X-Message-Number: 7580 Date: Tue, 28 Jan 1997 10:20:53 -0800 (PST) From: John K Clark <> Subject: Quantum Computers -----BEGIN PGP SIGNED MESSAGE----- Very recently there have been two new developments in the world of Quantum Computers. In the January 17 1997 issue of Science is a long article called "Bulk Spin-Resonance Quantum Computation" by Neil A Gershenfield and Issac L Chuang, there is also an editorial about the article in the same issue. It talks about hardware methods of isolating a Quantum Computer from the environment so you don't get Quantum decoherience and suggests using Nuclear Magnetic Resonance (NMR). The nucleus of atoms are like spinning bar magnets, and they can be in 2 states, spin up or spin down. If you send in a radio pulse of the exact frequency you can change the spin state of the atom from one to another. This frequency can change if an atom is coupled to another atom, so if you pick the correct frequency you could flip the spin of a carbon atom if and only if the Hydrogen atom it is coupled with is spinning up. This could make a controlled logic NOT gate. This would also be a Quantum logic gate, because a radio pulse can also put an atom in a superposition of states, both spin up and spin down at the same time, and anything coupled to that state would also be in a superposition of states. There are 4 advantages to using NMR to make a Quantum computer rather than other methods: 1) The quantum state of interest is in the nucleus and is protected from contamination from the outside world by the electron cloud so the quantum state lasts a long time. In most other quantum systems decoherence happens in pico or femtoseconds but with NMR it can last for thousands of seconds. 2) You don't have to worry about every single atom, if only 50.000001% point up, that's good enough. I quote Tim Havel, who independently discovered most of the thing in the article at the about the same time, "It turns out having a well-defined statistical excess of a single quantum state is enough to do quantum computing. You don't need to have every molecule doing exactly the same thing". 3) By using a large number of atoms to store a qbit it further protects it against unwanted external interactions. 4) You don't need expensive exotic gadgets, cheap off the shelf equipment will do for NMR. The authors actually hope to have a small (10 qbit) quantum computer in operation in a few years, possibly as soon as next summer. With such a machine you could factor (drum roll please) the number 15. Hey don't laugh, it would be a proof of concept and could serve as a test bed to develop better quantum algorithms, and remember, each time you add a qbit to your computer you double its power. Speaking of better quantum algorithms, we'll need some to go much higher than 30 qbits using NMR, and to quote Gershenfield "Somewhere between 50 and 100 qbits is where calculations start to get interesting". Still, people are optimistic, to quote Gershenfield again "If researchers can figure out how to extend the technology to larger qbit numbers the future is unlimited. In a separate development, on January 27 researchers at Xerox announced that they have made a small molecule of manganese, oxygen, carbon and hydrogen that acts like a powerful magnet. They said it might be possible to use this material to make a disk drive that could store information at the molecular level and hold millions of times as much data as we can today, but that's not what they emphasized. I think it's interesting that at the announcement they spent more time talking about how it could be used in a Quantum computer rather than a conventional one. The material has a property called Quantum Magnetic Hysteresis, it can be in a superposition of many different magnetic states at the same time, and that's just what a Quantum Computer is all about. For me It's still hard for me for it to sink in that an otherworldly object like a Quantum computer could actually exist in our worldly universe, but it's beginning to look like one can really be built. If so the world will never be the same. John K Clark -----BEGIN PGP SIGNATURE----- Version: 2.6.i iQCzAgUBMu5Axn03wfSpid95AQHe/ATwzvImlfKeLXVZ17f/lh4ujg1tsbqpmI27 CQ4SKnU/8QKIfMYYXb1aLhNpT9+rGPK4ci7x9yh/5blB7PWDHe+JADmERxWd2poQ oOp1Aj98HUNWPr3wqVxTsm1MvgP2RAARFXkFLxd52hCFQYTHQLWxgElRaiTs3ETT /3hB99QOkFeYkYnHi0lmyTzDDXvGDLseWNMxo+DoNxbobRv/0BU= =alJJ -----END PGP SIGNATURE----- Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=7580