quantum physics


New experiment on quantum erasure for advanced physics lab developed

A signature experiment highlighting the wave-particle duality, uncertainty, delayed-choice and quantum computing is developed at the Physlab. It will introduce students to the concepts using classical light from a laser. For more details visit the experiment website here.

By Azeem Iqbal | News

Mach-Zehnder Interferometry and Erasure of ‘Which-path’ Information

Student Manual

One of the most astounding and bewildering depictions of quantum mechanics is the single- particle interference experiment. If a source that emits only one particle (such as photons or electrons) at a time, is used in a classic double slit experiment, an interference patterns will emerge. This pattern is built up over multiple passes of identical particles. Interestingly, the pattern could be switched on and off even after light has emerged from the interferometer.

The Mach-Zehnder interferometer is a useful device for exploiting the wave nature of light and observing various aspects of interference. The goal of this experiment is to familiarize students with this kind of an interferometer. We will observe interference of light, and see conditions under which interference can be washed out, and ultimately recovered.

Sample ResultsErasure and recovery of which-path information inside a Mach-Zehnder interferometer
Variation of intensity pattern with the post-interferometer polarizer
Variation of optical intensity with the relative angle between the polarizers placed inside the arms of the interferometer
Fringes change their visibility as polarizers between the arms of the interferometer change their relative angles
Experiment Code2.21

Further Readings and References

Pictorial Procedure

IMG_2499 (Medium)IMG_2501 (Medium)

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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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Email: physlab.org@gmail.com Telephone: 111-11-LUMS (5867) Address: Lahore University of Management Sciences (LUMS), Sector U, DHA, Lahore Cantt. 54792, Pakistan