New experiments - real and imagined - are probing ever more deeply into the surreal quantum realm
In ancient Greece,Plato tried to think an talk his way to the truth in extended dialogues with his disciples.Today physicists such as Leonard Mandel of the University of Rochester operate in a somewhat different fashion.He and his students,who are more likely to wear t-shirts and laser proof goggles than robes and sandals,spend countless hours bent over a large metal table trying to align a laser with a complex network of mirrors,lenses, beam splitters and light detectors.
Yet the questions they address in their equipment-jammed laboratory are no less profound than those contemplated by Plato in his grassy glade.What are the limits of human knowledge? Is the physical world shaped in some sense by our perception of it? Is there an element of randomness in the universe,or are all events predetermined?
Mandel,being inclined toward understatement,offers a more modest description of his mission."We are trying to understand the implications of quantum mechanics," he says,"The subject is very old,but we are still learning." Indeed,it has been nearly a century since Max Planck proposed that electromagnetic radiation comes in tidy bundles of energy called quanta.Building on this seemingly tenuous supposition,scientists erected what is by far the most successful theory in the history of science.In addition to yielding theories for all the fundamental forces of nature except gravity,quantum mechanics has accounted for such disparate phenomena as the shining of stars and the order of the periodic table.From it have sprung technologies ranging from nuclear reactors to lasers.
Still,quantum theory has deeply disturbing implications.For one,it shattered traditional notions of causality.The elegant equation devised by Erwin Schrödinger in 1926 to describe the unfolding of quantum events offered not certainties,as Newtonian mechanics did,but only an undulating wave of possibilities. Werner Heisenberg's uncertainty principle then showed that our knowledge of nature is fundamentally limited - as soon as we grasp one part,another part slips through our fingers. The founders of quantum physics wrestled with these issues. Albert Einstein, who in 1905 showed how Planck's electromagnetic quanta, now called photons,could explain the photoelectric effect (in which light striking metal induces an electric current), insisted later that a more detailed, wholly deterministic theory must underlie the vagaries of quantum mechanics. Arguing that "God does not play dice," he designed imaginary, "thought" experiments to demonstrate the theory's "unreasonableness." Defenders of the theory such as Niels Bohr, armed with thought experiments of their own, asserted that Einstein's objections reflected an obsolete view of reality. "It is not the job of scientists," Bohr chided his friend, "to prescribe to God how He should run the world."
Until recently, the prevailing attitude of most physicists has been utilitarian: if the theory can foretell the performance of a doped gallium arsenide semiconductor, why worry about its epistemological implications? In the past decade or so, however, a growing cadre of researchers has been probing the ghostly underpinnings of their craft. New technologies, some based on the very quantum phenomena that they test, have enabled investigators to carry out experiments Einstein and Bohr could only imagine. These achievements, in turn,have inspired theorists to dream up even more challenging - and sometimes bizarre - tests.
The goal of the quantum truth-seekers is not to build faster computers or communications devices-although that could be an outcome of the research. And few expect to "disprove" a theory that has been confirmed in countless experiments. Instead their goal is to lay bare the curious reality of the quantum realm. "For me, the main purpose of doing experiments is to show people how strange quantum physics is," says Anton Zeilinger of the University of Innsbruck, who is both a theorist and experimentalist ."Most physicists are very naive; most still believe in real waves or particles."
So far the experiments are confirming Einstein's worst fears. Photons, neutrons and even whole atoms act sometimes like waves, sometimes like particles, but they actually have no definite form until they are measured. Measurements, once made, can also be erased, altering the outcome of an experiment that has already occurred. A measurement of one quantum entity can instantaneously influence another far away.This odd behaviour can occur not only in the microscopic realm but even in objects large enough to be seen with the naked eye.
These findings have spurred a revival of interest in "interpretations" of quantum mechanics, which attempt to place it in a sensible framework But the current interpretations seem anything but sensible. Some conjure up multitudes of universes. Others require belief in a logic that allows two contradictory statements to be true. "Einstein said that if quantum mechanics is right,then the world is crazy," says Daniel Greenberger, a theorist at the City College of New York. " Well, Einstein was right. The world is crazy."
The root cause of this pathology is the schizophrenic personality of quantum phenomena, which act like waves one moment and particles the next. The mystery of wave-particle duality is an old one, at least in the case of light. No less an authority than Newton proposed that light consisted of "corpuscles," but a classic experiment by Thomas Young in the early 1800s convinced most scientists that light was essentially wavelike. Young aimed a beam of light through a plate containing two narrow slits, illuminating a screen on the other side. If the light consisted of particles, just two bright lines should have appeared on the screen. Instead a series of lines formed. The lines could be explained only by assuming that the light was propagating as waves, which were split into pairs of wavelets by the two-slit apparatus. The pattern on the screen was formed by the overlapping, or interference, of the wavelet pairs. The screen was bright where crests coincided and dark where crests met troughs,cancelling each other out.
