Light-Based Computers Using Quantum Principles

Maddog2030 cites a story at Science Daily, writing: "Here's an interesting twist to all the news on quantum computing. A computer running similarly to a quantum based computer, except it runs on light at similar speeds for particular tasks. It also rids itself of the many complications introduced by quantum computing."

Re:Sounds very simple

Because it's cheating. First of all, because it has got nothing to do with quantum computing. Searching a database in parallel is only one aspect of quantum computing. Other aspects, like Shor's algorithm, totally depends on entanglement, which is something completely different from interfence. It's even worse, Shor's algorithm uses the fact that the combination of entangled states and partial measurements result in the destruction of interference. The second reason is that it will take you a rather long time to find out which of the 50 frequencies is altered. You have to do an interference measurements, which involves measuring light intensities with a moving detector. This is slower than a clasical computer.

Re:the cryptographic race begins again

No one has ever shown that quantum computers could break a symmetric encryption at more than twice as fast as brute force, and the actual clocking of quantum gates is not that fast.

What everyone is talking about with encryption is Public Key, where quantum mechanics may be able to reduce the brute force test to n operations given an n bit key, and n quantum gates. This is particularly true of those built on prime numbers.

This doesn't sound like quantum computing

This sounds more like holographic computing, rather than quantum computing. In QC, a search would involve manipulating the system until one state is left. In this article's example, you still have to iterate over the frequencies to find the frequency that changed. As I am given to understand QC, were this a true analog to QC you'd simple have one color standing alone, with no searching.

No link to quantum computers

The article describes a physical mecahnism that is faster than a algorithmic mechanism. This is nothing new. The one I leanred in first year CS was sorting spaghetti sticks. An algorithm will take O(n*log n) steps to sort them, whereas in the physical world it takes O(1) step: you pick them all up and tap the bottom of the cluster on a desk (lining them up, leaving the taller ones sticking out more).

Another example is finding the bounding convex polygon for a set of n points. I don't remember the runtime for the algorithm, but for the real world it's O(n): you get a board, nail in the n points, then find a rubber band and wrapp it around the nails.

The article describes another one of these problems that is solved faster with a physical process, in this case looking up a record in a database. By physically encoding data differently, you can find a record in a large set in a single step (well, maybe not since you still have to FFT the light to find the frequency, so I'm still not sure how this is faster than the O(log n) of an index, remember FFT is also O(log n) where n is the number of frequencies, and you need the number of frequencies to be the same as the number of records so it seems equivelent to me, but there may be some other way of determining the frequency of the altered light).

This can't be used to crack RSA, and it's not a general method of algorithmically running through a large number of possiblilities concurrently, which we get with quantum computers. There may be a way to crack RSA generically with a physical process (didn't Shamir come up with an optical process for 512 bit RSA). But this has nothing to do with that.

Does anyone think anymore?

That article was insipid to the point where I couldn't finish it. A few points in the first few paragraphs are worth mentioning:

1) Quantum speeds? WTF is that? There's no such unit, not even associated with quantum computing.
2) The device "mimics quantum interference". No, it's light; it displays quantum interference. Light is photons, quantum particles. Dur.
3) "performs some tasks a billion times faster". This is what I call a 'crazy number' since it's not based on any sort of measurement and thrown in only for show-value.

Don't get me wrong, I'm active in QC research and I like what the folks at Rochester are doing, so, too, the folks in an optics group at Los Alamos. But whoever wrote that Science Daily article is whacked out. It cheapens everything.

the cryptographic race begins again

So, it looks like this thing can do computations about a billion times faster then conventional methods....that's gotta knock a hole in current key lengths and security...I wonder how fast this thing has to get before brute forcing a 256 bit key becomes feasible. (big math people out there anywhere?)

Fortunately, that should also offer a slew of new possibilities for encryption schemes that were previously too slow or bulky.

On your mark, get set, encrypt

Holographic storage, yes.

This looks exactly like the idea of holographic storage. What is stored in the memory material is the interference pattern of the data and the address beams. Then you can either light it with an address beam (the address can be either a direction, or frequency, or maybe something else) and out comes the data content, or vice versa (grep). This is excellent for database and memory technology, but I see no connection to quantum computing here.

Disclaimer/shameless plug: I've recently compiled a semi-technical paper [ucam.org] on some of the theory behind quantum computing, as a project in our undergraduate physics course.

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Light Interference can't match Quantum

AFAIKS, this isn't scalable, which is the key to making a real useful Quantum Computer. The amplitude of the output, a single frequency component, must surely drop away with the number of entries in the database. So if you increase the number of entries by a factor of a million, the output beam is a million times weaker, and almost undetectable.

In analysing this sort of thing, the "size" of the problem is usually taken as the logarithm of the number of entries (ie the number of bits required to label each item). Since the strength of the output beam decreases linearly with the number of entries, it falls off exponentially with problem size.

Now, it can be shown that even with a Quantum Computer, the best we can do is to speed up the search by the square root of the number of entries. So 10^6 entries takes 10^3 searches, and so on. This isn't an exponential speedup (which is impossible for Unordered Search), but I can't see that this "light interference" method could match a quantum machine.

And it certainly couldn't match the exponential speedups on Factoring, the killer app for Quantum Computing.
StuP

Optical vs. Quantum Computing...

Yes but quantum computing Sounds So Much Cooler....

In all seriousness, this is the sort of situation where the Internet is more a hinderence than a help. Over time discussions such as this will polarize the lay community either for or against a particular area of research, wher two areas of research strive to achieve similar goals.

Public Opinion greatly influences funding of research, so I hope that premature dabates of which technology is superior, won't shape decisions to fund one or the other, since ther is the possibility that one or the other area of research might hit a brick wall at some time in the future, at which point it wll be nessecery to pursue the other area of study. It would be bennefitial to all to have continued both areas of research in parrelel.

Don't get me wrong. I don't believe that discussions like this alone will influence the course of research, but merely that the colaborative enviroment the Internet offers will promote (suprisingly) colaboration to the point where only one research path will be pursued by both teams, working together, rather than competing, as it were.This is an area whewre competition is a positive thing in academic research. I merely question the degree to which the Internet actually contributes to this.

--CTH

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Parallel data retrieval... dear god...

*smirks*

Let's look at the story for a second here folks.

The scientist set up a data-storage device (in this case an acoustically massaged medium), then an information retrieval was carried out against the medium. This retrieval was carried out in parallel. Now this is fairly exciting news, but it has some serious distance to go before it manages to become something general enough to threaten the intellectual-share of true quantum-entaglement computing schemes.

The promises for the device so far seem to be in determining data returns along mulitple paths. In effect, the thing is performing the many many calculations (in this case actually only data-retrievals). However, it's performing them in parallel.

In addition, I'm curious as to how the data is retrieved. If the recombinant beam must be compared to the original beam along all the frequency divisions, there's another indivisible operation requiring some length of time.

But....

It is an interesting method of encoding/decoding data from a medium to a laser without transducers. I'd say that this technology has great promise as a method to be derived from to create all-optical switching fabrics that are actually data-sensitive (how'd you love it if you could decode, process, and filter packet data from the very laser transmission that carried it down the fat fiber pipe...?)


Nietzsche on Diku:
sn; at god ba g
:Backstab >KILLS< god.