadays quantum dots (QD) technology:
      12 May 2000
      PhysicsWeb - Quantum dots detect single photons
      *Researchers at Toshiba Research Europe in Cambridge, UK, have developed a
      single-photon detector based on quantum dots. It is the first time quantum
      dots have been used to detect individual photons at visible or near-infrared
      wavelengths, Andrew Shields of Toshiba told the CLEO conference in San
      Francisco this week. The quantum dot device consists of a transistor made of
      different layers of gallium arsenide and aluminium gallium arsenide. *

      During 7th Conference of Laser Technology at Swinoujscie (Poland) held on
      Sept 23-27, 2002,
      I have met info on this year first industrial implementations of QD lasers
      (A. Jelenski: Lasery z kropkami kwantowymi [Lasers with Quantum Dots], the
      material will be published in SPIE soon). It does mean it is rapidly
      developing technology, really.
      Prof. Andrzej Jelenski of Instytut Technologii Materialow Elektronicznych,
      Warsaw, Poland:
      *Advantages given by a discrete energy spectrum and efficient overlap of
      electron and hole wave function were already recognised for several years
      and first papers on the utilisation of quantum dot arrays for light
      appeared already in the 80-ies. However because of technological
      difficulties more than a decade was needed for manufacturing the first
      quantum dot (QD) lasers, and first industrial implementations will take
      place only this year. (...) many major laboratories around the world work
      actually on research and development of QD laser.*

      Actually the horizontal (edge) QD lasers are manufactured - not suitable for
      the goals of single-photon interferometry. But the vertical versions of QD
      lasers should make possible to achieve development of QD lasers with a
      single quantum dot. When we will have single QD lasers then, having
      single-photon QD detectors, the true fully controlled direct strong
      verification of 2 slit experiment will be possible, finally.


    • manyworlds
      And here is another pack of relevant comments: Talking physics with the Dalai Lama 4 August 1998 *(...)Zeilinger had invited the Dalai Lama to his laboratory
      Message 2 of 8 , Nov 13, 2002
        And here is another pack of relevant comments:
        Talking physics with the Dalai Lama
        4 August 1998
        *(...)Zeilinger had invited the Dalai Lama to his laboratory following a
        meeting at Dharamsala in Northern India last October at which he and four
        other physicists had, over the course of five days, discussed physics and
        cosmology with the Buddhist leader. In Dharamsala, Zeilinger had
        demonstrated some basic quantum phenomena - such as wave-particle duality -
        using a laser-based double-slit experiment with a photomultiplier tube
        connected to a loud-speaker. The Dalai Lama's visit to Innsbruck allowed
        other quantum effects to be demonstrated for him.
        Zeilinger says that the Dalai Lama did not have a problem with photons
        having both particle and wave-like properties, but was reluctant to accept
        that individual quantum events are random. For example, he refused to accept
        that we cannot know which path a photon takes in a two-path quantum
        interference experiment. Zeilinger notes that continuity of existence is
        very important to Buddhists because it leads to reincarnation.
        However, observation plays a key part in what we can know in both quantum
        theory and Buddhism, and Zeilinger was surprised to learn that the Dalai
        Lama agreed that there are not only limits on what we can measure, but also
        limits on what we can know, even in principle.(...) *
        To see more:

        The many wavelengths of light
        Physics in Action: August 1999
        *(...)Sebastião de Pádua and colleagues at the Universidade Federal de Minas
        Gerais in Belo Horizonte, Brazil, have now measured the de Broglie
        wavelength of a two-photon wave packet in a Young's double-slit experiment
        (E J S Fonseca et al. 1999 Phys. Rev. Lett. 82 2868). They used a nonlinear
        crystal to "down-convert" 351 nm wavelength photons from an argon laser into
        pairs of 702 nm photons, which were then directed onto a double slit.
        Avalanche photodiodes were used to detect the fringes formed by the
        interference between the two paths from the source to the detector.(...)*
        More: http://physicsweb.org/article/world/12/8/7/1
        As we see, from our point of interest, they do vsio-na-abarot... Seemingly
        there is no point in conducting such an experiment but the demonstration of
        technical advancement... In other words: nobody doubts the results they
        should obtain before ... they were obtain. If only they could block out
        every one of pair and make parade of single photons... But they surely can
        not. There is still a beam of photons left.

        But there is no need in conducting the exact 2-slit experiment to anticipate
        its results.
        The number of interference experiments were reported lately in reference to
        Quantum Comps (QC).
        And they demonstrate the same in a slightly different, technically more
        convenient way:
        Fundamentals of quantum information
        Feature: March 1998
        *(...)Quantum interference can be explained by saying that the particle is
        in a superposition of the two experimental paths: passage through the upper
        slit> and passage through the lower slit>. Similarly a quantum bit can be in
        a superposition of 0> and 1>. Experiments in quantum information processing
        tend to use interferometers rather than double slits but the principle is
        the same (see http://physicsweb.org/box/world/11/3/9/world-11-3-9-1 ). So
        far single-particle quantum interference has been observed with photons,
        electrons, neutrons and atoms. (...)*


        Kandydatka 18: Ma³gorzata >>> http://link.interia.pl/f1679
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