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 pattern would develop after enough particles have passed through as shown in Figure 1. This suggests that the particles somehow pass through both slits simultaneously, just like waves.

Four squares labelled b, c, d, e showing the build up of electron diffraction pattern. The first square is black with a few white spots, and this builds up until picture e represents 5 vertical lines built up of many white dots.
Figure 1: Build up of electron diffraction pattern.

The catch, however, is that if you place a detector in front of one of the slits which told you whether the particle passed through that slit or not, the interference pattern disappears. We instead get just two bright spots on the screen, corresponding to the particles exiting through the two slits.

In other words, if we know the information about which particle goes through which slit, the interference pattern goes away, otherwise, it exists. One explanation for this behaviour is that the observation somehow messes with the particle and affects it in an irreversible way, hence causing a change in its behaviour.

So, can we design an experiment that can observe which slit the particle passes through without affecting the particle itself?

That is exactly what we try to do in the Quantum Eraser experiment. We place a block of a special material in front of the two slits so the photons (particles of light) exiting from the slits will have to pass through the block. The block will absorb the incoming photon and emit two different photons which travel in two different directions. We will call them photon x and photon y. A simplified, schematic diagram of the setup is shown in Figure 2.

Figure 2: A schematic diagram of the quantum eraser experimental setup.

At the end of photon x’s path is a screen that is used to develop an interference pattern. At the end of photon y’s path is a set of detectors that tell you which slit the original photon passed through. One would naturally believe that detecting photon y would have no effect on photon x’s behaviour. Wrong! Even though they are two different photons, as soon as we use the detectors on photon y to deduce which slit the original photon passed through, the interference pattern produced by photon x still disappears! If we turn the detectors off, the interference pattern returns.

Theoretically, the two photons produced by the block are what are known as quantum entangled. This means that some of the properties of the two photons are intricately linked in an inseparable way. Before detection, the state of a quantum entangled pair is in a superposition of all possible states, but measuring any one of the pair will cause the states of both to collapse into one of the possible states, regardless of their separation in space.

In this case, the two photons could have come from the first slit or the second one. Until we make a measurement, the photons behave as if they are in a superposition of both possible states (i.e. as if they’ve come from both slits, like a wave). But if we know photon y passed through one of the slits, we immediately know that photon x did as well. This then changes photon x’s behaviour from that of a wave into that of a particle, even though we only measured photon y and there is no interaction between the photons! It is what Einstein famously called ‘spooky action at a distance.’

So, what if we instead detect photon y after photon x already hits the screen? Then measuring photon y should not affect photon x, right? This is what the Delayed Choice Quantum Eraser Experiment does, however, the same result occurs!

This seems to suggest a reversal of cause and effect at first glance, however, there are possible relativistic and even classical explanations to this result that does not involve retrocausality.

However, it's safe to say that these experiments highlight the non-intuitive nature of quantum particles.

Further Reading:

1. Paul Sutter. 'What is Quantum Entanglement?' Live Science. Online. 26/05/21.

2. Wikipedia contributors. "Delayed-choice quantum eraser." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopaedia, 13 Mar. 2022. Web.

3. Jennifer Harrison. 'The Delayed Choice Quantum Eraser Experiment Does Not Rewrite The Past.' Medium. Online. 01/09/21.