Microwave gates for trapped ions

Short answer: Microwave-driven gates use the strong magnetic field gradient close to a microwave current in a trap electrode, instead of laser beams, to entangle trapped-ion qubits. A 2011 Nature paper reports entanglement with fidelity 0.76, a 2016 arXiv preprint reports a two-qubit gate fidelity of 99.7%, and a 2019 npj Quantum Information paper reports 98.2% with a conductor embedded in a surface-electrode trap. These are laboratory demonstrations on small numbers of ions.

Why microwaves

The 2011 Nature authors explain that trapped-ion motion and internal states can be coherently manipulated with laser light, but that similar control with radio-frequency or microwave radiation is difficult, because coupling internal states to motion needs significant field changes across the extent of the atoms' motion, which freely propagating fields do not give. An exception is the near field of microwave currents in structures smaller than the free-space wavelength, where stronger gradients arise.

2011: first demonstration

The authors manipulate the internal states of ions in a microfabricated trap coherently on timescales of 20 nanoseconds, with the controlling magnetic fields generated by microwave currents in electrodes integrated into the trap. They also entangle the internal states of two atoms with a gate suitable for general quantum computation, reaching an entangled-state fidelity of 0.76(3). They say integrating quantum control into the trapping device is scalable.

2016: a high-fidelity gate

The 2016 arXiv preprint demonstrates a two-qubit gate driven by near-field microwaves in a room-temperature microfabricated trap, with a measured fidelity of 99.7(1)%, which the authors say is above the minimum threshold required for fault-tolerant quantum computing. The gate acts on calcium-43 atomic-clock qubits with a coherence time of about 50 seconds, using the microwave magnetic field gradient produced by a trap electrode. The authors introduce a dynamically decoupled gate method that stabilizes the qubits against fluctuating a.c. Zeeman shifts and avoids needing to null the microwave field. As a preprint, it is not itself a peer-reviewed record in this form.

2019: an embedded conductor

The 2019 npj Quantum Information paper reports a two-qubit Mølmer-Sørensen gate on a magnetic-field-insensitive hyperfine transition in beryllium-9 ions, using microwave near-fields from a single conductor embedded in a surface-electrode trap. The conductor was optimized to give a high oscillating field gradient at the ion position. The measured fidelity is 98.2 plus or minus 1.2%, which the authors say is limited by technical imperfections, as confirmed by a numerical error analysis. They describe the conductor as potentially simplifying multi-qubit gates and as a self-contained, scalable module for entangling gates in the quantum CCD architecture.

Limits

The three experiments use different ions and traps, so the fidelities are not a single trend. Each involves two ions, and the claims about scalability are the authors' views about future architectures.

FAQ

Why not use lasers?

The papers describe microwave control as an alternative that can be integrated into the trap itself, which the 2011 authors say is scalable.

What fidelity was reported?

0.76 in 2011, 99.7(1)% in the 2016 preprint and 98.2 plus or minus 1.2% in 2019.

Is 99.7% enough for fault tolerance?

The 2016 authors say it is above the minimum threshold required for fault-tolerant quantum computing, which is their statement about the threshold.

Which ions were used?

Calcium-43 in the 2016 work and beryllium-9 in 2019. The 2011 abstract does not name the ion in the text used here.

Is there a large machine built this way?

None of the three papers reports one.