Quantum Computer Demoed, Plays Sudoku

prostoalex writes "Canadian company D-Wave Systems is getting some technology press buzz after successfully demonstrating their quantum computer (discussed here earlier) that the company plans to rent out. Scientific American has a more technical description of how the quantum computer works, as well as possible areas of application: 'The quantum computer was given three problems to solve: searching for molecular structures that match a target molecule, creating a complicated seating plan, and filling in Sudoku puzzles.' Another attendee provides some videos from the demo." Anyone want to guess how long before "qubit" gets compressed to "quit" (as "bigit" became "bit" in the last century)?

Comments

Traveling Salesman

[Short Circuit]

Does this mean we'll be able to solve the Traveling Salesman problem soon? That would lead to a revolution in efficiency of everything from travel to mass transit to shipping.

I imagine the USPS and other shipping organizations will be the first to buy commercial versions of these.

Re:Traveling Salesman

[Ed Avis]

Nobody is going to use Travelling Salesman in the real world to plan journeys. You can already quickly run an algorithm which will get you a journey plan that's maybe 99% as good as the optimum. Besides, real things (even salesmen) don't just have to travel in straight lines between points in space. There are other factors like lunch breaks and the location of good restaurants, which the problem doesn't account for.

Travelling Salesman is NP-hard, which means (I think) that if you find a polynomial time algorithm for it, any problem in NP can be done in polynomial time (hence P=NP). But I believe that even this quantum computer can't calculate TSP in polynomial time, except for instances small enough to fit inside the 16-qubit memory. Can anyone comment on this? By contrast, a conventional computer can always calculate TSP in the same time complexity (exponential time) no matter how large the input - you just have to hook up enough memory.

Re:Traveling Salesman

[tbo]

Disclaimer: I am a physicist who works on quantum computing and also has a computer science background

Nobody is going to use Travelling Salesman in the real world to plan journeys. You can already quickly run an algorithm which will get you a journey plan that's maybe 99% as good as the optimum.

Actually, I think there is a theorem that finding an algorithm that efficiently produces highly-accurate approximate solutions to arbitrary problems in NP-hard is about as hard solving NP-complete problems exactly.

All this aside, it's worth noting that D-Wave is only claiming to provide a square-root speed-up for NP-complete problems, and there is some doubt as to whether they can even deliver that, as they scale up to larger numbers of quantum computers. While it's technically possible that P=NP, most people believe P!=NP, and it seems almost as likely that BQP != NP (BQP is the class of problems efficiently solvable with a quantum computer).

For an excellent discussion of what D-Wave has done and just how skeptical you should be, visit Scott Aaronson's blog. (No, I am not Scott Aaronson, but I do know him and can vouch for him being an extremely smart guy. I am also not the Quantum Pontiff, aka Dave Bacon.)

Curious

[vlad_petric]

I'm actually curious - for how long do the 16 qubits stay coherent? You can only do quantum computations while the qubits remain coherent. Furthermore, IIRC coherence times where (at best) in the range of a few microseconds.