快猫短视频

SUPPOSE you lived in a world where you could make a car run uphill by shoving
it gently back and forth, or send a pool ball straight into the pocket of your
choice just by shaking the table.

Such a world wouldn鈥檛 seem odd to a small band of researchers in an area that
is new even by the standards of frontier physics. The days will soon be over,
they believe, when electrons rolled predictably downhill, away from the negative
terminal in any circuit. They have discovered how to make electrons move around
without any directed voltage.

This is the new science of the quantum ratchet. With an oscillating or
randomly varying signal, you can produce useful, directable motion from what
seems like chaos. 鈥淵ou can make electrons go round in circles, or up or down,
you can make them run uphill. We can do everything with electrons that we do
with cars and buses in a city鈥攊t鈥檚 almost like a child鈥檚 game,鈥� enthuses
one of the leading players in the field, Peter H盲nggi of Augsburg
University in Germany.

By making electrons leap from one electrical component to another, we could
build electronics without connecting wires. And single electrons shunted around
at will could be used to store quantum information, and specially designed
compartments could form the logic gates of a generation of quantum computers. As
a bonus, quantum ratchets might even help us understand how our muscles turn
unfocused chemical energy into directed motion.

Any ratchet produces motion in one direction from a cyclical force. For
example, twisting a ratchet screwdriver back and forth drives a screw
relentlessly inwards. This relies on a ring of lopsided ratchet teeth: twisting
one way drags a sprung peg over the shallow side of each tooth, but twisting in
the other direction brings the peg up against the steep side of a tooth, pushing
the whole ratchet around. Ratchets appear in bicycle transmissions, turnstiles
and the escapements of pendulum clocks, which turn the pendulum鈥檚 swing into the
one-way motion of the hands.

If you build ratchets on a much smaller scale, things start to get weird. The
simplest sort don鈥檛 even work. Richard Feynman wondered whether a microscopic
ratchet would be moved in one direction by the random thermal movements of air
molecules. If so, you鈥檇 be able to build a perpetual motion machine. Feynman
proved, reassuringly, that such a ratchet would move at random back and
forth鈥攏o directed motion, so no useful work.

But when you actually put some effort into driving a microscopic ratchet, it
should work. In 1997, H盲nggi and his Augsburg colleague Peter Reimann
calculated what should happen to electrons confined in a series of lopsided
wells, like the teeth of a ratchet (Physical Review Letters, vol 79, p
10). If you add a voltage across the whole series of wells, it pulls on the
electrons, raising the potential of the wells at one end and lowering it at the
other. And when the wells are lopsided, their shape changes too
(see Diagram). A
vertical wall, like the left-hand ones in the diagram, isn鈥檛 affected by the
voltage; a sloping wall is made steeper or shallower.

Applying a positive voltage to the ratchet in the diagram makes the
right-hand side of each well shallower. The highest energy electrons can then
spill rapidly over into the next well. But a negative voltage only makes the
right wall steeper, and then electrons are trapped. So if you apply an
alternating voltage, the electrons shuffle step by step to the right.

So far, this is just like a classical ratchet, where a peg slips over the
shallower slope. But transferred to the electronic world, it could be useful.
Electrons powered by AC signals could run against a static electric field. 鈥淵ou
can make electrons go `uphill鈥�,鈥� says H盲nggi.

Then H盲nggi and Reimann discovered that quantum theory can turn things
upside down鈥攐r rather, back to front. At low temperatures, when the
electrons sit near the bottom of each trough, they can鈥檛 get over either wall.
Classical physics says they should be permanently trapped.

Escape route

But according to quantum theory, they can sneak out. Because an electron is a
probability wave, without a well-defined position, it can never be entirely
contained by the walls of the potential. So electrons have a small probability
of finding themselves on the other side of a barrier, leaking through in a
process called tunnelling.

Electrons can tunnel in both directions through the ratchet. But tunnelling
is much more probable through a thin barrier than a thick one. So at low
temperatures, H盲nggi and Reimann calculated, the overall current must be
dominated by electrons leaking through the thin part of the tooth to the left
when the voltage is negative
(see Diagram). Again, there is net electron
movement, but on the other half of the voltage cycle, and in the other
direction.FIG-mg22224301.JPG

That鈥檚 the theory. Last month, Heiner Linke of the University of New South
Wales in Sydney, and his colleagues from Lund University in Sweden, confirmed it
(Science, vol 296, p 2314).

Linke started by making a string of triangular quantum dots, each about a
micrometre long. A quantum dot is an area of semiconductor which acts like a
well for electrons鈥攊ts walls hold them inside the dot. Because Linke鈥檚
dots are triangular, electrons get squeezed together at the narrow end. That
confinement increases their energy鈥攊n other words, it makes the potential
higher at the narrow end. So the electrons feel the string of dots as an
asymmetrical sawtooth.

Sure enough, Linke saw a current that reversed when he raised the
temperature.

But a real surprise, and a potentially very useful one, appears at much lower
temperatures. Towards the end of 1998, Linke, then at Lund, and colleagues from
the Niels Bohr Institute in Copenhagen, were tinkering with a triangular quantum
dot a micrometre across, which they had cooled to just 0.3 kelvin.

With a gentle alternating signal, the researchers saw a directed current. But
when they slightly adjusted the strength of the signal, the current went into
reverse. Somehow, the small change made the electrons abandon their previous
route and come out on the other side. It is as if you could make your clock run
backwards by giving its pendulum a nudge.

