Prof. Dr. Michael Drewsen - Long-Term Coherence of a 138Ba+ Optical Qubit via Continuous Dynamical Decoupling

A qubit coherence time much longer than the duration of individual gate operations is essential for scalable quantum computing. In trapped-ion systems, coherence is often limited by fluctuations in the addressing laser frequency and intensity, as well as in ambient magnetic fields. Continuous dynamical decoupling via frequency modulation (FM) generated double-dressed states highly suppress all these noise channels by nesting the qubit inside two protective energy gaps [1,2]. Here, we apply this protocol to an optical-frequency qubit in ¹³⁸Ba⁺, extending the Rabi coherence time from ~ to ~, an increase of more than three orders of magnitude and corresponding to over 10,000 coherent Rabi oscillations. 
We further demonstrate motional sideband coupling in the double-dressed basis, where the phase modulation that creates the second dressing simultaneously generates sideband transitions between the double-dressed states and the motional mode, without requiring a separate laser detuned to the motional sideband. This sideband coupling is a prerequisite for implementing a recently proposed scheme [1] for robust two-qubit entangling gates, where geometric phase form state-dependent displacement loops produce entanglement while inheriting the noise resilience of the double-dressed basis. 

References 
[1]        M. Nünnerich et al., Phys. Rev. X 15, 021079 (2025). 
[2]        I. Cohen et al., New J. Phys. 17, 043008 (2015).