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Physical Review A

Long-lived metastable-qubit memory

Xiaoyang Shi, Jasmine Sinanan-Singh, Kyle DeBry, Susanna Todaro, Isaac L. Chuang, John Chiaverini

Published Feb 18, 2025• 15 citations
Publisher

Abstract

The authors demonstrate an experimental realization of a long-lived quantum memory using the optical-frequency--metastable-state--ground-state architecture in a trapped ion, where the qubit is stored in the metastable states while an ancillary ion is used for sympathetic cooling. A dynamical decoupling sequence and leakage detection are employed to extend the coherence time to approximately four times the natural lifetime of the metastable state.

DOI
10.1103/physreva.111.l020601

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The authors demonstrate an experimental realization of a long-lived quantum memory using the optical-frequency--metastable-state--ground-state architecture in a trapped ion, where the qubit is stored in the metastable states while an ancillary ion is used for sympathetic cooling. A dynamical decoupling sequence and leakage detection are employed to extend the coherence time to approximately four times the natural lifetime of the metastable state.
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 5D5/25D_{5/2}5D5/2​ state of a single 137^{137}137Ba+^++ 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.
Quantum memory device capable of storing quantum information for a long period of time is one of the fundamental ingredients to realize large-scale quantum computation and quantum communication. Comparing with other quantum computation platforms, one of the advantages of the trapped-ion system is the long intrinsic coherence time. Before our work, the longest single-qubit coherence time in trapped-ion systems has been achieved to be less than 1 minute. It is discovered that the main limitation for the coherence time is the motional mode heating and the environment noise that includes the contributions from the magnetic field fluctuation and the phase noise of the microwaves. In a hybrid trapping system simultaneously trapping 171Yb+ and 138Ba+ ions, single-qubit quantum memories with coherence time longer than 10 minutes can be realized by applying sympathetic cooling and dynamical decoupling. This technique may have some value as the building blocks for quantum cryptography protocols and hybrid quantum computation platforms.
A quantum memory is an essential element for quantum computation, quantum networks, 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 a different species. However, the frequent random position hopping between the ions in the room-temperature trap limits the technique to be applicable only 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 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 ultrastable frequency reference for the stored qubit, and it has a preferable scalability owing to the same mass of the metastable-state memory ions and the ground-state coolant ion.
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.
Long-lived metastable states in the 4f$^{13}5d6sconfigurationofYb5d6s configuration of Yb5d6sconfigurationofYb^+$
We study the occurrence of long-lived metastable states in the 4f$^{13}5d6selectronconfigurationofYb5d6s electron configuration of Yb5d6selectronconfigurationofYb^+$. By optical pumping of a single trapped ion on the 2F7/2o→(7/2,0)7/2^2F^\text{o}_{7/2}\rightarrow (7/2,0)_{7/2}2F7/2o​→(7/2,0)7/2​ transition at 377.5 nm, we prepare a wide range of metastable electronic states. We use a co-trapped control ion to sympathetically cool the spectroscopy ion, allowing us to accurately time its subsequent decay. We record a strong decay signal corresponding to a lifetime of 0.92(8) s, a weaker decay signal with lifetime 9.8(+2.9, -2.0) s, and find evidence for a much longer lifetime, >>> 30 s. We identify the metastable states with these lifetimes qualitatively, and corroborate our results with atomic structure calculations that support the observed lifetimes and decay paths. These long-lived states provide new opportunities in qubit and qudit state detection and optical clocks.
Mid-circuit ground-state cooling and ancilla readout in the omg\textit{omg}omg architecture
The trapped-ion optical-metastable-ground (omg\textit{omg}omg) architecture for quantum processors promises the full functionality of two-species experiments, including sympathetic cooling and non-destructive ancilla readout, without the corresponding hardware overhead. We confirm that we can cool a global motional mode of a mixed metastable-ground state Coulomb crystal to the motional ground state via dissipative operations on the ground (g\textit{g}g) qubit without disturbing coherence of the metastable (m\textit{m}m) qubit. This enables quantum logic spectroscopy to non-destructively readout the state of the m\textit{m}m qubit using fluorescence detection of the g\textit{g}g qubit. Extensions of these demonstrations to larger system sizes should enable the mitigation of motional heating after ion shuttling and syndrome extraction for quantum error correction, both crucial primitives for future fault-tolerant quantum computers based on trapped ions.