ENTANGLED photons have been successfully transmitted to opposite banks of the river Danube, in the first ever demonstration of this bizarre quantum property over long distances without optical fibre cables. The technique could be a vital step towards ultra-secure quantum cryptography, using satellites to beam entangled photons to Earth.
Quantum entanglement is a property that allows two particles to behave as one, no matter how far apart they are. Measuring the state of one particle instantly determines the state of the other. Physicists have shown the effect could be used to transmit secure encryption keys.
To make use of entanglement, you need to generate an entangled pair of photons and transmit each particle to a different point in space. Until now, entangled photons have only been sent via optical fibres, but eventually these particles get absorbed by the fibre, limiting the distance they can be transmitted to about 150 kilometres.
Advertisement
Researchers have considered using satellites to beam entangled photons to different places on Earth. But no one had ever transmitted such particles across free space on Earth, let alone from a satellite. Now Markus Aspelmeyer of the University of Vienna, Austria, and colleagues have sent entangled photons across the Danube, working in blustery night-time winds and sub-zero temperatures.
The team built a miniaturised, robust version of the entangled photon generators routinely used in labs. It produces pairs of photons entangled in terms of a quantum property called polarisation. For instance, if one particle鈥檚 polarisation is measured to be +45 degrees, the other鈥檚 will always be -45 degrees. After weatherproofing the generator, the team鈥檚 next challenge was to beam their photons to receivers on opposite banks of the river in Vienna (see Graphic).
For this delicate task, the researchers used telescopes on both sides. Transmitting telescopes 鈥渇ocused鈥� the photons on similar receiving telescopes on the opposite bank, which focused them onto photon detectors. The researchers measured the polarisation of the photons entering each detector and established that each pair was still entangled. 鈥淭his experiment deserves all sorts of praise,鈥� says quantum physicist Pieter Kok of the California Institute of Technology in Pasadena. 鈥淎t 600 metres, it鈥檚 a significant achievement.鈥�
Currently, the detectors work only at night, as their signal is overwhelmed by other photons after sunrise. However, another researcher, Richard Hughes at Los Alamos National Laboratory in New Mexico, has recently managed to transmit and receive non-entangled photons in daylight. Aspelmeyer now plans to use the same techniques to transmit entangled photons during the day.
If successful, the technique could work for quantum cryptography. Satellites could beam a sequence of entangled photon pairs to two receivers on Earth, which would measure their polarisation. Measuring polarisation at one receiver fixes the polarisation of the photon at the other. So a key based on the sequence of polarisations could be used to encode and decode messages transmitted between the two receivers.
The advantage is that if someone tried to intercept the photons and measure their polarisation, they would destroy the entanglement. This would be detected by the receivers, raising the alarm. 鈥淚t鈥檚 virtually impossible to eavesdrop on this,鈥� says Kok.