Delayed Choice Quantum Eraser Experiment

 BY: Arjit Mishra

It is widely said that "efforts in the present decides our future" but what if I say actions in future affects past events! Yes you heard it right. As we aren't able to travel back in time ; it's hard to digest the fact.

But with the help of an experiment in Quantum physics it is possible!(At Quantum level). this experiment is entitled as Delayed Choice Quantum Eraser Experiment, It is basically an extension of Double Slit Experiment.

[NOTE: If you haven't read my double slit experiment blog, visit:https://theoarmi.blogspot.com/2020/05/double-slit-experiment.html otherwise you wouldn't get the concept behind the experiment]

We can conclude from double slit experiment that electrons show particle nature in the presence of an observer machine and show wave nature in the absence of same; Which indicates electrons somehow know whether they are being observed or not so that they can act accordingly.

That's Scary right! but there's a scientific explanation to the fact that doesn't completely but somehow justify this weird behaviour. Physicists say that there is a fuzzy space between creation and detection where any type of well defined particle doesn't exist at all. Copenhagen interpretation tell us that in this space a particle exist in the form of wave function[it is basically a distribution of all possible properties of a particle], this Probability is wave only which shows wave nature and form interference pattern but when an observer is placed the wave function collapse and the wave converts into particle which shows particle nature and form just two lines on the back wall.

 In double slit experiment we placed an observer machine to detect which electron passes from which of the two slits but we failed in doing so thus Physicists executed Delayed Choice Quantum Eraser Experiment.

Before we go through delayed choice quantum eraser experiment we should first understand wheeler's Delayed Choice Experiment.

In Double Slit Experiment we kept the observer machine before the slits but in wheeler's Delayed Choice Experiment it is kept near the wall[on which patterns appear] and tried to observe the electrons exactly before when they collide so that they can't alter their behaviour accordingly. But the observations were shocking and weird, when electrons were observed they behave as particle and only for two lines ensuring particle nature and when they weren't observed they show wave nature and form interference pattern. Since the observer was observing the electrons just microsecs. before collision; this can be true only if electrons time back in time and decide their behaviour depending upon observance and non-observance. This experiment has again proved that Quantum Physics is weird.

Now, we shall see Delayed Choice Quantum Eraser Experiment:

In this experiment a Non-Linear optical crystal is placed behind the double slit which create entangled pair of electron passing through slit A and slit B moreover one particle of the entangled pair is allowed to move towards the backwall while other one is send towards the detector D3 and D4 in order to predict that electron has passed through which slit(either A or B).

[NOTE: If you don't know about entangled pairs, you should read my blog on Quantum entanglement and EPR Paradox, visit:https://theoarmi.blogspot.com/2020/08/quantum-entanglement-and-epr-paradox.html]

The Entangled Particles are entitled as "twin brothers". whatever happens with one, it also happens with other. If electron's second entangled partner is detected near D4 then electron must have passed through slit B but if second entangled partner is detected near D3 then electron must have passed through slit A. These Detectors are positioned in such a way that the second entangled particle should reach them after the collision of first one on the back wall. Don't be perplexed at all; the big picture is that we can accurately predict electron has passed from which of the slit(either A or B). But in this experiment electrons only show particle nature and form just two lines on the backwall. It seems like second entangled partner of electron which is detected(either by D3 or D4) send information to first one that observer knows the passage of electron and thus ensure particle nature.

Later on some additions were made in the experiment; Half silvered mirrors were placed in the paths of detectors which ensure 50 percent chance of reflection and 50 percent chance of transmission. After that a Quantum Eraser was used which means now electrons can be detected near detectors D1 and D2 too. when electrons were detected near D1 and D2 then there was no possibility to predict the passage of electron(which means we don't know electron has passed from which of the slits ). Now, in this case electrons show wave nature and form interference pattern on the back wall. It seems like second entangled partner of electron which is detected(either  by D1 or  D2) send information to the first one that observer doesn't know the passage of  electron thus ensure wave nature.

This is very weird right! but the weirdest thing is that when second entangled partner send information to the first one, till that time first entangled partner has already collided with the wall. Inspite of  this the alterations in the patterns on the back wall due to observance and non-observance indicates that electrons travel back in time and decide their behaviour accordingly.

Yeah, Quantum Physics is really very weird but nevermind; In future we might get our answers.

"QUANTUM PHYSICS MAKES ABSOLUTELY NO SENSE"