The Many Varieties of Nonlocality
Enrique Galvez's lab at Colgate University is about the size of a two-car garage and, like most people's garages, jammed with stuff. Along the walls are workbenches loaded with toolboxes, electronic gear in various stages of disrepair, and, on the left side as you enter, the most frequently used piece of equipment: the coffee pot. In the middle of the room are a pair of optical benches: industrial-strength steel platforms, each the size of a dining-room table, covered with a pegboard-like grid of holes for attaching mirrors, prisms, lenses, and filters. "It's like playing with Erector sets all over again," says Galvez, a mellow Peruvian who looks remarkably like Al Franken.
If anyone has taken it on himself to show the world what quantum entanglement looks like, it's Galvez. Entanglement is the best known of several types of nonlocality that modern physicists have observed, and the one that spooked Einstein. The word "entanglement" has connotations similar to a romantic entanglement: a special and potentially troublesome relationship. Two particles that are entangled with each other are not literally intertwined, like balls of yarn; rather, they have a peculiar bond that transcends space. You can see this effect by creating, deflecting, and measuring beams of light — not ordinary flashlight beams, but beams of entangled photons. The earliest versions of the experiment, done in the 1970s at Berkeley and Harvard, involved mad-scientist contraptions of broiling-hot ovens, stacks of glass panes, and clattering teletypewriters. Galvez has taken advantage of Blu-ray lasers and optical fibers to miniaturize the setup, so that it now fits on a classroom desk.
Most experimental physicists I've met are tinkerers at heart, as fascinated by cool stuff as by the mysteries of the universe. An experimentalist at the Centre for Quantum Technologies in Singapore told me that, in his lab, incoming students have to pass a test. There's not a single physics question on it. Instead, they have to tell the story of how they took apart some household appliance and managed to get it back together, hopefully before their family found out. Apparently, clothes washers are a popular choice. Galvez, for his part, says his childhood passion was chemistry — of the blowing-up variety. Growing up in a middle-class neighborhood in Lima, he and some friends once tried to make gunpowder. All they got was a smoke bomb, which is perhaps just as well. "It was much more fun than something exploding," Galvez recalls. "It probably wasn't very healthy."
Galvez says he found his calling as a nonlocality crusader almost by accident. In common with the majority of physicists, he didn't give much thought to the phenomenon until the late 1990s, when a colleague stopped by his office with some dramatic news: the Austrian physicist Anton Zeilinger and his lab mates had used entanglement to teleport particles from one place to another. Teleport?! No fan of Star Trek could fail to be impressed. Although Zeilinger's team had beamed only single photons rather than an entire starship landing party, the coolness factor rivaled that of smoke bombs. And the procedure was straightforward. Suppose you want to teleport a photon from the left side of your lab to the right. First, you prime the teleporters by creating a pair of entangled photons and positioning one on each side of the lab. Then, you take the photon you want to beam and let it interact with the left particle. Because the entangled particles have a special bond between them, the interaction is immediately felt on the right, allowing the photon to be reconstituted there. (Some quibble whether the procedure should really be called teleportation; they consider it closer in spirit to identity theft. The experimentalists strip the left particle of its properties and thrust those properties onto the right particle. But a particle is nothing more than the sum of its properties, so these two characterizations amount to the same thing.)
Galvez and his colleague already had all the gear, and before long, they were beaming particles across their lab, too. "We were trying to figure out teleportation just for the fun of it," Galvez says. Another colleague suggested they design an entanglement experiment that even a physics-for-poets class could do. It doesn't do teleportation, but achieves the first and most important step in the process — namely, creating and distributing the entangled photons. As simple as the apparatus looks now, the team sweated over it for two years. Galvez began to run summer workshops for ALPhA, a physics-education group, to show teachers how to do the experiment, and he posted his instruction manuals online so that do-it-yourselfers can entangle particles in their basements. The former president of ALPhA, David Van Baak, exclaims: "We're past the stage where entanglement is a research-university-only affair. It's getting out to the masses."
