For a few minutes each night in certain parts of China, the brightest light in the sky is the lurid glow of the Micius satellite, shooting a green laser down to Earth as it swings through space 500 kilometres above. When conditions are right, you might also see a red beam lancing back through the darkness from one of the ground stations that send signals in reply. Micius is not your average telecommunications satellite. On 29 September 2017, it made history by accomplishing an astonishing feat, harnessing the mysterious qualities of quantum entanglement – what Einstein called ‘spooky action at a distance’ – to ‘teleport’ information into space and back again. In doing so, it enabled the first intercontinental phone call – a video call, in fact, between Beijing and Vienna – that was completely unhackable.
The weird science of quantum physics that powers Micius is at the heart of a technology arms race. On one side are quantum computers, still in their infancy but with enormous potential once they grow in power. Among their most prized, and feared, applications is the capacity to cut through the complex mathematical locks that now secure computer encryption systems – the ones that mean you can confidently conduct financial transactions over the internet. On the other side is the only sure defence – encryption techniques that also rely on the laws of quantum physics.
Until recently scientists had managed to make quantum encryption work only across distances of a hundred kilometres or so. The Chinese scientists behind Micius have now reached around the world. It brings the ultimate prize tantalisingly closer. “I envision a space-ground integrated quantum internet,” says Pan Jianwei, whose team became frontrunners in the quantum communications race after Micius switched on.
That quantum internet will be both unquestionably secure and disconcertingly strange, opening new windows for science and computing.
Pan is used to thinking small. The Chinese physicist made his name with groundbreaking explorations of quantum entanglement, that curious kind of telepathy between subatomic particles that Einstein famously derided.
At the same time Pan thinks very big. He has led China’s massive quantum technology program for more than a decade.
After Micius launched from Jiuquan spaceport on the remote plains of Inner Mongolia in August 2016, it began to perform a series of experiments that steadily escalated in complexity. At their core was a crystal-based gadget that produces pairs of entangled photons and sends them via tightly focused laser beams to receiving stations on the ground.
Pan’s team first established long-range entangled connections between ground stations inside China. Then they succeeded in transmitting the quantum state of a particle – so-called quantum teleportation, which will be a vital technique for quantum computers to communicate. An extraordinary year was capped with the unhackable international videoconference, in which dignitaries from the Chinese and Austrian academies of science exchanged congratulations.
Pan has no shortage of resources at his disposal. Quantum technology is a key research priority for the Chinese government, as for many others.
The best estimate of the scale of global efforts comes from consulting firm McKinsey & Company. It reported in 2015 that about 7,000 researchers worldwide were working in the field, with about US$1.5 billion a year being spent. Those numbers are undoubtedly bigger now, and will only grow as governments and corporations chase the advantages of quantum technology.
High on their list of motivations: protecting secrets. “Security is the big selling point,” says Jacq Romero, a photonics expert at the University of Queensland. A quantum network could also be used to realise more exotic proposals, such as super-telescopes that combine light from multiple telescopes to massively enhance astronomical observations.
The work Pan and other scientists are doing now is part of what some call “the second quantum revolution”. The first quantum revolution began in the early decades of the 20th century with the discovery of the bizarre laws of the subatomic realm – in which an object can be both a wave and a particle – by pioneering scientists like Heisenberg, Schrödinger and Einstein. Applied to technology, these ideas ushered in the era of modern electronics with devices such as the transistor, the laser and the solar cell.
In the second quantum revolution, scientists are applying the quantum rules to the basic ideas of information technology.
Classical computing relies on binary information, represented by bits that are either 1s or 0s. Quantum information uses quantum bits, or qubits, which can be in both the 1 and 0 states at the same time. This can be done using the magnetic spin of electrons, for example, which can be ‘up’ , ‘down’ or some combination of up and down.
This combination quantum state, known as a ‘superposition’, is the first of several concepts that form the foundation of the second quantum revolution. A qubit only ‘chooses’ one state or the other – at random, though the probability depends on how much up and down are in the superposition – when it is measured. Until then qubits inside a quantum computer can effectively perform multiple calculations simultaneously.
The second important concept is entanglement, where the behaviour of distant particles can be inextricably connected – or ‘entangled’. When one entangled particle is measured – and hence ‘chooses’ a state – its partner is immediately bound by that choice, no matter how far away it is. Entanglement is the key to quantum communication.
The third concept is the ‘no-cloning theorem’, which says the information in a quantum particle can never be fully copied without changing the state of the particle. A hacker can make a copy of your email now without you ever knowing; a hack of a quantum system, however, is bound by the laws of physics to leave traces.
Together, these phenomena pave the way for quantum computers able to crunch through big data problems that involve finding optimum solutions from vast numbers of options. That includes efficiently reverse-engineering the encryption keys that protect your internet banking sessions. At the same time, they make possible hack-proof quantum communication, in which eavesdropping can always be detected.
