# Unifying paradigms of quantum refrigeration: A universal and attainable bound on cooling

## TL;DR

This paper derives a universal, attainable bound on quantum cooling that applies across all control paradigms and machine sizes, providing a fundamental limit in the steady state regime.

## Contribution

It introduces a universal bound on quantum cooling applicable to any control paradigm and machine size, valid in the steady state limit, and proves its attainability for qubits.

## Key findings

- Universal bound depends on a single parameter of the refrigerator.
- Bound is achievable in a single cycle for qubits.
- Bound applies to autonomous machines and all control paradigms.

## Abstract

Cooling quantum systems is arguably one of the most important thermodynamic tasks connected to modern quantum technologies and an interesting question from a foundational perspective. It is thus of no surprise that many different theoretical cooling schemes have been proposed, differing in the assumed control paradigm and complexity, and operating either in a single cycle or in steady state limits. Working out bounds on quantum cooling has since been a highly context dependent task with multiple answers, with no general result that holds independent of assumptions. In this letter we derive a universal bound for cooling quantum systems in the limit of infinite cycles (or steady state regimes) that is valid for any control paradigm and machine size. The bound only depends on a single parameter of the refrigerator and is theoretically attainable in all control paradigms. For qubit targets we prove that this bound is achievable in a single cycle and by autonomous machines.