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Coherent Manipulation of Rotational States of Single Molecules via Direct Frequency Comb Excitation

Periodic Reporting for period 1 - COMAMOC (Coherent Manipulation of Rotational States of Single Molecules via Direct Frequency Comb Excitation)

Reporting period: 2018-05-01 to 2020-04-30

The increasing ability to manipulate physical systems at the most fundamental quantum level is currently leading to a wave of new technologies that have superior performance and capabilities for many applications in metrology, sensing, communication, simulation and computing. In this context, trapped and laser-cooled atomic ions have proven to be an outstanding system. Their exploitation, is however, typically limited by their coupling to time-varying stray magnetic fields which shortens the time available to perform high precision measurements, quantum simulations or to process quantum information. In this perspective, molecular ions are very promising because they can be made almost insensitive to magnetic field fluctuations. Their complex internal structure, however, makes quantum control of molecular ions very challenging. The action COMAMOC (“COherent MAnipulation of rotational states of single Molecules via direct frequency Comb excitation”) aimed at achieving full deterministic coherent control over the quantum states of a single molecular ion by demonstrating, on a test molecule MgH+, frequency comb driven manipulation of its rotational states. This innovative approach is today made possible by the development of frequency comb laser technologies and by the invention of a refined non-destructive state detection technique named “quantum logic spectroscopy”.

To achieve this goal, the scientific objectives of this Marie Skłodowska Curie Action (MSCA) have been to 1) achieve fast manipulation of both the electronic and motional quantum states of a single atomic ion with a frequency comb laser, 2) to implement quantum logic spectroscopy and 3) to combine the two techniques to perform frequency comb manipulation of pure quantum states of a molecular ion. A parallel goal of the MSCA Individual Fellowship was 4) to foster the development of the individual researcher.

During the project, objective 2 has been reached; objective 1 was almost completed, while important steps have been taken towards the final objective 3. As for objective 4, the action has without any doubts greatly enhanced the development and the career prospects of the individual researcher.
Work was conducted via 5 Work Packages (5 WPs). WP1 comprised setting-up a frequency comb laser. This laser was demonstrated to fulfil all the requested requirements in terms of optical power, spectrum and stability to reach fast and coherent quantum state manipulation of ions. A final experiment on a test atomic ion of Ca+ remains yet to be done. In parallel, spurious heating of the ion’s motion by voltage noise from the trapping electric sources has been investigated and reduced. This allows for less stringent requirements on the speed for quantum state manipulation. Within WP2, quantum logic spectroscopy was demonstrated on a test system consisting of two co-trapped atomic ions (Ca+ - Mg+). In particular, motional ground-state cooling has been achieved. Also, experimental spectra obtained on a dipole transition in Mg+ have been well reproduced by numerical simulations. Although the final objective of WP3 has not been reached yet, single MgH+ ions are now routinely created and trapped in the Paul trap. Efficient motional ground-state cooling of one MgH+ via laser cooling on one Ca+ atomic ion has also been demonstrated. A quantum cascade laser has been set-up to laser-cool the MgH+ to its rotational ground state although the cooling transition is yet to be found. In parallel, the expected quantum logic spectra have been simulated and narrow transitions that are nearly insensitive to magnetic field fluctuations have been identified. WP4 involved training of the individual researcher. Scientific training was achieved via transfer knowledge from Prof. Michael Drewsen’s group at Aarhus University (AU), the host institution. Teaching training was done by teaching part of the Laser & Optics course at AU and supervising (under-)graduate students in the laboratories. The individual researcher also got trained in proposal writing, in project and team management. WP5 was dedicated to the dissemination and communication of the project results. This yield to date to, amongst other, 2 forthcoming publications, 4 talks given in international conferences, 3 national seminars, 1 presentation addressed to the general public.
Through COMAMOC action, numerous challenges have been addressed towards achieving full deterministic coherent control of molecules through frequency comb driven manipulation of molecular rotational states. More specifically, major technical challenges were overcome such as: reaching stability performances of a solid-state bulk frequency comb laser that are similar to state-of-the-art fiber-based frequency comb lasers, developing state-of-the-art low-noise electronics, achieving sideband cooling of ions of very different masses, or demonstrating a more general form of quantum logic spectroscopy that is particularly powerful for the search of molecular lines. These achievements go beyond the scope of the COMAMOC project since they relate to experimental developments in laser technologies, electronics, quantum control of trapped particles that are relevant not only to the field of ion trapping but to quantum physics and quantum engineering in general. Although the scientific objective 3 was not reached, it is not due to any fundamental issues but rather due to the limited timeframe of the project. In fact, a similar experiment has been recently demonstrated in the group of Prof. Dietrich Leibfried, NIST, USA, on a CaH+ molecular ion [Chou et al., Science, Vol. 367, Issue 6485, pp. 1458-1461, 2020]. COMAMOC contributed to boost the experimental research on molecular ions which will open up for many fundamental investigations and technological achievements ranging from tests of Quantum Electro-Dynamics to the use of molecular ions as qubits in ion-based Quantum Technologies.

This MSCA-IF allowed the individual researcher (the Fellow) to acquire many new scientific skills which highly contributed to diversify his research profile. Besides, by collaborating with Prof. Michael Drewsen within the action, the Fellow gained a certain number of responsibilities and developed new competences in team and project management. The Fellow also gained more teaching experience via the teaching training actions during the project. Moreover, COMAMOC action was a unique opportunity for him to broaden his international network via the different dissemination actions and collaborations. In summary, this MSCA has without any doubts greatly enhanced the potential and future career prospects of the individual researcher.
The MgH+ molecular ion is cooled and detected via its interaction with a laser-cooled Ca+ ion.