ANTON Zeilinger raps his knuckles on the wooden table in front of him. He thinks the table is there, passively sitting on the floor of his office at the University of Vienna in Austria. But he can’t be sure. “Reality seems to be immediate: I can touch this table,” he says. “However, if you think carefully about it, all I have is information getting into my brain.” From that, he says, he constructs some kind of reality. But the fact remains that he has nothing more than pure information to work with.
This philosophical discussion has been triggered by a few photons that Zeilinger and his collaborators have produced in their experiments. The photons come from hefty molecules such as carbon-70 buckyballs that the team have heated up. As they leave, Zeilinger says, photons carry away information about the molecule.
And it is not just any old information. The photons flying out from the team’s experiment provide a crucial insight into a deep mystery in quantum mechanics: the question of what exactly determines the boundary between the classical and quantum world.
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One of the key differences between the familiar classical world we inhabit, and the exotic quantum world, is the weird phenomenon of “superposition” – the ability of quantum things to exist simultaneously in two different states. Electrons, atoms and molecules can, for instance, be in two places at once. Yet what switches something from being in a quantum superposition to exhibiting normal classical behaviour?
Zeilinger’s latest set of experimental results, published in Nature earlier this year (vol 427, p 711), have convinced him that he has the answer, not to mention prompted him to ponder the nature of reality. By firing the C70 fullerene balls at 190 metres per second towards punctured pieces of gold foil, Zeilinger’s experimental team led by Markus Arndt, has manipulated the distinction between quantum and classical behaviour.
The key to the Vienna experiment is something called the de Broglie wavelength. Every moving particle can be considered as a wave, and its wavelength, as determined by physicist Louis de Broglie, is found by dividing the Planck constant by the particle’s momentum (which depends on its mass and velocity). You can think of the wavelength as roughly the scale at which the object’s interactions with other things change from being classical and particle-like to quantum and wave-like.
If an object’s wavelength is of a similar order to the size of the objects around it, the wave nature comes to the fore. The wavelength of a moving car, for example, is something around 10−38 metres, so on the road it behaves like a classical object. It would take some pretty tiny objects to expose the car’s wavelike properties. Your own wavelength is similarly small, which is why you aren’t generally seen in two places at once.
In the Vienna experiment, which takes place in a vacuum, the slits in the foil form gratings that look like the bars on a prison cell, with gaps of 475 nanometres (see Diagram). The fullerene balls are 1 nanometre across, but their de Broglie wavelength is about 1000 times smaller at 3 picometres, so when they hit the first grating, the apertures engage their wave-like characteristics. The particle then acts like a “matter wave”. After 38 centimetres, this wave then hits a second, identical grating.
When the matter wave passes through apertures in the first grating it “diffracts”, radiating out as if each aperture were the source of the wave. The result is that when it hits the second grating it has become spread out, and passes through multiple apertures side by side. It diffracts again as it passes through each slit, and the emerging waves interfere with each other wherever they overlap. When wave crests meet, the wave gets stronger; where crest meets trough, the waves cancel each other out. After another 38 centimetres, a detector registers where the matter wave ends up, ie where the particle emerges.
If you send light waves through this kind of apparatus, the classic interference pattern you produce is alternating vertical stripes of dark and light – dark where the waves cancel out, and light where the power of the waves combines. The stripes are spread across the width of the detector like a zebra crossing.
But what about matter waves? In the Vienna experiment the detector is effectively like a target on a rifle range, showing where the projectile ends up. As particles are fired into the interferometer, one after the other, they emerge at the other end and build up a pattern of impacts. But the spray of “bullet-holes” is far from random. If the particles were white paint balls and the target were black, you would see distinct vertical stripes of white on black, just as with interference in light.
