Free-space quantum communication using lasers could form the basis for a global network that’s impossible to hack or tap
Forget fibre optics, and for that matter, forget wireless. The biggest shake-up in interpersonal communication since Alexander Graham Bell is on the cards – and South Africans are at the forefront of the research. It’s all about exploiting quantum mechanics, the somewhat alien and frequently contrary rules that apply to life at the submicroscopic level.
At the CSIR’s National Laser Centre, a team of researchers is pursuing Free Space Quantum Communication: transmitting optical signals by using the quantum properties of laser light. The aim – according to principal researcher and project leader Dr Stef Roux – is to provide secure and safe ways of communication using lasers as opposed to fibre optic cables.
Quantum “teleporting” operates on the principle that appropriately entangling a pair of photons interlinks their quantum states. Change one photon’s quantum state, and you change its partner’s – even if one of the photons has been beamed off and is now at a distance.
“We want to communicate through the atmosphere from one tower to another using light,” says Roux. “We are trying to use quantum properties of light to communicate in a secure way.”
In trying to achieve the aim of transmitting information vast distances without depending on a conventional signal, scientists in various parts of the world have succeeded in teleporting information between photons at ranges of up to about 15 kilometres. First demonstrated in 1997, the effect has until recently only been feasible using a fibre optic transmission line.
However, subsequent experiments have successfully sent them through free space with excellent delity. This holds tremendous potential for transmission with satellites. What’s got everybody excited is that the theoretical upper limit on photon entanglement, as reported in the journal Science, is 100 km – limited by line of sight. This has the potential for use in a satellite network as the basis for a global quantum communication system.
Fibre optics have revolutionised hard-wired communications, but they can be tapped or tampered with. There’s a significant benefit to using quantum methods: tapping into an encrypted quantum communication would (a) destroy the data being communicated, and (b) be registered as an intrusion. No fiddling or tampering can be executed without the system being able to detect this. Roux said that this technology “can be used in any form of communication intended for transmission in a secure fashion”.
“People can always, in principle, tamper with or damage cables if they can get to them,” Roux says. “The aim of quantum communication is to protect the communication against such interception of the light. In other words, even if somebody succeeds in tapping the fibre, this person will be unable to obtain the information that is being sent. Every bit of light that is thus detected would represent useless information as far as the legitimate communicating parties are concerned.”
Since the advent of the Internet, myriad information security vulnerabilities have emerged, notably in the banking sector. Currently, the eThekwini Metropolitan Municipality is already using a quantum cryptography system to communicate.
The eThekwini quantum cryptography system was implemented by a group led by the University of KwaZulu- Natal’s Professor Francesco Petruccione before the Laser Centre became involved in this field. “We do have a collaboration with the UKZN group. The aim is to make some improvements to the quantum protocols that are being used in the eThekwini quantum cryptography system,” Roux says. “For that, actually quantum entanglement is required (the current system does not use quantum entanglement).”
“Quantum entanglement has to do with quantum properties of light and there is no classic equivalent of it,” Roux notes.
At the quantum level, light consists of photons. “One can entangle these photons, which means that we set them up in such a way that by fiddling with one, the other is affected even when they are far apart. Such entangled photons are used in our quantum communication scheme. Should an eavesdropper fiddle with one of them, the other would alert the sender and/or the receiver.”
What cost implications would there be for quantum communication?
“It is very early to say and it depends on the specific technology that is used,” says Roux. “If the quantum communication system is implemented in fibre, one may be able to use the existing optical fibre network, which would save much of the cost. The biggest cost would then be the source and detection systems.” If new optical fibre cables are required, the cost would increase significantly, he adds.
If, on the other hand, the quantum communication system is implemented through free space, line-of-sight communication towers would be necessary. But there again, it might be possible to use existing towers. “In (that) case the cost would again be determined by the source and detection systems.”
Roux makes the point that, as with any new technology, the initial cost could be high. However, as the technology matures, costs should reduce.
Although the project is only about a year old, Roux has made some headway. “We already have theoretical results,” he says. For instance: when entangled photons are sent through a turbulent atmosphere, they lose their entanglement through a process called decoherence. A theoretical framework has been derived with which they hope to predict how far the entanglement can be maintained amidst the turbulence in the atmosphere.
“It (turbulence) seriously affects the entanglement of photons in that variations in the temperatures of the atmosphere cause variations in the refractive index of the air and affects the photons,” he says. To mitigate the decoherence, it may be possible to use shorter relay links in the free space quantum communication system with the aid of quantum teleportation.
On the experimental side, he notes, “We are still in the process of getting a proper laser and to get entanglements.”
The laser source required must have a particular wavelength, optical power and pulse duration to optimise the efficiency with which entangled photons are produced. “However, this particular laser source is required because we are still in an early stage of the project,” Roux explains. The good news: the laser sources that would eventually be required to implement the technology should be less sophisticated. The hardware involved includes an optical system to perform the quantum operations that are required at both ends of the communication channel. “ These systems are required to set up the required quantum states and to detect them. As with any communication system, there would also be electronic control hardware to handle the protocols and other logistics.”
Besides the collaboration with the University of KwaZulu- Natal, a possible tie-up with the University of Freiburg in Germany is on the cards, and he is also in discussions with the University of Ottawa in Canada. In fact, he’ll be visiting Ottawa in the next two months to compare notes with ”topclass experts in free space communication”.
The key role players in this field are many and varied.
“In the world, there are many groups working towards this goal. These are mostly academic people. I’m not aware of any commercial company currently involved with this technology.”
In South Africa, Professor Petruccione’s group at the UKZN has been working in this field for quite a while, and the Laser Centre group is fast getting up to speed.
Free space quantum communication, Roux says, is an international buzzword. “If we in South Africa jump on this bandwagon and make a significant contribution, it will have a huge impact for South Africa and position us as a key role player for this technology.”
A man who knows what he’s talking about
Stef Roux has two doctor’s degrees: one in electronic engineering from the University of Pretoria and another in theoretical particle physics from the University of Toronto in Canada.
Roux, 47, says he became interested in optics while at UP. He joined the CSIR Aerotech (now known as the CSIR Defence, Peace, Safety and Security’s Aeronautics Systems Competency) in the late 1980s and later moved to Potchefstroom University (now North-West University) to work as a researcher/lecturer.
“While there I developed an interest in particle physics,” he says. He couldn’t resist the hankering to study particle physics (after all, who can?) so he left to study further in Canada. Having fulfilled his ambitions, he returned to optics and went to work for a company called JDS Uniphase Corporation in Ottawa, focusing on the design of optical fibre components.
“While at this company, there was a telecom bust and the timing was bad, so I left again – this time for the University of Ottawa,” he says.
Roux eventually returned to South Africa in 2004 and went to work at UP before joining the CSIR’s Laser Centre in 2009.
Computing’s bleeding edge
In a related project, Roux is working towards the new supercomputer. A quantum computer is a device for computation that makes direct use of quantum mechanical phenomena such as superposition and tanglement to perform operations on data. “At the moment we are at an early stage of this research – the technology development level,” reveals Roux. “We are trying to figure out how to implement the elementary processes necessary to do quantum computations.”
“We are working with light and we need two things: flying qubits – a unit of quantum information that moves with light – and stationary qubits such as ion traps. In our experiment, we focus on the flying qubits.”