top of page

The Mystery of Quantum Entanglement

By Vijay Damodharan - Natural Sciences Student @ Christ College, Cambridge


The Young’s Double Slit experiment is not only famous but has also been extremely important in proving that particles have wave-like properties. Over the years, many variations to the double-slit experiment have been designed to further illustrate some of these properties or discover completely new ones. One such variation is called The Quantum Eraser Experiment, and it demonstrates a key feature of Quantum Mechanics – Quantum Entanglement.

Before we delve into details, it is first worth mentioning what the difference between particles and waves is from a physics point of view. The main difference is that particles such as electrons are objects that (are supposed to) exist at a particular point in space. They are localised. However, waves, such as water waves, can exist over a large area simultaneously. They are not localised.

When we say particles have wavelike properties, it means that they sometimes behave as if they exist in a localised point in space, and at other times behave as if they exist in many points at the same time. The discovery of this contradictory behaviour was one of the preludes to quantum mechanics.

In a standard double-slit experiment, waves are incident on a plate with two small slits and pass through them simultaneously. The waves from the two slits ‘interfere’ such that the amplitude of the waves becomes very large at some points, and disappears at others. When observed on a screen, this corresponds to points of very high brightness, and points that are completely dark, as shown in the last image of Figure 1.

The same experiment can be done by passing particles such as electrons or photons through the slits one at a time. Over time, a similar interference p