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Quantum Satellite Micius Challenges Einstein

Quantum Satellite Micius Challenges Einstein

By Paul Edwards

Quantum cryptography experiments onboard a new Chinese Earth satellite foreshadow secret communications on a global scale with security guaranteed by the laws of physics.

Albert Einstein was never happy with the more bizarre predictions of the quantum theory that he had pioneered. In particular he was worried about what he called “spooky action at a distance” – the prediction that a measurement made at a remote location, no matter how distant, could instantaneously change the results of a local measurement. He realised this quantum entanglement violated the concept of a deterministic universe having an independent reality in which the speed of light limited the propagation of all physical effects through space.

Nevertheless, following the work of Irish physicist John Bell, Einstein’s spooky action at a distance has been demonstrated and confirmed by increasingly rigorous experiments up to distances of hundreds of kilometres. With the launch of the Chinese earth satellite Micius in August last year into a Sun-synchronous low-Earth orbit 500 km high, the bizarre property of quantum-entangled pairs of light photons is now being tested in space as a means of implementing secret worldwide communication.

Micius represents a major advance in the field of quantum cryptography, aiming to extend the secure distribution of private quantum keys to global distances for the first time. It is the culmination of several decades of international research, including early Australian work on the feasibility of quantum cryptography with LEO satellites.

Quantum Experiments at Space Scale

The Micius satellite is named after the 5th century BC Chinese philosopher scientist Mozi who, among other things, is said to have invented the pinhole camera. Micius is to be the first of a series of satellite experiments aimed at demonstrating and implementing an uncrackable international communication network, with absolute security guaranteed and protected by the fundamental laws of quantum physics.

The Quantum Experiments at Space Scale (QUESS) project also aims to investigate quantum entanglement in space in order to test the fundamentals of quantum mechanics on a global distance scale. Following 4 months of testing the satellite and its associated acquisition and tracking technology, handover took place on 18 January 2017, and planned experiments have reportedly commenced.

Specifically, Micius aims to distribute quantum keys between the satellite and pairs of ground stations in China. The objective of this quantum key distribution (QKD) demonstration is to transfer quantum key bits at a rate of 10 kb/s over much longer distances than current optical fibre links, and some 1000 times faster.

A long-range QKD demonstration between China and Austria is also planned, foreshadowing a global satellite-based network. Not only does the success of this demonstration and its global extension depend on technological issues but also critically upon Einstein’s spooky action at a distance, which still remains to be demonstrated on a global scales.

This is a major question to be addressed by the QUESS team. Is quantum entanglement preserved over long distances, such as the 7500 km between Vienna and Beijing, as is predicted by quantum mechanics and deemed essential for the realisation of worldwide QKD? If this is not the case it would signal a fundamental failure of quantum mechanics, with immense theoretical and conceptual consequences.

Quantum Cryptography

QUESS plans to distribute quantum keys by generating polarised photon pairs on board the satellite and then transmitting these by line-of-sight to selected pairs of ground stations to two recipients usually referred to as Alice and Bob (see box: Qubits, Entanglement and Quantum Keys). Their shared key then serves as a private one-time pad. One-time pads have been used for a century by secret agents and clandestine groups, most notably by Ché Guevara to communicate with fellow Latin American revolutionary Fidel Castro.

A truly random one-time pad is a “perfect cipher” because it enables Alice to encrypt a plain text message in a provably secure manner so that it can only be deciphered by Bob using his shared copy of the secret key. Maintaining absolute security then becomes the task of distributing the key so that it cannot be covertly intercepted by Eve.

This is the unique advantage of quantum key distribution: any eavesdropping by Eve will be detected by Alice and Bob because of the peculiarly elusive and fragile character of qubits.

Historical Background

The idea that secure cryptographic keys constructed from qubits could be distributed worldwide by satellite is not new. More than two decades ago, quantum physics and engineering research groups in the UK, Austria, the US (at Los Alamos National Laboratory) and elsewhere, including The University of Canberra, competed to demonstrate quantum key distribution over increasingly long “free space” atmospheric links. Some, including the Canberra group, investigated the limits imposed by the turbulent atmosphere on simulated satellite “free space” links.

In the early work, cryptographic keys were usually distributed by Alice, who transmitted sequences of weak, heavily attenuated pulses of laser light to Bob. Each pulse contained, on average, much less than one photon in order to suppress the emission of pulses containing two (or more) photons as this could give the game away to Eve. Efficient single-photon sources had not been developed, and a major advance occurred when French, Swiss and Austrian groups successfully distributed entangled photon keys over optical fibre and short-range atmospheric links.

Australian Research

In 1999 the Centre for Advanced Telecommunications and Quantum Electronics Research at The University of Canberra (CATQER) reported the first transfer of a quantum key in Australia. In 5 years of subsequent research CATQER extended its Free Space Quantum Communications Test Bed from the laboratory bench out to a distance of 43 km with the assistance of colleagues from Canberra-based optical instrumentation companies CH & AH Zelman and Electro Optic Systems, The University of NSW at the Australian Defence Force Academy, Monash University, the Canberra Institute of Technology, and the Australian Surveying and Land Information Group.

In 2004 we reported successfully transferring polarised infrared qubits at a rate of 100 bits per second with an error rate of less than 3% over a distance of 26 km using the B92 quantum key protocol invented by Charles Bennett of IBM Yorktown in 1992. In the final 2005 trials, Alice transmitted qubits from Little Burra in NSW, south of Canberra, to Bob at Wallaroo 40 km to the north. These simulations confirmed the feasibility of nocturnal satellite QKD at elevations greater than about 40° above the horizon, limited by atmospheric turbulence and attenuation of the qubit beam.

Shortly later, the CATQER group was disbanded when The University of Canberra closed its communications engineering school. Its work on a quantum key courier satellites ceased. Other satellite QKD proposals from European and US groups, notably the University of Vienna group, were also stillborn, although several groups including the University of Padua and the National University of Singapore have recently begun limited quantum satellite experiments.

Others successfully commercialised their research and developed optical fibre QKD links and related hardware and software products. An alternative method of key distribution called continuous variable QKD was developed at the Australian National University, and was later commercialised for local network use in banking and other secure working environments.

Now, more than two decades after the initial research into free-space quantum cryptography, Austrian and Chinese quantum researchers have successfully collaborated in a technological tour de force that promises to revolutionise cryptography on a global scale and, as well, to address fundamental aspects of quantum physics that challenge current conceptions of reality.

Of course, because of the heavy security classification of governmental research in this field, we do not know for sure that the Chinese QUESS group was the first to deploy a QKD satellite!

Em/Prof Paul J. Edwards is former Director of the Centre for Advanced Telecommunications & Quantum Electronics Research at The University of Canberra.