Teleportation is here, but we didn't expect

In 2005, the necrology of physicist Asher Peres in Physics Today magazine told us that when a journalist asked him if quantum teleportation could hold a person's soul as well as their body, the scientist replied: "No, not the body, only the soul." More than only a simple joke, Peres' answer gives a perfect description, embedded in a metaphor, of the nature of a phenomenon we've seen.

Teleportation in real science started to take shape in 1993, thanks to a theoretical analysis published in Physical Review Letters by Peres and five other researchers, setting the framework for quantum teleportation. Apparently, it was the idea of co-author Charles Bennett to equate the proposed phenomenon with the common concept of teleportation, but there is an essential difference between fiction and reality: in the latter, traveling is not important, but material, which transfers properties from the original matter to that of the destination matter.

Quantum teleportation is based on a hypothesis that physicist Albert Einstein and his colleagues Boris Podolsky and Nathan Rosen described as the EPR paradox in 1935. As a result of the laws of quantum physics, two particles could be obtained and separated in space to continue sharing their properties as two halves of a whole.

Therefore, an action on one of them (on A, or Alice, according to the nomenclature used) will immediately influence the other (on B, or Bob). This "spooky behavior at a distance," in Einstein 's words, would appear capable of exceeding the limit of light speed.

This phenomenon 's theory, called quantum entanglement, was later developed by John Stewart Bell in 1964 and supported by numerous experiments. Peres, Bennett and their collaborators' work proposed that a third particle might interact with Alice's and lose a quantum state—the value of one of Bob's physical properties—to be transferred to that of Bob, to restore that state.

By moving matter, the Bob particle would be turned into a duplicate of the interactive Alice particle, so there would never have been physical interaction between them.

TELEPORT QUBITS

Several experiments have achieved this quantum teleportation since 1998, initially using individual photons, then atoms and complex systems. At first, the phenomenon was demonstrated at a short distance, increasing to hundreds of meters and kilometers in subsequent studies.

The current record is the teleportation of photons 1,400 kilometers from Earth to the Micius satellite in Earth orbit, a successful achievement by the team led by Jian-Wei Pan at the Chinese University of Science and Technology in Hefei (USTC) in 2017.

Which is transmitted in these experiments is bits-coded information. A bit is a simple binary information unit that takes a value of 0 or 1. For example, the spin of a particle (a type of rotation) may contain information in its application to quantum states.

But in the quantity version of the bit, the qubit, its value may be either 0 or 1 or another value, such as 2, since quantity mechanics allow overlapping states. That's why quantum computing is seen as a more powerful technology than traditional computing, as it has much greater capacity to store and process information.

However, it is essential to stress that quantum teleportation does not serve to transmit data instantly or at speeds higher than light. The explanation is that Bob requires extra knowledge about Alice's measurements that is not transmitted through the enmeshed particle system and must thus be transmitted through another channel; two classic bits must be transmitted for each teleported qubit, and this can only be achieved through conventional means that at most can exceed the speed of light.

A QUANTURE NETWORK

Yet despite this constraint, quantum teleportation possibilities look increasingly exciting as new milestones are achieved. This year, two research teams first documented the transmission of qutrits, or three-dimensional information units (which can take three values, 0 , 1 and 2).

"Both studies showed qutrit teleportation. The key difference is the tool we used, "explains to OpenMind Bi-Heng Liu, physicist at UCTC and co-author of an as-yet-unpublished research.

There's still some controversy at play between the two teams, though. As explained to OpenMind by physicist Chao-Yang Lu, also from UCTC and co-author of the other research, published in Physical Review Letters, about his colleagues' work, "Teleportation's very quantum existence has not been confirmed."

Co-author of the same study Manuel Erhard of the University of Vienna also believes that in Liu's experiment, "measurements and results are not sufficient to claim genuine three-dimensional and universal quantum teleportation." Liu, for his part, defends his results: "We did numerical simulation and confirmed qutrit teleportation."

The debate also expands the possibilities of extending the device to more dimensions. According to Liu, "both schemes are scalable." For his part, Erhard argues that his own system can easily be extended to any dimension: "Technological development is about further enhancing dimensionality," he says. At the other hand, he's not sure if the program of his colleagues will say the same.

But what's the point of extending these studies into more dimensions? "High-dimensional quantum teleportation is possible in quantum networks," Erhard states to OpenMind. "So, we envisage a potential higher-dimensional alphabet-based quantum network. These come with the advantage of higher information capacities and, for example , greater noise resistance.

Going from qubit to qutrit, and from there to ququart, and so on, lays the basis for future quantum computing networks. Lu hopes that his method will achieve so-called quantum supremacy, the power of classical computing to solve unattainable problems: "We are introducing multi-photon multi-dimensional quantum computing experiments called boson sampling, and hopefully in the near future we expect to monitor 30-50 photons to achieve quantum supremacy."