How far is quantum computing from reality?

[petergreat]

When will quantum computers become useful? For example, cracking some real-world encryption, simulating quantum behaviour of molecules, or enhancing web search engines?
I somehow have the impression that this is a field of physics where theorists keep dreaming and experimentalists keep playing with toy systems.

 

[petergreat]

Is it possible to achieve realistic quantum computing by successively scaling up current systems and gaining experience along the way, or are we facing major hurdles and need a revolution in technology to do it?

 

[cgk]

I wouldn't hold my breath. Currently quantum computers are hip, but fusion power or healing cancer or aids has also been hip some time ago. And we know how far that went.

Also, even if such machines existed, they would only really be useful for a very limited set of problems (decryption being one of them). The reason for this is the quantum "no cloning" theorem, which people tend to ignore when speaking about quantum computers. Most algorithms we know from computer science would be impossible to implement on a quantum computer.

There is one "quantum computing" paper which used the technique to calculate a H2 molecule:
http://www.sciencemag.org/cgi/content/abstract/309/5741/1704
It was once described as the most expensive 2x2 matrix diagonalization in history :)

 

[Zarqon]

A full scale quantum computer (QC) is still very far away (decades most likely) but quantum technology in a broader sense can be used much sooner than a full scale QC. For example, quantum cryptography, which is already working in commercial forms.

It's also the case that even though running a full useful quantum algorithm requires a lot more qubits then are currently possible (new recent record from innsbruck ion trap QC is 14 entangled qubits), it will be possible to use QC's much sooner in the form of quantum simulations. A recent paper for example described how the Dirac equation can be simulated with a reasonable amount of qubits.

It's also important to state that the reason so many are interested in building a QC is that there are actually very many problems in physics and chemistry that is and forever will be impossible to calculate without one. Good examples include calculations of complex molecules for producing medicines. Because it's the only way to ever simulate many things, it is worth putting money into the effort of building one, even if it's a long term goal. There is also a bonus you get along the way towards a QC, in the form of general understanding of a quantum phenomena. Very soon, if not already, even classical computers are going to contain components that are so small that they will require full understand of the quantum phenomena in order to function properly.

 

[Claude Bile]

When will quantum computers become useful? For example, cracking some real-world encryption, simulating quantum behaviour of molecules, or enhancing web search engines?

Done and done, though admittedly there is plenty of scope to increase the complexity of the molecule.

http://www.msnbc.msn.com/id/35187725/ns/technology_and_science-innovation/

I somehow have the impression that this is a field of physics where theorists keep dreaming and experimentalists keep playing with toy systems.

You may be suprised just how quickly "impossible" turns to "realisable" in experimental physics, particularly in a fast-moving field such as this :).

Claude.

 

[petergreat]

Done and done, though admittedly there is plenty of scope to increase the complexity of the molecule.

http://www.msnbc.msn.com/id/35187725/ns/technology_and_science-innovation/

I will be impressed if someone manages to calculate the properties of a molecule to the same precision but with a lower cost than using classical computers, or simulate a molecule that is simply intractable by classical computers. As far as I'm aware of, neither of above was the case regarding the experiment you quoted. So that doesn't satisfy my definition of being "useful". Though I don't deny the possibility that the technology may improve in future.