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Spectroscopy of single trapped molecules

New method for state detection in molecular ions

PTB News 1.2016
Especially interesting for
  • the development of molecular optical clocks
  • fundamental research in physics
  • quantum chemistry
  • astronomy

The QUEST Institute at PTB has, for the first time, succeeded in proving the quantum state of trapped and indirectly laser-cooled molecular ions without destroying the molecule itself or its internal state. In this way, quantum jumps which were induced by thermal environmental radiation could be observed directly in a single molecule, and a new form of spectroscopy could be demonstrated. The new method enables precision spectroscopy of molecular ions with applications ranging from chemistry to tests in fundamental physics.

(a) Conceptual set-up of the experiment with MgH+(orange) and Mg+ (green) in a linear ion trap. The ionic crystal is cooled to the ground state via Mg+. An oscillating dipole force changes the motion state, depending on the rotation state of MgH+. This excitation is read out via Mg+. (b) Typical detection signal in which a quantum jump into the (J = 1) rotation state (transition from the red to the blue area) of the molecule and out of it (from blue to red) can be seen.

For the development of optical frequency standards and clocks, several preparation and detection techniques were successfully demonstrated in atomic systems. Prominent examples are laser cooling – to control motion – and the detection of quantum states based on state-dependent fluorescence. However, the resonant laser manipulation techniques successfully demonstrated with atoms cannot in general be transferred to molecules. Molecules have additional degrees of freedom in the form of rotation and vibration resulting in a large number of possible transitions from the excited state.

PTB researchers have now succeeded in reading out a selected quantum state of a trapped molecular ion (MgH+) non-destructively via the strong electrostatic interaction with a simultaneously trapped atomic ion (Mg+). To this end, the motion of the two ions along a direction is cooled down to the ground state via the atomic ion by means of lasers. Another laser is tuned in such a way that it exerts an oscillating optical force (similar to that of optical tweezers) onto the molecular ion only if the latter is in a selected state of rotation. The oscillation frequency of the force is adjusted to the period of a joint oscillation of the two ions in the trap which is thus resonantly enhanced. It is only when the molecular ion is in the selected state that the joint motional state of the two ions is excited. This motional excitation can be read out via the atomic ion. The thermal radiation of the environment interacts with the rotational states of the molecular ion and induces transitions between these states. These quantum jumps between the rotational states of the molecule were observed “live”.

Moreover, it was demonstrated that the dependence of the optical force on the detuning of the laser with respect to a resonance in the molecular ion can be used for a new form of spectroscopy. Combined with efficient state preparation procedures, the state detection method presented here allows precision spectroscopy on narrow-band transitions for molecular optical clocks and tests in fundamental physics such as a possible change of fundamental constants. Furthermore, the method could be applied in quantum chemistry and to interpret spectra of cold molecular ions in space.


Piet O. Schmidt
QUEST Institute at PTB
+49 (0)531 592-4700


Scientific publication

F. Wolf, Y. Wan, J. C. Heip, F. Gebert, C. Shi, P. O. Schmidt: Non-destructive state detection for quantum logic spectroscopy of molecular ions. Nature (2016), Doi: 10.1038/nature16513