A major goal of our group is to cool and detect a polyatomic molecule in its absolute ground state for the first time. Our proposal to do so, using a combination of established AMO methods and a few new tricks of our own, is described here.
The ability to prepare and readout the state of these molecules has the potential to unlock new frontiers in spectroscopy, precision measurement, and quantum information – and will let us start treating molecules like the perfect quantum system they are.
Our plan to cool these molecules begins with co-trapping the molecular ion of choice with a laser coolable atomic ion, likely Sr+. The internal state of the molecule is cooled to ~10K via cryogenic buffer gas cooling, and the motional state in the trap is cooled to ultracold regime via sympathetic cooling of the co-trapped atom. The internal state and the motional state of the ion can then be coupled by applying spatially non-uniform fields; for example, microwave-frequency modulated optical standing waves can couple molecule rotation and molecule motion. This method is closely related to Raman sideband cooling, a well established technique for laser cooling atoms. State readout is achieved by reversing this process – selectively heating the trapped ensemble, conditional on the internal state of the molecule. This readout could be described as a loose form of quantum logic spectroscopy – but works under far less exact conditions than high fidelity QLS requires
Why polyatomic molecules?
The last 2 years have seen enormous progress cooling diatomic molecular ions, with a particular emphasis on light diatomics, such as CaH+. It is natural to think that cooling polyatomic molecules would be considerably harder than cooling diatomics – in atomic systems, added complexity typically brings added difficulty. In fact, we believe the opposite is true: polyatomic molecules both provide access to significant new physics, and are in fact easier to control than diatomics.
Polyatomic molecules have rich spectra of connected states.
Our plans to cool large molecular ions begins with buffer gas cooling them to ~ 10 Kelvin. Polyatomic molecules and diatomics look very different at this temperature. Polyatomics exhibit a rich manifold of rotational states, all of which can be addressed via low frequency (< 20 GHz) synthesizers. Although addressing these levels requires substantial complexity, this complexity lies just where we want it – inside a modern, high frequency arbitrary waveform generator. In contrast, linear molecules exhibit sparse manifolds of states, and driving these transitions directly requires challenging THz electronics
Polyatomic molecules can be chiral
No molecule with 3 or fewer atoms – in fact no planar molecule – can be chiral. The ability to manipulate and detect the exact quantum state of a polyatomic molecular ion would be a major asset to the community looking for the predicted – but never observed – parity violating energy differences between otherwise equivalent right- and left- handed molecules. The tools we are developing will also realize single molecule, non-destructive chiral analysis – and molecule by molecule chiral separation – tools that lie currently far beyond the state of the art
Polyatomic molecules couple well to existing microwave technology
Light hydrides, such as CaH+, have rotational transitions in the 100s of GHz. While our ability to produce well controlled radiation in this challenging regime is improving rapidly, our agility in the 0-40 GHz regime is substantially greater; below 20 GHz, essentially arbitrary waveforms can now be reliably produced from off the shelf, direct digital synthesis based instruments.
Polyatomic molecules are also well suited to existing quantum microwave technology. In particular, superconducting striplines, which typically operate in the 5 GHz range, could be efficiently coupled to the rotational states of trapped polyatomic molecular ions.