A delicate quantum effect called interference is responsible. The electron
waves, with a wavelength almost as large as the dot, interfere with one another.
They combine to cancel out in some places and add in others, so an interference
pattern of peaks and troughs sits inside the quantum dot. A peak means there is
a high probability of finding an electron in that particular place; a trough
means a low probability. So if a peak coincides with one side of the quantum
dot, electrons are more likely to escape from that side, and a current will flow
in that direction.

A voltage affects how the waves bounce around inside the dot, says Linke,
changing the interference pattern. That can turn the current around by making
electrons leak from a different side of the dot.

Interference is so sensitive to other factors that Linke cannot even tell
which way the ratchet current will flow until he switches the device on. 鈥淚t鈥檚
difficult to construct a dot that will only direct the current in one direction.
Any little deviation in the shape of the dot on the scale of the electron
wavelength鈥�40 nanometres鈥攚ould affect the interference.鈥�

Linke is delighted at the fine degree of control that interference could
afford, in theory. Understand how the dot鈥檚 shape affects the electron current
and you could design dots to perform specific tasks. String such dots together
and you could build logic gates. Being controlled by very small voltage changes,
these gates would be faster and consume less energy than the gates we use today.
And because the full quantum state would be preserved, they would be ideal for
quantum computers.

But it鈥檚 not easy. Preserving quantum coherence in between the dots would
need very low temperatures, where the electrons have well-defined energies and
there is little confusion from vibrations in the semiconductor material. If the
dots can be made much smaller, they might work without expensive cooling, but
this is probably a long way off.

In the meantime, devices might be made from quantum ratchets that don鈥檛 rely
on interference. H盲nggi and his colleague Igor Goychuk think that two input
signals in a tunnelling ratchet might be better than one (Europhysics
Letters, vol 43, p 503). Like having two oars on a rowing boat, combining
two signals could let scientists steer an electron current. Alter the phase
difference between the signals, and, says H盲nggi, you can control the
direction of the motion in two dimensions. This makes many different outcomes
possible, so logic gates built on this principle would have a range of outputs,
reducing the total number needed.

Axel Lorke and his colleagues at the Ludwig-Maximilians University in Munich
are looking at another way to steer electrons. They created an array of
triangular 鈥渁ntidots鈥�, small areas of a semiconductor surface where electrons
cannot enter. Infrared radiation shakes the electrons so they crash against the
antidots rather like balls hitting the obstacles on a pinball table (they are
too warm to behave like quantum-mechanical waves). They tend to get funnelled
into the narrowing gaps between the dots. So the array turns the jiggling of the
infrared radiation into a well-directed beam of electrons.

This could lead to wireless electronics. Just arrange your blocks of antidots
to point in different directions, and bathe the lot with infrared. Electrons
will then whizz around, following the arrows whichever way they point鈥� and
you can even send several electron beams across one another. No more need for a
nightmare of connecting wires.

What鈥檚 more, it only takes one step to carve an array of antidots from a
piece of semiconductor, compared with the 20 or more needed to make the
complicated structures of modern electronics.

Lorke has already made an antidot rectifier that works at up to 77 kelvin.
And, because these devices are classical not quantum in nature, they should work
at even higher temperatures. 鈥淚f you think this is something that might find
uses in a market then you can start to tweak up the temperature,鈥� he says.

Charles Marcus of Stanford University in California believes that quantum
ratchets are a revolution waiting in the wings, but he says it鈥檚 too soon to
tell just how big it is going to be. 鈥淲e鈥檙e two steps away from making it
useful. We don鈥檛 even know what it is we鈥檝e got, and the rules of the game are
not yet known,鈥� Marcus says.

But electronics isn鈥檛 the only game in town. Linke points out that as
electrons carry heat, quantum ratchets could be used as heat pumps, perhaps for
cooling single microscopic components on a chip.

Quantum ratchets might even help researchers understand molecular motors.
These tiny engines are biology鈥檚 ratchets
(快猫短视频, 13 December 1997, p 38).
They take the directionless energy released in a chemical reaction,
and somehow produce motion in one direction. Our muscles are huge arrays of
molecular motors working in concert. Although it seems very unlikely that
muscles really are quantum ratchets, they may have quantum effects operating
within them.

Muscles contract when layers of two different protein fibres, actin and
myosin, slip over one another. Each myosin fibre has branching 鈥渉eads鈥� which
attach to sites along the actin fibre and walk along from site to site. To do
this, the myosin heads change their shape, driven by electron transfer inside
the protein. As all this happens at the atomic scale, quantum effects are
probably involved, says Linke.

Whatever the truth about real biomotors, H盲nggi is sure that those
looking to build machines on the nanoscale had better take note of quantum
ratchets. 鈥淎ny machine on a microscopic level cannot neglect quantum laws,鈥� he
says.

H盲nggi points out that we are now building ratchets of every size from
just a few micrometres to the human scale. Ratchets that work in the quantum
world could soon be used in electronics. Biologists are developing narrow
sawtooth channels to separate DNA fragments of different weight. H盲nggi is
building a ratchet that can separate different-sized microscopic particles in
suspension (ideal, he says, for segregating healthy cells from sick ones). And
then, in the macroscopic world, there鈥檚 your ratchet screwdriver.

Strangely enough, turning the screw relies on muscles whose mechanism might
be explained by the quantum ratchets. It鈥檚 a complete circle, you might say. But
it only turns one way.