Quantum theory is our most successful theory of physics. There is not one shred of experimental evidence that doesn’t fit with its predictions. So why, if it ain’t broke, is a growing number of researchers expressing a desire to fix it?
“Everything depends on whether you believe quantum mechanics is going to go on describing the physical world perfectly to whatever level you push it,” says Nobel laureate , who studies the quantum world at the University of Illinois at Urbana-Champaign.
Leggett thinks it won’t, that there are too many issues with quantum theory to think it anything more than an approximation of reality. “I’m inclined to put my money on the idea that if we push quantum mechanics hard enough it will break down and something else will take over – something we can’t envisage at the moment,” he says.
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The question is, how hard can we push it? Experiments have never had the sensitivity to pinpoint a weak spot in quantum mechanics. But thanks to a breakthrough earlier this year, that might be about to change. A new swathe of experiments is coming onto the scene that should be up to the job. Welcome to the dawn of the quantum machines.
Such machines are promising to patch a gaping hole in every experiment that has ever been used to back up our view of the quantum world. Take the simple process of measuring a photon’s spin. Thanks to the strange nature of the quantum world, it can actually be spinning in two directions at once, a phenomenon known as superposition. When we use a detector to measure the spin, however, the superposition disappears and we register a spin occurring in one direction or the other.
Quantum theory does not explain why this happens. “We don’t really understand the measurement process,” admits at the Institute for Advanced Study in Princeton, New Jersey.
If you want to know how little we know, ask a roomful of physicists what goes on when we measure a particle’s properties. All will be able to calculate the result of the measurement, but the explanation they give will differ wildly. Some will tell you that new parallel universes necessarily sprang into being. Others will say that, before a measurement is performed, talk of particles having real properties is meaningless. Still others will say that hidden properties come into play.
Another group will tell you that they deal with physics, not philosophy, and dismiss the question without giving you an answer. It has been thus for more than 80 years. “These conceptual challenges are still not understood at all,” says at the University of Vienna in Austria. “We’re still right at the beginning.”
Experiments investigating the quantum world have traditionally focused on what are known as interferometers. Researchers fire a single quantum particle, such as a photon, towards two apertures in a screen. Common sense says the photon has to go through one aperture or the other. However, as long as you don’t measure which aperture it went through, something remarkable happens.
At a screen on the far side of the twin slits, an interference pattern forms. This can only occur if the photon goes through both slits at the same time and interferes with itself. In other words, as long as nobody is watching, the photon exists in two different places at once.
A measurement changes everything, however. If you set up the experiment so you can see which slit the photon goes through, the interference pattern disappears; the photon will have gone through one slit or the other, but not both.
The situation is analogous to one of the most famous thought experiments in physics, that of Schrödinger’s cat. Here, an unfortunate feline is sealed in a box with a vial of poison and a lump of radioactive metal. When the metal emits a radioactive particle, it triggers a mechanism which will break the vial, killing the cat. But because the box is sealed, there is no measurement, and the particle remains in a superposition of emitted and not emitted. According to quantum logic, the cat is therefore alive and dead at the same time.
Schrödinger came up with this bizarre scenario to show that there was something wrong with quantum theory. There’s no way, he said, that something as non-quantum as a cat can be in a superposition of alive and dead – whether it is being observed or not.
Others beg to differ. at the University of Vienna has demonstrated that carbon-70 molecules can go through two slits at once, too. Though these ball-shaped molecules aren’t quite as substantial as cats, they can nonetheless be seen through a microscope.
Quantum tuning forks
Such interferometer experiments have been extremely useful in teaching us about what constitutes a measurement. It turns out that measurement doesn’t have to be a deliberate action. Experiments have shown that if conditions allow an observer to infer which slit the photon went through – if, say, there were stray photons in the apparatus that could bounce off the test photon and thus give away its position – the superposition will disappear. This destruction, or collapse, of the superposition is known as decoherence.
Exploring when decoherence occurs has allowed us to find out more about what makes the quantum world tick. However, there is still an enormous amount we don’t know. And here we are running up against a difficult logistical problem.
Pushing at the boundary between the quantum world and that of classical physics means using ever larger molecules to see where decoherence destroys superposition. But the bigger the molecule, the harder it is to control outside forces and stop them from disrupting the molecule’s delicate quantum state. For large molecules, uncontrolled decoherence effects rule, spoiling the very effect you want to measure.
This is where the quantum machines come in. At the moment, they don’t look like much. The most advanced of them is little more than a sliver of aluminium about 50 micrometres in length. It functions as an oscillator, something like a quantum tuning fork. The key is its mass. Even the relatively large clusters of carbon atoms Arndt sends through his interferometer are lightweights compared with the mass that quantum machines will have (see “Mass matters”). “They operate at masses which are orders of magnitude larger than even the most massive clusters we are using,” Arndt says.
This is useful because the mass of a quantum object plays an important role in several alternative explanations of how the quantum world works. For the past seven years, Dirk Bouwmeester at the University of California, Santa Barbara (UCSB), has been building a quantum machine to test an idea put forward by mathematician Roger Penrose of the University of Oxford. In 2003, Penrose suggested that gravity might cause superposition collapse. If this were so and heavy, or extremely close, objects were found to be unable to sustain being in two places at once, this could help us understand the inner workings of the quantum world and the measurement problem.
Testing such ideas, however, will require quantum machines of almost incomprehensible sensitivity. The necessary apparatus involves mirrors 10 micrometres across that weigh just a few trillionths of a kilogram. “It’s a difficult experiment – it will take maybe 10 years to complete,” Bouwmeester says.
