A quantum step to a great wall for encryption

Mon 19 Jun 2017

Quantum mechanics (QM) is the dark arts of physics. Though physics — in the Newtonian mould — tells us how every object will precisely behave when pushed and hurled, QM deals with the invisible world of subatomic particles, where counter-intuitive rules apply.

QM inhabitants such as electrons and photons live in zombie-like ‘undead’ states. The very act of observing them makes them beguiling tricksters. Though not always understandable, science knows, in bits and pieces, how they can be manipulated for purposes that benefit the visible world such as making integrated circuit chips and fibre-optic lines for global, instantaneous communication.

Transparency may be the shining ideal of modern society but countries and corporations are now infinitely more obsessed with secrecy than in the days of ancient Greece. On Friday, the world took a major leap in employing QM to the cause of secrecy.

How it works

China — as a study in the journal Science reports — has combined satellite technology and the elusiveness of quantum mechanics to demonstrate how secret information can be transmitted over a thousand kilometres — a tenfold increase over what has so far been achieved — with the guarantee that any unauthorised attempt to decipher it would be immediately discernible.

One of quantum mechanics’ cardinal principles, of Heisenberg Uncertainty, follows that no physical object can be known entirely. Measuring, say, its momentum with increasing precision reduces the accuracy with which you can determine its position. For long this was seen as a barrier imposed by nature to us fully comprehending a physical system but for a few decades now, the field of quantum cryptography has evolved around designing ‘keys’ or alpha-numeric codes exploiting quantum mechanical strictures.

Pairs of photons share their quantum properties no matter how long they are separated or how far they have travelled. These can even be created in a laboratory and are called entangled photons. Modern, electronic secrecy works by two parties encrypting the messages they want to exchange and sending each other ‘keys’ (which are chains of numbers) that can be used to decrypt the information. The trouble is that a third eavesdropper can intercept these keys. An “un-crackable” system would be one where both parties know if an intruder is trying to pry out information from the keys.

Enter the entangled photons of QM. Connected just like the ends of a see-saw, in that one going up necessarily means the other is going down, and using such photons to forge a key would mean that any change in their state indicates that someone’s been trying to manipulate them.

While this principle has been understood fairly well since the 1980s, it has been hard to transmit entangled photons through the atmosphere because they are extremely fragile and can disintegrate through contact with other particles in the air. Until last week the world record was a transmission of a few hundred kilometres.

Leading the way

The Chinese set-up transferred entangled photons through a satellite, called Micius, between two ground stations that were 1,200 km apart. According to a report in Science News , the researchers shot a laser beam into a light-altering crystal in the satellite. The crystal emitted pairs of photons entangled so that their polarisation states (or how they are oriented in space) would be opposite when one was measured. The pairs were split, with photons sent to separate receiving stations in Delingha and Lijiang, which are telescopes on mountains, 1,200 km apart. Both stations are in the high mountains of Tibet, reducing the amount of air the fragile photons had to traverse. This team then simultaneously measured more than 1,000 photon pairs. They found the photons had opposite polarisations far more often than would be expected by chance.

Only one out of six million photons sent could be recovered, which experts told Science News , was better than previous ground studies of entanglement but still not good enough for the moon-shot goal of sending secure keys using quantum mechanics principles. Doughty China has publicised plans for international collaborations and transmitting entangled photons in a trans-continental project. Were that to be successful, organisations and people reliant on online financial transactions — that are increasingly dependent on satellite-based Internet — and paranoid about security would take a shine to quantum satellite encryption technology.

The endeavour also underlines the extent to which the West’s domination of science has eroded. Today, the number of high-quality science publications out of China is second only to the U.S. Along with advances in the manipulation of stem cells, this latest step shows their command over the great symbols of the modern scientific age: the satellite, quantum and the Internet.



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