modern physics

Coincidence detection circuits for photon correlations

Coincidence detection is the simultaneous detection of two or more photons in different detectors. The electronics associated with coincidence-counting tend to be very costly. The aim of this project was to build a small and an affordable circuit based on fast logical AND Gates. The circuit includes a pulse compressor which, depending upon the setting of the multiplexer, can either shorten the input pulse width or let it pass through unchanged. The different available widths that can be obtained are as following:

  • 10 ± 0.2ns
  • 12.8 ± 0.2ns
  • 14.4 ± 0.2ns
  • Original Width  

After the desirable pulse width has been achieved, the output of the pulse compressor is input into a fast (SN74F08) AND gate from which twofold and threefold instances are measured. The circuit comprises of 4 such channels (A, B, C, D) and their complements. The coincidences available are as follows: AB, BC, AC, CD, BD, ABC, ABC.

The project, which was part of Ramish’s independent study was supervised by Dr. Sabieh Anwar and the circuit was built by Mr. Shafique, R.A. spin physics group.

 

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Determining the Planck’s constant

Omair1An interesting use of the ubiquitous light bulb is its utility in determining a fundamental constant of quantum mechanics – namely the Planck constant. The purpose of Omair qazi’s experiment was to measure Planck’s constant through the observation of black body radiation. Spectral analysis of the light was carried out verifying Planck’s radiation laws, all using filtered light from a bulb, a spectrometer and photodetectors.

By Azeem Iqbal | Research Stories
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Franck Hertz Experiment seen in a new light

The main fFhertzocus of this project was to come up with a new idea regarding the famous Frank-Hertz experiment. The experiment started with observing the well known Franck-Hertz pattern on an oscilloscope. Then the temperature dependence of the curve was demonstrated. We then sought to determine the dependence of he magnetic field on the curve. In order to describe these observations, a mathematical model is being developed.

 

By Azeem Iqbal | Physics Studio
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Magic eye

A magic eye is a simplistic triode tube that was used as a tuning indicator in radio receivers around the time of world-war II. It has a bowl shaped anode coated with a phosphorescent material that produces a green glow when electrons strike on it. This makes possible to measure the curvature of electrons’ trajectory when the tube is placed in a magnetic field. By measuring the radius of curvature, the applied voltage between cathode and anode and the magnetic field, charge-to-mass ratio of the electron can be measured.

By Azeem Iqbal | Modern Physics
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Electron diffraction

This interesting demonstration utilizes a highly evacuated electron diffraction tube to show the wave behavior of electrons. The electrons are emitted by the thermionic emission and accelerated towards target by applying a very high potential (2000-5000 V). The target is a micro meshed nickel grid on which a thin layer of graphite is deposited. The electrons being diffracted through the graphite satisfy the Bragg’s condition and produce an interference pattern consisting of two rings.

By Azeem Iqbal | Modern Physics . Solid State Physics
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Michelson Interfrometer

This demonstration uses the famous Michelson interferometer which is used either for precise distance measurements or the wavelength of the laser. An assembly of optical components including HeNe laser, mirrors and converging lenses is used. The interference pattern is produced by splitting the beam into two paths using a 50:50 beam splitter. The movable mirror is motor controlled and computer interfaced, a source of changing the path length and produce a interference fringes.

By Azeem Iqbal | Modern Physics . Optics
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Statistics of natural radioactivity

The classroom demonstrations uses a gamma ray source of Co-60 placed inside a lead container. The radiation is detected with a Geiger muller tube, whose data is brought into the computer. A histogram is built up showing a Poisson distribution. The distribution of decay times is readily observable and the statistical nature of the phenomenon could be explained.

By Azeem Iqbal | Modern Physics
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Optical tunneling

Optical tunneling is demonstrated by frustrated total internal reflection.

  • A green laser diode (from a laser pointer) falls on a glass prism. The angle of incidence is greater than a critical angle, resulting in light being totally internal reflection.
  • There is evanescent light leaking from the prism. But this light is difficult to tap off, unless a second prism is placed really close to the first one.
    the second prism is almost conjoined with the first prism using an index matching fluid (Cargille’s BK 7 Matching Liquid).
  • The fluid is able to mate the prisms really well, yet it keeps a small gap between the prisms. Light can tunnel across this classically forbidden region and transmit into the second prism. Total internal reflection has been frustrated.
By Azeem Iqbal | Modern Physics . Quantum Physics
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