Sunday, February 29, 2004

Quantum Computation

Well, I haven't said anything about quantum computation yet. Honest, if I had noticed anything in the news, I'd comment on it. Right now, there's plenty of buzz in the physics world, but there's just not much that would be very interesting to a layman. Here, in any case, is a quick explanation of what quantum computation is, so that my blog isn't completely devoid of anything about it:
In classical computers, information is stored in the form of bits which hold a value of either 1 or 0. Quantum computers have qubits which can be both 1 and 0 at the same time, in differing proportions: half 1 and half 0, or two-thirds 1 and one-third 0, etc. A classical computer can store any number between 0 and 255 in a single byte, which consists of 8 bits. A quantum computer can store all the numbers between 0 and 255 at the same time on a byte of 8 qubits. Whereas a classical computer performs a calculation on its byte and produces a single answer for the single number on that byte, a quantum computer can perform one calculation and get all the answers for all the numbers on the byte at the same time. This gives you massive parallelism. Of course, it's not as easy as that--getting information out of a quantum computer is a lot harder than putting it in. Thus it's only really good at answering questions that have sharply "peaked" answers, meaning questions where you put in a lot of inputs while lookng for only one answer, such as an unordered search.

(Adapted from The Donald S. Crankshaw Newsletter, Vol. 24, Iss. 1.)

Meanwhile, I have a couple of publications on my specific work, which is on a superconducting qubit, called the persistent current qubit. If you're interested, you can find preprints of the papers online:

Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure
K. Segall, D. Crankshaw, D. Nakada, T.P. Orlando, L.S. Levitov, S. Lloyd, N. Markovic, S.O. Valenzuela, M. Tinkham, K.K. Berggren
Published in Physical Review B, 67, 220506, 2003.

DC measurements of macroscopic quantum levels in a superconducting qubit structure with a time-ordered meter
D.S. Crankshaw, K. Segall, D. Nakada, T.P. Orlando, L.S. Levitov, S. Lloyd, S.O. Valenzuela, N. Markovic, M. Tinkham, K.K. Berggren
Accepted for publication in Physical Review B

Energy Relaxation Time between Macroscopic Quantum Levels in a Superconducting Persistent Current Qubit
Yang Yu, D. Nakada, Janice C. Lee, Bhuwan Singh, D. S. Crankshaw, T. P. Orlando, William D. Oliver, Karl K. Berggren
Accepted for publication in Physical Review Letters

These articles describe a series of experiments done on a persistent current qubit fabricated in niobium (which goes superconducting at 9 K), demonstrating a high Q and, by all indications, a long coherence time. Ask me sometime and I'll explain what that means, but for now, just trust me that these are good things.

Update: Now that Doc Rampage has linked to me, I figured that I needed to clean up the post a bit for clarity.

New Post: So what is a qubit, anyway? I go into more detail here.

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