If a particle with a particular set of properties disappears at one location and one with exactly the same properties appears elsewhere, how can anyone say they are not the same particle?
The seeds of a quantum internet were first sown in the 1970s by a physicist named Stephen Wiesner. As a graduate student at Columbia University in New York, Weisner realised how the strange laws of quantum mechanics could be used for new kinds of communication.
Wiesner’s ideas were developed into a detailed protocol for secure communication in 1984 by Charles Bennett and Gilles Brassard. Many cryptographic schemes involve a piece of information – known as a key – that is shared by the sender and the recipient of a message, but by no one else. The Bennett and Brassard scheme sought to solve the problem of sharing the key itself in a secure way.
Their idea involved a sender (conventionally known as Alice in cryptography) sending a long string of 1s and 0s to a recipient (call him Bob) that is encoded in photons in such a way that if an eavesdropper (Eve, naturally) conducted any measurements on it, Alice and Bob would know (because measuring a quantum particle changes its properties). They would then throw out any affected 1s and 0s, and be left with an ideal cryptographic key – a long random number they both know but no one else does.
Quantum cryptography suddenly became more relevant in 1994, when mathematician Peter Shor showed that quantum computers might one day be able to use quantum indeterminacy to break through existing cryptographic schemes with alarming ease. Cracking such schemes – like the ones that keep your internet banking sessions safe from prying eyes – involves finding the factors of extremely large numbers. Shor showed that a quantum computer would be able to do it much more quickly than a classical one.
Meanwhile, further developments in the theory of quantum communication – the practice was still some years off – made use of the even stranger phenomenon of entanglement, which can bind together the fates of objects separated by any distance.
This quantum connection turns out to be very handy for Alice and Bob in their quest to have a quiet chat without Eve interrupting. A pair of entangled particles is in a sense a single entity, no matter how far apart they are. This insight was extended to its logical yet absurd conclusion by theorist David Bohm, who noted that, as a consequence of quantum mechanics, “the entire universe must, on a very accurate level, be regarded as a single indivisible unit”.
In 1991, Oxford physicist Artur Ekert figured out exactly how entanglement could improve on the Bennett-Brassard scheme. Suppose Alice generates a stream of entangled photons and keeps one of each pair for herself, sending the other to Bob. She measures the polarisation of her own photons, and writes down a 1 every time it is horizontal and a 0 every time it is vertical. Eventually she will have a string of numbers. Thanks to entanglement, if Bob has done the same measurements he will have the identical string. If Eve has intercepted any photons, if will make detectable changes to the correlations between Alice and Bob’s measurements.
Another use for entanglement was discovered in 1993, when Bennett and Brassard, along with others, figured out that it could be used to transport the quantum state of a particle – a qubit, essentially – from one place to another. If Alice has a photon in some unknown superposition – the particular combination of 1 and 0 states – this ‘quantum teleportation’ technique lets her send information to Bob so he can create an identical photon. To collect this information, Alice must destroy the quantum state of her photon. Bob then uses that information to create a photon with the same attributes as Alice’s, including any entanglements.
Physicists call this teleportation because the properties of a subatomic particle, such as its position, momentum, polarisation and spin, are all there is to know about it. If a particle with a particular set of properties disappears at one location and one with exactly the same properties appears elsewhere, how can anyone say they are not the same particle?
This kind of weirdness highlights the deep connections between cryptography, information theory and fundamental physics that the quantum internet will exploit. Anton Zeilinger, an Austrian physicist who was Pan Jianwei’s mentor and is now his collaborator, put it bluntly in a 2005 essay in Nature: “the distinction between reality and our knowledge of reality, between reality and information, cannot be made.”
While the theory behind the quantum internet is mind-bending, building it is largely an engineering exercise. Even John Stewart Bell, the Belfast-born physicist who dreamed up the entanglement experiments that killed the idea of any kind of common-sense reality beneath quantum mechanics, described himself as a “quantum engineer”, and said he only had time to contemplate principles on Sundays.
So it is for today’s practical quantum scientists. Devices must be calibrated, experiments must be refined, noise must be reduced. Questions of why give way to figuring out how.
It is the ability to solve those discrete engineering problems that impresses Vikram Sharma, head of Quintessence Labs, a company based in Canberra, Australia, that builds quantum security systems.
Quintessence Labs is putting quantum technology to use in a network security system built around a device that uses quantum unpredictability to spit out a billion random numbers a second.
One of the company’s key achievements is to shrink the device. “We used to do it on an optics table with lasers and electronics and all kinds of equipment,” Sharma explains. “It was probably a metre by a couple of metres. Now we have reduced it to about the size of a cell phone.” He says it with an engineer’s pride. “It just slots in to a standard server.”
Next on the agenda is to “fully mature” a secure system that uses the properties of a whole laser beam to transport encryption keys, rather than single photons, making it a little less fragile. Sharma says he hopes to have a version on the market in early 2019.
