• Work is progressing on experimental small-scale quantum internet nodes and networks… 
• … while the upscaling of quantum data transmission proceeds quickly
• However, many problems remain be solved before a robust quantum internet becomes a practical reality
• ‘Entanglement swapping’ and teleportation seen as important developments

Has the quantum internet already been invented? Well, it all depends on how you look at it, as Erwin Schrödinger might have pointed out, were he not very definitely 63 years dead (and not just possibly dead or possibly alive, like his theoretical cat). The notion of the quantum internet is that at some time in the indeterminate future there will exist a network (or networks) of quantum computers that will send, compute and receive information encoded in quantum states. The quantum internet will not be a replacement for the ‘classical internet’ but will augment it with new functionalities, including quantum cryptography and quantum cloud computing. Hopes are that, eventually, a global quantum internet will be in place to be of service to all humanity, but given the geopolitical realities of these troubled days, that altruistic dream seems unlikely to come true any time soon.

Nonetheless, experimental work on the development of small-scale quantum internet nodes and networks is underway in various parts of the world. For example, as the University of Chicago news website reports, back in 2017 a team at the University of Science and Technology in Beijing, China, managed to use laser technology to transmit entangled photons from an orbiting satellite down to ground stations on earth some 700 miles beneath. The experiment showed that it may be possible to use satellites as an integral part of a quantum network, but as the system trialled could recover only one photon out of every 6 million transmitted, the communication itself was rendered unintelligible. 

Then, in April 2019, scientists from Brookhaven National Laboratory, Stony Brook University and the US Department of Energy’s nationwide Energy Sciences Network achieved entanglement over 10 miles on a fibre optic cable. That link has since been extended to 80 miles and includes a quantum network platform on which to conduct tests. Nine months later, in April 2020, research teams from the University of Chicago and the US Department of Energy’s Argonne National Laboratory in Lemont, Illinois, in the Chicago suburbs, successfully tested a 54-mile quantum loop that used an existing fibre-optic cable running under the western suburbs of Chicago. It carried optical pulses with a delay of just 200 milliseconds thus demonstrating the core functionality that will be required to run a viable quantum internet network. In June 2022, a 35-mile extension was added and the Chicago network now comprises six nodes and 114 miles of fibre optic cable, testing the transmission of particles carrying quantum-encoded information between the Argonne National Laboratory and two sites in Chicago’s South Side. And, according to Nature magazine, three separate research groups based in the US, China and the Netherlands have recently successfully demonstrated entanglement over several kilometres of existing optical fibres in urban areas. 

So, advances are being made in upscaling quantum data transmission, while a great deal of research effort is being expended on stabilising the notoriously unstable and fickle qubits that are the basic units of information in quantum computing. The focus is on the use of qubits that can function at room temperature rather than at close to absolute zero. What’s more, if the quantum internet is to progress beyond what, currently, might be described as small regional networks, it will be necessary for quantum signals to be amplified as they continue their journeys.

Adding a repeater/amplifier to a traditional network technology to boost the signal and send it on its merry way is easy enough, but doing that with an entangled photon simply destroys it as well as the data it is carrying. Tian Zhong, an assistant professor at the Pritzker School of Molecular Engineering at the University of Chicago, is working on a method that will use quantum memories to protect quantum data from decoherence, essentially creating quantum relays to keep qubits intact until they reach their designated destination. It is immensely complex work but, if successful, might permit the development of a quantum internet able to transmit and receive information anywhere on earth.

“Entanglement swapping” is a way to make quantum systems that have never interacted in the past become entangled. Basically, it works by generating a single long-distance entanglement from many short-distance entanglements: The primary goal of quantum networks is to distribute entanglement between members of the network and entanglement distribution opens up the ability to transmit qubits. The joining of two different and separate entanglement links can be effected through quantum teleportation. 

Don’t try to beam me up, Scotty

Yes, teleportation, whereby quantum states (in qubits) are transferred to another quantum state. Teleportation does not physically transfer data but transfers the quantum state of the data over long distances. In quantum teleportation, a ‘piece’ of information actually goes from one particle to another without physically passing between them. It moves instantaneously but doesn’t travel anywhere. It is a very difficult feat to achieve even with a minuscule piece of information and the Star Trek notion of disassembling a human body into its constituent atoms, teleporting them and reassembling them in perfect order and without any data loss would take so much networked quantum computing power as to be utterly beyond the reach of science, probably forever. 

However, although the idea of teleportation brings to mind episodes of a 1960s science fiction TV series, teleportation has been demonstrated time and time again in experiments by different research teams in different parts of the world. In one example, teleportation between two nodes has been achieved via a free-space link over 143 kilometres along and across the River Danube and over a ground-to-satellite uplink. 

Another vital piece of the quantum internet jigsaw that needs to be slotted into place is the transmission of qubits from one region of the world to another. Current experimental systems rely either on satellite transmission, which is very expensive, or fibre-optic cables, which are readily available and a lot cheaper. Thus, the first large-scale quantum networks will exploit existing fibre-optic technology, though the challenge is that the intrinsic nature of that physical medium limits the distance over which fragile quantum data can be transmitted before data losses compromise the system and it collapses. 

That’s why much research is being dedicated to constructing city-wide quantum networks and generating entanglement across those links. Proving that such networks can work in real-life circumstances will be the necessary precursor to regional and, later, international quantum networks.

Einstein called quantum entanglement “spooky interaction at a distance”: It’s a barely understood phenomenon under which one atomic particle can instantly “know” something about another particle regardless of the distance between them, be that across nanometres or across the universe. Quantum entanglement seems to indicate that it is possible to exceed the velocity of light in a vacuum, which is exactly 299,792,458 metres per second, or close to 186,282 miles per second. This is the universal constant and speed limit that Einstein himself said it is impossible to exceed – but maybe it’s not. 

Quantum mechanics shows that subatomic particles behave in a completely different way to the laws that pertain in the macroscopic world, as quantum entanglement enables particles to communicate instantly. As proof of the longevity possible in quantum entanglement, scientists from the prestigious Massachusetts Institute of Technology (MIT) have conducted an experiment in which upwards of 100,000 pairs of provably entangled photons were detected and measured via optical telescopes looking out across cosmic distances. The most distant stars used in the experiment are some 600 light-years away from earth: In other words, the entangled photons were emitted 600 years ago. That’s about the time the Byzantine Empire collapsed after Constantinople fell to the Ottomans, when Henry VI was the King of England, and Dick Whittington died. It is not known if his famous cat was in any way entangled – now that’s spooky!

- Martyn Warwick, Editor in Chief, TelecomTV