Progress Towards Mixed-Species, Laser-Free Quantum Logic Operation in a Surface Electrode Ion Trap

The use of dual species in trapped ion quantum processors enables sympathetic cooling and quantum-logic-based state preparation and readout operations, both of which are important for maintaining high fidelities and low crosstalk in scaled systems. Using microwave and RF magnetic fields and field gradients to perform quantum logic between the “data” and “helper” species widens the range of potential “data” species by reducing or eliminating the need for data species lasers.

We present progress towards sympathetic cooling, quantum logic state preparation, and quantum logic readout operations for a ²⁵Mg⁺ data ion using a co-trapped ⁴⁰Ca⁺ helper ion in a surface electrode trap. Direct laser cooling and detection for Mg⁺ requires UV laser light at 280 nm, which can be complicated to generate and can cause charging of trap surfaces. Instead, we can sympathetically cool and perform indirect state-preparation and readout using laser-free quantum logic operations with the co-trapped Ca⁺. Once the crystal is ground state cooled, quantum logic state preparation is implemented by sideband pumping the hyperfine manifold of Mg⁺ to increase population in a target state, with dissipation provided by re-cooling the helper ion. Radiofrequency/microwave-based spin-motion coupling for both species enables coupling of the data and helper ion states, and thus projection of the data ion state by measuring the helper ion, without suffering from spontaneous emission. Such measurements can also be used for general data qubit readout.