Routing-Aware Placement for Zoned Neutral Atom-based Quantum Computing

Abstract: Quantum computing promises to solve previously intractable problems, with neutral atoms emerging as a promising technology. Zoned neutral atom architectures allow for immense parallelism and higher coherence times by shielding idling atoms from interference with laser beams. However, in addition to hardware, successful quantum computation requires sophisticated software support, particularly compilers that optimize quantum algorithms for hardware execution. In the compilation flow for zoned neutral atom architectures, the effective interplay of the placement and routing stages decides the overhead caused by rearranging the atoms during the quantum computation. Sub-optimal placements can lead to unnecessary serialization of the rearrangements in the subsequent routing stage. Despite this, all existing compilers treat placement and routing independently thus far - focusing solely on minimizing travel distances. This work introduces the first routing-aware placement method to address this shortcoming. It groups compatible movements into parallel rearrangement steps to minimize both rearrangement steps and travel distances. The implementation utilizing the A* algorithm reduces the rearrangement time by 17% on average and by 49% in the best case compared to the state-of-the-art. The complete code is publicly available in open-source as part of the Munich Quantum Toolkit (MQT) at https://github.com/munich-quantum-toolkit/qmap.

• The paper introduces a novel routing-aware placement method that integrates atom movement constraints and reduces rearrangement steps by up to 49%.
• It employs an A*-based search with a multi-part heuristic, combining greedy grouping and look-ahead cost estimation for efficient gate scheduling.
• Experimental results on benchmark suites demonstrate substantial reductions in rearrangement time and enhanced circuit parallelism in zoned neutral atom architectures.

Routing-Aware Placement for Zoned Neutral Atom-Based Quantum Computing

Introduction and Motivation

This work introduces the first routing-aware placement methodology tailored to zoned neutral atom quantum computing architectures—a domain distinguished by the coexistence of entanglement and storage zones, where atom transport between zones is required for the execution of two-qubit operations. Previous compilation approaches for such systems have decoupled placement and routing, optimizing placements solely with respect to accumulated distance, consequently missing considerable opportunities to exploit parallelism during atom rearrangement phases due to the non-crossing and preservation constraints inherent to Acousto-Optic Deflector (AOD)-based atom movement. This paper identifies this as a significant suboptimality and proposes a fundamentally different approach that explicitly incorporates the downstream constraints of routing into the placement stage.

Technical Framework

The authors systematically dissect the existing workflow for quantum circuit compilation targeting zoned neutral atom architectures. The stages include gate scheduling, reuse analysis (identification of when reusing a qubit position across layers reduces swaps), placement (assignment of logical qubits to physical traps), routing (decomposition of position changes into valid, non-crossing, parallelizable rearrangement steps), and code emission.

A key insight is that minimization of atom travel alone is insufficient; it can in fact hinder routing by creating intricate conflicts that force serialization of atom moves. Instead, the optimization objective should be to minimize the number of required rearrangement steps, which directly impacts coherence-limited execution fidelity.

To formalize the improved objective, the authors define a cost function with two main terms:

1. A proxy for routing effort, using greedy grouping of atom moves into parallelizable sets based on the underlying hardware constraints.
2. A look-ahead term that incorporates future cost anticipations, avoiding shortsighted choices where, for example, aggressive atom reuse would drastically increase the subsequent layer’s routing burden.

Given the combinatorially vast space of candidate placements (e.g., for 16 atoms and a modest set of possible traps, over $10^{24}$ configurations), the authors leverage an A*-based search, with carefully-crafted admissible and non-admissible components in the heuristic to steer exploration toward globally superior placements under realistic time budgets.

Algorithmic Realization

The implementation employs a dedicated data structure for efficient insertion and compatibility checking of atom moves into “rearrangement groups” with respect to the non-crossing, preservation, and ghost-spot constraints. Compatibility checks leverage binary search trees over discretized source/target trap indices, supporting constant-time conditions for the group invariants (e.g., relative order preservation in rows/columns).

The heuristic function for A* is multi-part: the admissible term establishes a lower bound on cost-to-go by positing all future moves as group-compatible; an acceleration term penalizes placements likely to create future conflicts based on deviations from previous gap structures; and a look-ahead term folds in estimated costs of placement options for unplaced atoms, weighted appropriately. Pruning is enforced via a spatial window over candidate traps to restrict the search to relevant placement neighborhoods.

Experimental Results

The methodology is evaluated on two benchmark suites: QASMBench and MQT Bench, incorporating circuits with up to thousands of two-qubit gates and over a hundred qubits. Parameter sweeps (e.g., varying look-ahead weighting $\alpha$) empirically confirm that incorporating future cost estimation is critical for effectiveness.

Compared against the routing-agnostic approach of ZAC [linReuseAwareCompilationZoned2024], the routing-aware placement consistently reduces both the number of rearrangement steps and the total rearrangement time—two strongly fidelity-affecting metrics. On average, across benchmarks, rearrangement steps are reduced by 17%, and in the best case, by 49%. Rearrangement time is similarly reduced by up to 49%. These benefits are especially pronounced for circuits with high parallelism in their two-qubit gate layers. Meanwhile, increase in placement computation time is moderate and compatible with compiler workflows.

A summary comparison illustrates these improvements:

Approach	Rearrangement Steps	Rearrangement Time	Placement Time (typ.)
Routing-agnostic (ZAC)	Baseline	Baseline	Low (~ms)
Routing-aware (proposed)	−17% (avg), −49% (best)	−17% (avg), −49% (best)	Moderate (<1–5s)

Appropriate runtime/quality tradeoffs can be achieved via parameterization.

Implications and Future Directions

This work demonstrates that classical placement objectives are insufficient in the context of zoned neutral atom quantum architectures; explicit consideration of hardware-imposed atom movement constraints at placement time can nontrivially decrease overall circuit execution time and exposure to decoherence or atom loss. The routing-aware paradigm is especially relevant as device sizes and the scale of quantum circuits increase.

Practically, the proposed method is already publicly deployed as part of the Munich Quantum Toolkit (MQT), facilitating its adoption in quantum software pipelines. Theoretically, it underscores the value of cost modeling and joint optimization across multiple compiler phases, with further opportunities including tighter integration with routing algorithms, better anticipation of atom loss and reloading strategies, or application to other modular or zone-based quantum technologies.

As neutral atom quantum computers scale, it is likely that such joint placement-routing strategies will become standard, potentially augmented by machine-learned cost-to-go estimators or hardware-informed adaptation. Dynamically-aware and global optimization techniques will become increasingly important as other modalities (e.g., real-time error correction cycles, rerouting under failure) are incorporated.

Conclusion

The paper "Routing-Aware Placement for Zoned Neutral Atom-based Quantum Computing" (2505.22715) presents the first approach to unify placement and routing with awareness of movement constraints, demonstrating substantial quantitative improvements in rearrangement cost metrics for neutral atom devices. The techniques reveal a necessary evolution in quantum compiler methodology appropriate for the architectural realities of scalable, parallelizable quantum platforms. The work lays a foundation for more holistic and hardware-informed quantum software stacks, and invites further refinement in cooperative optimization across the quantum compilation pipeline.