Cooling a Qubit Using n Others
Jake Xuereb, Benjamin Stratton, Alberto Rolandi, Jinming He, Marcus Huber, Pharnam Bakhshinezhad · PRX Quantum · 2025
In the task of unitarily cooling a quantum system with access to a larger quantum system, known as the machine or reservoir, ? Focusing on the task of cooling a single qubit given access to n separable, thermal qubits with arbitrary energy structure, we answer these questions by giving two new perspectives on this task. First, we show that a set of inequalities related to the energetic structure of the n -qubit machine determines the protocol which cools the qubit to the coldest reachable state, which parts of the machine contribute to this protocol and give rise to a Carnot-like bound. Second, we show that cooling protocols can be represented as perfect matchings on bipartite graphs enabling the optimization of cost functions, e.g., gate complexity or dissipation. Our results generalize the algorithmic cooling problem, establish new fundamental bounds on quantum cooling and offer a framework for designing novel autonomous thermal machines and cooling algorithms.