Ytterbium Atoms Encode Quantum Data Using Two Independent States

Scientists at the University of Michigan, led by Chun-Wei Liu, have unveiled a new qubit encoding architecture for neutral-atom arrays utilising ¹⁷¹Yb atoms. The architecture employs a dual metastable-state approach, leveraging both nuclear-spin and hyperfine-spin qubits within distinct energy manifolds to enhance quantum information processing. This innovative design enables extended coherence times suitable for both quantum storage and arithmetic operations. It also facilitates swift Raman operations and direct, state-selective imaging facilitated by a hyperfine frequency of $2π\times 6.7~\mathrm{GHz}$. By seamlessly integrating mid-circuit measurements and rapid qubit operations on a single platform, the research presents a versatile framework with the potential to significantly advance the development of fault-tolerant quantum computing. The ability to perform complex quantum computations relies heavily on maintaining the delicate quantum states of qubits, and this new architecture addresses key challenges in that regard.

Dual-manifold Ytterbium-171 qubits enable rapid operations and scalable architectures

Yb-171 atoms now demonstrate a $2π\times 6.7~\mathrm{GHz}$ hyperfine frequency, representing a substantial improvement over previous qubit encoding schemes. This precise frequency control enables rapid Raman operations, which were previously constrained by spectral limitations inherent in single-manifold qubit systems. Raman operations, utilising the interaction of light with atomic states, are crucial for manipulating qubit states and implementing quantum gates. The utilisation of both the (6s6p),³P0 and (6s6p),³P2 manifolds creates spectrally distinct zones, simplifying qubit assignment and reducing the overall architectural complexity of the quantum processor. This separation of spectral zones minimises unwanted interactions between qubits, improving the fidelity of quantum operations. The choice of these specific manifolds is based on their suitability for both long-term coherence and rapid manipulation, offering a balanced approach to qubit design.

A dual-metastable state approach integrates mid-circuit measurements with fast qubit operations on a single platform, offering a flexible framework for scalable, fault-tolerant quantum computing. Coherent shelving, the controlled transfer of quantum information between manifolds, further enhances the system’s capabilities, allowing for dynamic resource allocation and improved error correction strategies. This dynamic allocation allows the system to adapt to changing computational needs, optimising performance and resource utilisation. Simulations of single and two-qubit gate fidelities within the ³P2 manifold suggest a flexible framework for future quantum computing, with resource estimation indicating the potential for scalable, fault-tolerant systems. The team modelled circuits with up to two logical qubits and varying syndrome-extraction schedules to comprehensively assess performance. However, current simulations do not fully account for erasure-type errors, such as atom loss, and a detailed examination of these effects is needed before practical implementation becomes viable. Atom loss, a significant challenge in neutral-atom quantum computing, occurs when atoms are lost from the trapping potential, disrupting the qubit array and introducing errors.

Ytterbium-171 Qubit Architecture via Dual Metastable-State Encoding

This advancement centres on a dual metastable-state qubit encoding, a technique representing quantum information using two distinct, yet linked, energy levels within ytterbium-171 atoms. A metastable state is a temporary, relatively stable energy level; analogous to balancing a pencil on its tip, it won’t remain stable indefinitely, but can be maintained for a useful duration. This stability is crucial for preserving quantum information over extended periods. The approach utilises the (6s6p),³P0 manifold for long-term storage and calculations, leveraging its inherent stability to maintain qubit coherence, while the (6s6p),³P2 manifold facilitates rapid operations via Raman operations, a method of manipulating atomic states with light, similar to tuning a radio. The selection of these manifolds is based on their differing properties, optimising the system for both storage and processing.

Coherently ‘shelving’ information between these manifolds enables a more streamlined and flexible architecture. This process allows for the temporary storage of quantum information in the long-lived (6s6p),³P0 manifold, protecting it from decoherence during periods of inactivity. Operating at 6.7GHz, the hyperfine-spin qubit’s frequency was chosen to simplify the architecture and potentially enable fault-tolerant quantum computing. This frequency allows for the integration of mid-circuit measurements with fast qubit operations on a single platform, and researchers are now investigating the impact of real-world imperfections, such as atom loss, on the viability of this approach for large-scale fault-tolerant computation. The 6.7GHz frequency also aligns well with existing microwave technology, simplifying the control and readout of the qubits. Furthermore, this frequency facilitates the implementation of error correction protocols, which are essential for building reliable quantum computers.

Ytterbium-171 atoms simplify quantum error correction via distinct energy level control

Neutral-atom arrays offer a promising route to scalable quantum computers, boasting long coherence times and flexible control. Achieving fault tolerance, however, demands repeated measurement of ancilla qubits, a process that introduces architectural overhead and challenges existing qubit encoding schemes. Ancilla qubits are auxiliary qubits used to assist in error correction, and their repeated measurement can disrupt the delicate quantum states of data qubits. This work proposes utilising two distinct energy levels within the ytterbium-171 atom to streamline operations and reduce the resources needed for practical, large-scale quantum computation. By carefully separating the functions of data and ancilla qubits, the architecture minimises interference and improves the overall performance of the quantum computer.

Confining operations to distinct spectral zones within the ytterbium atom mitigates the architectural complexity typically associated with mid-circuit measurements of ancilla qubits. Separated storage and processing functions are achieved by employing both nuclear-spin and hyperfine-spin qubits within the (6s6p),³P0 and (6s6p),³P2 manifolds, streamlining quantum operations and integrating rapid measurements with long-term coherence on a single platform. This architectural choice establishes a pathway towards more practical quantum computers, and scientists are currently focused on modelling the effects of atom loss to determine the feasibility of this approach for large-scale fault-tolerant computation. The ability to perform error correction without significantly impacting the coherence of data qubits is a crucial step towards building a robust and reliable quantum computer. Further research will focus on optimising the system for scalability and minimising the impact of environmental noise on qubit performance.

This research demonstrated a new qubit encoding scheme using ytterbium-171 atoms that utilises two distinct energy levels within the atom. This approach simplifies quantum error correction by separating data and ancilla qubit functions, reducing the architectural overhead typically associated with mid-circuit measurements. By employing both nuclear-spin and hyperfine-spin qubits, the system enables both long-term coherence and rapid measurements on a single platform. Researchers are now modelling atom loss to assess the feasibility of scaling this architecture for fault-tolerant quantum computation.