Showing papers by "Richard Phillips Feynman published in 1984"

Quantum-Mechanical Computers

Abstract: A computer is imagined in which all the parts are simple two-state objects obeying quantum-mechanical laws. A time-independent Hamiltonian for such a system is written and analyzed. The computation is done in logically reversible steps. It is used to study possible limitations on computers that are due to fundamental physical laws. The ideal machine can run ballistically, coasting through the calculation in a short time TB. If there are imperfections that are due to irregularities in construction or finite temperatures the forward-moving calculation is often reversed or scattered. External work must be done to drag the computer forward. The free energy lost in doing a calculation is kT ln 2 per logical step divided by turn factors. One is the mean free number of steps per scattering—measuring how perfectly the machine is built. The other is the mean time (divided by Tb) that the calculation actually takes, which measures how gently you urge it forward. There are no specific quantum limitations except that the exact time a calculation will be finished is unpredictable. When an acknowledge bit is found and held in the correct state the answer can be read. There is, of course, an additional entropy loss in performing this reading operation, which is analogous to a measurement.

The Dignified Professor

Abstract: Re-entry into academia after World War II brings on research block, social problems of mistaken identity, and the fear of doing something silly.