## Opening Quantum Computing To the Public 191 191

director_mr writes

*"Tom's Hardware is running a story with an interesting description of a 28-qubit quantum computer that was developed by D-Wave Systems. They intend to open up use of their quantum computer to the public. It is particularly good at pattern recognition, it operates at 10 milliKelvin, and it is shielded to limit electromagnetic interference to one nanotesla in three dimensions across the whole chip. Could this be the first successful commercial quantum computer?"*
## Still not easy to build at home (Score:3, Interesting)

FTFA : "These things [quantum computers] can be very small and

very cold, and they can be built out ofexotic materials" - emphasis mine.He makes this sound as a good thing.

## Re:What does this mean for encryption? (Score:4, Interesting)

it also seems pretty hard to add more bits to these quantum computers, so it looks like traditional encryption might be here to stay after all.That is exactly the point. Qhantum-computers scale much, much worse than traditional computers. The problem is that tweo of these do basically give you the same maximum problem size as one does. (for traditional computers you can break problems into smaller steps. For Quantum computers you cannot, without loosing all the advanatges.) So you cannot use just more to break encryption. You need to build one with more qbits that are all entangled wich each other. My present impression is that the effort of adding qbits grows quadratically or the like, as each qbit has to be entangled with each other qbit (that is n*n entanglements). If that is true, even 100 qbits are far out of reach. This means that all modern encryption is perfectly safe from this quantum nonsense.

## It is if you are the NSA (Score:3, Interesting)

To keep our security agencies happy, quantum computers need to be almost impossible to make. The inventor of a really simple, cheap one is unlikely to have a successful career selling them to Joe Public.

## D-Wave a bit of scam (Score:5, Interesting)

I work with the IQC, we specialize in quantum computing, quantum crypto, and many other things like that. We are also joined partially with the Perimeter Institute (and they do mostly theoretical physics). Anyway, when I first joined the institute, we had a discussion about d-wave. No one believed that it was real, and in fact considers d-wave to be bad for the field. Many of you will probably remember the cold fusion controversy. What happened was that experiment that could not be reproduced was published. This enraged the scientific community. Also, this led to massive funding cuts, and killed off the field. QC has a more stable base, but if d-wave keeps on been publicized like this, and they can never prove their claims (remember that all the experiments and functioning of the QC are considered "trade secrets", they let no one look at it), then we may end up with skepticism from the funders. Keep in mind that the ones who donate have usually no clue what is happening in the field (politicians, ceos, etc, so they are "stupid" enough to be affected by this. Everyone in the field is in the back of their head hoping that its real, but with that chance being so low, we want d-wave to be forgotten.

## Re:Still not easy to build at home (Score:5, Interesting)

Thanks to scam companies like this more qualification is needed when referring to "quantum computing".

This is only a little better than the quacks who talk about "quantum healing energy"; they're exploiting the vague term "quantum computing" and the small amount of understanding to try and make a quick buck from investors.

## Re:1st thing I'd get it to compute... (Score:3, Interesting)

I'd write a Jeopardy program and have the only clue be "42". I'd like to see what the thing churns out.

## Re:D-Wave a bit of scam (Score:4, Interesting)

One would think that it should be possible to design tests which they could pass if they possessed the working technology, without them having to reveal how exactly they achieved the result.This is actually quite hard (and I'm not sure any such test exists).

One can distinguish two scenarios: (1) a quantum computer tas a box that gets a classical input, processes it and outputs a classical result. Then the only distinction between classical and quantum is speed - or rather "computational complexity" in the sense that the number of required computational steps sclaes differently with the size (in bits) of the input - hence by sending a series of queries with varying length and plotting the scaling one might conclude "this device is better than any known classical machine". But there are two caveats. one needs to go to really large input to see such a scaling and there's no proof that there does not exist a clever classical algorithm with the same scaling.

(2) one can demand more of a quantum computer, namely the capability to perform a universal set of gates and therefor prepare a large class of quantum states. There are well-developed criteria to verify that such states have been produced and that certain gates have been performed. If a universal set of gates has been implemented with sufficient quality one knows that the device is capable of performing quantum computations (but maybe this capability is not needed for QC). To apply this criterion, however, one needs to "look into the box" and perform measurements on the qubits.

This problem could be circumvented, if their supposed quantum computer would also have a "quantum interface" that allows input and output of quantum information (e.g., I send them a bunch of photons, they map their state into their computer, perfom a set of operations I ask them to do and then they write back the state ofthe qubits to photons and send them back to me for analysis. Then I could verify (not me, but experimentalists with the proper equipment) if the desired operation has indeed been performed.

Of course, d-wave does not claim that their device is a "universal quantum computer" or that it can prepare these kind of states. How their claims can be verified without looking into their device, I don't know.

## Re:What does this mean for encryption? (Score:5, Interesting)

The simplest example of a quantum computing algorithm is Deutsch's algorithm.

Here is how it works. Consider a simple boolean function b_out = f(b_in). It takes an argument that can be 1 or 0 and returns a 1 or 0. There are four possibilities: always zero, always 1, the identify, and logical not.

Now imagine that I give you a black box that computes 'f'. However, it is very, very slow --maybe internally it is computing some NP-complete problem. If you want to know which of the four functions the box calculates, you need to run it twice, once for zero and once for one.

However, suppose you simply want to find out whether zero and one map to the same or different values, i.e., the parity of f. With classical computers, you are screwed. You still have to run the box twice to find that even though you only want to get a single bit of information.

However, you can do better if the black box I gave you is a quantum implementation of f(x). By feeding in a input state that is a superposition of 0 and 1, I can detect in a single evaluation plus some simple operations whether the function is constant or not. However, in doing so I get no information about the specific value. Effectively I can ask any one-bit question about f(x) as efficiently as a specific value.

It unlikely this will every be useful as stated. While it is known how to efficiently translate every classical computing algorithm into a quantum version it is unlikely a real implementation would be within a factor of 2 in speed or cost. I believe it illustrates the basic idea. The character of other quantum algorithms is similar, you often feed in a superposition of all possible inputs and read a single output which is the specific answer you want with high probability without having to ever compute the values you don't want.