Entangled Quantum States Held Stable for One Hour in Ion Trap

Scientists have demonstrated long-term preservation of quantum entanglement, a crucial element for advancing quantum technologies. Led by L. Zhang and Y.-L. Xu from the Center for Quantum Information, Institute for Interdisciplinary Information Sciences, Tsinghua University, and working with colleagues at HYQ Co., Ltd., this research details the successful storage of entangled logical states for approximately one hour. The team, which also included researchers from HYQ Co., Ltd. and Tsinghua University, encoded these states within decoherence-free subspaces of ions held in a cryogenic trap, employing innovative techniques to minimise errors and extend coherence. This achievement represents a significant step towards realising practical applications of quantum memories in areas such as quantum computing, quantum networks and high-precision measurement.

Extended entanglement storage via dual-type qubits and second-order decoherence-free subspaces

A one-hour storage lifetime for two-qubit entangled states was achieved, exceeding previous limits of single qubits or limited-capacity logical qubits by a substantial margin. This duration crosses a vital threshold for practical quantum technologies, as maintaining entanglement for such extended periods was previously unattainable. The states were encoded within ‘decoherence-free subspaces’, protective states shielding quantum information from disruptive environmental influences within four trapped ions cooled to extremely low temperatures.

Errors induced by ion collisions were discarded, further extending storage through a dual-type qubit encoding scheme and multi-state detection. Improved suppression of spatially nonuniform noise, compared to a first-order approach, was demonstrated by utilising a second-order decoherence-free subspace, enhancing stability. Measurements were taken at six distinct time points—2, 30, 60, 120, 240, and 960 seconds—yielding between 30 and 250 samples per observation to ensure statistical relevance.

Storage fidelity exceeding 80 per cent was verified even after 60 minutes, demonstrating minimal decay in the entangled state’s quality over this extended period. Analysis of two specific Bell states, |ψ+ L ⟩ and |φ+ L⟩, revealed storage lifetimes of about one hour, calculated with an unspecified confidence level. Error rates dropped significantly. Further experiments with a second-order decoherence-free subspace showed a storage lifetime of approximately one hour for entangled states, offering the advantage of suppressing spatially nonuniform noise over a first-order subspace.

While these results demonstrate the storage of two logical qubits, they do not yet show scalability to larger qubit numbers or address the challenges of maintaining fidelity during complex quantum computations. Prolonged stability within these protective states was demonstrated, but high-accuracy readout of the stored quantum states remains to be shown, indicating a limitation between storage duration and information retrieval.

Decoherence protection and sympathetic cooling of ytterbium ion qubits

Central to extending the storage of entangled quantum bits was the employment of ‘decoherence-free subspaces’ – a protective ‘bubble’ around quantum information shielding it from environmental disturbances that cause errors. This technique encodes quantum information in a way that makes it insensitive to certain types of noise, effectively isolating the qubits from disruptive influences.

Four ytterbium ions were trapped within a cryogenic trap, cooled to 6 Kelvin to minimise disruptive collisions and maintain stability during storage. A dual-type qubit encoding scheme was implemented, allowing for crosstalk-free sympathetic cooling; this process cools one substance using another without direct contact. Entangled logical states were stored for approximately one hour using this method.

The technique encoded four central ions into two logical qubits, protecting quantum information from environmental noise. Multi-state detection distinguished and discarded errors, contributing to the approximately one hour of entangled state storage. This prolonged coherence is vital for building more complex and powerful quantum computers and networks, enabling the development of more sophisticated quantum algorithms. Maintaining quantum entanglement—the fundamental link enabling quantum technologies—requires both lengthy coherence and sufficient storage capacity.

Prolonged qubit coherence demonstrated, but readout fidelity remains a key challenge

Maintaining quantum entanglement for an hour represents a key advance, yet practical quantum devices demand not just storage, but reliable retrieval of that information. No prior method matched this performance. This omission highlights a critical tension: extending storage time is insufficient without also solving the challenge of faithfully accessing the preserved quantum information.

Achieving hour-long quantum storage without immediately reading out the information feels like perfecting a beautifully insulated flask then leaving it on the shelf. Protective states shielded quantum bits from disruption and crosstalk-free cooling techniques underpinned this breakthrough. The success of this approach demonstrates the potential for building more complex quantum systems, but further research is needed to address the readout challenge and achieve scalability. Speed doubled in some tests.