Fast, robust and laser-free universal entangling gates for trapped-ion quantum computing*

Entangling gates are an essential building block of any quantum processor and can improve the performance of quantum sensing applications in various fields of physics. Ideally, they operate at high speed in a robust and scalable manner. Radio frequency (RF)-driven trapped ion quantum gates are particularly promising in terms of technological scalability of quantum processors. However, 2-qubit gate speeds of RF-controlled quantum gates have lagged behind their counterparts using laser-driven trapped ions.

Here, we introduce and experimentally realize a novel Mølmer- Sørensen-type entangling gate employing a continuous dynamic decoupling technique [1,2]. This gate is implemented with trapped ¹⁷¹Yb⁺ -ions exposed to a static magnetic gradient of 19 T/m, thus taking advantage of magnetic gradient induced coupling (MAGIC) (3). We implement double-dressing of the hyperfine states |0〉≡ | ²S_1/2, F=0, m_F=0〉and |1〉≡ |²S_1/2, F=1, m_F=-1 〉using only a single phase modulated RF field per ion. Using state tomography, we follow the time evolution of the 2-qubit gate, resulting in symmetric and antisymmetric Bell states after 300 µs, with fidelities better than 97 % (4). This gate is faster by an order of magnitude than previous RF-driven quantum gates without increasing the magnetic gradient. At the same time, double-dressing of the qubit states protects them against decoherence, resulting in a three order of magnitude improvement in coherence time. In future micro-structured ion traps we expect a further increase in gate speed by another order of magnitude assuming comparable trap frequencies.

[1] I. Cohen et al, New J. Phys., 17,043008 (2015)

[2] D. Farfurnik et al, Phys. Rev. A, 96, 013850 (2017)

[3] Ch. Piltz et al, Sci. Adv.2, e1600093 (2016)

[4] M.Nünnerich et al, arXiv:2403.04730 (2024)