Undisturbed excitation with pulsed light
PTB presents a new procedure to excite atoms and molecules using a laser
Stylized representation of the excitation of a single ion in a trap by means
of a "hyper" Ramsey pulse sequence.(Fig. PTB)
November 22, 2012
[ptb/es] The best method to obtain the most 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. Scientists of the Physikalisch-Technische Bundesanstalt (PTB) have now shown
experimentally how to prevent such "light shifts". This confirms the advantages of "hyper" Ramsey excitation that had already been
predicted theoretically. This method can make their optical ytterbium atomic clocks even more accurate. Furthermore, "hyper" Ramsey
excitation can be helpful in numerous applications where the focus lies on a precise, controlled interaction between atoms and
laser light. The results have been published in the current issue of the scientific journal "Physical Review Letters".
"Light shift" means that intense laser light modifies the position of the atomic energy levels; the 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. With the new procedure, which has been applied experimentally for the first time at PTB,
a sequence of judiciously selected laser pulses used to excite the atom eliminates the disturbing light shift effect.
The basic idea of using pulsed radiation to perform precise measurements goes back to Norman Ramsey, who was awarded the
Nobel Prize in physics in 1989 for this finding. With this method, a first laser pulse is shot at the atom, where it starts
a resonant excitation. Then the pulsation excited in the electron shell of the atom continues undisturbed "in the dark" until
eventually a second laser pulse completes the comparison between the resonance frequency of the atom and the laser frequency.
A similar approach is also usual in clock comparisons: two clocks are set to the same time, they are then left to run on and are
eventually compared again. The result shows which clock was faster or slower than the other.
The signal of the Ramsey excitation contains, due to the dark phase between the laser pulses, an averaging over the positions of
the states of the atom 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 (exclusively) during the pulses. This, however, would not bring great
improvement from a practical point of view as the precise information concerning the disturbance of the atom should be known
to begin with. In 2010, a group of scientists (also with PTB's participation) suggested a method they called "hyper" Ramsey
excitation in order to solve this problem. This theoretical consideration has now been confirmed experimentally for the first time.
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 which could occur due
to misjudgment as regards the size of the light shift and due to small variations in the laser intensity during the light pulses.
Realizing "hyper" Ramsey excitation experimentally succeeded 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. This opens up the possibility for the optical Yb+ clock to achieve even greater
accuracy. This method could also be interesting for other researchers trying to obtain a precisely controlled interaction between
atoms and laser light, for instance in the field of quantum information processing.
Huntemann, N.; Lipphardt, B.; Okhapkin, M.; Tamm, Chr.; Peik, E.; Taichenachev, A.V.; Yudin, V.I: Generalized Ramsey excitation
scheme with suppressed light shift. Phys. Rev. Lett. 109 (2012) 213002
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