On the day I visit Galvez's lab, one of his optical benches is given over to the entanglement experiment, the aim of which is not only to demonstrate entanglement, but also to explore what might be causing it. I recognize the setup as basically a high-tech Rube Goldberg coin flipper in which photons assume the role of coins. They are either "heads" or "tails" depending on whether they pass through a filter or not. The system is tuned so they have a 50-50 chance of getting through, like flipping a fair coin. The basic plan is to create a pair of these coins, flip both at the same time, see which sides they land on, create another pair, flip them, and so on. Repeat thousands of times and add up the statistics. It seems like a lot of effort for a predictable result, until you remember that we're talking about quantum coins. Clearly, thinking of particles as coins is a metaphor, but as long as you don't take it too literally, it's completely kosher. Physicists themselves understand phenomena in terms of metaphor.
To set the apparatus into motion, Galvez fires an ultraviolet laser through a series of optical elements that ensure proper alignment of the light. The beam strikes a small crystal of barium borate, a material discovered by Chinese scientists in the early 1980s, which splits the ultraviolet beam into two red beams. The splitting occurs particle by particle: if you could zoom in and view the beam as a stream of photons, you would see some of the ultraviolet photons hit the crystal and divide their energy into identical twin red photons. Voilà, coins. Located just upstream of the crystal is an optical element known as a waveplate, which Galvez uses to control the output of the crystal. Depending on how he sets the waveplate, the red photons are either entangled or not.
Once the red beams diverge, they cease to interact. Galvez aims each beam at a polarizing filter, much like the ones that photographers screw onto the front of their lenses to cut down on glare. The filter lets photons through or blocks them depending on their orientation — their polarization. Galvez can turn a dial on the side of the filter to control which photons make it. For this experiment, he sets both filters to the same setting, one that admits half the photons at random, thereby simulating coin flips.
Photons that make it through the filters are sent to detectors that convert them to electrical pulses. These detectors are the you-break-'em-you-bought-'em part of the system. Being sensitive enough to pick up individual photons, they run $4,000 apiece and are easily damaged by bright light. Even with the room lights off, the detectors pulse wildly, because the minutest sliver of light will set them off. Watching them gives me a new appreciation for how bright a supposedly dark room can be. We have to make sure our phones and laptops are fully off; a single glowing LED might spoil the experiment. "A while back we had to put black tape over anything that lit in the lab," Galvez says. "You would be surprised how many of those lights there are." He drapes a black velvet cloth over the devices and draws a thick curtain around the entire bench.
Finally, the detectors are wired into a meter with three digital readouts, located safely outside the curtain. Two show the number of photons that make it through the left and right polarizing filters. When Galvez switches on the laser, those numbers flash by like milliseconds on a stopwatch. The third readout shows the "coincidences" — when both photons in a pair make it through their respective filters. In terms of the coin metaphor, a coincidence means that both coins have landed on heads. These coincidences are Galvez's window into quantum nonlocality.
Having given me the tour, Galvez is ready to take some data. To verify that everything is working properly, he first simulates flipping ordinary coins by setting the waveplate to produce unentangled photons. The meter reads about twenty-five coincidences per second. For comparison, you'd get one hundred coincidences per second if every single photon in every single pair made it through the filters. So, the coincidence rate is about a quarter of its maximum possible value. This is just what you'd expect from the laws of chance. If you take two coins and flip them, each will come up heads about half the time, so both will be heads about a quarter of the time.
Now Galvez adjusts the waveplate to generate entangled photons. The coincidence rate jumps to about fifty per second. A change from twenty-five to fifty on a digital readout in a basement lab might not seem like much. But that's physics for you. It takes effort to peer beneath the surface of the world around us, and the clues are subtle, but they are no less dramatic for that. All those years of waiting and preparing for this moment have paid off, because when I see that fifty, I realize what I am seeing, and I shiver. The photons are behaving like a pair of magic coins. Galvez flips thousands of such pairs, and both always land on the same side: either both heads or both tails. That kind of thing doesn't happen by pure dumb luck.
If a friend of mine did this trick at a party — flip pairs of coins so that both came up heads twice as often as they rightfully should — I'd assume it was a practical joke. My friend might have gone to a magic shop and bought double-sided coins, which look the same on both sides, making the outcome of a flip preordained. Could an equivalent stunt explain the pattern I was seeing in Galvez's lab? To test for such trickery, Galvez uses a tactic proposed by the Irish particle physicist John Stewart Bell in the 1960s. He turns one of the filters by an angle of 90 degrees, which, like flipping a coin with your left hand rather than your right, doesn't alter the probability of a particle getting through; if the outcome really is predetermined, nothing should change. But this seemingly innocuous change does have an effect on the photons. The coincidence meter drops nearly to zero — meaning that if one photon gets through, the other never does. In other words, the magic coins have switched from always landing on the same side to always landing on opposite sides. A practical jokester would need some extra sleight of hand to pull off this trick. By making further refinements, Galvez rules out any conceivable chicanery.
