Minimum-energy pulses for quantum logic cannot be shared

We show that if an electromagnetic energy pulse with average photon number <n> is used to carry out the same quantum logical operation on a set of N atoms, either simultaneously or sequentially, the overall error probability in the worst case scenario (i.e., maximized over all the possible initial atomic states) scales as N^2/<n>. This means that in order to keep the error probability bounded by N\epsilon, with \epsilon ~ 1/<n>, one needs to use N/\epsilon photons, or equivalently N separate "minimum-energy'' pulses: in this sense the pulses cannot, in general, be shared. The origin for this phenomenon is found in atom-field entanglement. These results may have important consequences for quantum logic and, in particular, for large-scale quantum computation.