data_Long-time storage of a decoherence-free subspace.zip

Here we report a multi-ion quantum memory in a cryogenic trap based on the dualtype scheme, and demonstrate a coherence time above two hours for a logical qubit encoded in the decoherence-free subspace, i.e. two-ion entangled states, after correcting the dominant leakage error. Our scheme alleviates the necessity of an ultra-stable frequency reference for the stored qubit, and has a preferable scalability owing to the same mass of the metastable-state memory ions and the ground-state coolant ion. We also identify the origin of the leakage error to the nearby Zeeman<br>levels as the collision between the ions and the background gas molecules at a time scale of hundreds of seconds.<br><br>