Noise-aware Token Swapping for Qubit Routing
Asim Sharma, Avah Banerjee · 2023
In Noisy Intermediate-Scale Quantum (NISQ) era devices, the output fidelity of a quantum circuit, under plausible assumptions, decreases exponentially with its size and depth. As such, optimizing these aspects becomes paramount. This is especially true for sparsely connected physical architectures where added swap gates are essential to meet device connectivity requirements. In this work, we adapt the Approximate Token Swapping (ATS) algorithm, traditionally used for routing qubits, to handle noisy settings by giving priority to specific swap sequences over others. This priority is partially determined by the error rates of the couplings in the qubit connectivity graph. We refer to this algorithm as Priority-ATS (PATS). We provide theoretical justification for PATS's effectiveness over the ATS algorithm and experimentally demonstrate its ability to improve the output state's fidelity. In our simulations, we employ a depolarizing channel to represent two-qubit gate errors, with each neighboring qubit pair having a distinct CNOT gate error rate. By applying realistic error rates, our results prominently highlight an enhanced output fidelity of the quantum circuit when PATS is employed.