But more recent two-slit experiments suggest that Newton was also right. Modern photodetectors (which exploit the photoelectric effect explained by Einstein) can show individual photons plinking against the screen behind the slits in a particular spot at a particular time-just like particles. But as the photons continue striking the screen, the interference pattern gradually emerges,a sure sign that each individual photon went through both slits, like a wave. Moreover, if the researcher either leaves just one slit at a time open or moves the detectors close enough to the two slits to determine which path a photon took, the photons go through one slit or the other, and the interference pattern disappears. Photons, it seems, act like waves as long as they are permitted to act like waves, spread out through space with no definite position. But the moment someone asks where the photons are-by determining which slit they went through or making them hit a screen-they abruptly become particles.
Actually, wave-particle duality is even more baffling than this explanation suggests, as John A. Wheeler of Princeton University demonstrated with a thought experiment he devised in 1980. " Bohr used to say that if you aren't confused by quantum physics then you haven't really understood it," remarks Wheeler who studied under Bohr in the 1930s and went on to become one of the most adventurous explorers of the quantum world. In the two-slit experiments, the physicist's choice of apparatus forces the photon to choose between going through both slits like a wave or just one slit, like a particle. But what would happen,Wheeler asked, if the researcher could somehow wait until after the light bad passed the two slits before deciding how to observe it?
Five years after Wheeler outlined what he called the delayed-choice experiment, it was carried out independently by groups at the University of Maryland and the University of Munich. They aimed a laser beam not at a plate with two slits but at a beam splitter, a mirror coated with just enough silver to reflect half of the photons impinging on it and let the other half pass through. After diverging at the beam splitter the two beams were guided back together by mirrors and fed into a detector. This initial setup provided no way for the investigators to test whether any individual photon had gone right or left at the beam splitter. Consequently, each photon went both ways splitting into two wavelets that ended up interfering with each other at the detector.
Then the workers installed a customized crystal called a Pockels Cell in the middle of one route. When an electric current was applied to the Pockels Cell, it diffracted photons to an auxiliary detector. Otherwise, photons passed through the cell unhindered. A random signal generator made it possible to turn the cell on or off after the photon had already passed the beam splitter but before it reached the detector as Wheeler had specified. When the Pockels-cell detector was switched on, the photon would behave like a particle and travel one route or the other, triggering either the auxiliary detector or the primary detector, buy not both at once. If the Pockels-cell detector was off ,an interference pattern would appear in the detector at the end of both paths, indicating that the photon bad travelled both routes.
To underscore the weirdness of this effect, Wheeler points out that astronomers could perform a delayed-choice experiment on light from quasars, extremely bright, mysterious objects found near the edges of the universe. In place of a beam splitter and mirrors the experiment requires a gravitational lens, a galaxy or other massive object that splits the light from a quasar and refocuses it in the direction of a distant observer, creating two or more images of the quasar.
Psychic Photons The astronomers choice of how to observe photons from the quasar here in the present apparently determines whether each photon took both paths or just one path around the gravitational lens-billions of years ago. As they approached the galactic beam splitter the photons must have had something like a premonition telling them how to behave in order to satisfy a choice to be made by unborn beings on a still nonexistent planet. The fallacy giving rise to such speculations,Wheeler explains, is the assumption that a photon had some physical form before the astronomer observed it. Either it was a wave or a particle; either it went both ways around the quasar or only one way. Actually Wheeler says quantum phenomena are neither waves nor particles but are intrinsically undefined until the moment they are measured. In a sense the British philosopher Bishop Berkeley was right when he asserted two centuries ago that "to be is to be perceived."
Reflecting on quantum mechanics some 60 years ago, the British physicist Sir Arthur Eddington complained that the theory made as much sense as Lewis Carroll's poem "Jabberwocky" in which "slithy toves did gyre and gimble in the wabe." Unfortunately, the jargon of quantum mechanics is rather less lively. An unobserved quantum entity is said to exist in a "coherent superposition" of all the possible "states" permitted by its "wave function." But as soon as an observer makes a measurement capable of distinguishing between these states the wave function "collapses", and the entity is forced into a single state. Yet even this deliberately abstract language contains some misleading implications. One is that measurement requires direct physical intervention. Physicists often explain the uncertainty principle in this way:in measuring the position of a quantum entity, one inevitably blocks it off its course, losing information about its direction and about its phase, the relative position of its crests and troughs.
