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double slit experiment

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  • cemturgay
    I recently heard that, the double slit experiment with single electrons or photons was never been made actually because of technical difficulties. What always
    Message 1 of 8 , Feb 15, 2002
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      I recently heard that, the double slit experiment
      with single electrons or photons was never been made actually
      because of technical difficulties.
      What always been made was using ray of (multiple)
      electrons or photons.
      And, when scientists talk about double slit experiments
      and say "even a single electron passes thru both slits"
      is just a thought experiment based on calculated assumptions
      in QM theory, referring to some other experiments
      done about 70-80 years ago.

      Is this true? Or up to what part it is true?

      Or are there recent experiments, where single electrons/photons
      send thru double slits, one after another
      and have an acceptable interference pattern on the other side?

      Cem
    • Dan Bloomquist
      From John Baez: Fourier transform land In 1910 or thereabouts, Geoffrey Taylor placed a very weak light
      Message 2 of 8 , Feb 17, 2002
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        From John Baez: Fourier transform land
        <http://math.ucr.edu/home/baez/photon/fourier.htm>

        In 1910 or thereabouts, Geoffrey Taylor placed a very weak light source
        in a box on one side of a two-pinhole screen. He placed a photographic
        film on the other side, then he went sailing for a few months. He
        calculated that there was (with overwhelming probability) never more
        than one photon at a time in the box. Nonetheless, when he returned from
        his topsails and spinnakers to develop the film, he found the familiar
        two-point-source diffraction pattern--- the very same diffraction
        pattern that Thomas Young observed over a hundred years before.

        Best, Dan.

        cemturgay wrote:

        >I recently heard that, the double slit experiment
        >with single electrons or photons was never been made actually
        >because of technical difficulties.
        >What always been made was using ray of (multiple)
        >electrons or photons.
        >And, when scientists talk about double slit experiments
        >and say "even a single electron passes thru both slits"
        >is just a thought experiment based on calculated assumptions
        >in QM theory, referring to some other experiments
        >done about 70-80 years ago.
        >
        >Is this true? Or up to what part it is true?
        >
        >Or are there recent experiments, where single electrons/photons
        >send thru double slits, one after another
        >and have an acceptable interference pattern on the other side?
        >
        >Cem
        >
        >
        >
        >To unsubscribe from this group, send an email to:
        >bell_bohm-unsubscribe@egroups.com
        >
        >
        >
        >Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
        >
        >
        >
      • cemturgay
        Yeah, thanks. But 100 years... Nobody else tried up to now ??? Cem ... source ... photographic ... more ... from ... familiar ...
        Message 3 of 8 , Feb 18, 2002
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          Yeah, thanks.
          But 100 years...
          Nobody else tried up to now ???

          Cem


          --- In bell_bohm@y..., Dan Bloomquist <dan@l...> wrote:
          > From John Baez: Fourier transform land
          > <http://math.ucr.edu/home/baez/photon/fourier.htm>
          >
          > In 1910 or thereabouts, Geoffrey Taylor placed a very weak light
          source
          > in a box on one side of a two-pinhole screen. He placed a
          photographic
          > film on the other side, then he went sailing for a few months. He
          > calculated that there was (with overwhelming probability) never
          more
          > than one photon at a time in the box. Nonetheless, when he returned
          from
          > his topsails and spinnakers to develop the film, he found the
          familiar
          > two-point-source diffraction pattern--- the very same diffraction
          > pattern that Thomas Young observed over a hundred years before.
          >
          > Best, Dan.
          >
          > cemturgay wrote:
          >
          > >I recently heard that, the double slit experiment
          > >with single electrons or photons was never been made actually
          > >because of technical difficulties.
          > >What always been made was using ray of (multiple)
          > >electrons or photons.
          > >And, when scientists talk about double slit experiments
          > >and say "even a single electron passes thru both slits"
          > >is just a thought experiment based on calculated assumptions
          > >in QM theory, referring to some other experiments
          > >done about 70-80 years ago.
          > >
          > >Is this true? Or up to what part it is true?
          > >
          > >Or are there recent experiments, where single electrons/photons
          > >send thru double slits, one after another
          > >and have an acceptable interference pattern on the other side?
          > >
          > >Cem
          > >
          > >
          > >
          > >To unsubscribe from this group, send an email to:
          > >bell_bohm-unsubscribe@e...
          > >
          > >
          > >
          > >Your use of Yahoo! Groups is subject to
          http://docs.yahoo.com/info/terms/
          > >
          > >
          > >
        • Charles
          ... The location that a single particle is observed behind the double slit cannot indicate that this single particle could have gone through both slits by
          Message 4 of 8 , Nov 5, 2002
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            --- In bell_bohm@y..., "cemturgay" <cem.turgay@u...> wrote:
            > I recently heard that, the double slit experiment
            > with single electrons or photons was never been made actually
            > because of technical difficulties.
            > What always been made was using ray of (multiple)
            > electrons or photons.
            > And, when scientists talk about double slit experiments
            > and say "even a single electron passes thru both slits"
            > is just a thought experiment based on calculated assumptions
            > in QM theory, referring to some other experiments
            > done about 70-80 years ago.
            >
            > Is this true? Or up to what part it is true?
            >
            > Or are there recent experiments, where single electrons/photons
            > send thru double slits, one after another
            > and have an acceptable interference pattern on the other side?
            >
            > Cem

