Researchers created entangled photons that are less susceptible to disturbances such as sunlight.

Protection against signal loss: Physicists have made quantum entanglement less susceptible to interference.

Quantum entanglement less susceptible to interference.
Quantum entanglement less susceptible to interference.

This makes quantum signals detectable even in daylight and noisy channels - an important prerequisite for long-distance transmissions or the transmission of quantum messages through the air. 

The researchers have achieved these more robust signals by interweaving spatial and temporal characteristics of the light particles. The basis for quantum communication and the transmission of keys for quantum cryptography is entanglement.

Quantum signals detectable even in daylight.
Quantum signals detectable even in daylight.

In this phenomenon, the states of two particles are interconnected in such a way that the change of state of one automatically causes that of the other - instantaneously and over long distances. Researchers have already succeeded in transmitting quantum signals via fiber optic cables, through the air and even in orbit.

More dimensions for noise reduction

But there is a catch: the quantum entanglement is very sensitive to disturbing influences. Environmental influences, interfering light from the sun and the loss of too many photons cause the particles to lose their entanglement. The content of the quantum message then disappears in the noise. 

Therefore, these interference effects limit the range of quantum signals both in fiber optic cables and in open-air transmission, which until now has only been possible at night without interferences.

But there is a solution for this noise problem, as Sebastian Ecker from the Institute of Quantum Optics and Quantum Information (IQOQI) and his colleagues report. In their experiment, they entangled the light particles not only in one feature, but in several. 

Such a "multidimensional entanglement" not only enables the transmission of more than one bit of information per photon, it also makes the transmission more robust, as the researchers now demonstrate.

Entanglement in several planes

The physicists were investigating two variants of such multidimensionally entangled photons. In the first experiment, they used lasers and a photonic crystal to generate light particles that were entangled in terms of their polarization and the time at which they were generated. 

In the second experiment, the photons were entangled over several orbital angular momentum planes. In both tests, these photons flew through a distance with different levels of interfering light.

The result: Despite the strong noise, the researchers were able to successfully identify and read out the quantum signals at the receiver. 

"The reason for this is that entanglement has special correlations in many distinguishable states, which can still be clearly distinguished from classical correlations even with strong background noise," explains senior author Marcus Huber from IQOQI.

Important step towards the quantum Internet of the future.

According to the researchers, their experiment proves that such higher-dimensional entanglements could at least partially solve the problem of noise and signal degradation during quantum transmission. 

"With our experiment, we were able to show how entanglement can be made more robust. This is another important step towards the quantum Internet of the future," says Ecker.

The next step is to develop communication protocols for quantum transmission that make such higher-dimensional entanglements practical for quantum cryptography and other applications. 

Overcoming Noise in Entanglement Distribution

Sebastian Ecker, Frédéric Bouchard, Lukas Bulla, Florian Brandt, Oskar Kohout, Fabian Steinlechner, Robert Fickler, Mehul Malik, Yelena Guryanova, Rupert Ursin, and Marcus Huber. Phys. Rev. X 9, 041042 – Published 26 November 2019.


Sebastian EckerFrédéric Bouchard, Lukas Bulla, Florian Brandt, Oskar Kohout, Fabian Steinlechner, Robert Fickler, Mehul Malik, Yelena Guryanova, Rupert Ursin, and Marcus Huber.

Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.

Vienna Center for Quantum Science and Technology (VCQ), Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria.

Department of physics, University of Ottawa, Advanced Research Complex, 25 Templeton, Ottawa, Ontario, Canada, K1N 6N5.

Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Albert-Einstein-Strasse 7, 07745 Jena, Germany.

Abbe Center of Photonics-Friedrich-Schiller-University Jena, Albert-Einstein-Strasse 6, 07745 Jena, Germany.

Photonics Laboratory, Physics Unit, Tampere University, Tampere, FI-33720, Finland.

Institute of Photonic and Quantum Sciences (IPaQS), Heriot-Watt University, Edinburgh, Scotland, United Kingdom EH14 4AS.