RESUMO
In classical computation, a 'bit' of information can be flipped (that is, changed in value from zero to one and vice versa) using a logical NOT gate; but the quantum analogue of this process is much more complicated. A quantum bit (qubit) can exist simultaneously in a superposition of two logical states with complex amplitudes, and it is impossible to find a universal transformation that would flip the original superposed state into a perpendicular state for all values of the amplitudes. But although perfect flipping of a qubit prepared in an arbitrary state (a universal NOT operation) is prohibited by the rules of quantum mechanics, there exists an optimal approximation to this procedure. Here we report the experimental realization of a universal quantum machine that performs the best possible approximation to the universal NOT transformation. The system adopted was an optical parametric amplifier of entangled photon states, which also enabled us to investigate universal quantum cloning.
RESUMO
We show that the basic dynamical rules of quantum physics can be derived from its static properties and the condition that superluminal communication is forbidden. More precisely, the fact that the dynamics has to be described by linear completely positive maps on density matrices is derived from the following assumptions: (1) physical states are described by rays in a Hilbert space, (2) probabilities for measurement outcomes at any given time are calculated according to the usual trace rule, and (3) superluminal communication is excluded. This result also constrains possible nonlinear modifications of quantum physics.
