Thought Experiments Made Real

We're back from Boskone, which included lots of fun stuff, and not enough sleep. I also cleverly forgot to bring my lecture notes home from work, which means I need to go in early to figure out what the hell I'm talking about in class today, so there's not much time for blogging at the moment.

I would be remiss in my physics-blogging duties, though, if I failed to point people to this Physics Web story about a new single-photon interference experiment (you'll need a subscription to read the Science article). A French group including Alain Aspect (who else?) has done a beautifually clean realization of Wheeler's delayed-choice experiment:


The experiment in this: a beam froma single-photon source is split between two paths, which travel some 48 meters before coming back together. A beamsplittler can then be inserted to recombine the two beams, or the beamsplitter can be removed to allow the paths to fall on two separate detectors. If the beamsplitter is in, you see intereference between the two paths, and if it's out, you see which path the photon took.

The trick to the experiment is that you don't decide whether the beamsplitter will be there or not until after the photon is in flight. They have their single-photon source connected to a random number generator, which spits out a random number some time after the photon is emitted, and that number determines whether they put the "beamsplitter" (which is an electronically controlled polarization modulator) in the path or not. They still see exactly the results you expect: with the beamsplitter, they see beautiful interference fringes, and without it, they see nothing.

There's nothing really all that stunningly new, here. Similar experiments have been done in the past, and gotten the same results. The thing about this experiment that makes it interesting enough to make Science is what a clean realization of the thought experiment it is. They've done almost exactly what John Wheeler proposed, lo these many years ago, and with the random number generator and extremely long optical paths (they must've used optical fibers, though the paper doesn't specify), they've really separated the photon emission and the decision about which measurtement to make in a way that makes it almost impossible to find a loophole.

It's always really nice to see simple thought experiments brought to reality. It's not a paper that's going to change the world, but it's excellent work, and nicely readable.

More like this

Okay, so now let's test John Wheeler idea.

The cosmos is as it is because we recently decided to look carefully at its cosmological origin (background radiation anisotropy and polarization, etc.).

That forced the universe to retroactively decide what to have always looked like, by some sort of Heisenberg uncertanity coupling of t=o with now.


As summarized:

After a Short Delay, Quantum Mechanics Becomes Even Weirder

By Adrian Cho
ScienceNOW Daily News
16 February 2007

According to quantum mechanics, light can be either a graceful rippling wave or a hail of bulletlike particles, depending on how you look at it. Now, an experiment shows that an observer can make the choice retroactively, after light has entered a measuring apparatus. The result shows that reality is truly in the eye of the beholder.

A single dollop of light, or photon, must be described by a flowing quantum wave that gives the probability of finding it at any particular place and time. At the same time, the photon acts a bit like an indivisible bullet: When observed with a particle detector, it produces a distinct signal, like a pebble pinging off a car door. And things get weirder. The quantum wave can split in two and recombine, like ripples flowing around a stump in a pond, to create striking "interference" effects that determine which way the recombined wave flows. On the other hand, it's simply impossible to split a photon at a fork in the road. If there is no way to eventually put the pieces back together, the photon acts like a particle and goes one way or the other.

Even weirder still, the choice to allow the waves to recombine or not can be made even after the photon passes the fork where it should have split--or not. Famed physicist John Archibald Wheeler realized that nearly 30 years ago and dreamed up an experiment to prove the point. Now Jean-François Roch of the Ecole Normale Supérieure de Cachan in France and colleagues have performed the experiment. The researchers shot photons one by one at a half-silvered mirror, or "beam splitter," to cleave the quantum wave describing each photon. After traveling different distances, the two halves sloshed back together at a second beam splitter 50 meters away, which could recombine them. The experimenters could randomly switch this second beam splitter on and off electronically well after the photon had passed the first one.

If the second splitter was on, interference between the two pieces directed the recombined wave of probability toward one or the other of two detectors, depending on the difference in the path lengths. If the second beam splitter was turned off so the waves couldn't recombine, then the photon took one path or the other with 50-50 probability, and equal numbers of photons reached detectors. The results, reported this week in Science, prove that the photon does not decide whether to behave like a particle or a wave when it hits the first beam splitter, Roch says. Rather, the experimenter decides only later, when he decides whether to put in the second beam splitter. In a sense, at that moment, he chooses his reality.

