Microwave-driven quantum logic in Ca43+ at 288 Gauss

Magnetic field gradients, generated by microwave circuitry in the proximity of trapped ions, can couple the ions internal and motional degrees of freedom to implement two-qubit gates [1,2]. This approach presents many advantages with respect to laser-driven gates: the hardware is cheaper and more readily scalable, phase control is facilitated, and photon scattering errors are eliminated.

In the past, we have demonstrated gate fidelities of 99.7% [3], approaching the state of the art for laser-based gates. Critically, this number is above the minimum threshold of 99% required for implementing quantum error correction. But the drawback of microwave-driven quantum logic is that gate durations are more than an order of magnitude longer than their laser-driven counter-parts.

Here, we present a detailed characterization of quantum logic operations in our next-generation experiment, designed to improve on gate fidelity and speed. These gains are enabled by a novel ion trap design and qubit. Our surface trap features a simple single-electrode microwave geometry which passively minimizes the field amplitude whilst producing a large gradient. Operating Ca43+ at 288 Gauss detunes transitions to "spectator" states, whilst offering a π-clock transition which is more responsive to microwave fields. Finally, by cooling the trap to cryogenic temperatures, we are able to reduce anomalous heating of the ions motion, allowing a reduced distance between the microwave electrode and the ions and hence a more effective delivery of microwaves.

 

References:

[1] C. Ospelkaus et al., Phys. Rev. Lett. 101, 090502 (2008)

[2] C. Ospelkaus et al., Nature 476, 181 (2011)

[3] T. P. Harty et al., Phys. Rev. Lett. 117, 140501 (2016)