Reading quantum signals between Alice and Bob

The quantum world can be quite a strange one. Particles at opposite ends of a galaxy can instantaneously react to each other, and can exist in more than one place simultaneously. It now seems this world may be even more complicated, allowing communication to occur without a physical medium.

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.

"Mind-boggling"

The possibility to communicate without any physical support carrying the information is provocative

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.

But this is what Nicolas Gisin, a physicist from The University of Geneva, thinks will prove particularly interesting: "How do we quantify how much quantum stuff goes from the sender Bob to the receiver Alice?" he asks.

In theory, nothing goes from Bob to Alice. However, ensuring with a 100% certainty that the photon never ends up at Bob's side would require running this experiment for an infinite amount of time. Nevertheless, by using a closed loop thousands of times – which would only take very little time because of the speed of light – the researchers argue that it can in principle work more than 95% of the time.

Nevertheless Gisin believes the paper is likely to attract a lot of attention. "The possibility to communicate without any physical support carrying the information is provocative" he says.

Suhail Zubairy, another of the study's co-authors and member of Texas A&M's Institute for Quantum Science and Engineering, believes it would be possible to implement this technique in an actual experimental setting. "There is no serious issue about the possible experimental implementation of our counterfactual communication protocol," says Zubairy. Given the difficulty in controlling the setting and achieving the perfect level of reflectivity in the mirrors, he believes that a 70-80% counterfactuality should be possibly experimentally.

The research team hopes other scientists will attempt to implement their set-up soon. But even if successful, it is far from clear what practical applications might be exploited. It seems obvious to envision a use in information security – whereby there would be literally no information to steal to begin with – but, the authors are cautious about making any claims about its practical uses.

Indeed, more tantalizing for these researchers is not the promise of a particular technology based on this phenomenon, but understanding how it is that information can be communicated without any particle being transferred.

As Salih says: "I believe the question of how information gets from Bob to Alice is a deep one speaking to the heart of the debate about the reality of the quantum state: if physical particles did not carry information between sender and receiver, what did?"

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