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Daniel Lidar joined USC College last fall. He is preparing for the quantum future.
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Preparing for a Quantum Leap in Computing
Daniel Lidar foresees a future where
scientists put quantum physics to work, and he’s already figuring out
ways to keep quantum computing stable and safe.
By Tom Siegfried
March 2006
Imagine a place where anything possible always happens, like a TV screen that displays all the channels at once.
If that seems beyond imagination, you are not alone. The world of
quantum physics is so weird that even the scientists who study it say
it challenges everyday concepts of common sense. The field has grown
from a realization that at the smallest scale — the realm where atoms
and molecules roam — the classical equations that Isaac Newton used to
describe the physical world no longer apply. In this realm, matter
behaves differently, and many realities can co-exist. Particles like
electrons, for instance, occupy several locations at the same time,
behaving more like fuzzy waves than solid pebbles.
Fortunately, such weirdness mostly confines itself to the inner life of
atoms. But a new quantum world is coming, where scientists hope to
preserve the quirky diversity of the subatomic realm. This would allow
them to devise superfast computers, design new drugs and guarantee
security for sending secret messages.
Harnessing the power of the quantum realm requires coordinated planning
from experts in fields ranging from physics and chemistry to electrical
engineering. And that puts USC College’s Daniel Lidar in a perfect
position to help prepare for the quantum future. A physicist with joint
appointments in the departments of chemistry and electrical
engineering, Lidar is a leader in current efforts to transform quantum
physics from theoretical curiosity to cutting-edge information
technology.
As the son of two scientists (a biochemist and pharmacologist), Lidar
was constantly exposed to scientific thinking while growing up in
Israel and Holland. He earned his Ph.D. in physics from Hebrew
University in Jerusalem in 1997, and soon thereafter began exploring
the emerging field of quantum information theory.
After a postdoctoral position at Berkeley and several years on the
faculty at the University of Toronto, he migrated to USC last fall. He
was drawn by Southern California’s growing status as the world’s
leading region for the new quantum research enterprise.
“This is a real hub,” he said, noting that USC, Caltech and UC Santa
Barbara all boast strong programs. “Southern California is probably the
world capital of activity in my field.”
In the mid-1990s, Bell Labs mathematician Peter Shor initiated the
quantum information revolution by proving that a computer using quantum
programming could crack the toughest of today’s secret codes, used for
governmental, military and financial communication. About the same
time, other research showed that only another quantum system could
provide absolute protection against any illicit eavesdropping.
Work by Lidar and his collaborators has focused on how to protect the
delicate process of quantum computing from attack — by nature itself or
malicious hackers.
So far, quantum computations have been performed only in rudimentary
laboratory experiments. If feasible on a larger scale, quantum
computers could solve some difficult problems at a fraction of the
speed of today’s fastest supercomputers. The trick relies on those
multiple quantum realities. Like the TV screen showing every channel at
once, a quantum computer could process all the numbers in its memory
simultaneously, rather than one computation at a time. It’s a bit like
finding which of a thousand keys opens a lock; instead of trying one at
a time, you could just spin one key in the lock until it opened.
Certain problems that would tax a supercomputer for a trillion years
could yield to a quantum computer in minutes.
But such speed is available only as long as the multiple quantum
calculations can be protected from outside interference. And the same
process nature uses to make rocks and people solid, instead of fuzzy
like electrons, conspires to keep that time very, very short. That
process, known as quantum decoherence, is usually an immediate and
inevitable result of interaction with the environment — collisions with
atoms or mere particles of light can cause a frail ensemble of multiple
quantum realities to crash.
Lidar and colleagues have shown, though, that some quantum computing
set-ups are at least partially immune to the ravages of decoherence. By
designing an apparatus with “decoherence free subspaces,” quantum
information can be preserved in the face of environmental insults. The
solution is to make sure that external effects exert a symmetric effect
on the quantum storage sites. (If one bit of information is altered, so
is its partner, so the two together retain a record of the stored
information.)
A more difficult challenge may arise on a future “quantum internet”
where quantum computers share data. Nobody had considered the potential
for quantum viruses afflicting such a network until last year, when
Lidar and post-doc Lian-Ao Wu proposed a scheme for fighting such
“quantum malware” in a paper to be published in the journal Quantum
Information Processing.
“Essentially the proposal is to do the analog of backup,” said Lidar,
an associate professor hired as part of the College’s Senior Faculty
Initiative. Only legitimate users of a system would be told when “real”
data is being transmitted. During the remaining down time, the quantum
data could be stored on a secure device, off the network, while bogus
transmissions serve as a decoy for intruders. A hacker would never know
when the system was vulnerable, and constant intrusion attempts would
be easy to detect.
“It’s the first look at this problem,” Lidar said, and much further
work will be needed to devise foolproof protection and a quantum virus
cleanser if infection is successful.
For now, of course, quantum viruses are of no serious concern, as there
is no quantum network to attack. But Lidar foresees a growing
likelihood that quantum technology will soon play a significant role in
sending secure messages and eventually in computing.
“It’s a field that is likely to have a widespread impact in the context
of secure information transmission,” he said. “It is the most secure
method of information transmission that we know of.”
As for quantum computing, its advantages are limited to certain types
of problems; quantum computers are likely never to be good for word
processing. But they could prove valuable in economically important
realms such as designing drugs from scratch, by computing the quantum
rules governing how biological molecules interact. Any such uses
depend, of course, on effective hardware for building quantum computing
devices, which might require advances in nanotechnology approaches for
fabricating the necessary materials.
Thus while Lidar focuses on theory, he emphasizes the need to develop the experimental side of the field as well.
“My dream for USC would be to develop not only as a leading theoretical
place, which I believe it is . . . but also to strongly develop the
experimental capabilities here,” he said. “That would really put us on
the map.”
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