            The location that a single particle is observed behind the double
            slit cannot indicate that this single particle could have gone
            through both slits by 'interfering with itself', and thus the
            experiment cannot be conducted with just one particle. So, the idea
            behind the experiment is intrinsically one of the statistical
            behavior of many particles passing through one or the other slit and
            being observed with their locations behind the screen correlated. It
            turns out that the pattern of landings behind the slits is as if the
            particles, having apparently traversed independently through the
            double slit apparatus, at distinct times, did indeed interfere with
            each other.

            Weird,

            Charles
          • Jeff L Jones
            ... Sure it can... just keep recording where single particles land on the screen over time, and build a statistical graph from that. They can t be interfering
            Message 5 of 8 , Nov 8, 2002
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              On Tue, Nov 05, 2002 at 11:46:42PM -0000, Charles wrote:
              > The location that a single particle is observed behind the double
              > slit cannot indicate that this single particle could have gone
              > through both slits by 'interfering with itself', and thus the
              > experiment cannot be conducted with just one particle. So, the idea

              Sure it can... just keep recording where single particles land
              on the screen over time, and build a statistical graph from
              that. They can't be interfering with each *other* if they don't
              go through at the same time.

              Jeff
            • manyworlds
              ... Weird, indeed... ... On contrary - all the evidences show they can! It seems that we people are still tip-toping around the same place, here: If
              Message 6 of 8 , Nov 10, 2002
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                On Nov 08, 2002 at 12:44, Charles wrote:
                >The location that a single particle is observed behind the double
                >slit cannot indicate that this single particle could have gone
                >through both slits by 'interfering with itself', and thus the
                >experiment cannot be conducted with just one particle. So, the idea
                >behind the experiment is intrinsically one of the statistical
                >behavior of many particles passing through one or the other slit and
                >being observed with their locations behind the screen correlated. It
                >turns out that the pattern of landings behind the slits is as if the
                >particles, having apparently traversed independently through the
                >double slit apparatus, at distinct times, did indeed interfere with
                >each other.
                >
                >Weird,
                >
                >Charles

                Weird, indeed...

                On Nov 08, 2002 at 12:44, Jeff L.Jones wrote::
                >Sure it can... just keep recording where single particles land
                >on the screen over time, and build a statistical graph from
                >that. They can't be interfering with each *other* if they don't
                >go through at the same time.
                >
                >Jeff

                On contrary - all the evidences show they can!
                It seems that we people are still tip-toping around the same place, here:
                If correlation is possible then only in space - we are stating readily,
                when referring to foton interference.
                And, meantime, the possible solution demands new revolutionary ideas.
                One such fresh idea, I personally not remember I could met before,
                seems to be contained within the Charle's suggestion:

                >(...)particles, having apparently traversed independently through the
                >double slit apparatus, at distinct times, did indeed interfere with
                >each other.

                Let us see if it occurs worthy of remembrance to wonder a little about this
                idea:
                What would be the physical sense of non-zero correlation-in-time, here?
                (By the way, we should speak about convolution rather then the correlation,
                here, I suppose. In any case, I will refer further to this more common
                colloquialism.)

