Light’s secrets

An experiment shows that quantum information transmitted by photons is resistant to air turbulence effects

The Starry Night by Vincent van Gogh 

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Osvaldo Farías’ interest in the influence of air turbulence on light propagation led him to visit an exhibition in New York a few years ago that included the painting The Starry Night, by Vincent van Gogh (1853-1890). The physicist at the Brazilian Center for Physics Research (CBPF), in Rio de Janeiro, wanted to see the famous painting up close — studies carried out by physicist colleagues who had analyzed its geometrical shapes had concluded that the vortices painted by the Dutch painter represent a mathematical order similar to that of turbulent currents of air, water and fluids in general. This parallel between art and science can be evoked to explain recent research carried out by the researcher.

Similarly to how one can recognize the distorted light of stars in the firmament portrayed by Van Gogh, Farías and an international team of physicists demonstrated that, if correctly codified according to quantum mechanics, the information transported by a light beam through Earth’s turbulent atmosphere can be recovered. The result of the study, published in February 2015 in the journal Scientific Reports, opens up the possibility of developing technology for transmitting confidential messages — theoretically espionage-proof — through laser sources on the ground or on ships, aircraft or satellites. “Our experiment was proof of principle,” explains Farías. “We generated and transmitted the light states needed to implement a quantum cryptography protocol.” Commercial quantum cryptography systems exist today, but they use a fiber-optic network rather than the atmosphere to transmit data.

Quantum cryptography is considered more secure that traditional cryptography. It is almost impossible to read or copy a cryptographic key transmitted via the quantum proprieties of photons, the particles that make up light. Unlike classical cryptography, the quantum version allows the recipient of the key — which is later used to decode a secret message — to identify any attempt at interception. Despite theoretical inviolability, the strategy is not fully espionage-proof. In recent years, researchers have been able to break the code of commercial systems using quantum cryptography.

The information to be transmitted in these messages can be written in a binary code similar to that of computers using a quantum property of photons called polarization. This property can be visualized as an arrow pointing in a specific direction, for example up or down. Thus, a photon with an arrow pointing upwards could represent a bit of information of type 0, while a photon with an arrow pointing downwards could represent a bit of type 1. The laws of quantum mechanics also allow a photon to be in more than one state at the same time. In the context of cryptography, this phenomenon, which differentiates the classical and quantum worlds, would make it impossible for a spy to identify which state was sent.

There is, however, a problem with using photon polarization in this way. Both the sender and the recipient of the message need to agree exactly on the definitions for “up” and “down.” “Imagine a wartime situation in which someone on land needs to send a secret message to a ship at sea,” suggests physicist Stephen Walborn, of the Federal University of Rio de Janeiro (UFRJ), a colleague of Farías. “The ship’s rolling motion will result in errors when the message is received.”

058-059_Luz Torcida_236To avoid this problem, Walborn and the physicist Leandro Aolita, also at UFRJ, proposed a new way to code quantum information in 2007. They noted that they could prepare two different photon states to represent the bits 0 and 1, whose appearance does not change when the message recipient turns or bounces in relation to the sender. A 0 would be encoded by a photon whose spatial phase follows a spiral trajectory, turning clockwise, while its polarization turns counter-clockwise, in the same proportion. The 1 would be encoded by a photon whose phase and polarization gyrate in directions opposite to those of the 0 photons. “These states do not change when subject to rotation,” explains Walborn.

The idea remained a theoretical possibility until 2011, when Aolita and Walborn met the physicist Fabio Sciarrino, of the Sapienza University of Rome, Italy. Sciarrino’s group has been carrying out experiments with photons with different types of gyrating phases. These photons are prepared in this way when a laser beam passes through a special filter called a q-plate, developed by physicist Lorenzo Marrucci, of the University of Naples Federico II, also in Italy. The researchers decided to collaborate on an experiment using q-plate filters to both generate the photons proposed by Walborn and Aolita and detect them.

Farías contributed to the experiment by developing a way to simulate the effect air turbulence would have on the photon beam carrying the quantum information in the laboratory. “The turbulence caused by fluctuations in air temperature and density acts like a lens that distorts the light beam in a random manner, like mirages over hot asphalt,” explains Farías. “I built a machine that mixes air heated electrically with the cold air of the laboratory, using fans. The greater the temperature difference between the hot and cold air, the greater the degree of turbulence. Thus, the machine simulates the effect of light propagation through a few kilometers of air.”

With the experiment, the researchers proved that Walborn and Aolita’s scheme works. They found that, while the turbulence distorts the laser beam, the q-plate filter receiver is able to capture photons whose quantum information is preserved. “We showed that the information detected is reliable,” says Farías.

“The possibility of transmitting quantum information encoded in states that do not depend on the relative alignment of the sender and recipient is interesting for applications involving mobile stations,” comments Carlos Monken, a specialist in quantum optics and turbulence at the Federal University of Minas Gerais.

Scientific article
FARÍAS, O. J. et al. Resilience of hybrid optical angular momentum qubits to turbulence. Scientific Reports. February 2015.