# Overhead in Quantum Circuits with Time-Multiplexed Qubit Control

## TL;DR

This paper analyzes the trade-offs of using time-multiplexed control lines in quantum processors, showing that it can reduce hardware complexity with manageable overhead for many algorithms.

## Contribution

It provides a quantitative assessment of control line multiplexing, demonstrating its advantages and limitations for scalable quantum computing architectures.

## Key findings

- Fewer control lines can be used without significant overhead for many algorithms.
- Grouping two-qubit gate couplers on common lines is feasible up to connectivity limits.
- Serialization overhead for single-qubit gates scales logarithmically with qubit count.

## Abstract

When scaling up quantum processors in a cryogenic environment, it is desirable to limit the number of qubit drive lines going into the cryostat, since fewer lines makes cooling of the system more manageable and the need for complicated electronics setups is reduced. However, although time multiplexing of qubit control enables using just a few drive lines to steer many qubits, it comes with a trade-off: fewer drive lines means fewer qubits can be controlled in parallel, which leads to an overhead in the execution time for quantum algorithms. In this article, we quantify this trade-off through numerical and analytical investigations. For standard quantum processor layouts and typical gate times, we show that the trade-off is favorable for many common quantum algorithms $\unicode{x2014}$ the number of drive lines can be significantly reduced without introducing much overhead. Specifically, we show that couplers for two-qubit gates can be grouped on common drive lines without any overhead up to a limit set by the connectivity of the qubits. For single-qubit gates, we find that the serialization overhead generally scales only logarithmically in the number of qubits sharing a drive line. These results are promising for the continued progress towards large-scale quantum computers.