Each time a particle goes in, a particle comes out. But repeat the process enough times, and the physical spread of emerging particles looks exactly as if each particle is interfering with others inside the apparatus, even though it is alone. Somehow, the particle is interfering with itself: smearing itself out into a superposition of simultaneously taking many different paths through the apertures of the interferometer. It then pulls itself back together to appear on the detector at just one location. After a few thousand impacts, the detector shows the perfect arrangement of interference stripes.
It is, however, possible to disrupt that neat pattern, and that is where the portentous photons come in. To fire the molecules into the detector, the team heated them to around 1200 °C. But as the team increased the heating power to around 2700 °C, it affected the emerging interference pattern.
Above about 1700 °C, the number of impacts in the “white” bands starts to fall, and the number at the “black” areas increases. As the temperature of the fullerenes rises further, the zebra pattern washes out towards an elephant grey.
The question is why. Zeilinger’s answer is that heating the molecules makes them radiate energy in the form of thermal photons, and those photons carry away crucial information about the molecule’s position. Most importantly, the shorter their wavelength, the more they reveal.
Hot matter radiates photons over a wide spectrum of wavelengths, with the exact distribution of wavelengths depending on the temperature. Below about 1700 °C, the fullerene is very unlikely to emit any short-wavelength photons during its flight through the apparatus. But by 2200 °C there is a high probability that the fullerene will emit at least three photons in the visible part of the spectrum – between 0.4 and 0.8 micrometres.
These wavelengths are short enough to expose the true location of the molecule – potentially revealing which slit it passes through. Roughly speaking, a photon will let you distinguish between two points if its wavelength is no more than twice the distance between them. “Since the grating has a period of 1 micrometre, a 2-micrometre photon would be sufficient to give away which-path information,” Arndt says.
Of course, it depends where the molecule emits its photon: 2 micrometres is enough if it was emitted close to the second grating, since here the different paths are well spread out. But nearer to the first grating, the paths are harder to distinguish. “Close to the first grating, shorter photons are required,” Arndt says.
So in principle, the short-wavelength photons could reveal which of the many apertures in the grating the molecule actually travelled through. And if you know that the molecule took one path, it cannot be behaving like a wave. It becomes “localised”, which is why the interference pattern starts to fade.
Klaus Hornberger, the group’s theorist, used this idea to calculate exactly how the temperature of the molecules ought to affect their interference pattern. The match with what the team found in experiments was extraordinary (see Diagram). Every two extra photons emitted at visible wavelengths reduced the contrast of the interference pattern by a factor of two. Emitting these photons stopped the molecules behaving like waves. As the photons carried away information, the wave properties disappeared.
Radiating photons isn’t the only way to lose information, of course. In experiments led by Lucia Hackermuller, also at Vienna, Zeilinger’s group has shown that reducing the quality of the vacuum also strips fullerene molecules of their wave-like character. As more and more gas molecules collide with the fullerenes, the interference pattern fades out.
Other groups have also made significant inroads into the problem of defining what causes “decoherence” – the name for the process by which quantum behaviour turns classical. “There are now half-a-dozen experiments that test various aspects of decoherence in various settings,” says Wojciech Zurek of the Los Alamos National Laboratory in Albuquerque, New Mexico, who is a pioneer of the study of decoherence.
David Pritchard’s group at the Massachusetts Institute of Technology, for instance, has shown that if single atoms emit photons as they pass through a double slit in an interferometer, the photon’s wavelength determines whether the interference pattern will be washed out. “When the photon had a sufficiently short wavelength it ‘knew’ where it originated,” says Zurek. “That killed the interference.” And experiments at the École Normale Supérieure in Paris, France and at the National Institute of Standards and Technology in Boulder, Colorado, are making similar advances.
So far, all the experimental results fit perfectly within the framework of quantum theory. But what actually causes the decoherence? In the Vienna experiments, for example, how does emitting photons destroy the wave-like properties of a molecule?
