How to Teach Physics to Your Dog (21 page)

The first Aspect experiment. An excited calcium atom emits two photons with entangled polarizations. Each photon heads toward a single detector with a polarizing filter in front of it, set to an appropriate angle.

Physicists like to deal with numbers, and for the specific configuration they used, a local hidden variable treatment predicts that their results should boil down to a number between -1 and 0. When they did the experiment, they measured a value of 0.126, with an uncertainty of plus or minus 0.014.
*
The difference between the maximum LHV value and their measurement is nine times larger than the uncertainty in the measurement, meaning that there’s a one in 10
³⁶
probability of this happening by chance.
^†

So, that’s the end of LHV theories, right? It looks just like our imaginary experiment above, and that’s an astonishingly small probability of this happening by accident. Why did they need to do a second experiment, let alone a third?

Unfortunately, there’s a loophole in their result that allows some LHV theories to survive. In our thought experiment, we imagined Truman and RD with photon detectors that were abso
lutely perfect, because they’re very good dogs. Aspect and his coworkers are only human, though, and so were stuck using detectors with limited efficiency. On rare occasions, a detector would fail to record a photon that was really there.

This is a problem, because their experiment recorded a “0” when they expected a photon and didn’t see one—they assumed that those photons were blocked by the polarizers. But because their detectors sometimes failed to detect photons, it’s conceivable that the first Aspect experiment just
looked
like it violated the LHV prediction. If some of their “0”s really should’ve been “1”s, that could confuse their results.

For LHV theories to slip through this loophole the universe would need to be somewhat perverse, but it’s possible, so they did a second experiment, published in 1982, using two detectors for each photon.

They closed the detector efficiency loophole by directly detecting
both
possible polarizations, and only counting experiments where they detected one photon on each side of the apparatus. They replaced the polarizers with polarizing beam splitters that directed each polarization to its own detector. If one of the detectors failed to record a photon, that run of the experiment was discarded.

Their measured value in the second experiment exceeded the
LHV limit by an astonishing 40 times the uncertainty, and the odds of that happening by chance are so small it’s ridiculous. So, why did they do the third experiment? As impressive as the second experiment was, it still left a loophole, because something could have passed messages between their detectors and their source.

The second Aspect experiment. The entangled photons leave the source and head toward a pair of detectors with a polarizing beam splitter in front of them. These beam splitters direct the “0” polarization to one detector and the “1” polarization to another, ensuring that no photons are missed in the experiment.

To test Bell’s theorem, it needs to be impossible for the measurement at one detector to depend on what happens at the other detector without some faster-than-light interaction. If there’s a way to send messages between the detectors at speeds less than that of light, all bets are off. In the first two experiments, they chose the detector settings in advance, and left them set for much longer than it took light to pass between the source and the detector. Something might have communicated the polarizer settings from the detectors to the source, which then sent out photons with definite polarization values chosen to match the quantum predictions. When the experimenters changed the angles, the new values would be sent to the source, which would change the polarizations sent out. Their results
seemed
to prove quantum theory, but they might have been the victims of a cosmic conspiracy.

The third experiment found an ingenious way to close that loophole, as well. Aspect and his colleagues ruled out any possibility of some sort of universal conspiracy mimicking the quantum results by changing their detector settings faster than light could go from the source to the detector.

They replaced the beam splitters with fast optical switches that could direct the photons to one of two detectors, each set for a different polarization. The switches flipped between the detectors every 10 nanoseconds, while it took the photons 40 ns to reach the detector. In effect, which detector a given photon would hit was not decided until
after
the photon had already left the source.

The third experiment’s results exceeded the LHV limit by five
times the uncertainty. The chances of such a result happening by accident were about one in a hundred billion—better than the chances for the other two experiments, but still low enough to be convincing.

The third Aspect experiment. The two entangled photons leave the source, and head toward fast optical switches that send each photon toward one of two different polarizers, with the choice not being made until after the photons have left the source.

Even the third experiment doesn’t close every loophole,
*
but Aspect stopped there, because the experiments were extraordinarily difficult. A number of people have repeated these experiments, using more modern sources of entangled photons,
^†
and a 2008 experiment has even tested Bell’s theorem using entangled states of ions instead of photons, but no loophole-free test has been done. As a result, there are still a few people who argue that LHV theories have never been completely ruled out.

These few die-hard theorists aside, the vast majority of physicists agree that the Bell’s theorem experiments done by Aspect and company have conclusively shown that quantum mechanics is nonlocal. Our universe cannot be described by any
theory in which particles have definite properties at all times, and in which measurements made in one place are not affected by measurements in other places.

