Quantum signals with Several Bits per Photon sent

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For the first time researchers have been able to transmit Quantum signals with several bits per photon through the air. The special feature of the experiment: The researchers have sent the manipulated photon over a distance of 300 meters above the roofs of the city of Ottawa.

Ottawa (Canada). Presumably, quantum communication will be the future. Also in the field of quantum cryptography is currently being intensively researched, because this encryption method is considered almost unhackbar. Another advantage of this communication technology is that the entanglement of photons causes the signals to reach the receiver almost in real time – even over long distances and even between earth and orbit .

Two Bits per Photon

So far, one bit per photon has been transmitted for quantum communication – ie a one or a zero. But if several properties of the photon are manipulated, it can also be transmitted several bits per photon. Ebrahim Karimi of the University of Ottawa and his colleagues has now succeeded in successfully sending out such a manipulated photon and reading it again at a distance of 300 meters.

As the researchers in the journal Optical Society report, the researchers have installed their laboratory instruments, which among other things generate the entangled photons, on rooftops in the city of Ottawa. They first manipulated the quantum states so that each photon can carry two bits, four possible signals: 00, 01, 10, or 11. This technique is also called 4D quantum cryptography.

Quantum signals communication across Ottawa

Under laboratory conditions, the procedure has already been tested. Now the researchers wanted to find out whether the quantum information would survive the transport through the city of Ottawa – just because the state of entanglement is very susceptible to interference. The slightest turbulence in the atmosphere can cancel the entanglement. “Some colleagues thought our experiment was impossible,” says Karimi.

Despite the headwinds, the researchers managed the experiment – and even better than expected. “For the first time, we have successfully sent messages using high-dimensional quantum encryption under real city conditions,” explains Karimi.

A later analysis showed that the error rate was only eleven percent. Thus, it was well below 19 percent, where a secure transfer of information is no longer possible.

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