Shining laser light onto atoms and molecules is the best way to obtain precise information about their inner structure, but light above a certain intensity can fundamentally change their energy levels. A group in Germany has demonstrated how to prevent such “light shifts,” and their method could make optical atomic clocks even more accurate. Scientists at Physikalisch Technische Bundesanstalt (PTB) applied selected laser pulses to excite the atoms, attaining a ten-thousandfold suppression of the light shift. Their method confirms the 2010 theoretical prediction of “hyper” Ramsey excitation, which uses pulsed radiation to perform precise measurements. Stylized representation of the excitation of a single ion in a trap by means of a “hyper” Ramsey pulse sequence. Courtesy of PTB. The idea was conceived by Norman Ramsey, who received a physics Nobel Prize in 1989 for this finding. In the method, a laser is shot at the atom, where it starts a resonant excitation. The pulsation excited in the electron shell of the atoms continues undisturbed “in the dark” until a second laser pulse completes the comparison between the resonance frequency of the atom and the laser frequency. Because of the dark phase between the laser pulses, the signal of the Ramsey excitation contains an averaging over the positions of the states of the atoms with and without a light shift. In principle, it would be possible to compensate for the light shift by modifying the laser frequency by exactly this quantity during the pulses. However, this would not bring great improvement from a practical point of view because precise distance of the atoms should already be known. In the case of “hyper” Ramsey excitation, a third laser pulse of the same intensity and the same frequency, but with an inverted phase, is inserted into the dark phase. This third laser pulse automatically compensates for possible errors that could occur because of misjudgment of the size of the light shift or small variations in the laser intensity during the light pulses. The study was published in Physical Review Letters (doi: 10.1103/PhysRevLett.109.213002). For more information, visit: www.ptb.de/index_en.html