I go over and look at the optical bench again. Those filters are separated by the width of my hand. Experiments by Zeilinger and others have stretched the distance to one hundred miles, and researchers at the Centre for Quantum Technologies are working on a space-based version that will go even farther. For a tiny particle, that might as well be the other side of the universe. The photons manage to coordinate their behavior across that gap. They are not in contact, and no known force links them, yet they act as one. When Galvez dials the polarizer filter on the left side of his lab bench and a photon passes through, the photon will be polarized in the same direction as the filter. Its entangled partner follows in lockstep: it acquires the same polarization and will respond accordingly to its own filter. So, what happens on the left affects the photon on the right, even when there's no time for any kind of influence to cross the gap. Indeed, such an influence would need to travel from left to right instantly — that is, infinitely fast, which is plainly faster than light, in apparent defiance of the theory of relativity. This is one of the many mysteries posed by nonlocality. Physicists have commented that it is as close to real magic as they've ever seen. "Students love it," Galvez says. "The good students say, 'I want to figure this out.'"
Shut Up and Calculate
Is nonlocality just a carnival freak show — fun to ooh and aah over, but having no broader implications — or does it belong on the center stage of physics? For most of the twentieth century, physicists treated it as a freak show, and as a student I adopted this attitude, too. It wasn't until years later, when I delved into Tim Maudlin's book Quantum Nonlocality and Relativity, that I appreciated the depth of the mystery.
Sitting in his George Nakashima–furnished living room, Maudlin tells me he'll never forget the moment he learned about quantum nonlocality. One day in the fall of 1979, while a physics major at Yale University, he opened up the latest issue of Scientific American magazine. The cover story was about dung beetles, but he flipped past it and landed on an article on the early entanglement experiments. For particles to act as if by magic stunned Maudlin. "I remember the day when I read that article," he says. "My roommates remember that day. I walked around and around my room. The world wasn't what I thought it was. It bugged the hell out of me."
It also bugged him that his physics professors, like mine, had never once mentioned this phenomenon. When he probed them about it, they blew him off. Once, Maudlin recalls, he raised his hand in class and asked whether quantum theory might not give way to a deeper theory in which the seeming contradictions would make perfect sense. The professor dismissed the idea and went back to scribbling Greek letters on the blackboard. "He didn't offer any explanation at all of why not," Maudlin says. "So he shut down the question without answering it."
* * *
To appreciate the mind block that Maudlin and I ran into, you have to go back to the famous debates between Einstein and another of the founders of quantum mechanics, the Danish physicist Niels Bohr, in the 1920s and '30s. Einstein worried that nonlocality would contradict his theory of relativity and argued that it had to be a kind of illusion, reflecting our ignorance of some essential aspect of nature. Bohr argued ... well, nobody is quite sure what Bohr argued. His reasoning gave "tangled" a whole new meaning, and his missives have been interpreted as either championing or contesting nonlocality. To the extent that anyone does understand what he said, he was asserting that it didn't matter what weirdness lay behind the scenes, as long as the theory could predict what experiments saw.
As anyone who has watched an American presidential debate knows, judgments about "win" or "lose" often have little to do with what the debaters actually say. Most physicists just wanted the Bohr-Einstein debate to be over, so they could get on with applying quantum mechanics to practical problems. Because Bohr promised closure, they rallied around him and wrote off Einstein as a has-been. One later wrote that Einstein's "fame would be undiminished, if not enhanced, had he gone fishing instead."
Over the subsequent decades, physicists used quantum theory to do all sorts of useful calculations. They figured out transistors, lasers, and other mainstays of the modern world. So the collective decision to set aside questions about the theory's deeper meaning seemed justified. Whenever those conceptual questions did come up, physicists deemed them "philosophical," which wasn't intended as a compliment, but as a way to deny that the questions were even worth asking. The English physicist Paul Dirac wrote, "It is only the philosopher, wanting to have a satisfying description of nature, who is bothered."