Quantum-logic precision spectroscopy and control of trapped molecules

Trapped ions are among the best-controlled quantum systems. However, for molecules, a similar degree of control currently lacks due to their additional rotational and vibrational degrees of freedom, severely complicating their energy-level structure. Quantum-logic protocols in which atomic ions serve as probes for molecular ions promise quantum-control over these degrees of freedom. Here, I will describe our experimental results in achieving >99% fidelity in the quantum-nondemolition state detection of the nitrogen ion's electronic, vibration, and rotation ground state [1], thus making a crucial step towards the coherent manipulation of molecular quantum states. We further enhanced our quantum-logic protocol for hyperfine and Zeeman resolved state identification and preparation in a complex region of the molecular spectrum, mimicking the situation encountered with the richer spectrum of polyatomic molecules [2].

The quantum control of rotation and vibration of molecules will improve precision-spectroscopy of molecules by a few orders of magnitude. It will open up opportunities for creating new time standards in the THz domain [3], testing beyond-standard-model theories such as possible time variation in the proton-to-electron mass ratio, and encoding quantum information in molecular qubits at telecom frequencies for quantum-communication applications. With the increasing size and complexity of the molecule studied, it is an open question to what extent quantum control of these complex systems can be maintained.

[1] Sinhal, ZM, Najafian, Hegi, Willitsch, Science 367, 1213 (2020).

[2] Najafian, ZM, Sinhal, Willitsch, Nat. Commun. 11, 4470 (2020).

[3] Najafian, ZM, Willitsch, Phys. Chem. 22, 23083 (2020).

[4] ZM, Hegi, Najafian, Sinhal, Willitsch, Faraday Discuss. 217, 561 (2019).

Figure: Left) Radio-frequency ion trap used for quantum-logic state detection and preparation of rovibrational molecular states in nitrogen molecular ion [4]. 
Right) Quantum-logic protocol. The strong coupling between trapped atomic and molecular ions is used for non-destructive and quantum non-demolition state detection of the molecular-ion state.