08 March 2017 Coherence in Distributed Scenarios

Hierarchy of incoherent operations in distributed scenarios.

A study in PRX reveals the hierarchy of classes of incoherent operations, essential in manipulating coherence, in distributed scenarios. Quantum coherence and entanglement are among the most puzzling concepts in nature. The former deals with the notion of superposition, a basic tenet of quantum mechanics, the latter studies nonlocal correlations, a phenomena that Albert Einstein described as ’spooky action at a distance’. Although these concepts were discovered in the early days of quantum physics, it is only very recently that researchers started to understand how entanglement and coherence can be applied to quantum technology.

In the last two decades, most of the theoretical research was essentially focused on understanding quantum entanglement, and developing a rigorous operational framework to exploit entanglement as a resource. However, recent results show that all features of a quantum system cannot be described solely by the presence of entanglement, but require other perceptions of nonclassicality, such as quantum discord and coherence.

In “Towards Resource Theory of Coherence in Distributed Scenarios”, recently published in Physics Review X, former ICFOnian Dr. Alexander Streltsov in collaboration with ICFO researchers Dr. Swapan Rana, Dr. Manabendra Nath Bera, led by ICREA Prof. at ICFO Maciej Lewenstein, report on the study of the resource theory of coherence in distributed scenarios, i.e., the situation of two (or more) spatially separated parties who are allowed to share classical information and are restricted to local operations in their labs which cannot create coherence.

The most fundamental question in this context concerns the possible transformations a quantum state can undergo in this process. The ultimate goal is to characterize all transformed states for a given input state, and conversely, all possible input states for a given output state. While those transformations are difficult to characterize in general, the work carried out by the researchers shows that a larger class of transformations has a simple mathematical form, yet preserving the main features of the process. This superclass is shown to be useful in several scenarios, including a generalized version of assisted coherence distillation or incoherent quantum state merging. In the former, one party can enhance coherence distillation with the help of a second party, while in the latter, one is aiming to send parts of a quantum state to this second party while preserving correlations with a third party. In addition, they have also shown that the standard quantum teleportation does not require local coherence.

The tools presented in this work not only pave the way to a rigorous resource theory of coherence for distributed scenarios, but can also be implemented in quantum thermodynamics, processes that respect symmetries, as well as other related research fields.