Source: Charles Marcus, Stanford University
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Normally, when you want to move electrons you apply a voltage and the electrons begin to flow. That is the basis of Ohm's Law: Electrical current equals voltage divided by resistance.
But a team of physicists from Stanford and the University of California-Santa Barbara (UCSB) report in the March 19 issue of the journal Science that they have invented a device that moves electrons without relying on voltage differences to push them around.
The device -- a "quantum electron pump" -- operates according to the laws of quantum physics, which describe what goes on in the sub-atomic world, rather than classical physics, which describes what happens in the everyday world. This means it may play an important role in a new field, called quantum information technology, that could provide the basis for new kinds of computers and other electronic devices in the next millennium.
Quantum physics is much different from classical physics. In quantum mechanics, particles do not always behave as solid particles, but can appear as probability waves. That means they have a certain probability of being in a number of different places at any given time. It is only when a particle is observed (not just by people, but also by other particles), that its presence is pinned down to a specific location.
At the atomic scale, particles such as electrons tend to behave as waves, whereas at larger scales, like the human scale, electrons, atoms and other particles are constantly interacting. As a result, they continually provide information about their properties to each other.
"That is why two coffee cups don't pass through each other," says Charles M. Marcus, assistant professor of physics at Stanford, who headed the research effort. His collaborators were graduate student Michael Switkes at Stanford and Arthur C. Gossard, professor of materials science at UCSB, and graduate student Kenneth Campman.
Midway between the atomic and human scales, at the nanometer to micrometer scale, it is difficult to keep particles coherent -- that is, capable of behaving as waves -- but not impossible. So the growing ability to create nanoscale structures has allowed researchers to create devices that operate according to the laws of quantum physics and so can exhibit radically new modes of operation.
Take the case of a quantum computer. In an ordinary computer, the primary quantity of information is the bit, which can take one of two values, "0" or "1." In a quantum computer, the bit is replaced by a "qubit," which can be "0", "1", or both "0" and "1" at the same time. Exactly what computers based on qubits will look like, and how well they will work, is not yet clear, but considerable effort is being expended to develop them.
Researchers are exploring the use of electrons, photons and atoms as the basic elements of quantum information systems. Before electrons can be used, however, researchers need some way to move them around without loosing coherence. That is where the quantum electron pump comes in.
The microscopic pump is a special kind of quantum dot. A quantum dot is a spot of electrically conducting material surrounded by non-conducting material that is smaller than an electron in its wave guise. Because of its small size, the dot constrains the electron's motion in all three dimensions. Some quantum dots are completely closed, but those that the Marcus group studies have openings that allow electrons to enter and exit.
The pump has two openings on one side with a gate in between that controls access to them. The other side of the dot contains two electrodes that change the "shape" of the dot. Its shape is not material, but is created by electrostatic forces that are generated by microscopic gates fabricated by the same techniques used to produce computer chips.
Slight changes in the shape of the dot cause electrons to enter or leave the device. When electrons enter the tiny cavity, they do so like water waves entering an enclosed bay. They bounce off the walls of the enclosure and overlap to form a complex "interference" pattern. Slight changes in this pattern cause electrons to enter or leave the area.
"If you want, you can picture the electrons as a liquid," says Marcus. "But it's a liquid that carries charge and, crucially, it's a liquid that interferes with itself."
The quantum dot becomes an electron pump when the researchers vary the charge on the two electrodes in a way that alters the amount that the dot's shape is distorted in a regular cycle. The researchers have operated the pump at frequencies ranging from a few million to about 20 million cycles per second and have determined that 20 or so electrons pass through it in a typical cycle. The direction that the pump pushes the electrons is random, and can be changed by small variations in an external magnetic field. Such randomness is a signature that the pumping is caused by properties of quantum waves, rather than classical physics, Marcus says.
Next, Marcus' research group will attempt to measure the degree of quantum coherence that survives the pumping action. The physicist expects to prove it possible to use this kind of pump to move charge around a chip without destroying its quantum mechanical properties, but the extent to which this can be done remains to be studied.
"These experiments are all carried out at a few hundredths of a degree above absolute zero, so don't look for a product in your dashboard next year," Marcus cautions. "We are mostly trying to figure out the new rules that take effect as chips get smaller and smaller, and how these rules can be used to advantage." The research was supported by the Army Research Office, the National Science Foundation, and the Air Force Office of Scientific Research.
Other relevant material:
Prof. Marcus home page:
http://www.stanford.edu/~cmarcus
Marcus research group web page:
http://www.stanford.edu/group/MarcusLab/
Journal
Science