Sustaining high-fidelity quantum logic in neutral-atom circuits via mid-circuit operations

Sustaining High-Fidelity Quantum Logic in Deep Neutral-Atom Circuits via Mid-Circuit Operations

Motivation and Context

Neutral-atom platforms have emerged as a leading architecture for scalable quantum processing, exploiting flexible connectivity and the ability to assemble defect-free atomic arrays at large scale. Recent advances have enabled thousand-qubit systems and initial logical operations, but the practical realization of fault-tolerant quantum computing critically requires not only increasing qubit counts, but sustaining high gate fidelities over deep circuit executions. In neutral-atom circuits, entangling operations mediated by Rydberg interactions are susceptible to motional heating and atom loss, which is exacerbated with circuit depth due to repetitive gate operations and photon scattering. These cumulative effects, described as “thermal decay,” force current implementations to rely on out-of-circuit resets, precluding continuous operation and imposing a fundamental bottleneck for large-scale quantum error correction.

Architecture Overview and Mid-Circuit Refresh Strategy

The paper presents a hardware-efficient scheme that directly addresses sustained high-fidelity operation by embedding mid-circuit refresh protocols throughout quantum circuit execution. The fundamental components are:

• In Situ Raman Sideband Cooling (RSC): Active entropy management is achieved by driving red-sideband transitions between motional levels and dissipative optical pumping, cooling atoms close to the three-dimensional ground state. The protocol operates directly between hyperfine clock states $|1,0\rangle$ and $|2,0\rangle$, providing robustness against light shift and magnetic field inhomogeneity, and enabling uniform cooling across atom arrays.
• Qubit Re-Initialization: Mid-circuit optical pumping ensures qubits are reset to low-entropy internal states, closing the RSC cycle and providing a combined “cool-and-reset” operation.
• Non-Destructive, Loss-Resolved Measurement: Employing a two-stage imaging process (state-selective and state-independent fluorescence), atom survival and qubit state can be resolved without destructively removing atoms. This process achieves 98.9% state-resolved fidelity and 99.7% atomic survival probability, and utilizes AI analysis for robust real-time classification.

These integrated mid-circuit operations allow continuous entropy removal, active management of both internal state and motional degrees of freedom, and conversion of atom loss to erasure errors, each with known locations that can be handled efficiently in QEC codes.

Gate Performance and Loss Correction

Entangling gates are executed via a time-optimal CZ pulse, phase-modulated on the 420-nm Rydberg laser, yielding a two-qubit CZ gate fidelity of 99.60(1)% under global echoed randomized benchmarking. The theoretical error model identifies that dominant infidelity arises from residual Rydberg population and black-body-radiation-induced state transitions, manifesting as atom loss expelled by the tweezers.

By post-selecting only those gate operations where both atoms survive (ascertained from mid-circuit measurement), gate fidelity is increased to 99.81(1)%. This substantial improvement demonstrates the efficacy of erasure conversion, where loss events are rendered detectable and correctable—relaxing fault-tolerance thresholds compared to conventional unknown leakage.

Sustained Performance in Deep Circuits

The scheme’s capability to preserve high-fidelity logic over deep circuits is validated by repetitive benchmarking protocols. Without active cooling, successive rounds exhibit a $\sim$ 0.3% drop in gate fidelity due to cumulative heating. Local gray molasses cooling alone is insufficient to mitigate this decay, underscoring the necessity for targeted RSC.

With mid-circuit RSC in place, mean phonon number is maintained at $n \sim 1$ in radial and axial modes across five consecutive rounds, as evidenced by sideband spectra and benchmarking. The loss-corrected CZ gate fidelity remains stable at $\sim$99.8% throughout, with no observable degradation, marking a transition from “single-shot” demonstrations to sustainable circuit-level operation. This persistent performance enables repeated syndrome extraction cycles necessary for continuous quantum error correction in topological codes.

Practical and Theoretical Implications

The in-situ refreshable architecture establishes the essential prerequisite for large-scale fault-tolerant quantum computation in neutral-atom systems. Practical implications include:

• Topological Codes: Continuous entropy reset and loss-resolved measurement directly support syndrome extraction in surface code and related architectures.
• Zoned Layouts: The entropy removal protocol can be extended to zoned topologies, allowing independent refresh of ancilla and data qubits through coherent transport and localized addressing.
• Teleportation-Based Replacement: The approach is compatible with teleportation-based qubit replacement, which enables efficient refresh of logical qubits by transferring information onto freshly prepared atomic sites.

Theoretical implications center around relaxed threshold criteria enabled by erasure conversion, greater robustness to motional decoherence, and the pathway to autonomous, continuous error correction at scale.

Future Directions

Future developments may explore:

• Scaling the refresh protocol to multi-qubit gate operations and larger arrays.
• Integration with fault-tolerant logical devices, measuring full quantum error correction performance under sustained deep operation.
• Application of advanced AI-driven state discrimination for real-time control feedback.
• Extension to teleportation-based architectures and dynamic ancilla replacement.

Conclusion

The paper demonstrates a sustainable neutral-atom quantum architecture capable of maintaining high-fidelity quantum logic across deep circuits by embedding mid-circuit cooling, qubit re-initialization, and loss-resolved measurement. The ability to actively manage both internal and motional entropy mid-circuit removes core limitations imposed by cumulative heating and atom loss, enabling stable execution of repeated error correction routines and continuous deep quantum circuits. The architecture provides a direct pathway toward practical, large-scale quantum processors, establishing new operational benchmarks and advancing the theoretical landscape for fault-tolerant quantum computation in neutral-atom platforms (2603.01612).