Edition: U.S. / Global

Far Apart, 2 Particles Respond Faster Than Light

Published: July 22, 1997

IT was as if some ghostly bridge across the city of Geneva, Switzerland, had permitted two photons of light nearly seven miles apart to respond simultaneously to a stimulus applied to just one of them.

The twin-photon experiment by Dr. Nicolas Gisin of the University of Geneva and his colleagues last month was the most spectacular demonstration yet of the mysterious long-range connections that exist between quantum events, connections created from nothing at all, which in theory can reach instantaneously from one end of the universe to the other.


Since there was no way for the photons to communicate with each other, ''classical'' physics would predict that their independent choices would bear no relationship to each other. But when the paths of the two photons were properly adjusted and the results compared, the independent decisions by the paired photons always matched, even though there was no physical way for them to communicate with each other.

Albert Einstein sneered at the very possibility of such a thing, calling it ''spooky action at a distance.'' Scientists still (somewhat shamefacedly) speak of the ''magic'' of ''quantum weirdness.'' And yet all experiments in recent years have shown that Einstein was wrong and that action at a distance is real.

The idea behind Dr. Gisin's experiment was not new. Since the 1970's, physicists have been testing a prediction of quantum theory that ''entangled'' particles continue to communicate with each other instantaneously even when very far apart.



''In principle, it should make no difference whether the correlation between twin particles occurs when they are separated by a few meters or by the entire universe,'' he said in an interview.

''This research is interesting not only from a scientific and philosophical point of view, but because of a very practical consequence: we can now create a completely secure code. A quantum key, which is now within reach, would allow banks to carry out transactions with each other over optical fibers, completely safe from all possible code-breaking methods and from eavesdropping or interference.''

The idea for such a system, he said, originated with Dr. Artur D. Eckert at Oxford University in England.

Details of the Swiss experiment will be described in a forthcoming technical paper, Dr. Gisin said, and he is working with the Swiss telecommunications agency to develop a cryptographic system based on entangled particle ''twins.'' Identical random-number sequences generated simultaneously by pairs of widely separated twins would serve as cipher keys equivalent to the ''one-time pads'' used by spies and governments to encode and decode ultra-secret messages.

The receiver and sender of a secret message based on a one-time pad each must have a copy of the pad, which contains a random sequence of numbers. The sequence defines a series of mathematical operations used to encipher the message, and the reverse sequence is used to decipher it. The key pads of sender and receiver are used for only one message and then destroyed; this means that every letter of every message is enciphered by its own unique key and is therefore completely immune to cryptanalysis.

One of the leading experimentalists in quantum optics, Dr. Raymond Y. Chiao of the University of California, Berkeley, hailed the Geneva experiment as ''wonderful.''

But an underlying enigma of quantum mechanics remains unfathomed.

The connections that persist between distant but entangled particles are ''one of the deep mysteries of quantum mechanics,'' Dr. Chiao said in an interview. ''These connections are a fact of nature proven by experiments, but to try to explain them philosophically is very difficult,'' he said.

Quantum events obey the laws of quantum theory, which governs the behavior of minute objects like atoms and subatomic particles, including photons of light. By contrast with the laws of ''classical'' physics (which apply to the relatively large objects of the everyday world), quantum physics often exhibits behavior that seems impossible.


Physicists call this a ''collapse of the wave function.'' The amazing thing is that if just one particle in an entangled pair is measured, the wave function of both particles collapses into a definite state that is the same for both partners, even separated by great distances.

Among several proposed explanations of all this is the ''many worlds'' hypothesis: the notion that for every possible pathway or state open to a particle, there is a separate universe. For each of 10 possible pathways a quantum particle might follow, for example, there would exist a separate universe.

Since the 1970's, Dr. John F. Clauser of the University of California at Berkeley, Dr. Alain Aspect at the Institut des Optics in Orsay, France, and others have been experimenting with pairs of entangled particles.

