The Delayed Choice Quantum Eraser, Debunked

A lot of you have asked me to do a video  about the delayed choice quantum eraser,

an experiment that supposedly rewrites the  past. I haven’t done that simply because

there are already lots of videos about  it, for example Matt from PBS Space-time,

the always amazing Joe Scott, and  recently also Don Lincoln from Fermilab.

And how many videos do you really need about the  same thing if that thing isn’t a kitten in a box.

However, having watched all those gentlemen’s  videos about quantum erasing, I think they’re all

wrong. The quantum eraser isn’t remotely as weird  as you think, doesn’t actually erase anything,

and certainly doesn’t rewrite the past.  And that’s what we’ll talk about today.

Let’s start with a puzzle that has  nothing to do with quantum mechanics.

Peter is forty-six years old and he’s captain of a  container ship. He ships goods between two places

that are 100 kilometers apart, let’s call them  A and B. He starts his round trip at A with the

ship only half full. Three-quarters of the way to  B he adds more containers to fill the ship, which

slows him down by a factor of two. On the return  trip, his ship is empty. How old is the captain?

If you don’t know the answer, let’s  rewind this question to the beginning.

Peter is forty-six years old and he’s captain  of a container ship. The answer’s right there.

Everything I told you after that was completely  unnecessary and just there to confuse you.

The quantum eraser is a puzzle just like this.

The quantum eraser is an experiment  that combines two quantum effects,

interference and entanglement. Interference  of quantum particles can itself be tested by

the double slit experiment. For the  double slit experiment you shoot a

coherent beam of particles at a plate with  two thin openings, that’s the double slit.

On the screen behind it, you then observe several  lines, usually five or seven, but not two.

This is an interference pattern created by  overlapping waves. When a crest meets a trough,

the waves cancel and that makes a dark  spot on the screen. When crest meets crest

they add up and that makes a bright spot. The amazing thing about the double slit is

that you get this pattern even if you let only  one particle at a time pass through the slits.

This means that even single particles act like  waves. We therefore describe quantum particles

with a wave-function, usually denoted psi. The  interesting thing about the double-slit experiment

is that if you measure which slit the particles  go through, the interference pattern disappears.

Instead the particles behave like particles again  and you get two blobs, one from each of the slits.

Well, actually you don’t. Though you’ve  almost certainly seen that elsewhere.

Just because you know which slit the wave-function  goes through doesn’t mean it stops being a

wave-function. It’s just no longer a wave-function  going through two slits. It’s now a wave-function

going through only one slit, so you get a one-slit  diffraction pattern. What’s that? That’s also an

interference pattern but a fuzzier one and indeed  looks mostly like a blob. But a very blurry blob.

And if you add the blobs from the two individual  slits, they’ll overlap and still pretty much

look like one blob. Not, as you see in  many videos two cleanly separated ones.

You may think this is nitpicking, but it’ll be  relevant to understanding the quantum eraser,

so keep this in mind. It’s not so relevant for  the double slit experiment, because regardless

of whether you think it’s one blob or two, the  sum of the images from both separate slits is

not the image you get from both slits together.  The double slit experiment therefore shows that

in quantum mechanics, the result of a measurement  depends on what you measure. Yes, that’s weird.

The other ingredient that you need for the quantum  eraser is entanglement. I have talked about

entanglement several times previously, so let  me just briefly remind you: entangled particles

share some information, but you don’t know  which particle has which share until you

measure it. It could be for example that you know  the particles have a total spin of zero, but you

don’t know the spin of each individual particle.  Entangled particles are handy because they allow

you to measure quantum effects over large  distances which makes them super extra weird.

Okay, now to the quantum eraser. You take  your beam of particles, usually photons,

and direct it at the double slit. After the double  slit you place a crystal that converts each single

photon into a pair of entangled photons. From each  pair you take one and direct it onto a screen.

There you measure whether they interfere.  I have drawn the photons which come from

the two different places in the crystal  with two different colors. But this is

just so it’s easier to see what’s going on,  these photons actually have the same color.

If you create these entangled pairs after  the double slit, then the wave-function of

the photon depends on which slit the photons went  through. This information comes from the location

where the pairs were created and is usually  called the “which way information”. Because

of this which-way information, the photons on  the screen can’t create an interference pattern.

What’s with the other side  of the entangled particles?

That’s where things get tricky. On the other side,  you measure the particles in two different ways.

In the first case, you measure the which-way  information directly, so you have two detectors,

let’s call them D1 and D2. The first detector is  on the path of the photons from the left slit,

the second detector on the path of  the photons from the right slit.

