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Re: Light pulses crack security codes within seconds DO READ DJF
From: David Farber <dave () farber net>
Date: Sat, 18 Aug 2007 16:14:14 -0400
I keep pointing out that University PR departments are hyping things out of reason. At leats this is an advance Many are all hot air
Begin forwarded message: From: Rod Van Meter <rdv () sfc wide ad jp> Date: August 18, 2007 11:39:47 AM EDT To: dave () farber net Subject: Re: [IP] Light pulses crack security codes within seconds http://www.tgdaily.com/content/view/33425/118/ Wow, that's one of the most egregious quantum computing-related articles I've ever seen. I'm not even sure where to start. First off, let's point at the real research paper: http://www.sciencemag.org/cgi/content/abstract/317/5840/929 Coherent Optical Spectroscopy of a Strongly Driven Quantum Dot Xiaodong Xu, Bo Sun, Paul R. Berman, Duncan G. Steel, Allan S. Bracker, Dan Gammon, L. J. Sham I read it. It's an advance, but does not yet mean anything at all is practical. Their work is on the optical properties of self-assembled quantum dots. There are two major categories of quantum dots in semiconductors, self-assembled and lithographically created (and within each of those, many types). The self-assembled dots are a compound grown on top of a substrate of a different kind. Differences in the crystalline structure mean that the deposited material "beads up", like water on a freshly-waxed car. The quantum dot itself then is a place where the motion of electrons can be confined to a small two-dimensional area at the interface between the materials, creating a place where quantum wave functions can behave like an "artificial atom". The work presented in the paper is some of the first solid experimental work on the optical properties of self-assembled dots that I have seen, though I'm not an expert. Various groups, including that of my adviser, Kohei M. Itoh ( http://www.appi.keio.ac.jp/Itoh_group/ ), have been working for years on the growth and mechanical characteristics (stress/strain, size and shape, etc.) of self-assembled dots. All of that has been very hard work, and as far as I know no one has a reliable way to grow the dots in a given place. I wish they had a micrograph of the device, I'd like to see it. But the TG article talks only a little about the research itself; it's mostly breathless pie-in-the-sky reporting on the possibilities of quantum computers. "Light pulses crack security codes within seconds," the title reads. Wow. Well, first off, it can't be done yet, and won't be done for years, despite the present tense. Second, saying it's done with light pulses is like saying we compute today with electrons. It's true, but tells you nothing about transistors or computer architecture. Third, "crack security codes" is as vague and non-technical as it gets, not to mention outright wrong (we'll come back to that). Fourth, "within seconds" presumes many things about a quantum computer that are not yet defined to any level of precision. This topic is the focus of my research: how do you build a large-scale quantum computer out of a given technology? No one really knows yet. Which security codes does a paper on the spectroscopy of a quantum dot break? Well, none, really. But where they're headed with that is obviously Shor's algorithm for factoring large numbers on a quantum computer. If the algorithm can be efficiently implemented, it is theoretically capable of breaking RSA public-key cryptography and elliptic curve crypto. HOWEVER, the advantage may well be with the defenders on this one. Shor turns a super-polynomial problem (factoring) into a polynomial one. Not coincidentally, the complexity of running Shor is similar to the complexity of doing the encryption in the first place. And running an algorithm of the same computational class on a quantum machine will probably always be harder than running an algorithm on a classical computer. So, raise your key length and you might be okay. Shor does nothing to affect symmetric key cryptography, or any system not dependent on the factoring problem. I hesitate to mention this, for fear it will be misinterpreted, but in my opinion there is still some small doubt about whether Shor can in practice be scaled to large sizes, on theoretical grounds, let alone the practical difficulties of building using any given technology. The problem is the quantum Fourier transform (QFT) that is the key to Shor requires, in the abstract, exponentially precise gates as the problem size grows. Most researchers believe that the QFT can be truncated at some reasonable level and will still have a high probability of success. However, the several papers on the topic (including one by a collaborator of mine) in the last decade have taken different approaches to the calculation, and come up with substantially different answers, making different assumptions about the problem. The theorists seem confident, but I will give only provisional assent until I see it implemented. Perhaps I'm just not smart enough to fully grasp the arguments in the papers. Breaking a code in seconds really depends on both the problem and the machine. A major factor is how many levels of quantum error correction (QEC) are necessary, which is directly dependent on the quality of the physical implementation. QEC is a major topic of research; USC is even sponsoring a conference on the topic in December ( http://qserver.usc.edu/qec07/ ). Physical and logical clock speed, as well as the amount of parallelism in the system, determine how long it will take to run the algorithm on a particular problem, of course. This fact has gotten too little attention from both the experimentalists and the theorists, in my opinion. See http://arxiv.org/abs/quant-ph/0507023 . Enough for now. I can talk about this topic all day, including what the boundaries of my knowledge are; take everything I say with a grain of salt. --Rod -- Rodney Van Meter Assistant Professor of Environment and Information Studies Keio University, Shonan Fujisawa Campus, Japan http://web.sfc.keio.ac.jp/~rdv/
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- Re: Light pulses crack security codes within seconds DO READ DJF David Farber (Aug 18)