Even Scientists Marvel At 'Spooky' Behavior Of Separated Objects
October 14, 2005; Page B1
The crystals in Paul Kwiat's physics lab have none of the sparkling facets and points that New Agers believe heal the body and "enhance the life force." But his chunks of beta-barium borate do something even more magical.
The crystals produce special particles of light that, no matter how far apart they ever travel, will always have an eerie connection to one another across the vastest reaches of time and space. If a scientist makes a measurement on one particle, the other will "feel" it instantaneously.
Einstein called this "spooky action at a distance," and the very idea gave him fits. But in the 70 years since the great man insisted that spooky action had to be wrong, and dreamed up thought experiments to disprove it, evidence for it has gotten only stronger. Studies such as Prof. Kwiat's, at the University of Illinois at Urbana-Champaign, are closing the loopholes in earlier research. And although spooky action gives philosophers a lifetime's worth of enigmas to ponder, it may also become the basis of revolutionary technologies, such as quantum computing and quantum cryptography.
Spooky action is more properly called entanglement. To produce entangled particles, Prof. Kwiat shines ultraviolet light onto two back-to-back crystals. The crystals split some of the light particles, or photons, into two infrared daughter particles. The daughter photons emerge and, obeying the laws of physics, vibrate in the same direction as one another.
But which direction? You can't predict by studying the crystals. You have to measure the daughters directly, using polarizers like those found in your better sunglasses. So Prof. Kwiat does, but only one of the photons. That makes the measured photon, which had been a fuzzy, indeterminate mix of vibrations in all directions, settle into one, definite mode.
This is where the magic enters. The other daughter, which like its sibling has been a mix of wiggles, settles into the exact same motion at exactly the same time, even though Prof. Kwiat never got near it with his measuring device.
This is what gave Einstein fits. He hated the idea of particles not having real traits (such as which way they're wiggling) until someone measures them. And he really hated the idea that measuring one particle affects an entangled particle, even one clear across the universe.
Prof. Kwiat's instruments don't reach quite that far. But quantum physics says that entangled particles should have the same spooky connection whether they are across the cosmos or across the lab. Every experiment has confirmed that. But every experiment has had loopholes.
One is the conspiracy loophole. Maybe when the crystal creates the entangled particles they decide how they'll vibrate when a physicist later measures them. "If that guy with the beard comes at you with a polarization detector, vibrate horizontally." (Minus the anthropomorphizing, this "locality" loophole means the particles somehow signal one another to synchronize vibrations before they part.)
This loophole closed in 1999 when physicists produced entangled particles that were too far apart to exchange signals. Even without conspiring, the particles showed spooky action at a distance: When one was measured, the other instantly showed the same kind of vibration.
Then there is the ones-who-got-away loophole. Devices that detect entangled particles typically snare something like 1% of them. Maybe the 1% were aberrant, and many of the 99% that got away failed to show entanglement. Prof. Kwiat, however, thinks that by next year he'll be able to close the "detection loophole" by snaring 95% of the photons.
One by one, says physicist Daniel Greenberger, of City College, New York, "the loopholes are getting shut." And with that, things are getting more bizarre rather than less. Physicists have entangled particles that, unlike the photons from Prof. Kwiat's crystals, "have never met, and share no previous history," says Prof. Greenberger.
Still, it isn't easy accepting spooky action at a distance, even for physicists. "There are still some holdouts," says Prof. Kwiat. "Even those of us who are doing these experiments usually just 'shut up and calculate.' But in our off-hours we do think about what it means" for our understanding of reality.
If it's any consolation, embracing quantum weirdness and spooky action opens the door to quantum technologies. Take quantum cryptography. Let's say Alice sends Bob a secret key for decrypting subsequent messages. The key is a series of photons; each is entangled with a photon Alice keeps in her lab. If a spy intercepts the key to learn Alice's code, the interference instantly causes a change in the particles Bob receives. Bob and Alice detect the change, realize their key has been compromised, and get a new one.
Quantum computation also requires entanglement. An operation performed on one of the quantum bits in the system would instantly affect all the bits with which it is entangled, enabling computations that, on an ordinary computer, would take longer than the age of the universe.
"I don't think we've even scratched the surface of what entanglement can do," says Prof. Greenberger. "Talk about mind boggling. We'll see all sorts of crazy things."
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