Quantum technologies have strong emotions in store for us. Of that there is no doubt. In October 2019, Google claimed to have achieved quantum supremacy because one of its research teams had managed to solve with a quantum computer in just 200 seconds a problem that would have taken 10,000 years for a classic supercomputer. Months later this milestone is still being discussed by some competing researchers, but what is really interesting is that it brings to the table the enormous potential that not only quantum computing in particular has, but also quantum technologies in general.
And it is that this field of study does not only have strict applications in computing: some of the surprising phenomena that govern the quantum world, the world of the very small, of particles, can also be used for other things. And one of these applications is to transmit encrypted messages through communication networks that are impossible to violate. Having this technology is crucial for the great world powers, which has caused the United States, the European Union and China to embark on a race to see which of them manages to fine-tune their own large-scale quantum communications infrastructure sooner. And at the moment it is the Asian country that is in first position.
What China has achieved and why it matters
A team of Chinese researchers today published in the prestigious scientific journal Nature an article describing the procedure that has allowed it to transmit an encrypted message that is impossible to hack between two land stations separated by a distance of 1,120 kilometers. And to make this possible they have resorted to an essential property of quantum systems: entanglement. This phenomenon has no equivalent in classical physics, and is that the state of the quantum systems or particles involved, which can be two or more, is the same.
This means that these objects are actually part of the same system, even if they are physically separated. In fact, distance doesn't matter. If two particles, objects or systems are intertwined by this quantum phenomenon, when we measure the physical properties of one of them, we will be instantly conditioning the physical properties of the other system with which it is intertwined. Even if it is at the other end of the Universe. It sounds like science fiction, it's true, but no matter how strange and surprising this phenomenon may seem to us, it has been empirically proven. In fact, if it were not really given, quantum computers would not work. And these Chinese scientists would not have achieved what they have achieved.
This is not the first time that information has been transmitted using a quantum communication system. The three powers that I have mentioned a few paragraphs above, and perhaps some others as well, have done so before. But China has gone further and has managed to guarantee the total invulnerability of its communications (at least this is what the Asian researchers expose in their article published in Nature). Distance in this context is essential because to fine-tune a global quantum communications network, which is the ultimate goal to which all powers aspire, this technology needs to work perfectly over very long distances.
The procedure they have used is very ingenious: they use quantum systems to generate private keys in different parts of the planet by sending them interlaced photons emitted by Micius, a satellite that orbits Earth at about 500 or 600 kilometers high. It seems very complicated, but if we ignore the most complex technological details, it is not difficult to understand how this technology works. In reality, the entangled photons that the satellite sends to ground stations do not encode the encrypted message; what they contain is the key that allows you to decrypt the message when it has been collected at its destination. In fact, the message can be sent from one point to another using any other communication channel.
The heart of this technology lies in the fact that each pair of entangled photons encodes one bit of key information. Their entanglement guarantees that if one of the photons is altered, for example, because someone has managed to observe it, its physical properties change instantly and the entanglement is broken, so the encrypted message cannot be violated. The biggest limitation that this technology has in practice is that transferring photons over very long distances is not easy, hence the fact that communication between the Micius satellite and Chinese ground stations has worked correctly is a very important achievement. There is undoubtedly much more to be done to deploy a global quantum communications network, but it appears that China's effort in particular is paying off.
The European Union has also been launched with an initiative called the European Union Quantum Communication Initiative, in which Spain participates, which aims to create quantum crypto networks for infrastructure. Curiously, this technology was born in Europe, and a decade ago our continent had a very clear scientific advantage in this field, but we have lost it. The United States and, above all, China, are taking the lead in this area, so it is clear that the European Union needs to invest if it wants to catch up so as not to lag behind as has happened in the field of microelectronics.