**(Phys.org)** Researchers at the University of Oxford show how the phenomenon of indefinite causal order—in which different orders of events are quantum superposed—suggest this could actually work for quantum systems and be put to use in a type of quantum refrigeration.

The results follow reports of the effects of indefinite causal order in quantum computation and quantum communication. “People were asking—is the quantum circuit model a complete description of every possible quantum ordering of events?” explains David Felce, a Ph.D. student at the University of Oxford, as he describes how research into indefinite causal order has emerged over the past 10 years.

The researchers considered the effects of indefinite causal order. Among the “weird” effects they found was the possibility of thermalizing a quantum state with two thermal reservoirs at the same temperature with indefinite causal order and ending up with the state in a different temperature.

Felce plans to look into ways of implementing the indefinite causal order refrigerator. So far, experimental implementations of indefinite causal orders have used control qubits in a superposition of polarization states. A polarization-dependent beamsplitter would then send a photon through a circuit in a different direction depending on the polarization, so that a superposition of polarization states leads to a superposition of the order in which the photon passes through the circuit elements. Felce is also interested in looking into generalizing the results to more reservoirs.