Harvard and MIT take a decisive step for quantum internet

Scientists in the United States have taken a decisive step towards developing a quantum internet capable, for example, of shielding messages from 'kacker' attacks, improving the accuracy of GPS or facilitating quantum computing in the cloud. This Monday, the journal Nature published a study by experts from Harvard University and the Massachusetts Institute of Technology (MIT), in which they presented a prototype quantum node ready to capture, store and interlace bits of quantum information.

“For the past two decades,” the authors explain in a statement, “efforts to create such a quantum network have encountered the difficulties of transmitting quantum signals over long distances without any loss”. This prototype quantum node, they point out, corrects that loss of signal, common today in any type of long-distance communication technology, whether it be from the first telegraph to the current fiber optic Internet.

Consequently, it could become the definitive component to create a practical quantum internet and contribute to the development of long distance quantum networks. “This demonstration represents a conceptual breakthrough that could extend the longest possible range of quantum networks and allow, in principle, many new applications in a way that is impossible with existing technologies,” says Mikhail Lukin, co-director of the Harvard Quantum Initiative (HQI).

In today's communication networks, messages sent, for example, by two people from distant points “travel” on a more or less linear infrastructure and, on their way, pass through repeaters that read and amplify the signal and correct errors. That process, experts warn, is vulnerable and exposed to attack.

In contrast, if these two people want to send each other a quantum message, the process changes because their networks use quantum particles of light (individual photons) to communicate light states over long distances. In addition, unlike traditional networks, these quantum networks have so-called “interlacing” capability, which allows bits of information to be perfectly correlated across any distance.

Thanks to “interlacing”, the authors point out, the messages are invisible if there are no changes and they cannot be spied on or intercepted, which opens the door to the development of quantum cryptography applications. However, they point out, long-distance quantum communication can also be affected by conventional photon loss and this has been, to date, the main obstacle to creating a large-scale quantum internet.

Furthermore, the same physical principle that allows the development of high-security quantum communication prevents the use of conventional repeaters to cope with the deterioration of information, since, if they cannot detect the invisible signal, they cannot amplify or correct it either. The solution to this problem has been found at HQI and MIT with the development of a quantum repeater.

Unlike traditional repeaters, which amplify the signal through an existing network, quantum ones create a network of interlaced particles on which a message can be transmitted. This way, at each point in the system, the repeater can capture and process quantum bits of quantum information to correct errors and store them for as long as necessary so that the rest of the network is ready.

However, experts have not been able to implement this until now due to two factors: the difficulty in trapping individual photons and the fragility of the quantum information, which complicates its processing and storage over long periods of time.In this regard, Lukin and his collaborators have worked with systems that can carry out these two tasks, the so-called “silicon vacancy colour centres in diamonds”.

These centres, he details, are the tiny defects present in the atomic structure of diamonds that, by absorbing and radiating light, give rise to the characteristic bright colour of this mineral. So the experts integrated individual color centres into diamond cavities made with nanotechnology to confine the information carried by the photons and force them to interact with each centre.

Then, they introduced this device into a dilution refrigerator, with temperatures close to absolute zero, and sent individual photons through fiber optic cables to the interior of the refrigerator, where they were trapped by the color center. This device can store quantum information for milliseconds, long enough for it to be transported across thousands of kilometers.

“It combines the three most important elements of a quantum repeater, a large memory, the ability to effectively capture photon information and how to process it locally,” adds Bart Machielse of the Harvard Nanoscale Optics Laboratory.