At PTB, a procedure, which had until then only been predicted theoretically, has now been confirmed experimentally; this procedure allows so-called “light shifts” to be prevented and, thus, PTB's optical ytterbium atomic clock to be made even more accurate. Furthermore, “hyper” Ramsey excitation, which is the subject of this investigation, can be helpful for numerous applications where the focus lies on a precise, controlled interaction between atoms and laser light.
The best method to obtain extremely precise information on the inner structure of atoms and molecules is to excite them by means of resonant laser light. Unfortunately, just this laser light (above a certain intensity) can lead to measurable modifications within the atom's electron shell. In the event of such a “light shift”, the position of the atomic energy levels is modified; the extent of this shift depends on the intensity and the wavelength of the laser used. If one is seeking the properties of the atom as an undisturbed quantum object, this shift must be either prevented or corrected.
During the so-called "hyper" Ramsey excitation, a series of laser pulses with a specific pattern of frequency and phase shifts allows the undisturbed transition frequency to be rendered directly. This had recently been demonstrated theoretically within the scope of a cooperation between Russian, American and PTB scientists. The “hyper” Ramsey excitation has now been realized at PTB in an atomic transition which allows very slight frequency variations to be detected and, at the same time, exhibits a large light shift, since a high laser intensity is necessary for its excitation. It is an electrical octupole transition in the Yb+ ion which is being investigated as a basis for an optical clock. The experiment confirmed the theoretical predictions concerning the advantages of “hyper” Ramsey excitation and attained a 10 000-fold suppression of the light shift.
By combining the maximum spectral resolution currently achievable with an optimized control of the experiment, it is possible to reduce the contribution of the light shift to the systematic uncertainty of the ytterbium clock to less than 10–17, the clock thus becomes clearly more accurate. Furthermore, this method could also be interesting for other investigations aimed at obtaining a precisely controlled interaction between atoms and laser light, for instance in the field of quantum information processing.
Ekkehard Peik
Department 4.4 Time and Frequency
Phone: +49 (0)531 592-4400
ekkehard.peik(at)ptb.de
N. Huntemann, B. Lipphardt, M. Okhapkin, Chr. Tamm, E. Peik, A.V. Taichenachev, V.I. Yudin: Generalized Ramsey excitation scheme with suppressed light shift. Phys. Rev. Lett. 109, 213002 (2012)