Mirror, mirror on the wall, what is the largest quantum object of them all? Aspelmeyer, Schwab, and Zeilinger have teamed up to find out.
August 15, 2008
University of Vienna
Credit: Jaqueline Godany
They say that bigger is better. Keith Schwab, Anton Zeilinger, and Markus Aspelmeyer certainly agree. They are applying the adage to both quantum devices and scientific collaborations.
Defying the standard wisdom of the nanotechnology crowd, the team wants to scale their nanotools up. They hope to create quantum effects in ever larger objects, to test how far they can push the boundary between the quantum and classical realms.
But thatís only part of it. Stretching across continents, the collaboration itself is an exercise in bigger and better. Schwab and a small army of grad students operate out of his lab at Cornell University in Ithaca, New York, while Aspelmeyer, Zeilinger, and another crew of grad students work in Austria, at the University of Vienna. The group leaders rarely see each other face-to-face, yet they regularly share ideas, students, and even lab equipment.
Quantum effects are mysterious, says Schwab. "Why do they work on a small scale but not at a big scale?" This, he says, is the major question that the group plans to answer: "Can we start to observe quantum behavior in bigger and bigger objects?"
To investigate where quantum mechanics breaks down and classical mechanics begins, the team is investigating two weird quantum properties: entanglement
. When two particles become entangled, they become inextricably intertwined, so that changing the properties of one has an immediate effect on the properties of its partner. Superposition is another feature that is peculiar to quantum systems. Before a quantum object is measured, it does not have definite characteristics. Instead, it exists in a superposition of multiple mutually contradictory states—allowing it to be in two places at once, for example.
Can we start to
behavior in bigger
and bigger objects?
- Keith Schwab
If the team can create a quantum effect in an object that is big enough to see with our eyes, it would have major implications for our understanding of how the laws of physics work.
In the quest to do this, Aspelmeyer is using micro-mirrors, just half a millimeter across and a hundredth of a millimeter wide, to pick up quantum effects. That may sound small, but one micro-mirror contains billions of atoms, making the system several orders of magnitude larger and more complex than anything that has been used to test quantum mechanics before. The team will bounce entangled photons off the mirrors. The trick is to see if the photons transfer their quantum nature to the mirror.
"If one photon hits the mirror, it kicks the mirror, which then oscillates with a certain amplitude," says Aspelmeyer. He likens the situation to the motion of a diving board. "If there were two people on the diving board it goes up and down more strongly," he says. Similarly, you can work out how many photons have hit the mirror, by how much it vibrates.
Sounds straightforward. But what happens if the number of photons that hit the mirror is not an exact number?
That would be the case if a superposition of an indefinite number of photons hits the mirror. Now the diving board doesnít know exactly how many people are standing on it. "The mirror canít decide how far it will move, and youíve created an entangled state," says Aspelmeyer. And a quantum effect would have been demonstrated on a larger scale than ever before.
For the experiment to work, the researchers need to keep their cool—or at least, keep their apparatus at a low temperature because heat causes the mirror to shake on its own. This is where Schwab comes in. Not only will Schwab build the micro-mirror at his lab, he has also spent years using cryogenics and laser cooling trying to get tiny mechanical systems as cold as possible. One of the techniques used to cool particles (and small mechanical devices) is to hit them with laser light. When hit with just the right frequency of laser light, the particles temporarily absorb the light, gaining energy. However, they quickly lose this again, releasing more energy than they absorbed in the process, and cooling down to a temperature that is lower than where they started.
Using this technique and others, Schwab has cooled mechanical devices to just above their lowest possible energy level, known as the ground state. "We want to hit some basic quantum mechanical milestones like cooling mechanical devices with light down to ground state," said Schwab. Zeilinger was the matchmaker who brought Schwab and Aspelmeyer together. He believes they work so well together not only because they share the same interest but also because "their personalities get along well together."
"Coming from different backgrounds there are so many different things to share and exchange," says Aspelmeyer. "We have learned a great deal from Keith already concerning mechanical systems and low temperature physics, and there were many good discussions where the quantum optics from our side brought new perspective to the experiments."
We want to hit
some basic quantum
- Keith Schwab
Schwab also agrees that collaboration is the key to success. "The techniques we use are pretty deep in the amount of years it takes to master each," he says. "Combining forces is the only way to do this."
, of McGill University in Montreal, Canada thinks the work will have far-reaching applications. The fragility of quantum states has been a big ob-stacle for building quantum computers. Understanding the quantum-classical crossover could help us learn how to prevent quantum effects from breaking down, says Clerk.
Nobel Laureate Tony Leggett
, of the University of Illinois at Urbana-Champaign, is keenly watching the teamís work. "It’s important to probe quantum mechanics on various frontiers, and one frontier not probed enough is in the direction towards the real world," he says.
Itís one thing to share theories and ideas with researchers overseas, but how tough is it to carry out cross-continental experiments? No stranger to international scientific collaboration, Zeilinger has worked with scientists in New York and Boston, while being based in Vienna. He believes the ability to work for the common good, rather than competing, is a simple but essential element.
One frontier not probed
enough is in the
direction towards the
- Tony Leggett
"International collaborations work well as long as both sides appreciate the importance of the other and that working together is more important than whether the other side is slightly ahead," he says.
Currently, students from Vienna regularly travel to New York to fabricate the nanotools and learn some cooling techniques, while some Cornell students will also begin to travel to Vienna soon to do some measurements. Technologies such as Skype and email also allow the group to stay connected despite being spread across a big area geographically.
"I really think this is the best that could happen when people with very different backgrounds meet and collaborate," said Aspelmeyer. "Itís been great discussing ideas with Keith. Huge fun."
And the fruits of their collaboration will begin to ripen as temperatures continue to drop.
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NARENDRA NATH wrote on December 19, 2009
Mesomorphic region experiments are required to be conducted. It may be a difficult regime but human beings are smart enough to generate new design ideas in the minds of prospective physicists to conduct such experiments in the near future. What about High energy Compton scattering using nanostructured crystal of silver or gold! Where to see the effect of change in value of Planck's constant, compton scattered electron or the scattered photon?
TEJINDER SINGH wrote on November 28, 2009
argues that quantum theory is intrinsically nonlinear, and goes to the standard linear limit for microscopic objects. THe nonlinear theory goes to the classical limit for large objects, but departs from linear quantum mechanics for mesoscopic objects.
Because of the nnlinearity, the lifetime of two superposed states is no longer infinite. It decreases as the number of atoms in the object under study increases, going from an...
NARENDRA NATH wrote on November 7, 2009
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This bold attempt at a tough problem is highly commendable. My personal interest has arisen in this experiment from the desire to test the valdity of possible variation in the value of h above 0 limit for classical objetcs/processes. The interst follows an attempt by Tejinger Singh of TIFR, Mumbai, India who has developed a theory for such a mesomorphic region where neither classical nor quantum mechanics is strictly valid.
I have yet to grasp the full import of the planning being done by...