While atom and ion chip technologies are now well advanced, there are only a few examples of chip-scale structures for gas-phase molecules. We aim to develop 3D chip-based electric traps, and load them from a cold molecular gas prepared above the surface. We will integrate coplanar waveguides that deliver microwave photons to prepare the quantum state of single molecules and small ensembles. The same chip could also support integrated photonic structures. Optical tweezers are ideal for bringing molecules into the chip traps or bringing them close enough to the surface for strong coupling with microwave circuits or photonic structures. The methods developed using tweezers for controlling the dynamics of small tightly-trapped ensembles can be carried over to molecules trapped on the chip.

Microstructured Electric Traps

We will build microstructured electric traps on chips suitable for confining molecules in the weak-field-seeking states. These electric traps could be used with any of our diatomic molecules, giving us flexibility over our approach to developing and testing them. Our preliminary chip design is based on previous work and is shown below. It consists of two central L-shaped electrodes that together form a square, and two side electrodes.

(a) Possible design for an electric trap on a chip. The wires (blue) and side patches (red) are electrodes on an insulating surface (green). (b & c) Potential of a CaF molecule in the (1, 0) state versus position along the lines (y = 0, z = 2.7 μm) & (x = 0, y = 0). The origin is on the chip surface at the trap centre, with z along the surface normal. The potentials are calculated for Vwire = 15 V, Vpatch = 180 V.

We will fabricate these electric surface traps with a range of size scales, and learn how to load molecules into them, starting with the larger ones and progressing towards smaller scales. An attractive, deterministic loading method is first to capture molecules in a tweezer trap, and then to move these molecules into the chip trap. The tweezer and chip traps can be mode-matched, potentially making it possible to cool the molecules to the motional ground-state of the chip trap using Raman sideband cooling, as will be done in the tweezers. Our plan is to develop that cooling method in the tweezer traps, and then consider whether it is feasible and useful to apply it in the chip trap.

Integrated Microwave Waveguides

We will integrate microwave waveguides that will provide good coupling of microwaves to molecules in the chip trap. A good geometry is the coplanar waveguide which consists of a single conducting track fabricated on the chip surface together with two return conductors on either side, with the field concentrated above the gap between the conductors. This gap can be micron-sized, concentrating the field to a small volume. These waveguides are used extensively in monolithic microwave integrated circuits (MMICs) and in the field of circuit quantum electrodynamics, and have already been integrated with atom chips. Another simple geometry is a microwave ‘slotline’ formed of two coplanar plates separated by a small gap. The trap electrodes could be accommodated in the gap where the microwave field is concentrated.

We will first study how best to integrate the structures needed to make the chip trap with those needed for the microwave waveguides. Then, with molecules loaded into the chip traps, we will drive the rotational transition with microwaves propagating in the waveguides, tuning the dc field to Especial so that the broadening is minimized. We will study the linewidth of the transition and the fidelity of the Rabi oscillations driven in this way. With trap frequencies of ∼100 kHz, it will be straightforward to resolve the motional states, while the strong spatial gradient of the molecule-microwave coupling makes it feasible to drive a motional sideband of the rotational transition. This would demonstrate the ability to control both the internal and motional modes of molecules trapped on a chip.