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Qubit Reuse Beyond Reorder and Reset: Optimizing Quantum Circuits by Fully Utilizing the Potential of Dynamic Circuits

Damian Rovara, Lukas Burgholzer, Robert Wille

TL;DR

The paper tackles the scarcity of qubits on near-term quantum devices by fully leveraging dynamic quantum circuits. It develops a framework that moves measurements earlier in circuits and replaces gates with classically controlled equivalents, thereby breaking qubit interdependencies and enabling aggressive qubit reuse. Across QPE, QFT, VQE, and random circuits, the approach achieves significant reductions in required qubits—up to two for QPE and one for QFT, and up to 95% for sparse random circuits—often with modest depth increases. These results outperform existing qubit-reuse methods and come with an open-source MLIR-based implementation, underscoring practical impact for implementable, hardware-efficient quantum computation.

Abstract

Qubit reuse offers a promising way to reduce the hardware demands of quantum circuits, but current approaches are largely restricted to reordering measurements and applying qubit resets. In this work, we present an approach to further optimize quantum circuits by fully utilizing the potential of dynamic quantum circuits-more precisely by moving measurements and introducing dynamic circuit primitives such as classically controlled gates in a way that forges entirely new pathways for qubit reuse. This significantly reduces the number of required qubits for a variety of circuits, creating new opportunities for running complex circuits on near-term devices with limited qubit counts. We show that the proposed approach drastically outperforms existing methods, reducing qubit requirements where previous approaches are unable to do so for popular quantum circuits such as Quantum Phase Estimation (QPE), Quantum Fourier Transform~(QFT), and Variational Quantum Eigensolver (VQE) ansätze, as well as leading to improvements of up to 95% for sparse random circuits.

Qubit Reuse Beyond Reorder and Reset: Optimizing Quantum Circuits by Fully Utilizing the Potential of Dynamic Circuits

TL;DR

The paper tackles the scarcity of qubits on near-term quantum devices by fully leveraging dynamic quantum circuits. It develops a framework that moves measurements earlier in circuits and replaces gates with classically controlled equivalents, thereby breaking qubit interdependencies and enabling aggressive qubit reuse. Across QPE, QFT, VQE, and random circuits, the approach achieves significant reductions in required qubits—up to two for QPE and one for QFT, and up to 95% for sparse random circuits—often with modest depth increases. These results outperform existing qubit-reuse methods and come with an open-source MLIR-based implementation, underscoring practical impact for implementable, hardware-efficient quantum computation.

Abstract

Qubit reuse offers a promising way to reduce the hardware demands of quantum circuits, but current approaches are largely restricted to reordering measurements and applying qubit resets. In this work, we present an approach to further optimize quantum circuits by fully utilizing the potential of dynamic quantum circuits-more precisely by moving measurements and introducing dynamic circuit primitives such as classically controlled gates in a way that forges entirely new pathways for qubit reuse. This significantly reduces the number of required qubits for a variety of circuits, creating new opportunities for running complex circuits on near-term devices with limited qubit counts. We show that the proposed approach drastically outperforms existing methods, reducing qubit requirements where previous approaches are unable to do so for popular quantum circuits such as Quantum Phase Estimation (QPE), Quantum Fourier Transform~(QFT), and Variational Quantum Eigensolver (VQE) ansätze, as well as leading to improvements of up to 95% for sparse random circuits.

Paper Structure

This paper contains 16 sections, 3 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Background
  3. Dynamic Quantum Circuits
  4. Reducing Qubit Count Using Dynamic Quantum Circuits
  5. Motivation
  6. Related Work
  7. General Idea
  8. Enhanced Qubit Reuse by Moving Measurements
  9. Measurement Commutation Rules
  10. Dynamic Circuit Transformations
  11. Applying Qubit Reuse
  12. Evaluation
  13. Evaluation on QPE and QFT Circuits
  14. Evaluation on VQE Circuits
  15. Evaluation of Random Circuits
  16. ...and 1 more sections

Figures (3)

  • Figure 1: Quantum phase estimation using three counting qubits.
  • Figure 2: Iterative quantum phase estimation using a single counting qubit.
  • Figure 3: The process of employing enhanced qubit reuse. Measurements are moved using multiple commutation rules and further transformations eliminate the gates, making the qubits independent and allowing for a reduction in resource requirements.

Theorems & Definitions (2)

  • Example 1
  • Example 2