Aspect’s experiments represent a resounding defeat for the view of the world favored by Einstein and presented in the Einstein, Podolsky, and Rosen paper in 1935. But while the EPR paper is wrong, it’s brilliantly wrong, forcing physicists to grapple with the philosophical implications of nonlocality. Exploring the ideas raised in the paper has deepened our understanding of the bizarre nature of our quantum universe. The idea of quantum entanglement exploited in the EPR paper also turns out to allow us to do some amazing things using the nonlocal nature of quantum reality.

“Physicists are really weird.”

“Yeah, nonlocality is strange.”

“Not that, the loopholes. Do physicists really believe that there are messages being passed back and forth between different bits of their apparatus? What would carry the messages?”

“I’m not sure anybody ever suggested a plausible mechanism, but it really doesn’t matter. They could be carried by invisible quantum bunnies, for all the difference it makes.”

“Quantum bunnies?”

“Invisible quantum bunnies. Moving at the speed of light. Don’t get your hopes up.”

“Awww . . .”

“Anyway, the third Aspect experiment pretty much rules out any means of carrying messages between parts of the apparatus, involving bunnies or anything else. The point is, prior to that, it was at least possible in principle for there to be another explanation. And in science, you have to rule out all possible explanations, even the ones that seem really unlikely, if you want to convince anybody of an extraordinary claim.”

“Even the ones involving bunnies?”

“Even the ones involving bunnies. And anyway, the idea that distant particles can be correlated in a nonlocal fashion isn’t all that much weirder than quantum bunnies would be.”

“Good point. So, what’s this good for?”

“What do you mean?”

“You dropped a really broad hint in that last paragraph that this entanglement stuff is good for something. What’s it good for, sending messages faster than light?”

“No, you can’t use it for faster-than-light communication, because the detections are random. There are correlations between particles, but the polarization of each pair will be random. I can’t send a message to somebody else using EPR correlations—all I can send is a random string of numbers.”

“So what good is it?”

“Well, random strings of numbers can be useful for quantum cryptography, making unbreakable codes. And the idea of entanglement is central to quantum computing, which could solve problems no normal computer can tackle. And there’s quantum teleportation, using entanglement to move states from one place to another. There’s all sorts of stuff out there, if you look for it.”

“Ooh! Teleportation sounds cool! Talk about that.”

“Well, that’s next . . .”

*
The process of decoherence (described in
chapter 4
) involves the interaction of a single quantum particle with a much larger environment, but we care only about the state of the single particle.

*
We’re assuming that Truman’s photon is measured first, for the sake of clarity. The result is the same if we assume RD’s photon is the first one measured.

†
The horizontal photon has a 75% chance of passing through the polarizer to the detector in either position. A “0” to match Truman’s result happens only when RD’s photon is blocked, a 25% probability.

*
This raises the question of whether a sufficiently clever experiment might distinguish between, say, the Copenhagen interpretation and the many-worlds interpretation. This is a much harder problem than distinguishing between quantum and LHV theories. Some future John Bell may yet come along and find the right test, but no one has managed yet.

*
John Clauser and a couple of other people had done earlier tests, but the Aspect (pronounced “As-PAY”) experiments had better precision, and so are regarded as the definitive tests.

*
This uncertainty is a technical limitation based on the details of their experiment, and not anything to do with the Heisenberg uncertainty principle.

†
10
³⁶
is a billion billion billion billion, a number so large that it might have made even Carl “Billions and Billions” Sagan blink.

*
The third experiment actually reopens the detector efficiency loophole, because they used only one detector for each polarizer.

†
One experiment by Paul Kwiat (who was part of the Innsbruck–Los Alamos team doing quantum interrogation experiments in
chapter 5
) and colleagues at Los Alamos saw an effect a mind-boggling 100 times larger than the uncertainty.

*
Werner Heisenberg, who developed the uncertainty principle while working with Bohr, once described Bohr as “primarily a philosopher, not a physicist.”

†
In almost all of those cases, Bohr’s argument depended on the effect of measurement on the system. Something in the process by which Einstein proposed to measure the position would cause a change in the momentum (as in the case of the Heisenberg microscope thought experiment discussed in
chapter 2
[page 38]), or vice versa. Measuring the system requires an interaction, and that interaction changes the state of the system in a way that introduces some uncertainty in the quantities being measured.

*
Bohr was somewhat famous for the opacity of his writing, but he outdid himself in this case. The crucial paragraph of his paper refers to the quantum connection between distant objects as
“an influence on the very conditions which define the possible types of predictions regarding the future behavior of the system”
(italics in original), and declares that the quantum view “may be characterized as a rational utilization of all possibilities of unambiguous interpretation of measurement, compatible with the finite and uncontrollable interaction between the objects and the measuring instruments of quantum theory.”

†
This light-speed limit is one of the main consequences of Einstein’s theory of relativity, and thus very important to his conception of physics.