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Quantum Physics Meets the Qubit
by Mark K. Anderson

2:00 a.m. Jan. 9, 2001 PST

AMSTERDAM -- Information, say the politicians and pundits, is the
natural resource of the New Economy.

But hold that thought. To what kind of information do we refer? Is it
the abstract concept of "0" and "1" -- entities, like God himself,
that never bow to the physical world but just are?

Scientists are convening in Amsterdam this week to continue a quest
begun two decades ago by Nobel laureate Richard Feynman, a quest to
dispel this Platonic fantasy. The Fourth Workshop on Quantum
Information Processing will be drawing scores of information
researchers from around the globe.

These scientists and mathematicians investigate the manifold ways
everything -- from computing to cryptography to the nature of
information itself -- changes when quantum mechanics, that 20th
century camel, sticks its nose into the tent.

Information, or at least the only kind of information computers can
work with, is innately physical. It must be represented in some
physical system -- say, the millions of on-off switches representing
"1" and "0" in a computer chip. And it must be manipulated and
processed within that physical system -- say, through the logic gates
of a computer's microprocessor.

No sand and wires, no info.

The distinction may seem inconsequential, but then again so did the
niggling that spawned Einstein's Theory of Relativity (minor
discrepancies in the measured speed of light).

Perhaps because of the extreme pace of miniaturization and sheer
number-crunching might over the past several decades, an invincible
mindset has emerged that sees information as exempt from the physical
universe around it.

"These [computers] -- the most complex things produced by the human
mind -- can be made indefinitely small because of a crucial
distinction," writes George Johnson in a Dec. 31 New York Times
editorial. "While ordinary machines work by manipulating stuff,
computers manipulate information, symbols which are essentially
weightless.

"A bit of information, a one or a zero, can be indicated by a pencil
mark in a checkbox, by a microscopic spot on a magnetic disk or by the
briefest pulse of electricity or scintilla of light.

"The special nature of information confers another advantage. The
power of computation can be leveraged and leveraged again. Design a
computer and then use it to help you design a better computer, ad
infinitum."

Well, sort of.

Charles Bennett of IBM's Thomas J. Watson Research Center studies the
fundamental, physical nature of information and information processing
and says that the time is long overdue for a re-think not only of the
computer but also of all the 0s and 1s that flow through its veins.

And we can start with those two little numbers.

"When this abstraction was successfully made -- in the 1930s to the
'50s -- the people who did it had left out the principles of quantum
mechanics, which have to do with distinguishability," Bennett said.

The early computer innovators set up a framework that continues to
this day. They thought of the bit as theoretically identical to zeros
and ones written on long strips of paper. A zero can forever be zero;
it will only change to one if the computer or computer operator makes
it so; it cannot be both zero and one at the same time; everyone who
wants to can see it; and observing it does nothing to alter its
"zeroness."

An obviously productive idea, although it's also one that has little
relevance to the elemental laws of physics that our ever-smaller and
faster computer chips now brush up against.

"(John) von Neumann and (Claude) Shannon, all those people thought of
information as classical, even though they knew better," Bennett said.
"They were aware of quantum mechanics. And indeed, von Neumann had
contributed fundamental things to quantum mechanics. But they thought
it was somehow irrelevant to information processing, like it would
just make the information less reliable. It would be a nuisance."

So "quantum information processing" is, in a sense, where all
computers will be headed when the IT size and speed scales improve by
several more orders of magnitude. It's time we get used to it: as far
as the laws of nature are concerned, zero and one are quaint
abstractions.

What comes closer to mirroring the physical universe is not the bit
--robust, unambiguous and infinitely manipulatable -- but rather the
quantumbit or "qubit."

Start computing with individual atoms, molecules, photons or nuclei,
and it quickly becomes evident how fragile, ephemeral and intricately
entangled the fundamental units of information can be.

To be discussed at QIP 2001 includes the kinds of programs that can be
run on a computer composed of qubits; the kinds of ways qubits can be
manipulated to do things no conventional computer can; the kinds of
quantum leaps in security and communication that qubits bring; and the
systems that need to be devised to make it all happen.

No small task, and one that will undoubtedly open more hidden doors
and passageways than the ones that are already known.

But at the root of it all is the qubit itself.

"What I say to people who don't like to think about many-dimensional
spaces is that classical information is like the information in a book
and quantum information is more like the information in a dream,"
Bennett said.

"If you have a dream and somebody asks you about it, there's a certain
privacy to it. Pretty soon, you're remembering your explanations
rather than what the dream originally was. So in the course of making
it public and making many copies of it, the original content of it is
altered in an unpredictable way.

"Of course, dreams are terribly inexact. But there's an exact
mathematics for handling this dreamlike information -- and it was
known even in the '20s. But ... it wasn't thought of as something that
had to do with the central notion of information. And that's what our
insight over the last several years is. That it should have been
included in our central notion of information, and that we are just
beginning to discover all that can flow from that."

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