One possible answer is that the photon is entangled – or informationally linked – with the molecule that emitted it, and also with whatever absorbs it, most likely the chamber’s inside wall. Establishing entanglement involves redistributing the information that the photon carries about where the molecule is, so that the fullerene molecule is no longer an isolated system. Through the absorption of its emitted photons its fundamental properties become linked to the rest of the world, and it starts to behave classically.
A simpler idea is that recoil destroys the superposition: the act of jettisoning a photon or colliding with a gas molecule causes an uncertainty in the position of the fullerene molecule, blurring the interference pattern. The shorter the wavelength of the emitted photon, the stronger the recoil.
But no one has a definitive answer for how the photons switch between quantum and classical behaviour. “The interpretation question still remains with us,” Zeilinger says. “I still find it mind-boggling.” Yet he feels everything in the experiments fits with expectations. “The theory is a complete theory, and the experiments fit nicely with the theoretical idea,” he says.
What is more, the experiments show that the size of an object is no barrier to quantum-like behaviour. “Nothing in quantum theory puts any limit on the size,” says Zeilinger. “Whether an object goes into superposition depends on the experimentalist. It’s down to the choice of equipment.”
So, could you be turned quantum? It all comes down to information – or the flow of it. This determines how you interact with the world, and whether you can, for example, be in two places at once. If you want to be quantum, you have to isolate yourself from your surroundings by making sure there is no way for you to leak information.
That means it’s going to be awfully hard to send humans through an interferometer. It would involve creating apertures on the scale of a person’s de Broglie wavelength, which is around 10−35 metres. “It’s not even clear what space and time are doing on that scale,” Arndt points out. And once you have worked out a way to fire someone at your grating, you have to stick them in a near-perfect vacuum and make sure they don’t radiate any photons at wavelengths similar to the aperture size. Arndt says that a person collides with about 1028 external bits of matter per second: enough to localise them. They also emit thermal photons. Prevent all those interactions, though, and there is no reason, in principle, why you couldn’t turn quantum.
Although humans are not yet on their agenda, the Vienna team is certainly aiming bigger than fullerenes. They are preparing to fire small chains of amino acids through the interferometer, and they have plans to send haemoglobin and insulin too. They even talk about developing a way to beam rhinoviruses, which are more than 20 nanometres across.
Firing and controlling these objects will be tricky because they are not charged, so you can’t use an electric field to control them. The fullerenes are easily ionised by passing through an intense laser beam. But the team is sure it will see interference between the matter waves of these giants. “We are very confident about that,” Zeilinger says. Practically speaking, he thinks it shouldn’t be a problem to “quantise” objects up to about 108 times the mass of a neutron, which is more than a million times the mass of the fullerene molecules. “For larger objects I am also confident, but I wouldn’t dare to give a quantitative estimate,” he says.
So what about living things? Zeilinger says people often argue that living things won’t show interference because they depend on interactions with their environment. Fine, he says, why not include an environment in the experiment? “Nothing forbids you to build a nanochamber where you put your bacterium and enough oxygen and water and whatever it needs to survive to fly through the apparatus,” he says. “There’s no reason why such a system should not be in a quantum superposition of being both here and there. It’s just a practical issue – though with gigantic challenges.”
Intriguing though these possibilities are, it is the deeper issues that really capture Zeilinger’s attention – particularly the implications about the importance of information. He feels they hint at a shortcoming in quantum theory: the theory doesn’t explain what is so special about the information, or tell us how it underpins the world. “What quantum physics does, in my eyes, is put information into mathematical formalism,” he says. “The quantum state to me is nothing more than a representation of information.” Something else – some other as yet undiscovered theory – will have to explain what role that information plays (91av, 17 February 2001, p 26).
But if you’re thinking this new theory will finally put us out of our collective misery over the weirdness we see in quantum experiments, think again. Zeilinger is sure it will only get worse. “There’s no indication that it shouldn’t get weirder,” he says. “This new theory will be so much stranger that the people attacking quantum mechanics now will long to have it back.”