Speedy communication takes a quantum leap towards reality
Quantum computing is expected to revolutionize electronics over the course of the next few decades, but a number of outstanding issues still remain. One such problem is that "qubits," the basic building blocks of quantum information, are very fragile and can be easily destroyed when sent on a fiber optics cable, due to the surrounding noise. Working on this issue, a team from Stockholm's KHT University, led by Magnus Rådmark, has developed a new method for combining six photons to obtain a robust qubit that is resistant to noise and is, therefore, able to travel long distances without interference.
Small imperfections in the cable, electromagnetic waves coming from nearby sources and small changes in temperature are just a few of many possible sources of noise that, when they add up, can corrupt the information traveling on a cable. Digital electronics deals with it by periodically regenerating the signal and adding redundant data (checksums, parity bits, etc) to identify and fix possible errors.
With quantum computing, however, the problem becomes much more complicated. When a single photon is sent through an optical fiber, the information is encoded in terms of the particle's polarization, which could be, for instance, horizontal or vertical. Adding a second photon makes it possible to generate many more useful combinations, but it then become impossible to know which photon has which polarization — it's only possible to observe the relationship between the two.
Quantum entanglement, a property of quantum mechanical systems according to which the state of one part (the polarization of one photon) can't be described without the mention of its remaining parts (the polarization of the remaining photons) is the cause of this further complication But it's also what makes quantum computing so attractive to scientists and engineers, because it allows for massive parallelism in data processing — when an operation is performed on one photon, the entire system is simultaneously affected.
With their work, the researchers managed to build a quantum state formed by six photons that can easily travel long distances in optical fibers, even when subject to mechanical stress or interference, allowing for reliable data transmission from one end to the other.
What's the catch? Unfortunately, while the team has successfully shown that its design would perform well, they still lack the technology to actually encode information on this six-photon configuration and then read it back. Once this issue is resolved, though, science will be very close to obtaining fast and highly reliable quantum communication.