Quantum Communication Demonstrated in Real-Life City Conditions
A quantum-encrypted message containing more than one bit of information in each particle of light was beamed through the air between two buildings in a real-life city for the first time, a demonstration that could simplify quantum communication and make it more viable in the future, according to a recent study.
Scientists previously demonstrated in laboratory conditions that a single particle of light, or photon, could encode multiple bits of information. But until now, the experiment had never been demonstrated in a real-world scenario.
"So far, people have done quantum communication in such a way that they can send either zero or one: one bit of information," said study lead author Ebrahim Karimi, an assistant professor in the Department of Physics at the University of Ottawa.
"The problem is that for every single letter, you need to send eight signals — eight zeros or ones. And that's really difficult," Karimi told Live Science. "One signal can get lost and then the entire letter, the entire message, is lost."
Encrypting information into the quantum states of particles such as photons in multiple dimensions would therefore considerably simplify the whole process, according to Karimi.
"Instead of sending many photons, or many electronic signals, I can send you a single pulse, which contains a file of information," he said. "That would be amazing. This is what we call superdense coding."
The method tested by Karimi and his team can reduce the number of photons required to transmit a message by 50 percent, according to the study.
During the experiment, Karimi and his team successfully sent photons containing two bits of information between two buildings at the University of Ottawa that were located 984 feet (300 meters) apart.
According to Karimi, using high-dimensional encoding would also bolster security, making the quantum-communication channel more resistant to "noise" from weather or other external influences.
"In one-dimensional quantum communication, if the noise reaches to 11 percent [of the signal], the channel is no longer secure," Karimi said. "However, the limit will increase to 19 percent, if you work with four dimensions."
The researchers would now like to experiment with sending and receiving high-dimensional quantum-encrypted messages at distances of up to 3.5 miles (5.6 kilometers), in order to be able to use the technique on the city scale. However, there are significant challenges that will need to be overcome.
"The biggest difficulty is turbulence as the light transmits through the atmosphere," Karimi said. "In our experiment, we are sending a single photon, so that is really difficult. You need to send it to go under a certain angle and use a complicated telescope with sophisticated electronics."
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