11/11/2025 -- Kihwan Kim -- A trapped-ion qubit with a coherence time exceeding ten hours

UMD Physics Colloquium on Nov. 11, 2025. Prof. Kihwan Kim from Tsinghua University presented his research on

A trapped-ion qubit with a coherence time exceeding ten hours

Quantum systems promise to revolutionize technology in the realms of computing and sensing. Yet they are limited by decoherence mechanisms. In theory, trapped atomic ions should be perfect quantum memories, supporting million-year coherence times based o n spontaneous emission. Yet experimental demonstrations have fallen many orders of magnitude short. This gap raises the question of whether we face fundamental physical limitations or addressable technical challenges.

We combine clock-state qubits with decoherence-free subspace (DFS) encoding to achieve coherence times exceeding ten hours. Using 171Yb+ ion pairs sympathetically cooled by 138Ba+, we demonstrate this without magnetic shielding or enhanced microwave stabilization. DFS encoding cancels common-mode magnetic fluctuations across micrometer separations while clock states provide environmental insensitivity. Our exponential fits yield a coherence time of 37,750 ± 10,890 seconds.

These results experimentally verify that trapped-ion qubits are limited by technical factors rather than fundamental decoherence, establishing a pathway toward ultimate quantum memories for scalable information processing. I will also discuss our related work on scaling up qubit numbers using two-dimensional ion crystals.