Quantum logic spectroscopy of molecular ions

Short answer: Molecular ions have no simple cycling transitions for laser cooling or state detection, so researchers pair them with a co-trapped atomic ion that acts as a probe, an approach called quantum logic spectroscopy. A 2016 Nature paper reports non-destructive state detection of a single molecular ion, a 2017 Nature paper reports preparing and detecting states of a calcium hydride ion, and a recent arXiv preprint reports coherent manipulation of rovibrational transitions in a nitrogen molecular ion. These are laboratory experiments on single ions.

The problem

The 2016 abstract says precision laser spectroscopy of cold, trapped molecular ions is a powerful tool in fundamental physics, used for determining fundamental constants, testing their possible variation, and searching for an electron electric dipole moment. The absence of cycling transitions in molecules makes direct laser cooling and control of quantum states difficult. Earlier detection techniques based on photodissociation or chemical reactions are destructive and therefore inefficient, which limits the resolution of laser spectroscopy.

2016: non-destructive detection

The authors demonstrate non-destructive detection of the quantum state of a single trapped molecular ion through its strong Coulomb coupling to a well-controlled co-trapped atomic ion. An algorithm based on a state-dependent optical dipole force changes the internal state of the atom according to the internal state of the molecule. They show individual molecular quantum states can be distinguished by the strength of their coupling to the force, observe quantum jumps between rotational states induced by black-body radiation, and implement a variant of quantum logic spectroscopy of a molecular resonance. They say the technique is relevant to a wide range of molecular ions, including for state-controlled quantum chemistry.

2017: preparation and coherence

The 2017 abstract says the vibrational and rotational degrees of freedom make controlling molecular quantum states a challenge. The authors use quantum-logic spectroscopy, which maps quantum information between two ion species, to prepare and non-destructively detect states in molecular ions. They develop a general technique for optical pumping and preparation of the molecule into a pure initial state, which lets them observe high-resolution spectra in a single calcium hydride ion and coherent effects such as Rabi flopping and Ramsey fringes. They say the protocol needs a single far-off-resonant laser that is not specific to the molecule, so other molecular ions, including polyatomic ones, could be treated the same way.

Recent: nitrogen molecular ion

The arXiv preprint notes that electric-dipole-forbidden transitions in atoms underpin many implementations of quantum computers, atomic clocks and sensors, and that coherently addressing such transitions in molecules has been a long-standing challenge. The authors report searching for, observing and coherently manipulating electric-quadrupole rotational-vibrational transitions in single trapped molecules with a quantum-logic-spectroscopy protocol. They identify individual hyperfine-Zeeman-rotational components of the fundamental vibrational transition of the nitrogen molecular ion and perform coherent population transfer between levels. They say the work opens perspectives for precision molecular spectroscopy, high-fidelity qubits in molecular rotation and vibration, infrared molecular clocks and searches for new physics. Being a preprint, it has not completed peer review in this form.

Limits

All experiments use single ions and specific species. The applications named in the abstracts, including quantum computing and tests of fundamental physics, are stated directions, not demonstrated results.

FAQ

What is quantum logic spectroscopy?

In the 2017 paper, a method that maps quantum information between two ion species so a well-controlled atomic ion can read out a molecular ion.

Why not detect molecules directly?

The 2016 abstract says molecules lack cycling transitions, and older methods destroy the molecule.

Which molecules were used?

A calcium hydride ion in 2017 and a nitrogen molecular ion in the recent preprint. The 2016 abstract does not name the molecule in the text used here.

Is this quantum computing?

The abstracts list quantum computing as a possible application. The experiments are spectroscopy and state control.

Is the nitrogen result peer reviewed?

It is an arXiv preprint.