The Quantum Eraser and the Delayed Choice

veer vishal dubey
7 min readApr 24, 2021

This article is a part of a series of articles on time. This is the final part, part 5/5. Check out the previous article here!

Let us continue our journey of Time through the Quantum field. In the last article, we have seen what past is in the Quantum field. Let us now turn to more intriguing questions- Can you Erase the past? Can you say that the future determines the past? If yes, why and how do these things happen? LEts find out.

The Quantum Eraser

Remember the double slit experiment? The one where you got an interference pattern? The Quantum Eraser experiment, first suggested by Martin Scully and Kai Drühl in 1982, uses such an apparatus, but with slight modifications.

The Original Double Slit

Drühl and Scully, modified the apparatus by placing a marker before and after the slits, (the mechanism isn’t of importance, but if you want to know, it basically influences the spin of the particle going through the left slit about a particular axis to one direction, and the particle going through the right slit’s spin in the opposite direction about the same axis.) They then proceeded with this incomplete apparatus, and found the usual double slit pattern on the screen. There was no interference. Now, they placed a Quantum Eraser just before the screen, (again the mechanism is not of importance, but if you want to know, the eraser just erases the mark, or randomises the spin of the particles, so you don’t know which particle went through which slit or influences the spin of all particles to be the same about the same axis.) and proceeded with the experiment. This time, they did find an interference pattern in all the trials. Amazed by the results, they suggested this:- What if you place the eraser closer to the screen, so close that it nearly touches it, and do the experiment, but, right before the Photon(in their case, you can use whatever particle you like) touches the detector screen, what if you erase the which path information(or the information about the route the particle took)? What if you extract the which path information from a detected particle, will all the histories(check that out here) come back into play, causing rememrgence of the interference pattern? That is why the screen and the eraser were placed so close to each other. But unfortunately, technology required to keep the eraser not influencing the detector was not available at the time.

Scully and Drühl’s quantum eraser experiment

In an experiment carried out later by Raymond Chiao, Paul Kwiat and Aerphraim Steinberg, the same apparatus was used. Since the eraser was placed right in front of the screen, subsequent examination of the spin yields no information on the slit the particle passed through. Therefore, the “which-path” information has been erased. Remarkably, the erasure does force the interference pattern to reappear! This, could mean, if the light source was a billion light years away, and if we place the at 500 million light years, and the eraser just on the surface of the earth, we are erasing past that happened millions of years ago, even before the first creatures appeared on the planet. How do we accept it? Keep in mind Drühl and Scully knew this would happen, because their knowledge of quantum physics and mechanical mathematics confirmed it would work. As usual case in Quantum Mechanics, it does not pit theory against experiment, it pits theory, confirmed by experiment, against our sense of reality.

The Delayed Choice

Another interesting experiment, is the Delayed choice experiment. Suggested by John Wheeler in 1980. (Wheeler was the teacher of Richard Feynman!) This, just like the quantum eraser, modifies another apparatus, this time of the beam splitter experiment. The apparatus is given below. If you’re wondering how does one split a beam, a beam splitter is a half silvered mirror, which allows half the light to pass through, and reflects half the light.

The original beam splitter experiment

The changes Wheeler made were interesting to say the least. He adjusted the laser to fire one photon at a time, and put a photon detector in one of the paths, so that you can see which path the photon took, if a photon is detected by the detector, it took that path, if the detector doesn’t show a reading, it took the other path. Now, since the detector provides “which path” information to us, it destroys the photon’s wavelike properties, and show it to be a particle, therefore destroying the interference pattern. If the detector is turned off, the “which path” information is not extracted, therefore the interference re-emerges. Seeing this, Wheeler asked:- What if I increase the distance between the photon detector and the beam splitter, and thus forcing the photon to act like a particle, but after the photon has passed through the beam splitter, I turn off the detector? In other words, What if I change the future of the particle, after it has made a fundamental decision, and influence the past by influencing the future? Or, does the past depend on the future?

The Delayed Choice Experiment, where O is the photon detector.

This is what happens in the delayed choice experiment. If we do the experiment so that the photon detector is on, we get the usual pattern. But, if we keep the detector on when the photon passes the splitter, and then turn it off, the interference comes back! This is extraordinary, as the particle made the right decision at the beam splitter, even though they were uncertain about their future! To complicate things further, in Wheeler’s experiment, the light source is a Quasar (it is basically a supermassive black hole, at the center of a galaxy, that is surrounded by a massive disc of gas and gives off light as a result.), and an intervening galaxy is the beam splitter, it’s gravitational field acting as a lens to bend the light two ways, and detectors, super massive ones, here at earth, above each screen. If we turn the detectors off, the interference emerges, if we switch it on, it disappears, and if we switch it off again, it re-emerges. This is staggering, since the origin of the photon is billions of light years away. The photon has been travelling at the speed of light from a time before the detector, or the earth, or even the galaxy we live in came to be. Somehow, it predicted, that from an infinite number of probabilities, the photons would end up here. The future, does influence the past, as has been confirmed theoretically, mathematically and experimentally!

The Delayed Choice Quantum Eraser

This part may confuse some reader. If you are confused, then skip to the last paragraph, but I suggest you read it, since the results are amazing.

The combination of the previous two experiments, The Delayed Choice Quantum Eraser experiment was also suggest by Scully and Drühl. It begins with the original beam splitter experiment, modified by inserting down converters. A down converter is device that takes in one photon, and produces two photons with half the total energy, one travels along the path the original photon would take (the signal photon) and the other one is directed to another path all together,(the idler photon). Use the figure below to advance further.

The delayed choice quantum eraser

Now if you look at the figure, Two beams, are repeatedly split by the splitter. A beam has equal probabilities to go through the Lens, Beam Splitter a and beam Splitter b. Then, each photon has many other chances to go through each down converter, and get deviated, and get bounced off into the detectors at the coincidence counter. But, the “which path” information, is only available for unique photons which took the paths of D0, D3, and D4. But we don’t have the “which path” for the paths through D2 and D1. So the question they asked is- Does erasure of “which path” information, even though no signal photons have been touched, mean that the interference can be recovered? It does, but it is complicated. If we look at the counter, we see no interference pattern. But, if we look at the subset of the data in detectors D1 and D2, we do get an interference pattern! And nothing we have done in this experiment entails distance. Nothing in our experiment is dependent on whether an idler is detected after or before or at the same time as it’s counterpart. If they are 50 light years away, and you record the signal photons one by one, you won’t see the interference. But, 50 years later, when all the idler photons arrive, and you see which paths they went through, and highlight all the photons that went through D1 and D2, you get an interference pattern!

As this experiments, theory and story forcefully champions, the explanation to the signal photon data depends significantly on the idler observation you record 50 years later. Let me emphasize that future measurements do not change anything you observe today, but they can influence the kinds of details you have to tell to explain today’s story. We thus see the that the future helps to shape the story you tell of the past.

This is it for the series. Thank You for reading, and I leave you with a hint of what my next series is going to be about (coming up on 27/04/21; there will be a one off article somewhere in between):- Symmetry and Cosmology. Thank you once again and see you soon!

Edit:- Thank you to Quantum Wormholes to help explain something that was missed out in this article.
Check them out here-