
Quest for Quantum Computers Heats Up - jonbaer
http://www.scientificamerican.com/article/quest-for-quantum-computers-heats-up/
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mturmon
People have been searching for the applications of quantum computers --
excluding the adiabatic QC's associated with D-Wave, which are somewhat in a
different class. I found this part of the article very interesting regarding
applications:

"[Mathias] Troyer has spent the past three years looking for an answer—a
'killer app' for quantum computing that would make the development costs
worthwhile. The two classic examples, code-cracking and searching databases,
are not good enough, says Troyer. Shor's algorithm will require thousands of
qubits to do any serious factorization, he says, and there are other forms of
encryption that a quantum computer would do nothing to solve. And although
quantum computers may search databases faster, they are still limited by the
time it takes to feed the data into the circuit, which would not change.

"Troyer thinks that a much more fruitful application for the near future is
the modelling of electrons in materials and molecules—something that quickly
becomes too difficult for today's supercomputers. At first, this, too, seemed
a long shot. His early estimates suggested that it would take a quantum
computer as long as 300 years to simulate the molecular dynamics of even a
small molecule—such as the iron sulphide inside the ferredoxin proteins that
are involved in nitrogen fixation in plants. “Clearly, that was on the border
of being science fiction,” he says. But by rewriting the software, he brought
the figure down to 30 years—then to just 300 seconds. “Just like in classical
computing, where one has to sit down and optimize the algorithm,” he says,
“the same is needed for a quantum algorithm.”

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Xcelerate
This is very exciting to me! It's a great time to be interested in quantum
mechanics.

Quantum computers would pave the way for the holy grail of computational
physics and materials science: perfect QM simulations. Imagine performing a
simulation on a computer that matches up exactly* with experiment. Now that
would be exciting. You could actually develop new materials right from the
comfort of your chair at Starbucks (SSHing into a supercomputer of course).
Your simulations would even be _more_ accurate than a lot of experiments
because of the absence of environmental and experimental errors.

Recalling history, it's amazing to me how quickly the idea of a nuclear weapon
went from conception to implementation. Also, it only took like 9 years to get
a person to the moon. You basically had a bunch of the world's greatest
scientists all gathered in one location with the motivation to get the task
done. I wonder what would happen if a similar situation was applied to quantum
physics research.

* Leaving out plenty of details here. Quantum computers can tackle the NP hard fermion sign problem, which is the current main obstacle in high accuracy QM simulations, but there are still some lingering questions about whether the process of inputting a particular system into a quantum computer is itself an NP hard task. Not to mention that some heavier elements require relativistic treatment, the fact that I'm talking about quantum chemistry rather than particle physics, and the fact that even with a polynomial time algorithm, quantum computers may still be too slow to simulate a large system in a practical amount of time. But I can dream.

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tfgg
Their application to quantum chemistry/condensed matter is certainly pretty
exciting. Currently the practical method for realistic system sizes, density-
function theory (DFT), is barely quantitatively accurate for many important
systems/properties. I've heard it suggested that current DFT codes could use a
quantum computer as a co-processor to handle strongly correlated parts of
systems, which is an interesting idea. Just speeding up post-Hartree Fock
methods would be nice, and they're already polynomial but impractical!

Do you have any references for the hardness of putting the quantum system into
the quantum computer? I haven't heard of that issue before. I wouldn't have
thought relativity is much of a problem, that's just a tweak to the
Hamiltonian.

~~~
mathgenius
This is still very much work in progress:
[http://arxiv.org/abs/1406.4920](http://arxiv.org/abs/1406.4920)

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buhrmi
"A 30-year slog to develop a useful quantum computer may finally be on the
verge of paying dividends"

Hahaha... Define "useful"