If you measure the photons with detectors  D1 and D2, you see no interference pattern.

But alternatively you can turn off the first  two detectors, and instead combine the two

beams in two different ways. These two white  bars are mirrors and just redirect the beam.

The semi-transparent one is a beam splitter.  This means half of the photons go through, and

the other half is reflected. This looks a little  confusing but the point is just that you combine

the two beams so that you no longer know which  way the photon came. This is the “erasure” of the

“which way information”. And then you measure  those combined beams in detectors D3 and D4.

A measurement on one of those two detectors does  not tell you which slit the photon went through.

Finally, you measure the distribution of  photons on the screen that are entangled

partners of those photons that went to D3.  These photons create an interference pattern.

You can alternatively measure the distribution of  photons on the screen that are partner particles

of those photons that went to D4. Those  will also create an interference pattern.

This is the “quantum erasure”. It seems you’ve  managed to get rid of the which way information

by combining those paths, and that restores  the interference pattern. In the delayed

choice quantum eraser experiment, the erasure  happens well after the entangled partner particle

hit the screen. This is fairly easy to do just  by making the paths of those photons long enough.

If you watch the other videos  about this experiment on YouTube,

they’ll now go on to explain that this  seems to imply that the choice of what you

measure on the one side of the experiment  decides what happened on the other side

before you even made that choice. Because the  photons must have known whether to interfere

or not before you decided whether  to erase the which-way information.

But this is clearly nonsense. Because, let’s  rewind this explanation to the beginning.

Because of this which-way information,  the photons on the screen can’t create

an interference pattern. The photons on the screen

can’t create an interference pattern. Everything  I told you after this is completely irrelevant.

It doesn’t matter at all what you do on the  other side of the experiment. The photons on

the screen will always create the same pattern.  And it’ll never be an interference pattern.

Wait. Didn’t I just tell you that you do get an  interference pattern if you use detectors D3 and

D4? Indeed. But I’ve omitted a crucial part of  the information which is missing in those other

YouTube videos. It’s that those interference  patterns are not the same. And if you add them,

you get exactly the same as  you get from detectors 1 and 2.

Namely these two overlapping blurry blobs. This  is why it matters that you know the combined

pattern of two single slits doesn’t give you  two separate blobs, as they normally show you.

What you actually do in the eraser experiment, is  that you sample the photon pairs in two groups.

And you do that in two different  ways. If you use detector 1 and 2

you sample them so that the  entangled partners on the screen

do not create an interference pattern  for each detector separately. If you

use detector 3 and 4, they each separately create  an interference pattern but together they don’t.

This means that the interference pattern  really comes from selectively disregarding

some of the particles. That this is possible  has nothing to do with quantum mechanics.

I could throw coins on the floor and then  later decide to disregard some of those

and create any kind of pattern.  Clearly this doesn’t rewrite the past.

This by the way has nothing to do with the  particular realization of the quantum eraser

experiment that I’ve discussed. This experiment  has been done in a number of different ways,

but what I just told you is generally  true, these interference patterns

will always combine to give the  original non-interference pattern.

This is not to say that there is nothing weird  going on in this experiment. But what’s weird

about it is the same thing that’s weird already  about the normal double slit experiment. Namely,

if you look at the wave-function of a single  particle, then that distributes in space. Yet

when you measure it, the particle is suddenly  in one particular place, and the result must

be correlated throughout space and fit to the  measurement setting. I actually think the bomb

experiment is far weirder than the quantum eraser.  Check out my earlier video for more on that.

When I was working on this video I thought  certainly someone must have explained this before.

But the only person I could find who’d done that  is… Sean Carroll in a blogpost two years ago.

Yes, you can trust Sean with the quantum stuff.  I’ll leave you a link to Sean’s piece in the info.

This video was sponsored by Brilliant.  Yes, quantum mechanics is a little weird.

But it isn’t as incomprehensible as  most physicists want you to believe.

If you want to understand quantum mechanics  in more depth, Brilliant is a great starting

point. Brilliant is a website and app that  offers courses on a large variety of topics

in science and mathematics. Whether you want to  learn something new or freshen up your knowledge,

Brilliant is a simple and fun way to do  it. All their courses are interactive,

so you’re challenged with questions and  can check your understanding along the way.

For this video, for example, I recommend their  courses on linear algebra and quantum objects.

To support this channel and learn more about  Brilliant, go to Brilliant dot org slash Sabine

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20 percent off the annual premium subscription. Thanks for watching, see you next week.