Breaking the limits of purification: Indefinite causal order enhances heat-bath algorithmic cooling

Quantum technologies require pure states, which are often generated by extreme refrigeration. Heat-bath algorithmic cooling is the theoretically optimal refrigeration technique: it shuttles entropy from a multiparticle system to a thermal bath, thereby generating a quantum state with a high degree of purity. Here, we show how to surpass this hitherto-optimal technique by taking advantage of indefinite causal order. Our protocol can create arbitrary numbers of pure quantum states without any residual mixedness by using a recently-discovered device known as a quantum switch, thereby fulfilling the dream of algorithmic cooling.