What We Hear in This Lecture Wave-Particle Duality Double-slit experiment Delayed Choice Experiment Quantum Eraser
Wave-Particle Duality Antiquity (Egypt, Greece): particles towards or from the eye (Epicure, Aristotle, Euclid) from the eye (Epicure, Aristotle, Euclid) XVIIth cent.: Waves (as “riddles on water”) Huyghens Newton (Opticks, 1702): particles (of various colours)
XIXth cent. The triumph of waves Young, Fresnel (1822): interference, diffraction, polarisation: light is a transverse wave Maxwell (1870): light is an electromagnetic wave
Early XXth: Photons (particles come back) Einstein (1905). Light made of quanta, elementary grains of energy and momentum (named “photons” in 1926 only). Quantitative predictions for the photoelectric effect E = hν p = hν / c How to reconcile the particle description with typical wave phenomenon of diffraction, interference, polarisation? Particle or wave? Ideas not accepted until Millikan’s experiments on photoelectric effect (1915). Nobel award to Einstein (1922) for the photoelectric effect Compton’s experiments (1923): momentum of photon in the X ray domain
Wave particle duality Light is both waves (capable to interfere) and an ensemble of particles with defined energy and momentum… Similarly particles such as electrons behave like a wave (diffraction, interference) Einstein 1909 Louis de Broglie 1923
Delayed Choice Experiment Proposed by John Archibald Wheeler in 1978 Proposed by John Archibald Wheeler in 1978 The method of detection in this experiment can be changed after the photon passes the double slit, so it is possible to delay the choice of whether to detect the path of the particle, or detect its interference with itself The method of detection in this experiment can be changed after the photon passes the double slit, so it is possible to delay the choice of whether to detect the path of the particle, or detect its interference with itself Father of black hole
This experiment is done with a single photon Time interval between of two successive emission is long enough In this case each photon is detected with one of these detectors. So photon behaves like a particle Semi -transparent mirror The arrangement is set in such a way that this part is destructive The arrangement is set in such a way that this part is constructive Now photon behaves like a wave It seems that causality is reversed!!? Maalool taghaddom bar ellat darad!!?????????
An implementation of the experiment in 2007 showed that the act of observation ultimately decides whether the photon will behave as a particle or wave. An implementation of the experiment in 2007 showed that the act of observation ultimately decides whether the photon will behave as a particle or wave. Cho, Adrian. After a Short Delay, Quantum Mechanics Becomes Even Weirder. ScienceNOW Daily News. 16 February 2007
Quantum Eraser Experiment The Which-way information that once established can be erased. The Which-way information that once established can be erased. The idea was first proposed by Marlan O. Scully and K. Druhl.(1982) The idea was first proposed by Marlan O. Scully and K. Druhl.(1982) M. O. Scully and K. Druhl, Phys. Rev. A, 25,2208 (1982)
Marlan O. Scully 1963: Graduates from Yale University with M.S. in Physics 1966: Graduates from Yale University with Ph.D. in Physics 1969: Professor of Physics and Optical Sciences, University of Arizona, through 1980 1980: Distinguished Professor of Physics, University of New Mexico, through 1992 1992: Professor of Physics, Texas A&M University, through 1996 2004: Professor of Chemical Engineering, Texas A&M University, through present 2005: Professor of Mechanical and Aerospace Engineering and Materials Science, Princeton University, through present
These photons are correlated in that they are created simultaneously, and they both have the same polarization (Here vertical to the plane of the experiment).
Assume that the photons have the same frequency and they are indistinguishable, so: Intensities at the two detectors are identical, and are proportional to
The which-way effect can be seen when the upper mirror is removed. In that case there is no reflected s photon associated with the reflected i photon, which allows us to distinguish between the two i photons. Analogous to knowing which of the two slits a photon passes through. (good start!!?)
Plate that rotates the polarization from V to H No interference Because, we have a which-way situation Distinguishing the reflected i photon then distinguishes the reflected s photon, since these are all produced in pairs.
By expressing the photon state in terms of states rotated through ±45 degree, We have:
Plate that rotates the polarization from V to H By putting a 45 degree filter in front of the upper detector, We have, So the which-way information is erased & We have fringes!!?