There’s something poetic about the idea that two objects separated by vast distances can share a singular fate. This is the simplified premise of quantum entanglement, which describes how two particles are governed by a single equation that makes it impossible to describe one without the other. That is, the properties – such as spin and momentum – of two entangled particles are apparently linked, no matter how far apart they are physically.
For a long time, quantum entanglement loomed as a source of fascination, bewilderment and even scepticism. Adding to its mystique was the fact that quantum mechanics was principally concerned with the behaviour of things unobservable to the human eye, such as atoms and atomic subparticles.
For years, scientists knew that entanglement existed, but they didn’t know why it occurred. It was something of a black box, an unwieldy cosmological mystery. As Lancaster University’s Professor Robert Young notes, the phenomenon wasn’t necessarily understood or even predicted by physicists. “It was basically just a result of some equations that were thought to describe what happens at the atomic scale,” he says. “Quantum mechanics was proposed as a probabilistic theory.” Albert Einstein, somewhat dismissively, described the entanglement phenomenon as spukhafte Fernwirkung: “spooky action at a distance”. According to the law of special relativity, he argued, such a binding influence was supposedly impossible, and irrefutable proof that the underlying theories of quantum mechanics were flawed or unworkable.
This presented a conundrum known as the EPR (Einstein–Podolsky–Rosen) paradox. It was at odds with core tenets of Einstein’s theory of relativity, which suggested that nothing could travel faster than the speed of light – if that was so, entanglement was impossible, as information couldn’t travel quickly enough between distant entangled particles to exhibit their instantaneous link. Einstein believed that the paradox was generated from the incompleteness of quantum mechanics; that is, it did not account fully for the nature of one’s lived reality. In a letter to his friend and fellow physicist Max Born, Einstein expressed his convictions emphatically: “God doesn’t play dice with the world.”
Yet Einstein’s penchant for determinism would turn out to be misguided. Classical physics, Young says, is about the observations we make on a daily basis – it concerns the world we see and the forces we interact with. It is governed by the principles of locality (an object can only be influenced by its surrounding environs), reality (the idea that reality exists independently of our minds), and causality (an effect can’t take place before its cause). Quantum mechanics, on the other hand, upended these principles. In 1964, the Northern Irish physicist John Bell found a way to test the EPR paradox, and proved that Einstein’s insistence on quantum mechanics’ incompleteness was, in fact, wrong.
What was once a rare occurrence has now become a routine experiment in laboratories all over the world. Still, distributing entangled subatomic particles outside of laboratory confines can be challenging, which makes successful experiments in the world at large all the more impressive. In August last year, a team of physicists from the Chinese Academy of Sciences led by Pan Jianwei launched Micius, the world’s first quantum-enabled satellite, named after the ancient Chinese philosopher who made striking observations about mechanics and motion. Pan’s team created pairs of entangled photons by splitting a single photon in two with a crystal made from beta-barium borate. Their experiment bore fruit when it was revealed that the team had managed to use a satellite to distribute these entangled photons between three base stations on Earth, about 1200 kilometres apart. This was a significant achievement for a number of reasons. Theoretically, entangled photons can be conjoined across any distance, but in reality, separating and moving photon pairs around can interrupt the entanglement process. Pan’s team not only achieved the farthest entanglement in terms of distance, it was also the first time such an experiment had been conducted between Earth and space.
What applications might all this have? The aim of China’s Micius satellite is to establish a communications network encrypted by quantum technology. Such encryption would be far more protected than anything currently offered, and could curtail the likelihood of damaging large-scale hacking, as seen earlier this year in the Petya and WannaCry ransomware attacks. Currently, digital cryptography provides the sender with a key with which to encode sensitive information, while their recipient uses another to decode it. It’s not foolproof – the risk of electronic eavesdropping, especially through the use of so-called Trojan programs, remains high.
Employing quantum entanglement in what’s called quantum key distribution, however, could present a potential breakthrough for cybersecurity. The process involves sending information about the encryption key through photons that have undergone a process of random polarisation. The message can’t be decoded unless the receiver has the specific quantum key. While conventional cryptography uses mathematics to keep confidential information secure, quantum key distribution, as the name suggests, is governed by the laws of quantum physics, which are, by orders of magnitude, much harder to crack. If a hacker tried to acquire the key – which would require measuring the properties of entangled photons – their attempts would generate detectable errors that would alert users that the key had been compromised. There are fewer intermediaries between parties and therefore fewer vulnerabilities. Entangled photons could theoretically provide an inviolable chain of communication.
Other practical applications of the technology include super-resolution imaging. Young says that under classical physics, the resolution of an ordinary optical microscope is related to the “wavelength of lights or the colour of light used to essentially illuminate the sample to make that measurement”. But quantum physics “actually has a law of resolution in it”, he says, which is divided by the number of entangled particles you use. “If I use a microscope that uses 10 entangled particles/entangled photons/particles of light to make a measurement,” Young says, “it can actually have 10 times better resolution than a classical microscope. So that could be very exciting for medical imaging.”
Similarly, quantum computing may be able to solve problems once considered too complex for classical computers. While classical computers work by encoding information into bits – expressing data with binary values of zero and one – quantum computers store information as qubits – quantum bits. Qubits allow information to be categorised in binary terms, but in any superposition of those values. As IBM Q, the company’s quantum computing arm, explains on its website: “superposition means that each qubit can represent both a 1 and a 0 at the same time”, while “entanglement means that qubits in a superposition can be correlated with each other; that is, the state of one (whether it is a 1 or a 0) can depend on the state of another”. In practical terms, this means quantum computing can, as Young puts it, “essentially make a very complex process very, very fast. It can parallel processing. There’s a promise that in the future we’ll be able to do some very complicated numerical tasks, like sorting databases or factorising products of prime numbers, very quickly.” These tasks are merely the tip of the iceberg. “We know all the few hundred quantum algorithms that we think are interesting for quantum computers, but there are probably hundreds and hundreds of thousands that we’ve yet to discover. So there’s a huge unknown world waiting for us there.”
Young believes the quantum revolution will easily rival the digital one, shaping our realities in unexpected ways, and perhaps even encouraging us to recalibrate our view of reality itself. Is it our gaze that is poorly defined, or is it the case that reality, as nebulous and indeterministic as it is, demands a kind of formlessness and uncertainty?
These questions are fascinating philosophical quandaries and are likely to have tangible scientific implications, too. “Most of the applications of this new branch of physics have yet to be discovered,” Young says. “A lot of the very simple ones that people are talking about – quantum communications and quantum computation, for example – are really just very almost trivial analogues to the digital revolution, and they’re probably red herrings. They’re probably the least exciting – or I hope they’re the least exciting. But we’ll see.”
This article was first published in the print edition of The Saturday Paper on Sep 9, 2017 as "Quantum leap". Subscribe here.