Long-time storage of a decoherence-free subspace logical qubit in a dual-type quantum memory

TL;DR

This paper demonstrates a multi-ion quantum memory in a cryogenic trap with a coherence time exceeding two hours, utilizing a decoherence-free subspace to improve stability and scalability for quantum information applications.

Contribution

It introduces a cryogenic dual-type quantum memory scheme that achieves long coherence times and scalability, overcoming limitations of previous room-temperature setups.

Findings

Coherence time of over two hours for a logical qubit.

Effective correction of dominant leakage errors.

Enhanced scalability due to identical ion masses.

Abstract

A quantum memory is an essential element for quantum computation, quantum network and quantum metrology. Previously, a single-qubit quantum memory with a coherence time of about an hour has been realized in a dual-species setup where a coolant ion provides sympathetic cooling for a memory ion of different species. However, the frequent random position hopping between the ions in the room-temperature trap limits the technique there only applicable to single-qubit storage. Here we report a multi-ion quantum memory in a cryogenic trap based on the dual-type 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.