                The only possible logical conclusion seems to be:
                Nothing more or less then an existence of some interaction between photons.
                But fotons as bosons do not interact one with another - seemingly the
                another
                wall... And maybe just another prejudice, only?

                Because: What else if not the simple result of interaction under peculiar
                phase in space conditions (i.e. refering to correlation in time)
                the interference true meaning is?

                And if so: In principle it should be possible to determine the character
                of such an interaction by determining the dependence of time correlation
                on the time intervals between each pair of fotons incoming one after
                another.
                It does mean that an experimental verification is possible here.
                We simply should try to measure not only the possibility of interference
                of single photons sequentially travelling through double slit layout in some
                time intervals,
                but the degree of such an interference in dependence of the length of these
                time intervals.
                As I know, up to now, no such wide reaserches were conducted, yet.
                (Dimly I remember description of some fresh experiment of this kind -
                depicted somewhere in the net;
                In any case I will try to find the adress in the pile of my archive CDs, to
                address Cem's inquiry and
                to refresh my memory in respect to that possible development of the proponed
                extended experiment;
                I will know you all if I find out anything, of course.).

                And possibly [more speculative remar, f only we are able to indicate the
                degree of
                speculative character of all presented here remarks]: To achieve this,
                fotons interfering in such a way should exchange some travelling in time
                particles (moving in the weirde sense dt/dx,
                it is, in some sense, not(-only) in space but (most possibly - exclusively)
                in time).
                (In such a case we should wonder about new designum for best suitable signum
                tachion, here - if only I have proper intuition of the one the tachions have
                now; and better understanding of space-time designum in general.)
                It would mean fotons feel the pattern of their tachion-in-new-sense fields
                spreading in time and *react* as well mutually as onto themselves in
                accordance to this pattern in dependence of the place they actually are.

                If the alternative would be some kind of unknown interaction (field
                spreading in space)
                then the interference between separated in time photons would not be
                possible, indeed.
                What would be left would be the auto-interference. And the only known to me
                explanation
                of this very upset fact seems to be delivered by MWI.

                So, in conclusion, we have two possible alternatives for explanation of
                (auto-)interference :
                I) Interaction in time (only?) giving rise to time correlation (convolution)
                of photons interfering in time.
                II) Interaction above the time and beyond the space pointing directly at the
                validity of MWI.

                Zbig

                PS.: As this is my first active post to the bell_bohm group, I should
                introduce myself.
                But in place of giving not interesting details concerning me personally,
                I prefer to give the address of my page, where I had archivised a vital for
                me part
                of the discussion at another yahoo forum I have been more active some time
                ago (I mean FoR):

                http://manyworlds.w.interia.pl/mw/html/forumen.html

                This deals, among other things, with some aspects of double slit experiment.
              • manyworlds
                Discussion of thought experiment with separate electrons and a nice Java applet may be find at :
                Message 7 of 8 , Nov 12, 2002
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                  Discussion of thought experiment with separate electrons and a nice Java
                  applet may be find at :
                  http://www.colorado.edu/physics/2000/schroedinger/two-slit3.html
                  http://www.colorado.edu/physics/2000/schroedinger/electron_interference.html
                  *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:
                  http://physicsweb.org/article/news/3/7/11/1
                  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
                  gate.*

                  http://physicsweb.org/article/news/03/12/14
                  *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
                  http://www.quantum.univie.ac.at/research/c60/index.html
                  *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
                  divergence.
                  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
                  grating)
                  (25 counts/s + 2 x 13 counts/s)
                  Is this enough to speak about C60 balls parade through the slits of the
                  grating?
                  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
                  C70:
                  http://www.quantum.univie.ac.at/research/stehwelle/standinglightwave.html
                  *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
                  Technology
                  operated atom interferometer of interest here:
                  http://rleweb.mit.edu/rlestaff/p-prit.htm
                  *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:
                  http://physicsweb.org/article/news/4/5/5/1
                  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,
                  http://www.stl7.ps.pl/
                  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
                  generation
                  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.

                  Cheers,

                  Zbig
                • 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 8 of 8 , Nov 13, 2002
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                    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:
                    http://physicsweb.org/article/news/2/8/14/1

                    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. (...)*
                    More:
                    http://physicsweb.org/article/world/11/3/9/1

                    Zbig




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