A long-standing quest for science and technology has been to develop electronics and information
processing that operate near the fastest timescales allowed by the laws of nature.
A promising way to achieve this goal involves using laser light to guide the motion of electrons
in matter, and then using this control to develop electronic circuit elements—a concept
known as lightwave electronics.
Remarkably, lasers currently allow us to generate bursts of electricity on femtosecond timescales—that
is, in a millionth of a billionth of a second.
Yet our ability to process information in these ultrafast timescales has remained elusive.
Now, researchers at the University of Rochester and FAU have made a decisive step in this
direction by demonstrating a logic gate—the building block of computation and information
processing—that operates at femtosecond timescales.
The feat, reported in the journal Nature, was accomplished by harnessing and independently
controlling, for the first time, the real and virtual charge carriers that compose these
ultrafast bursts of electricity.
The researchers’ advances have opened the door to information processing at the petahertz
limit, where one quadrillion computational operations can be processed per second.
That is almost a million times faster than today’s computers operating with gigahertz
clock rates, where 1 petahertz is 1 million gigahertz.
In recent years, scientists have learned how to exploit laser pulses that last a few femtoseconds
to generate ultrafast bursts of electrical currents.
This is done by illuminating tiny graphene-based wires connecting two gold metals.
The ultrashort laser pulse sets in motion, or “excites,” the electrons in graphene
and, importantly, sends them in a particular direction—thus generating a net electrical
current.
Laser pulses can produce electricity far faster than any traditional method—and do so in
the absence of applied voltage.
Further, the direction and magnitude of the current can be controlled simply by varying
the shape of the laser pulse (that is, by changing its phase).
The research groups have been working for several years to turn light waves into ultrafast
current pulses.
In trying to reconcile the experimental measurements with computational simulations, the team had
a realization: In gold-graphene-gold junctions, it is possible to generate two flavors—“real”
and “virtual”—of the particles carrying the charges that compose these bursts of electricity.
“Real” charge carriers are electrons excited by light that remain in directional motion
even after the laser pulse is turned off.
“Virtual” charge carriers are electrons that are only set in net directional motion
while the laser pulse is on.
As such, they are elusive species that only live transiently during illumination.
Because the graphene is connected to gold, both real and virtual charge carriers are
absorbed by the metal to produce a net current.
Strikingly, the team discovered that by changing the shape of the laser pulse, they could generate
currents where only the real or the virtual charge carriers play a role.
In other words, they not only generated two flavors of currents, but they also learned
how to control them independently, a finding that drastically augments the elements of
design in lightwave electronics.
Using this augmented control landscape, the team was able to experimentally demonstrate,
for the first time, logic gates that operate on a femtosecond timescale.
Logic gates are the basic building blocks needed for computations.
They control how incoming information, which takes the form of 0 or 1 (known as bits),
is processed.
Logic gates require two input signals and yield a logic output.
In the researchers’ experiment, the input signals are the shape or phase of two synchronized
laser pulses, each one chosen to only generate a burst of real or virtual charge carriers.
Depending on the laser phases used, these two contributions to the currents can either
add up or cancel out.
The net electrical signal can be assigned logical information 0 or 1, yielding an ultrafast
logic gate.
The researcher says that it will probably be a very long time before this technique
can be used in a computer chip, but at least we now know that lightwave electronics is
practically possible.
This new, potentially transformative technology arose from fundamental studies of how charge
can be driven in nanoscale systems with lasers.