Short answer: Trapped-ion gates are usually driven by lasers, but microwaves can do the same job when the field gradient is large, which happens near electrodes built into the trap. A 2011 Nature paper reported the first entangling gate this way with a Bell-state fidelity of 0.76, and a 2016 Physical Review Letters paper reported a two-qubit gate with a Bell-state fidelity of 99.7% in a room-temperature surface trap. These are laboratory demonstrations with a small number of ions.
Why microwaves are hard
The 2011 abstract explains that for trapped ions, quantized motional and internal states can be coherently manipulated with laser light. Similar control with radio-frequency or microwave radiation is difficult because the coupling between internal states and motion needs significant field changes across the extent of the atoms' motion, and such changes are negligible at these frequencies for freely propagating fields. An exception is the near field of microwave currents in structures smaller than the free-space wavelength, where stronger gradients can be generated.
2011: the first gate
In the Nature paper, the authors first manipulate the internal quantum states of ions in a microfabricated trap coherently, on timescales of 20 nanoseconds. The controlling magnetic fields come from microwave currents in electrodes integrated into the trap structure. They also generate entanglement between the internal states of two atoms with a gate operation they describe as suitable for general quantum computation. The entangled state has a fidelity of 0.76 with an uncertainty of 0.03 as standard error of the mean. They say integrating the control mechanism into the trapping device in a scalable manner could be applied to quantum information processing, simulation and spectroscopy. That is their outlook.
2016: a 99.7% Bell state
The Physical Review Letters abstract reports a two-qubit logic gate driven by near-field microwaves in a room-temperature microfabricated surface ion trap. The authors introduce a dynamically decoupled gate method, which stabilizes the qubits against fluctuating energy shifts and avoids the need to null the microwave field. They use the gate to produce a Bell state with fidelity 99.7% with an uncertainty of 0.1, after accounting for state preparation and measurement errors. The gate is applied to calcium-43 hyperfine clock qubits, which have a coherence time of about 50 seconds, using the oscillating magnetic field gradient from an integrated microwave electrode.
What changed between the two
The gate fidelity moved from 0.76 to 99.7% across the five years. The 2016 authors attribute their result in part to the dynamically decoupled gate method and to using long-lived clock qubits, as the abstract describes. The abstracts do not compare the two experiments side by side, since they use different ions, traps and corrections, and the 2016 figure is corrected for state preparation and measurement errors.
Limits
Both experiments entangle two ions, and neither abstract reports a larger circuit. The 99.7% figure is after accounting for state preparation and measurement errors, so it is not a raw measurement. The scalability point in the 2011 abstract is an aim of the approach and not a demonstrated result.
FAQ
How can microwaves drive an ion gate?
In the near field of microwave currents in small electrodes, strong field gradients couple internal states to motion.
How good was the 2016 gate?
A Bell state fidelity of 99.7%, after correcting for state preparation and measurement errors, per the authors.
What were the qubits?
Calcium-43 hyperfine clock qubits with a coherence time of about 50 seconds.
Why is this attractive?
The control mechanism is built into the trap, which the 2011 authors say could be scalable.
Is this a large quantum computer?
No. The abstracts describe two-ion gates.
All field notes