Global reflection: In the not-too-distant future, information encrypted in packages of light could be bounced off satellites and transmitted around the world.
Credit: NASA
SYDNEY: Scientists have managed to establish a 'quantum link' between the Earth and space by reflecting laser pulses off a satellite, a technique that might pave the way to quantum communication on a global scale.
According to the researchers, information encrypted in packages of light could one day be bounced off satellites and transmitted around the world. Banks, governments and intelligence agencies would welcome this advance, since quantum-encryption is a theoretically uncrackable system for encoding information.
Ultra-secure communication
According to the laws of quantum mechanics, if we try to determine the unknown physical properties of a particle they will change, rendering any measurement futile. This could offer an ultra-secure method of communication, because any attempt to decipher encrypted data will corrupt the information, making it unreadable.
To date, quantum-encrypted communication has only been proven possible up to a distance of 144 km, using either optical fibres or telescopes. Now, in a paper published in the New Journal of Physics, Paolo Villoresi from the University of Padova, Italy, and colleagues describe how they were able to exchange quantum information between a satellite and an Earth-based station.
"This experiment represents a crucial step toward future space-to-ground quantum communication," write the authors. "This is the very first time a quantum link between satellite and ground [has been] simulated using a satellite."
The team used a telescope at the Matera Laser Ranging Observatory of the Italian Space Agency in Italy, to send a rapid sequence of weak laser pulses towards the Japanese satellite Ajisai, in orbit almost 1,500 km above the Earth's surface. A small fraction of the photons in these pulses was reflected back to the telescope, as if an emitting quantum source were positioned on the satellite.
Packages of light
For a source to be 'quantum' it must send out only one photon per laser pulse emitted; quantum communication uses single packages of light (photons) to transmit the 1s and 0s of modern information systems. These photons are known as quantum bits, or 'qubits' and, remarkably, can exist as a combination of both 1 and 0 at the same time - a phenomenon known as 'superposition'.
"We are convinced that some time in the future all information will be handled as quantum information," said co-author Thomas Jennewein from the Institute for Quantum Optics and Quantum Information at the Austrian Academy of Sciences, Austria. "Quantum physics offers the potential of doing information processing protocols that fundamentally outperform classical protocols."
There are several hurdles to overcome in the use of free-space quantum communication over long distances, including atmospheric turbulence, the Earth's curvature and physical obstacles. While the use of satellites can eliminate many of these problems, scientists still face the challenge of detecting the few reflected photons among the huge background signal. But Villoresi's team have been able to prove that the photons received at their ground-based station are the same as those emitted.
"This paper is a proof-of-principle experiment that shows very weak, short optical pulses sent at specific times can be detected using existing satellites and ground-based telescopes," said Andrew White, head of the quantum technology lab at the University of Queensland in Brisbane, who was not involved in the research. "This is very good news."
He goes on to describe how the ability to transmit photons around the world will have significant implications for the burgeoning field of quantum computing: "If or when quantum computers are built, sending information between them will require high quality quanta that can be sent long distances, [that is] photons."
Quantum key
The next step for scientists is to apply these techniques to the mode of quantum communication known as quantum key distribution (QKD). In QKD the quantum properties of photons are manipulated using a process known as 'entanglement', which inextricably links particles so that any disturbance to one affects the other, even over vast distances. QKD provides a secure method for sharing a secret key between two parties, who can then use the key to encrypt and decrypt a message.
The experiment also paves the way for new tests of quantum theory. According to the authors, "Extending quantum communication to space environments would enable us to perform fundamental experiments on quantum physics as well as applications of quantum information at planetary and interplanetary scales."
