Qplacer: Frequency-Aware Component Placement for Superconducting Quantum Computers

TL;DR

This work proposes QPlacer, a frequency-aware electrostatic-based placement framework tailored for superconducting quantum computers to alleviate crosstalk by isolating these components in spatial and frequency domains alongside compact substrate design, and motivates a general approach to systematically resolving multifaceted crosstalks in a limited substrate area.

Abstract

Noisy Intermediate-Scale Quantum (NISQ) computers face a critical limitation in qubit numbers, hindering their progression towards large-scale and fault-tolerant quantum computing. A significant challenge impeding scaling is crosstalk, characterized by unwanted interactions among neighboring components on quantum chips, including qubits, resonators, and substrate. We motivate a general approach to systematically resolving multifaceted crosstalks in a limited substrate area. We propose Qplacer, a frequency-aware electrostatic-based placement framework tailored for superconducting quantum computers, to alleviate crosstalk by isolating these components in spatial and frequency domains alongside compact substrate design. Qplacer commences with a frequency assigner that ensures frequency domain isolation for qubits and resonators. It then incorporates a padding strategy and resonator partitioning for layout flexibility. Central to our approach is the conceptualization of quantum components as charged particles, enabling strategic spatial isolation through a 'frequency repulsive force' concept. Our results demonstrate that Qplacer carefully crafts the physical component layout in mitigating various crosstalk impacts while maintaining a compact substrate size. On various device topologies and NISQ benchmarks, Qplacer improves fidelity by an average of 36.7x and reduces spatial violations (susceptible to crosstalk) by an average of 12.76x, compared to classical placement engines. Regarding area optimization, compared to manual designs, Qplacer can reduce the required layout area by 2.14x on average

Figures (15)

• Figure 1: System infidelity due to crosstalk impacts versus the area required for accommodating an equal number of qubits and other quantum components using different placement strategies. Qplacer is designed to optimize layout area while maintaining low infidelity.
• Figure 2: a): Physical layout of a transmon qubit. b): Circuit diagram of a fixed-frequency transmon qubit featuring a capacitor, Josephson junction, and microwave control line. c): Energy levels of transmon, Josephson junction transforms energy potential from quadratic (dashed dark gray) to sinusoidal (solid blue), leading to distinct energy levels $|0\rangle$ and $|1\rangle$ for computational use, with energy separation $\hbar \omega_{01}$.
• Figure 3: Circuit diagram of two coupled transmon qubits via a resonator; Two-qubit gates (CZ, controlled Phase gate) are implemented by applying/removing an off-resonant pulse to the resonator.
• Figure 4: Coupling strength between two directly connected transmon qubits via a capacitor. $\omega_n$ for qubit $n$. $\omega_1$ is held constant while $\omega_2$ is varied. The peak coupling strength occurs when the two transmons are resonant ($\omega_1 = \omega_2$), depicted in blue shadow. As $\omega_2$ diverges from $\omega_1$, the residual coupling gradually diminishes. Coupling strength $g$ is typically around $20 \sim 30$MHz (gray dash line).
• Figure 5: a): Separation distance $d$ between transmon qubits. b): Variation in coupling strength $g$, effective coupling strength $g_{\text{eff}}$ and parasitic capacitance $C_p$ with the distance $d$ between two transmon qubits, indicating increased capacitance and coupling strength as $d$ decreases.