Author: Haroon Khalil
-
AN ENTANGLED-PHOTON SOURCE
Before we go any further, we want to warn you that many of the experiments described are out of the budget of many enthusiasts. This is because applications involving quantum entanglement are in the early stages of development, so the specialized crystals and detectors are not yet mass-produced. However, we feel that it is worthwhile…
-
BELL’S INEQUALITIES
The debate between Einstein and Bohr continued for decades regarding what “reality” meant in the context of quantum mechanics. Einstein and his followers insisted that an objective reality exists whether it is observed or not. Their most powerful argument was explained in the EPR paper, in which Einstein and his colleagues proposed that “elements of…
-
Introduction
The quantum eraser experiment, which seems to indicate that the photon “knows” when we—the observers*—are watching. Indeed, the photon behaves very differently when we—the observers—can say (at least in principle) which path it has traveled. The thought that an objective reality does not exist independently of an observer troubled Einstein very much. In opposition to…
-
THE QUANTUM ERASER
Let’s now see how we would actually go about determining the leg of the interferometer taken by a photon. Let’s insert a third polarizer between the output of beam splitter 2 and the ground glass screen, as shown in Figure 134b. This polarizer is the equivalent of the analyzing polarizer of Figure 121. At an angle of…
-
“WHICH-WAY” EXPERIMENTS
Just as with the two-slit experiments, the Mach–Zehnder interferometer builds up an interference pattern even when shooting photons one at a time. Remember that we also learned that Tonomura was able to do the same thing using single electrons, and more recently the team at the University of Vienna demonstrated two-slit interference using a collimated…
-
THE MACH-ZEHNDER INTERFEROMETER
Let’s now complicate the setup just a tiny bit, as shown in Figure 129. Here, photons are split just as before by a first beam splitter into T and R photons. Each of these are then reflected 90° so they will meet at a second beam splitter. Trace the possible paths of the photons and you…
-
WHO ROLLS THE DICE?
Beam splitters can be classified according to the mechanism used to split the incident light beam. As shown in Figure 127a, some beam splitters are made by depositing reflective elements over a transparent substrate. The reflective elements could be fully reflective mirrors arranged in a polka-dot pattern or fine metallic particles dispersed in a random manner.…
-
BEAM SPLITTERS
Another way in which physicists commonly prepare quantum states is by using partial reflectors as a way of placing a photon in a superposition of states at two different positions. Look out at night through any window in your house and you are essentially looking through a beam splitter. This is because you can see light…
-
SCHRÖDINGER’S CAT IN THE LAB
It is very important to remember that Schrödinger’s cat thought experiment was proposed as an absurd extrapolation of the Copenhagen Interpretation. Although we are ignorant about the boundary between quantum and classical systems, quantum physics has very little to do with cats or any other macroscopic system. For all of the reasons that we have…
-
MANY-WORLDS INTERPRETATION
The leading alternative interpretation was developed in 1957 by American physicist Hugh Everett, who proposed that the universe splits every time there’s an event with more than one possible outcome. Each different universe evolves with one of the possibilities realized. As shown in Figure 120, the bizarre, but logically consistent Many-Worlds Theory interprets the Schrödinger cat thought experiment…