# Mathematical Digest

### Short Summaries of Articles about Mathematics

in the Popular Press

"Quantum Computing with Molecules," Neil Gershenfeld and Isaac L Chuang.*Scientific American*, June 1998.

Today's computers are faster and more powerful than ever, but there arelimits to what they can do, and to how they can be improved. With this in mind, researchers are trying to develop a new kind of computer--one which exploits the laws of quantum mechanics.

Current computers use bits to encode information, so an n-bit binaryword is described by a string of n zeroes or ones. Alternatively,quantum computers use quantum bits, or qubits, which storeinformation differently--qubits can exist simultaneously as 0 and 1, with the probability of each state given by a numerical coefficient.In general, n qubits demand 2^n numbers, so if n is 50, about 10^15numbers are required to describe all the states of the quantum machine--a number that exceeds the capacity of the largest conventional computer.Better yet, the laws of quantum physics allow a quantum computer to be inmultiple states at once, making these computers immensely powerful.

Initial experiments in quantum computing have already yielded stunningresults. For example, factoring an arbitrary large number, a job that would take even the fastest classical supercomputer billionsof years, could be done in a year or so with a quantum computer, therebydefeating many sophisticated encryption schemes in use today.

While theoretical results such as this have generated great excitement,no practical quantum computer has been built. Researchers are tryingto use the principles of magnetic resonance in this effort, but currentlythey cannot involve over 10 qubits before the computer loses coherenceand falls apart. But even though there are still huge obstacles, researchers believe that constructing a physical quantum computer is a challenge worthpursuing.

*--- Ben Stein*