Long-lived metastable-qubit memory

TL;DR

This paper demonstrates a metastable-qubit memory in a trapped ion with a coherence time of over two minutes, highlighting its potential for fault-tolerant quantum computing and architectural benefits.

Contribution

It introduces a method for long-lived quantum state storage in metastable states of a trapped ion, with active error monitoring and sympathetic cooling.

Findings

Achieved 136 seconds coherence time in metastable $5D_{5/2}$ state.

Dephasing identified as the main error source after erasure errors are mitigated.

Model based on noise spectroscopy explains the dephasing behavior.

Abstract

Coherent storage of quantum information is crucial to many quantum technologies. Long coherence times have been demonstrated in trapped-ion qubits, typically using the hyperfine levels within the ground state of a single ion. However, recent research suggests qubits encoded in metastable states could provide architectural benefits for quantum information processing, such as the possibility of effective dual-species operation in a single-species system and erasure-error conversion for fault-tolerant quantum computing. Here we demonstrate long-lived encoding of a quantum state in the metastable states of a trapped ion. By sympathetically cooling with another ion of the same species and constantly monitoring for erasure errors, we demonstrate a coherence time of 136(42) seconds with a qubit encoded in the metastable $5D_{5/2}$ state of a single $^{137}$Ba$^+$ ion. In agreement with a model based on empirical results from dynamical-decoupling-based noise spectroscopy, we find that dephasing of the metastable levels is the dominant source of error once erasure errors are removed.