It is impossible to build a practical quantum broadcaster

The sharing of quantum information in the same way that we broadcast television or radio programs can be unaccomplicated – even for mathematical patterns which bypass the limits posed by quantum physics.

We have long known that quantum copying machines could never exist because the laws of quantum physics prohibit any quantum information in duplicate, a rule called the theorem without cloning. But the physicists then began to explore if they could avoid breaking this law and distributing – or disseminating – copies of quantum information to many receptors.

To do this, researchers should allow quantum copies to postpone slightly and add new stages of information processing for receivers. Now Zhenhuan Liu at Tsinghua University in China and his colleagues have shown that these programs can be prohibitive.

“There is no” Ctrl + C “in the quantum world,” explains Liu. “If you want to send quantum information to several receivers, there is no effective shortcut – you just need to prepare enough copies and send each of them.”

The researchers focused on a protocol previously proposed for “virtual quantum diffusion”, where the information is manipulated so that different states are correlated with each other but is not direct physical replicas with each other. In this case, the message transmitted to each receiver would not be an exact copy, but copies would share enough properties to be useful. It is comparable to a situation in which a television network simultaneously broadcasts a slightly different serial drama with each household, but maintains the story, on average, the same. Although this protocol certainly works, explains Xiangjing Liu, member of the team, at the National University of Singapore, the researchers wanted to know if it were effective.

They quantified the amount of effort that receptors should pass for the information that reaches them as useful, although it is not identical. This mathematical analysis has led them to conclude that practical quantum broadcasting may not exist.

Contre -up, even this modified version of the quantum diffusion approach – similar to sending a group text where everyone receives a message at a time – would require more resources than a technique more as writing an individual letter to each receiver from zero, explains Yunlong Xiao to the Agency for Science, Technology and Research in Singapore.

“If your only objective is simply to distribute quantum states in different places, I think that looking at virtual quantum dissemination is certainly a bad approach,” said Seok Hyung at the National Institute of Sciences and Technologies of Ulsan in South Korea. He says that the protocol has always been conceived as an investigation into fundamental constraints on the processes of quantum information theory, rather than a practical recipe for quantum communication.

Paolo Perinotti at the University of Pavia in Italy says that the team’s work is laudable as a mathematical effort, but that it thinks that it is unlikely to have an immediate impact on quantum technologies.

In the future, researchers are also interested in theoretical lessons in their current analysis. This could help us understand what correlations – what is between quantum states distributed in space, or sent one after the other – are authorized and can be manipulated. Xianjing Liu says that work can be part of a new framework to understand the quantum processes, which separates time and space less than traditional approaches.