> > CB: I didn't know about the chemically bonding contamination. How
> much is contaminated ? Sounds like a small percentage .
> >
> > How about taking a bunch of hydrogen and oxygen and combining it
to
> make new water ?
> ===========
> That's where the Star Trek technology comes in. You'd need a quantum
> computer capable of synthesizing probability amplitudes from the
> Planck scale; it's not even decidable whether it's possible yet, let
> alone if it would ever be technically and economically feasible.
>
> Ian
============================
For just a hint of how hard it will be to get started down that
road....
http://www.newscientist.com/hottopics/quantum/parallel.jsp
Parallel power
With their multiple personalities, quantum states could form the heart
of a massively parallel computer
WHAT would you get if you crossed a Bose-Einstein condensate with
Schroedinger's cat? One big, chilly cat? A litter of identical but
indeterminate kittens? In fact, no: you would get a quantum computer.
But let's take that a little more slowly.
A conventional computer is a decidedly classical machine. It relies on
electric currents acting as the 0s and 1s of a binary arithmetic
system, and those 0s and 1s have to influence each other in absolutely
predictable and dependable ways if they are to generate all manner of
complex calculations and manipulations. If some of those 0s and 1s
started behaving in an indeterminate, probabilistic way, what you
would have, as a rule, is a computer that makes random mistakes.
But quantum theory is not wholly random. It does obey rules. When
quantum states interact, they do so in an entirely predictable way.
It's only in the outcome of a measurement that unpredictability crops
up. Imagine a quantum computer in which the internal calculations and
operations take place through a series of entirely predictable
interactions of quantum rather than classical states. If no
measurements disrupt the system - and that's a big if, since any kind
of random, uncontrolled disturbance amounts to a measurement - nothing
unpredictable happens. The quantum computer can carry out a
calculation in a reliable way, just as a classical computer does.
The logic elements of a quantum computer could be, for example, the
"half-up, half-down" Schroedinger cat-state rather than the definite
up and down states a classical computer would use. And the computer
would resemble a Bose-Einstein condensate because you would need an
awful lot of these quantum states acting together coherently to
perform any sort of useful or interesting calculation.
Why go to all this trouble? The answer, as was most clearly pointed
out by David Deutsch of Oxford University in the mid-1980s, is that
quantum computing allows a kind of parallelism a classical computer
can't hope to match. In standard computing, any part of the internal
logic, whether a single 0 or 1 or a whole string of such bits,
represents a specific numerical state. But in quantum computing, an
electron spin or a photon polarisation - a "qubit" - can represent two
states simultaneously. A "half-up, half-down" state is both 0 and 1 at
the same time. What's more, when this state interacts with other
states, both parts of its dual identity participate in the
interaction. A quantum calculation lets 0 and 1 take part in the same
step, and at the same time.
With two "half-up, half-down" electron spins, for example, you can
have four different states - representing 00, 01, 10 and 11.
Similarly, with a string of 10 states you could simultaneously
represent all the numbers from 0 to 1024 (210). Two such states might
then interact in such a way as to produce an even more complex final
state which contains - again, all at once - representations of all the
numbers in the 1024 x 1024 multiplication table.
A conventional computer has to march through more than a million
individual calculations to work out all the numbers in this table.
Because a quantum computer explores all the possibilities
simultaneously, it reaches the same result in a single effortless
step.
So quantum computers could be enormously powerful. However, there are
two difficulties with exploiting their capacity for massively parallel
calculations. First, decoherence is hard to beat. It takes enormous
care and effort to create Bose-Einstein condensates and Schroedinger
cat atomic states in the lab. Any kind of random disturbance or
interaction will destroy the exquisitely delicate mix of coexisting
quantum states, forcing a single classical state to emerge instead.
Maintaining the integrity of a quantum computer would therefore be
incredibly hard. A whole array of complex and, importantly, different
states would have to be created, sustained, and made to interact in a
prescribed manner. (In a Bose-Einstein condensate, by contrast, all
the states are identical, which would be like having a quantum
computer whose bits are all permanently fixed at zero.)
The second problem is more subtle. How do you extract the result of
any calculation from a quantum computer? As soon as any measurement is
made on the quantum "multiplication table" state, for example, the
computer will respond with just a single answer out of the 1024 ¥ 1024
possible answers that could have emerged. All the rest are lost. Which
seems to defeat the purpose of doing the simultaneous quantum
calculation in the first place.
What this really shows is that quantum computers are likely to be
better at some kinds of calculations than others. The problem of
finding the prime factors of a large number, for example, has acquired
considerable urgency in the realm of code-making and breaking. The
security of many encryption systems hinges on how hard it is for a
conventional computer to find prime factors. It has to check all the
possibilities laboriously until it hits on the right combination. But
a quantum computer, in principle, could check on all possibilities at
once. What's more, this is a problem where the whole point is to have
a single answer emerge out of all possible (but wrong) answers -
something that a suitably defined measurement of the state could
achieve.
In a similar vein, Lov Grover of Bell Labs recently devised an
algorithm that would pick out a desired entry from a list of scrambled
entries in a time proportional to the square root of the number of
items in the list. Conventional searches would take a time
proportional to the number itself.
But how would you actually build a quantum computer? A number of
researchers have recently hit upon the idea of using the individual
atomic spin states within molecules. Single protons, the nuclei of
hydrogen atoms, have spins that can point up or down relative to the
spin of some other nucleus in a molecule. Pulses of radio waves with
the right frequency can flip these spins up and down, and the spins of
different nuclei on the same molecule interact in predictable ways.
This could form the basis of a quantum computer: the spins would store
the information and the radio waves would make them flip according to
plan, to carry out your calculations.
We already have some of the necessary technology. Nuclear magnetic
resonance imaging, now routine in medicine, maps out the position of
atoms by measuring their spins. A quantum computer's central
processor, its Pentium chip, could conceivably be nothing more than a
beaker of some suitable liquid, whose molecules would include a
variety of atomic spin states specially chosen to perform a set task.
Another possibility is to use a silicon chip etched with dots and
doped with atoms whose spins would be the computer's qubits. No
electric currents would flow in this chip: instead spins would nudge
each other along, dot to dot, passing along their message according to
the dictates of quantum logic.
