“Reading quantum signals between Alice and Bob” – Nature

Source: Nature.com Photo: Getty images
Source: Nature.com
Photo: Getty images

Hazem Zohny reports for Nature Middle East about new research into “reading quantum signals” via wave, not particle transfer.

Zohny states that, “In a quantum setting, scientists may be able to transmit information over vast distances without the need of a physical medium – at least theoretically.”

He writes about this new research:

It’s called counter-factual communication, and a group of researchers from Saudi Arabia’s King Abdulaziz City of Science and Technology (KACST) and Texas A&M University in the US, have just published a paper in Physical Review Letters demonstrating it – at least in principle1.

To illustrate the concept, the authors resort to the often highly unusual relationship between physicists’ favourite stand-in couple: Alice and Bob. The long-held assumption is that for information to travel from one to the other in empty space, physical particles have to be transferred. The authors challenge this assumption by proposing an experiment that includes a complex assortment of beam splitters, mirrors and detectors.

Imagine a communication channel between Alice and Bob, across which, normally something has to pass for communication to occur. But suppose Alice releases a photon – through an array of beam-splitters and mirrors – that Bob can choose to either block or not block.

What he does will rouse different detectors at Alice’s end. In this way, Alice can infer Bob’s action by checking her own detectors. But here is where it gets stranger: the photon didn’t even have to leave Alice’s side of the communication channel in order for her to know about Bob’s choice.

One of the authors of the paper, Hatim Salih, a physicist at KACST, notes that, “unlike most communication protocols, in ours it is Bob who sends a message to Alice, not the other way round.”

However, Salih says this description, in terms of Bob blocking the photon, is somewhat misleading, as no photon actually reaches Bob’s side to be blocked to begin with.

“Rather than blocking or not blocking the photon,” he says, “[Bob] only blocks or does not block the part of the photon’s superposition that comes his way.”

Superposition, which is a hallmark of the quantum world, refers to a particle’s capacity to also behave like a wave, mysteriously diffusing itself through multiple routes. Once detected, it then collapses back into a particle occupying a particular location. The key to the premise of this study is to ensure this collapse always occurs on Alice’s side, right where it started.

The researchers achieve this using something called the quantum Zeno effect – a phenomenon whereby repeated measurements of a quantum system leave it in its original state.

“This is a bit like Alice and Bob using pigeons to communicate – except that the pigeons never have to leave,” says Zhenghong Li, one of the paper’s authors.

And if none of this makes much sense, call to mind the assertion of renowned physicist, Richard Feynman, who said: “I think I can safely say that nobody understands quantum mechanics.”

The authors do not attempt to explain the deeper mechanism behind this means of transferring information without transferring any particles – something that Mohammad Al-Amri, one of the paper’s authors from KACST, calls “mind-boggling and highly counter-intuitive.”

According to the researchers, the study doesn’t show how this might take place: it doesn’t address the question of how information goes from Bob to Alice.

Read the full report, click here.


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