Heat Bath Algorithmic Cooling with Spins: Review and Prospects

TL;DR

This paper reviews Heat Bath Algorithmic Cooling (HBAC) in spin systems, highlighting its potential to improve qubit purity for quantum error correction in NMR and electron-nuclear systems, with insights into theoretical and experimental progress.

Contribution

It provides a comprehensive overview of HBAC techniques in spin systems, emphasizing prospects for using electron-nuclear coupled systems to enhance quantum error correction capabilities.

Findings

HBAC can effectively cool qubits below bath temperature.

Coupled electron-nuclear spins offer higher polarization than traditional NMR.

HBAC shows promise for enabling multiple round quantum error correction.

Abstract

Application of multiple rounds of Quantum Error Correction (QEC) is an essential milestone towards the construction of scalable quantum information processing devices. However, experimental realizations of it are still in their infancy. The requirements for multiple round QEC are high control fidelity and the ability to extract entropy from ancilla qubits. Nuclear Magnetic Resonance (NMR) based quantum devices have demonstrated high control fidelity with up to 12 qubits. On the other hand, the major challenge in the NMR QEC experiment is to efficiently supply ancilla qubits in highly pure states at the beginning of each round of QEC. Purification of qubits in NMR, or in other ensemble based quantum systems can be accomplished through Heat Bath Algorithmic Cooling (HBAC). It is an efficient method for extracting entropy from qubits that interact with a heat bath, allowing cooling below the bath temperature. For practical HBAC, coupled electron-nuclear spin systems are more promising than conventional NMR quantum processors, since electron spin polarization is about $10^3$ times greater than that of a proton under the same experimental conditions. We provide an overview on both theoretical and experimental aspects of HBAC focusing on spin and magnetic resonance based systems, and discuss the prospects of exploiting electron-nuclear coupled systems for the realization of HBAC and multiple round QEC.