Even when carefully engineered to maturity, however, Quintessence Labs’ system will be limited by an obstacle that is very difficult to work around, one that hinders all the competitors in the race to take quantum communications to the world.
The major obstacle that must be overcome to create a global quantum network is in the ‘global’ part: long distances are a real problem.
As entangled photons are beamed through air or an optic fibre, they are slowly picked off by encounters with other particles. After at most a couple of hundred kilometres, 99.99 percent will be gone and the signal will be too weak to use for communication.
One way around this is Pan Jianwei’s scheme: make connections via a satellite that orbits the world and fires photons down from space via laser beam.
Another approach is to use repeaters to retransmit faded signals. A ‘half-quantum’ system establishes quantum connections along a chain of ‘trusted nodes’ that decode and re-encode the signal. The longest such link in operation is a 2,000 km long pipeline running from Beijing to Shanghai via Jinan and Hefei, also built by Pan’s team. These trusted nodes are useful for key distribution – a potential hacker could only read the key by accessing a node itself. However, the nodes do not extend the reach of entanglement.
That will require the creation of a so-called ‘quantum repeater’: a device that can receive a quantum signal and transmit it again without destroying the quantum state, like a relay station that passes a package from a tired courier to a fresh one without opening it.
Some of the most promising research is being done at the Australian National University, where Matthew Sellars and Rose Ahlefeldt have found a way to use crystals doped with erbium atoms to store and release photons with a wavelength (about 1550 nanometres) that works neatly with existing fibre-optic cables.
When a photon is absorbed, its quantum state is mapped on to changes in the spin of the nucleus of the erbium atom. “If you put the information on a nuclear spin, it can hold for much longer,” Ahlefeldt says. This is because the nucleus of the atom is insulated from the outside world.
The atom can then be stimulated to release a new photon identical to the one that went in. “You can store the polarisation, the arrival time, the pulse shape, the direction,” Sellars says. “The photon that went in is the photon that comes out.”
Crucially, this includes any entanglement of the original photon. A chain of repeaters connected with optic fibre could extend entanglement indefinitely.
Sellars and Ahlefeldt are hoping to demonstrate the basic functions of a repeater in the next year or two. After that, says Sellars, “It becomes a case of engineering and how much money you throw at it.” One uncertainty is how much demand there will be: “No one’s had a quantum internet before.”
Similar technology will be needed to plug quantum computers in to the quantum internet. “If we set up this global-scale quantum network, we want to be able to connect things to it,” Ahlefeldt says.
Getting qubits out of a quantum computer – where they might be stored as electron spins or the magnetic flux of a superconducting loop – is a feat in itself.
“There are three problems to solve,” according to Sven Rogge, who works on quantum computers at the University of New South Wales. “First you have to be able to control one qubit and read it out. Then you have to couple two of them that are close together, for two-qubit operations inside the computer. Then you have to do that two-qubit operation over a much larger distance. That’s the holy grail, the really hard part.”
How long before a mature global quantum network is possible? Though many of the underlying technologies are still in prototype form, Pan believes that progress will be rapid. “Maybe it will take 10 years,” he guesses.
A team based at the Delft University of Technology in the Netherlands, however, hopes to have a small network connecting four Dutch cities – over distances in the tens of kilometres that will not require quantum repeaters – operating by 2020. After that? Even visionaries like Pan can only speculate about the the eventual uses of the quantum internet.
Right now secure communication is the killer app – the thing that makes governments and banks pour cash into research. Another likely use is connecting to quantum computers, which are expected to be expensive and cumbersome machines for some time to come. Much as people once dialled in to massive mainframes to get their computing done, a quantum link would allow remote access to quantum computers with the added twist of ‘blind computing’, in which the quantum computer can never know what calculations it has performed or what sensitive data it has handled.
Quantum communication will also allow distant clocks to be synchronised within 10–20 seconds, about a thousand times as precise as the best current atomic clocks. This precision will allow orbiting satellites to improve GPS systems, map Earth’s gravitational field in unprecedented detail and even catch the tiny ripples of passing gravitational waves.
Better optical telescopes are another potential fringe benefit. Radio telescopes such as the nascent Square Kilometre Array combine signals from distant dishes to effectively form a single, huge telescope. A quantum internet could make this possible for visible-light telescopes, too, by teleporting photons from distant telescopes.
Pan sees his work as part of a continuum in the human imperative to communicate and exchange information. It was, he says, the defining character of early Homo sapiens. “They created basic symbols and languages so that they could interact effectively and form a co-operative group. Information exchange is a key factor in human evolution”.
The next stage in that evolution will occur through the patient labour of small, incremental steps: improving the Micius technology to make the satellite work in daylight, replicating it in other satellites, learning how to make multiple satellites function together. He may have opened the door to a global quantum internet, but Pan still thinks in the measured terms of an engineer. “We will study how to build a more efficient network.”
This article appeared in Cosmos 78 – Autumn 2018 under the headline “The entangled web”