His experiment calls for the mirrors themselves to be put into a superposition – in other words, in two different places at once. Verifying that, however, will mean measuring how much they are deflected by a photon that is itself in a superposition. If Bouwmeester’s calculations are correct, the deflection will be less than a billionth of a millimetre.
Though Bouwmeester’s machine is years away from telling us anything, another quantum machine is ready to go: the aforementioned quantum tuning fork, created by Aaron O’Connell and his colleagues at UCSB. In March they reported that they had managed to get it to rest peacefully in its ground state ().
This was not easy to achieve – it has taken years of painstaking work – but it means that, like an atom, the oscillator is in a state in which it will absorb energy only in distinct incremental amounts, or quanta. Its movements are so minute that adding a single quantum of energy will affect its motion or change its position. And, also like an atom, it can exist in a superposition.
“Building the first quantum machine has taken years of painstaking work”
O’Connell’s team have managed to put their sliver of aluminium into a superposition of oscillating and not oscillating. They did this by putting the oscillator next to a tiny electric circuit that exhibits strange quantum behaviour, such as forcing current to move through it in two different directions at once. Being in the vicinity of this strange behaviour made the oscillator pick up similarly strange quantum states.
The experiment is a move towards a genuinely macroscopic case of something being in two places at once. If we can get there, it would be the mechanical analogue of a cat being alive and dead.
There is still some way to go before we are quite ready to resolve the paradox of Schrödinger’s cat. Though the oscillator is large enough to see with the naked eye, the difference between its static and oscillating positions is tiny, just 10-16 metres. To see a meaningful deviation from standard quantum mechanics, the separations would have to be at least an order of magnitude larger – possibly a lot more, Arndt reckons.
Nevertheless, says Arndt, the breakthrough is “really stunning”. He, like everyone else, is excited to see what might come of this new age of quantum machines. “The first key applications are improved tests of the foundations of quantum physics,” he says.
This could be done by watching the behaviour of such oscillators under various conditions, such as different temperatures and oscillation frequency. Placing it next to different kinds of quantum circuits and using various ways of protecting it from the environment might also expose hitherto unseen quantum behaviours. Because the quantum machine is so unusually large, it is possible that it will do things that quantum theory does not predict.
O’Connell’s group has not seen any deviations from the basic Schrödinger equation that describes the quantum world yet. “It seems to work just fine, from what we’ve seen so far,” says John Martinis, who leads the team.
But these are only the first of a new wave of tests. Last year, Adler and Angelo Bassi of the University of Trieste in Italy pointed out that a quantum machine could test a proposal made by a group of physicists at Trieste more than a decade ago: that the Schrödinger equation is only an approximation of a deeper theory, and that adding a term to it that describes random noise might make it more accurate.
In this “continuous spontaneous localisation” model, the random noise is linked to the mass of the objects involved and it is this rather than the measurement process that causes the decoherence. Adler and Bassi have now shown how this could be tested – and how it might reveal something new about the universe ().
What is this noise? Electromagnetic hiss? Chaotic Brownian motion of the particles? Something entirely unknown to physics? It’s not clear, Adler says, but the latter possibility is the most likely if their model turns out to be right. “There could be a new cosmological field,” he says. “At present it’s not detectable and it’s going to be very hard to detect.”
The idea of a new field permeating space sounds far-fetched, but Bouwmeester reckons we shouldn’t rule anything out. “We know a lot but we are far from having understood everything,” he says, pointing to the surprising conclusion from astronomical observations that the universe is filled with dark energy and dark matter. “There is room for some very big surprises.”
A deeper theory
And, as Adler points out, it’s no more fanciful an idea than saying quantum stuff is inherently incomprehensible – that it is a matter of mysterious measurement processes or new universes forming every time a particle is detected to be doing one thing and not another. “Make the superposition collapse a real process and you don’t have to explain it away through interpretations,” he says.
Another test on the horizon has been devised by Charles Wang and his colleagues at Aberdeen University in the UK. This quantum machine is not a microfabricated piece of aluminium, but a gas of super-chilled atoms designed to check whether the quantum uncertainty principle is responsible for the collapse of superpositions.
First formulated by Austrian physicist Werner Heisenberg, the uncertainty principle says that empty space is fizzing with particles that pop in and out of existence due to uncertainties in the energy of the vacuum. In 2000, Ian Percival of Queen Mary, University of London, suggested that these ghostly entities could affect the way subatomic particles form superpositions and interact with each other.
Wang and colleagues plan to test this theory. They propose that the energy inherent in empty space will affect in a particular manner the movements of an ultra-cold gas of atoms, called a Bose-Einstein condensate (). The result of Wang’s experiment – which he hopes to carry out in the near future – might reveal a previously unconsidered influence on the quantum world.
Any, or none, of the schemes could change the way we view the quantum world and the way we interact with it. If mass and gravity turn out to be affecting quantum behaviour, that might tell us something about the elusive theory of quantum gravity that physicists are trying so hard to create. Penrose reckons we will eventually be forced to combine Schrödinger’s equation describing quantum particles, an understanding of the measurement process, and the principles of Einstein’s theory of gravity into one theory, and that each of these three aspects of reality will then be seen as nothing more than an approximation to a deeper fundamental truth.
Bouwmeester is excited that, while all eyes are on the high-energy potential of the Large Hadron Collider at CERN, near Geneva, Switzerland, the quantum machines might tell us something more fundamental about the universe. “Most theorists would probably not expect big things to come out of the type of experiments we are doing,” he says. “They probably think we have to go to huge energies to resolve such fundamental issues.”
Though the road ahead looks to be a bumpy one, everyone is excited about quantum machines opening up a new vista. It’s a chance to probe the quantum world with a “new and unexplored regime”, Aspelmeyer says. “This is really testing quantum physics in its extremes.”
