One of the weirdest proposed forms of the strange beast called Quantum communication is counterfactual communication – the type of quantum communication with no physical particle transfer between two recipients. For years now, theoretical physicists have believed that such a form of communication was possible, but for the first time, researchers were able to experimentally confirm it in 2017 by transferring a black and white bitmap image from one location to another without physically sending any particles. This is quantum mechanics at its best and most complicated. But once broken down, counterfactual quantum communication isn’t actually as bizarre as it seems.
Firstly let’s see how this differs from regular quantum communication alias quantum teleportation that is also a form of particle-less information transfer. The two not the same.
Regular quantum teleportation is founded on the principle of entanglement where two particles become inextricably linked in such a way whatever happens to one will automatically affect the other irrespective of the distance between them. Einstein referred to this as “spooky action at a distance”, and scientists have been successful in demonstrating this to send messages over vast distances. But this form of quantum teleportation still depends on some form of particle transmission as the two particles typically need to be together at the starting point during entanglement before being transmitted to different locations before communication can occur between them. In an alternate approach, the particles can be entangled at a distance but usually with the help of another particle like photons to travel between them.
On the other hand, direct counterfactual quantum communication does not depend on quantum entanglement. It is based on a phenomenon called the quantum Zeno effect. Simply, this effect occurs when an unstable quantum system is measured repeatedly. In the quantum world, every time you look at or measure a system, it changes. And in this instance, unstable particles somehow never seem to decay while being measured, so the quantum Zeno effect produces a system that is effectively frozen with a very high probability.
This video gives a great explanation.
Based on this quantum Zeno effect Counterfactual quantum communication is defined as the transfer of a quantum state from one location to another without the transmission of any quantum or classical particle between them. This system requires a quantum channel running between two sites so there is always a small probability that a quantum particle will physically cross the channel. When this occurs, the system is discarded and a fresh one is set up. Researchers from the University of Science and Technology of China set up this complex system by placing two single-photon detectors in the output ports of the last beam of an array of beam splitters.
The system is frozen in a certain state due to the quantum Zeno effect making it possible to predict which detector would ‘click’ when photons pass through. To ensure the system doesn’t change, a series of nested interferometers measure its state. This set up is based on the principle that all light particles can be completely described by wave functions, rather than as particles in the quantum world. Thus, by embedding messages in light photons, the researchers were able to successfully transmit this message without ever directly sending a particle.
The team whose work was published in the journal Proceedings of the National Academy of Sciences explained that the basic premise for this set up is derived from holography technology.
The researchers wrote in the journal Proceedings of the National Academy of Sciences back in 2017, “In the 1940s, a new imaging technique – holography – was developed to record not only light intensity but also the phase of light. One may then pose the question: Can the phase of light itself be used for imaging? The answer is yes. The basic idea is this – someone wants to send an image to Alice using only light (which acts as a wave, not a particle, in the quantum realm). Alice transfers a single photon to the nested interferometer, where it can be detected by three single-photon detectors: D_0, D_1, and D_f. If D_0 or D_1 ‘click’, Alice can conclude a logic result of one or zero. If D_f clicks, the result is considered inconclusive”.
As Christopher Packham explained for Phys.org at the time, “After the communication of all bits, the researchers were able to reassemble the image – a monochrome bitmap of a Chinese knot. Black pixels were defined as logic 0, while white pixels were defined as logic 1 … In the experiment, the phase of light itself became the carrier of information, and the intensity of the light was irrelevant to the experiment.”
The team explains that not only is this a big step forward for quantum communication, it’s the technology that could also be used for imaging sensitive ancient artifacts that couldn’t survive direct light shined on them. The results will still need to be confirmed by external researchers to validate that these observations are a true example of counterfactual quantum communication.
Either way, this is a pretty cool demonstration of the bizarre and unexplored quantum world.