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Quivering Gizmo Ushers in Quantum Machines
17 March 2010 2:00 pm
The weird rules of quantum mechanics state that a tiny object can absorb energy only in discrete amounts, or quanta, and can literally be in two places simultaneously. Those mind-bending tenets have been amply demonstrated in experiments with electrons, photons, atoms, and molecules. Ironically, though, physicists have never observed such bizarre quantum-mechanical effects in the motion of a human-made mechanical device. Now, Andrew Cleland, John Martinis, and colleagues at the University of California, Santa Barbara, have taken a key first step in that direction by fashioning a vibrating, diving board-like gizmo a few dozen micrometers long and less than a nanometer thick that makes literally the slightest movement allowed by quantum theory.
The odd advance could open the way to new technologies that meld electronics, optics, and mechanics and might even lead to experiments that probe the origins of our everyday, non-quantum sense of reality. "This is the kind of result that will create a new field in itself," says Jack Harris, an experimenter at Yale University. "This is will be a door opening—a big one."
Ultimately, physicists would like to put the quantum into mechanics by making little thrumming devices that could be eased into quantum states in which, for example, they vibrate around two different positions at the same time. But to reach that grand goal, researchers must first coax such an "oscillator" into its simpler, least energetic "ground state." Even in that state, an oscillator cannot stand perfectly still. Thanks to quantum uncertainty, which makes it impossible to say simultaneously exactly where a thing is and exactly how fast it's moving, the widget must quake with an inextinguishable zero-point motion and possess an inextricable last half-quantum of energy. For years, physicists have been striving to reach the ground state and observe this minimal tremor.
Several years ago, a number of groups tried to reach the ground state in the most straight-forward way. Physicists would chill a tiny beam of semiconductor, which would vibrate like a guitar string, to suck out every possible quantum of energy. To detect the beam's motion, they would apply a voltage between it and a parallel electrode and monitor the oscillation in that voltage using an extremely sensitive electrical detector called a single electron transistor (SET).
This approach ran into problems, however. A beam's energy quanta are so small that to remove them all, physicists have to chill the beam to nearly absolute zero. Researchers could drive up the size of the quanta by increasing the beam's frequency. But that required making the beam stiffer, which reduced the size of its motion and makes it hard to detect. Moreover, electrons hopping through the SET would actually tug on the beam and jostle it. So in recent years physicists have turned to more-sophisticated techniques like laser cooling.
Cleland and Martinis have reached the ground state by putting several new twists on the old-fashioned approach. "I just said to myself, what's technologically the easiest way to do this?" Cleland says. The team's gizmo is a finger-like "cantilever" that vibrates at very high frequency—a whopping 6 gigahertz—and is cooled to 25 millikelvin. But rather than swinging side to side, it gets thinner and thicker. It also consists of so-called piezoelectric material that generates an oscillating electric field as it expands and contracts, making that motion easier to detect. To do that, Cleland and Martinis rely not on a SET, but on a widget called a "phase qubit," a strip of superconductor with a non-superconducting patch in it that acts a bit like a sandbar in a stream of free-flowing electrons.
The details aside, the phase qubit is itself a highly controllable quantum-mechanical system with a ground state and one higher-energy state. Researchers can ease the qubit from one state to the other—or even put it into both states at once—by applying microwaves of a specific frequency. Moreover, they can change that frequency by adjusting the current flowing through the qubit. So Cleland and Martinis can feed energy quanta into the oscillating beam one by one. They first put the qubit into its energetic state and then adjust the qubit's frequency to match that of the oscillator to shuffle the quantum of energy over. They can also run the process backwards to pull quanta out of the oscillator, the researchers report this week in Nature. And the team has pulled out every last one to reach the ground state.
"I would say that Andrew and John have achieved [the ground state]," says Keith Schwab, an experimenter at the California Institute of Technology (Caltech) in Pasadena. "We've gotten damn close, but these guys are deep into it."
The result opens up numerous possibilities, other researchers say. The ability to produce quantum states of motion could lead to tiny devices that meld quantized vibrations, or "phonons," with electronic and optical signals. These might find uses in the field of quantum optics or possibly in quantum information technologies. "You've got phonons, photons, and electrons" working together," says Oskar Painter, an applied physicists at Caltech. "That's where the revolution is going to come from."
Ultimately, if researchers can put a vibrating machine into a two-places-at-once state, it might allow them to explore a fundamental question: Why don't everyday objects like pennies and people behave quantum mechanically? Many physicists think that, in principle, objects much bigger than atoms and molecules could be put into such state, if they could be shielded from disturbances from their surroundings. Others argue that some as yet undiscovered principle prevents really large objects from being in such state. Physicist might be able to test the two ideas by trying larger and larger objects into two-places-at-once states, says Miles Blencowe, a theorist at Dartmouth College. "I think for a lot of us this is what we're aiming for." That goal is still a long way off. But it's a long stride closer than it was a few years ago.
For more on the race to achieve the first quantum machine, check out Adrian Cho's feature "Faintest Thrum Heralds Quantum Machines" in the 29 January issue of Science.