qGDP: Quantum Legalization and Detailed Placement for Superconducting Quantum Computers

TL;DR

The results indicate that qGDP effectively legalizes and fine-tunes the layout, addressing the quantum-specific spatial constraints inherent in various device topologies, and consistently outperforms state-of-the-art legalization engines.

Abstract

Noisy Intermediate-Scale Quantum (NISQ) computers are currently limited by their qubit numbers, which hampers progress towards fault-tolerant quantum computing. A major challenge in scaling these systems is crosstalk, which arises from unwanted interactions among neighboring components such as qubits and resonators. An innovative placement strategy tailored for superconducting quantum computers can systematically address crosstalk within the constraints of limited substrate areas. Legalization is a crucial stage in placement process, refining post-global-placement configurations to satisfy design constraints and enhance layout quality. However, existing legalizers are not supported to legalize quantum placements. We aim to address this gap with qGDP, developed to meticulously legalize quantum components by adhering to quantum spatial constraints and reducing resonator crossing to alleviate various crosstalk effects. Our results indicate that qGDP effectively legalizes and fine-tunes the layout, addressing the quantum-specific spatial constraints inherent in various device topologies. By evaluating diverse NISQ benchmarks. qGDP consistently outperforms state-of-the-art legalization engines, delivering substantial improvements in fidelity and reducing spatial violation, with average gains of 34.4x and 16.9x, respectively.

Figures (9)

• Figure 1: Impact of placement optimization stages on layout quality. Placement stages in sequence is global placement (GP, gray), legalization (LG, blue), and detailed placement (DP, light blue). The blue and red lines underscore the critical role of legalization. Despite its brief runtime, legalization considerably affects layout quality. Improper legalization can undermine the outcomes from GP, and these issues are often irreparable during DP.
• Figure 2: a) Transmon qubits coupled by resonators; two-qubit gates activated by applying/removing an off-resonant pulse to the resonator. b) Physical layout of a transmon qubit. c) Circuit diagram of a fixed-frequency transmon qubit featuring a capacitor, Josephson junction, and microwave control line.
• Figure 3: Airbridge diagram; A airbridge (blue) connects the signal lines of horizontal resonator from left to right, bridging over the vertical resonator. a) Top-view; b) Side-view,
• Figure 4: Qubit legalization, black box represent the layout border, qubits (blue) and resonator segments (gray) are color-coded by frequency; a): GP positions; b): Post-qubit legalization (red dot box depicts minimum spacing constraint, arrows show the displacement)
• Figure 5: pseudo connection in defining netlist a): Padded resonator with wirelength $L$ and padding length $l_{pad}$; b): Reshaped resonator from (a) into a compact rectangle, partitioned into segments of size $l_b$, retaining frequency consistency as indicated by color. Here $n=6$; c): Net connection of wire blocks (blue arrow indicates the connection between blocks, gray arrow indicates the connection between blocks and qubits); d): Enhanced net connection with pseudo connects (red dot arrow is the enhanced connections to lead a legalization friendly GP layout)