One way to create a pair of entangled twins is to start with a single photon of ultraviolet radiation and pass it through a peculiar artificial mineral called a ''down-conversion crystal.'' In the Swiss experiment, the crystal consisted of potassium niobate. The crystal splits the photon in two, producing two new photons that continue on in somewhat different directions, and whose combined energy equals the energy of their parent photon.


The astonishing consequence of this is that the particle's distant twin experiences exactly the same metamorphosis at the same moment, even though there is no physical link or signal between the two twins.

In 1935 a famous paper by Albert Einstein, Boris Podolsky and Nathan Rosen challenged the quantum theory prediction that entangled particles could remain instantly in touch with each other. One of their objections was based on the speed limit imposed by Einstein's Special Theory of Relativity: nothing can travel faster than the speed of light.

Einstein and his colleagues preferred a more intuitive explanation of the simultaneous correlation between entangled particles, based on the idea that the match between them is ordained by their identical antecedents. The behavior of each particle, they argued, is the product of hidden ''local'' factors, not by spooky long-distance effects.

But again and again in recent years, increasingly sensitive experiments have decisively proved that Einstein's explanation was wrong and quantum theory is correct.

In Dr. Gisin's experiment, as in earlier ones, no signal of any kind was transmitted between the photons, but despite this, one of the photons ''knew'' what happened to its distant twin, and mimicked the twin's response. This response took less than one ten-thousandth of the time a light beam would have needed to carry the news from one photon to the other at a speed of 186,282 miles per second. (In fact, the correlation between the two particles was presumably instantaneous. The Swiss experiment merely set an upper limit on the time required for the response as about three ten-billionths of a second.)

Dr. Gisin's experiment made use of a system of paired interferometers developed by Dr. James D. Franson of Johns Hopkins University, who is also a leading investigator of quantum effects.

Each interferometer, a device for separating and then recombining beams of light, consists of a complex arrangement of mirrors and ''beam splitters'' -- semi-opaque reflectors that randomly reflect some photons in one direction and transmit others in a different direction. In an interview, Dr. Franson explained the system:



Dr. Franson said of the correlation demonstrated over a seven-mile course by the Swiss experiment, ''It's pretty amazing.''

Whatever the nature of the connection between entangled particles may be, nearly all physicists agree that it cannot be used to transmit messages faster than the speed of light. All it can do is assure that a random choice by one entangled particle is instantly echoed by its distant partner. This is not the same thing as transmitting information, the experts say, and therefore it does not violate relativity theory.

But why is a numerical correlation between two particles different from information?

''That's a difficult question,'' Dr. Franson said, ''and I don't think anyone could give you a coherent answer. Quantum theory is confirmed by experiments, and so is relativity theory, which prevents us from sending messages faster than light. I don't know that there's any intuitive explanation of what that means.''

Another deep quantum mystery for which physicists have no answer has to do with ''tunneling'' -- the bizarre ability of particles to sometimes penetrate impenetrable barriers. This effect is not only well demonstrated; it is the basis of tunnel diodes and similar devices vital to modern electronic systems.


The probabilistic nature of quantum events means that if a stream of particles encounters an obstacle, most of the particles will be stopped in their tracks but a few, conveyed by probability alone, will magically appear on the other side of the barrier. The process is called ''tunneling,'' although the word in itself explains nothing.



''This is another great mystery of quantum mechanics.''


The late Rockefeller University physicist Heinz Pagels, like many other theorists, believed that quantum physics is a kind of code that interconnects everything in the universe, including the physical basis of life itself. In his book ''The Cosmic Code,'' Dr. Pagels, an ardent mountain climber, wrote:


Dr. Pagels was killed in a climbing accident in 1988.

Diagram: ''Tracking the Paths of 'Twin' Photons'' Interferometers like these were used in the Swiss experiment to detect instantaneous ''actions at a distance'' between photons of light seven miles apart. Pairs of ''entangled'' photons, split at the crystal, top, were sent their separate ways, with each one allowed to choose either the long or the short route, leaving either through an ''up'' port or a ''down'' port. Even with no physical connection or signal between them, each photon ''knew'' what choice the other had made, and matched its behavior to that of its twin. (Source: Scientific American)(pg. C2)