2021_11_04 Stefan Willitsch: "Quantum control and precision spectroscopy of single molecular ions"

The development of quantum technologies for molecules has remained a long-standing challenge due to the complexity of molecular systems. We have recently developed a quantum-non-demolition technique for the non-destructive detection of the internal quantum state of a single trapped molecular ion [1,2,3]. The method is based on the state-dependent coherent excitation of the motion of the molecular ion and subsequent measurement of the motional quantum state using a co-trapped atomic ion. This approach offers new perspectives not only for the detection, but also for the preparation and the manipulation of molecular quantum states on the single-particle level with a sensitivity several orders of magnitude higher compared to previously used destructive schemes. We present a characterisation of the technique using the homonuclear diatomic ion N2+ as an example and show how it can be used for non-invasive spectroscopic measurements on single molecules. We also discuss applications of this technique in the realm of precision molecular spectroscopy [4] using a newly established fibre network for the precise transfer of frequencies within Switzerland and their comparison to the Swiss primary standard at the Swiss Institute of Metrology METAS [5].
[1] Z. Meir, G. Hegi, K. Najafian, M. Sinhal and S. Willitsch, "State-selective coherent motional excitation as a new approach for the manipulation, spectroscopy and state-to-state chemistry of single molecular ions”, Faraday Discuss. 217 (2019), 561
[2] M. Sinhal, Z. Meir, K. Najafian, G. Hegi and S. Willitsch, "Quantum non-demolition state detection and spectroscopy of single trapped molecules”, Science 367 (2020), 1213
[3] K. Najafian, Z. Meir, M. Sinhal and S. Willitsch, "Identification of molecular quantum states using phase-sensitive forces”, Nat. Commun. 11 (2020), 4470
[4] K. Najafian, Z. Meir and S. Willitsch, ”From megahertz to terahertz qubits encoded in molecular ions: theoretical analysis of dipole-forbidden spectroscopic transitions in N2+”, Phys. Chem. Chem. Phys. 22 (2020), 23083
[5] D. Husmann et al., “SI-traceable frequency dissemination at 1572.06 nm in a stabilized fiber network with ring topology”, Opt. Expr. 29 (2021), 24592