Like atoms, nuclei have quantized energy states, which neutrons and protons fill according to the Pauli exclusion principle. Physicists have considered how laser techniques, similar to those used to study atoms, could directly probe the properties and transition frequencies of nuclei. However, exciting a nuclear transition would require an x-ray or gamma-ray laser with an intensity greater than Watts/cm.
Though they don’t operate at this extreme intensity, x-ray lasers, such as the new Linac Coherent Light Source at SLAC, and XFEL, a planned free-electron laser in Europe, are revitalizing the idea of nuclear quantum optics. In a paper appearing in Physical Review A, Ian Wong and his colleagues at Princeton University, New Jersey, demonstrate the theoretical feasibility of nuclear quantum state control, where an x-ray laser excites a nucleus from an arbitrary initial state to an arbitrary final state.
The probability of a laser-driven transition between two states has a complicated dependence on the temporal shape of the laser pulse. Experimentalists have been successful at using this dependence to optimize a laser pulse for exciting a particular atomic state. Wong and his colleagues investigate the “landscape” of transitions for nuclei and show mathematically that this optimization method should work in nuclear systems as well.
Isotopes in the lanthanide and actinide series of the periodic table, many of which have optically active transitions below electron volts, are likely the best candidates for laser-driven nuclear studies. – Jessica Thomas