Non-classicality created by quantum channels with indefinite causal order

Juan Gu, Zhimin Yin, Longsuo Li

Časlav Brukner

Jian Wei Cheong

Quantum compositions of channels can grant advantages over classical compositions in quantum information processing or thermodynamical tasks. One of such quantum composition is one with indefinite causal order, where multiple quantum channels are placed in a superposition of different causal or operation orders. The advantages of indefinite causal order were demonstrated in an experimentally realizable setup known as the quantum switch and were often attributed to this new quantum resource of indefinite causality, but some doubts and disagreements still remains as compositions without indefinite causal order can achieve the same advantages. In this work, we proposed that the quantum switch's uniqueness lies in its ability to exhibit coherent non-Markovianity, where memory effects acts across different paths of superposition. We showed that the quantum switch violates completely positive or CP-divisibility and thus has intrinsic non-Markovianity in its evolution, which allows for non-Markovian backflow of information from the environment. This backflow of information can then play a role in the many advantages seen in the quantum switch, and by extending the quantum switch operation such that the non-Markovianity is controllable, we showed that these advantages depend on the presence and amount of non-Markovianity, achieving the quantum switch case when the system is fully non-Markovian. Particularly, we demonstrated this for the quantum switch's advantages in communication capacities, work extraction via daemonic ergotropy, and refrigeration via heat extraction. Other quantum compositions that are often compared to indefinite causal order, such as the superposition of independent channels and superposition of trajectories, are also captured by this extended construction of the quantum switch, and we showed that their difference with the quantum switch lies in the presence or absence of non-Markovianity. Our work suggests that non-Markovianity is an important recipe for the quantum switch or indefinite causal order, and is a nonnegligible factor for its advantages.

Yu Toyama, Sachiko Kitajima, Masashi Ban

The dynamics of single-qubit coherence and two-qubit entanglement are investigated in the Bloch channels with the indefinite causal order, which is created by means of a quantum switch. Coherence and entanglement are quantified respectively in terms of l 1 -norm and negativity. It is shown that both coherence and entanglement with the indefinite causal order are larger than those without the indefinite causal order. In fact, the coherence remains finite even in the long-time limit. Contrary to expectations, it becomes larger as the environmental temperature increases. Furthermore, it is found that the entanglement becomes larger and survives longer than that in the absence of the indefinite causal order.

Giulio Chiribella

Jasleen Kaur, Shrobona Bagchi, Arun Kumar Pati