>
> 3/19/99
>
> CONTACT: David F. Salisbury, News Service (650)
> 725-1944;
>
> e-mail: david.salisbury@stanford.edu
>
> Breaking Ohm's law: Moving electrons without
> voltage
>
> 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 states
> 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 /
>
> -30-
>
> By David F. Salisbury
>
> The quantum electron
> pump is fabricated in
> the semiconductor
> gallium arsenide.
> Electrons, acting as
> quantum-mechanical
> waves, enter the
> central cavity through
> the gaps on the left
> side of the structure. From the point of view of
the
> electrons, the size of the cavity is determined by
the
> strength of the electrical fields around it. The
two
> electrode "plungers" in the right side distort the
> cavity's shape. When the electrodes operate in
phase,
> the pump does not produce a flow of electrons. When
> the electrodes operate out of phase, the electrons
> begin to flow.
>
> Source: Charles Marcus, Stanford University
>
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