Most experiments do in fact involve intrusive measurements. For example, blocking one path or the other or moving detectors close to the slits obviously disturbs the photons passage in the two-slit experiment as does placing a detector along one route of the delayed-choice experiment. But an experiment done last year by Mandel's team at the University of Rochester shows that a photon can be forced to switch from wavelike to particlelike behaviour by something much more subtle than direct intervention. The experiment relies on a parametric down-converter an unusual lens that splits a photon of a given energy into two photons whose energy is half as great. Although the device was developed in the 1960s, the Rochester group pioneered its use in tests of quantum mechanics. In the experiment, a laser fires light at a beam splitter. Reflected photons are directed to one down - converter, and transmitted photons go to another down-converter. Each down-converter splits any photon impinging on it into two lower-frequency photons one called the signal and the other called the idler. The two down-converters are arranged so that the two idler beams merge into a single beam. Mirrors steer the overlapping idlers to one detector and the two signal beams to a separate detector.
This design does not permit an observer to tell which way any single photon went after encountering the beam splitter. Each photon therefore goes both right and left at the beam splitter, like a wave, and passes through both down-converters, producing two signal wavelets and two idler wavelets. The signal wavelets generate an interference pattern at their detector. The pattern is revealed by gradually lengthening the distance that signals from one down - converter must go to reach the detector. The rate of detection then rises and falls as the crests and troughs of the interference wavelets shift in relation to each other, go in and out of phase.
Now comes the odd part. The signal photons and the idler photons, once emitted by the down-converters, never again cross paths; they proceed to their respective detectors independently of each other. Nevertheless, simply by blocking the path of one set of idler photons, the researchers destroy the interference pattern of the signal photons. What has changed? The answer is that the observer's potential knowledge has changed. He can now determine which route the signal photons took to their detector by comparing their arrival times with those of the remaining, unblocked idlers. The original photon can no longer go both ways at the beam splitter, like a wave, but must either bounce off or pass through like a particle.
The comparison of arrival times need not actually be performed to destroy the interference pattern. The mere "threat" of obtaining information about which way the photon travelled, Mandel explains, forces it to travel only one route. "The quantum state reflects not only what we know about the system but what is in principle knowable," Mandel says. Can the threat of obtaining incriminating information, once made, be retracted? In other words, are measurements reversible? Many theorists, including Bohr, thought not, and the phrase "collapse of the wave function" reflects that belief. But since 1983 Marlan O. Scully, [Isn't that just the correct name?-LB] a theorist at the University of New Mexico, has argued that it should be possible to gain information about the state of a quantum phoenomenon, thereby destroying its wavelike properties, and then restore those properties by "erasing" the information.
Several groups working with optical interferometry, including Mandel's, claim to have demonstrated what Scully has dubbed a "quantum eraser." The group that has come closest, according to Scully, is one led by Raymond Y. Chiao of the University of California at Berkeley. Earlier this year Chiao's group passed a beam of light through a down-conversion crystal, generating two identical photons. After being directed by mirrors along separate paths, the two photons crossed paths again at a half-silvered mirror and then entered two detectors. Because it was impossible to know which photon ended up in which detector, each photon seemed to go both ways.As in Mandel's experiment, the interference pattern was revealed by lengthening one arm of the interferometer; a device called a coincidence counter showed the simultaneous firings of the two photon detectors rising and falling as the two wavelets entering each detector went in and out out of phase.
Then the workers added a device to the interferometer that shifted the polarization of one set of photons by 90 degrees- If one thinks of a ray of light as an arrow, polarization is the orientation of the plane of the arrowhead. One of the peculiarities of polarization is that it is a strictly binary property; photons are always polarized either vertically or horizontally.The altered polarization served as a tag; by putting polarization detectors in front of the simple light detectors at the end of the routes, one could determine which route each photon had taken. The two paths were no longer indistinguishable, and so the interference pattern disappeared. Finally, Chiao's group inserted two devices that admitted only light polarized in one direction just in front of the detectors. The paths were indistinguishable again, and the interference pattern reappeared. Unlike Humpty-Dumpty, a collapsed wave function can be put back together again.
Spooky Action Following up another proposal by Scully, Chiao has even suggested a way to delay the choice of whether or not to restore the interference pattern until after the photons have struck the detectors. The simple polarizing filters in front of the detectors are replaced with polarizing beam splitters, which direct photons with opposite polarization to different detectors. A computer then stores the data on the arrival times of all the photons in one file and the polarization of all the photons in another file. Viewed all at once without regard to polarization, the arrival times show no interference pattern.But if one separates differently polarized photons, plots them independently , two distinct interference patterns emerge.