Others had tried to perform Wheeler's experiment but had lacked the single-photon source and other elements to really do it right, says Arthur Zajonc, an experimenter at Amherst College in Massachusetts. "This is the experiment you wanted to do, but it was too hard," he says. The experiment will likely become a classic cited in textbooks, Zajonc says: "It's going to be seen as a kind of a landmark."

Great post! We've been talking about this very experiment that Wheeler proposed so long ago. (I had asked whether it was a thought experiment or a real one. Now it's real.)
When the journalist Andrian Cho says "the second beam splitter," he really means the split-recombiner, so to speak, right? If I understand correctly....
I've recommended you at my blog and you'd have a pingback (if I could make it work).

Please note that the random number generator determines how the photon behaves (as a wave or particle), so this removes the oft-quoted importance of an "observer". Nature-itself does the observing (collapses the wave function). The Moon exists, regardless of whether anyone is looking. Nature is looking. Perhaps the bottom line is a reminder that humans are simply part of nature.

By Don Garlick (not verified) on 20 Nov 2009 #permalink

I'm currently reading Brian Greene's "Fabric of the Cosmos" where he refers to the Delayed Choice Quantum Eraser experiment.

It's a double-slit experiment which by means of a "down converter" can take a copy of each photon (so-called "idler photons") exiting each of the slits, which are then either checked for which slit they originate from, or their locality information is erased again, while the original "signal" photons may interfere (or not). This check is done after the photons have passed the double slit. The above link shows the setup and explains the experiment. As everyone expects, there is no interference pattern for those signal photons, whose corresponding idler photon's path is checked, while there is interference of those signal photons, whose idler photon's path information is erased, which is verified with a coincidence counter. This fits well to the Wheeler experiment explained here, but it is even more subtle.

The trick is, the idler photons provide a means to check, what the signal photons have done, even after the double slit has been passed.

The following modification of this experiment came to my mind and I would like to know, if anybody can explain to me what would happen in this case.

My idea is to vastly extend the path of the idler photons, so much that the signal photons have already interfered (or not) on the screen or detector before their corresponding idler photons have been analysed or even reached the analyser. If we set up the experiment so that at the start, the setup would erase the localization information of the idler photons, then let the signal photons interfere, and only then quickly switch the setup to a configuration, where the idler photons are checked for their path, what would happen?

Confer the lower picture in the Wiki article. If we

1) just extended the path from the double slit/BBO to the prism PS vastly, maybe by copying the path information of the idler photons onto another particle type that we can store for a moment before retransmitting them, or by sending them on a long detour, then the experiment should still work as before, or shouldn't it? Except for the coincidence counter, which we wouldn't need, we could actually photograph the interference pattern (ok, we couldn't really, because in the original experiment, 50% of the idler photons are path-checked, and their signal photons would overlay the interference pattern; but the photons should show the same behaviour as before, in principle).

2) The first modification would be to remove the elements that check the origin of the idler photons (D3 and D4) by removing the beam splitters BSa and BSb. Now, since the path information of the idler photons is always erased, we always expect to see an interference pattern of the signal photons in detector D0 that we could photograph, wouldn't we?

3) Now for the mean part: when the interference at D0 has happened and has been recorded, we quickly introduce fully deflective mirrors in place of beam splitters BSa and BSb, so every idler photon gets checked for its origin, in the end. So there should be no interference pattern, but what if we recorded one already?

Has anyone thought of such an experiment? What would be the result? A time paradox? I suppose, steps 1) and 2) would already fail to produce interference, otherwise nature could be tricked out, which I'm certain it can't, but where is the problem in those steps? Any idea, anybody?

By Dieter Kreuer (not verified) on 28 Jun 2011 #permalink

No experts around who could comment on my above post??
Or any suggestion whom I should contact alternatively?

Here's a short recap of the post:

The Delayed Choice Quantum Eraser is a double slit experiment, where copies of the interfering photons (or better: signal photons) can be evaluated to determine which actual slit the signal photons have passed beforehand, or their path information can be erased. If the copy-photons are checked, the interference disappears for the associated signal photons, while it stays intact for copy-photons, whose path information is erased.

What would happen, if this check or erasure is delayed until after interference should have occured already? this is an attempt of creating some kind of quantum time paradox. This will certainly fail, but why?

By Dieter Kreuer (not verified) on 01 Jul 2011 #permalink