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‘Quantum repeaters’ could extend secure communication

by Robin Yee May 4 / 15

Scientists have proposed a new method to transmit secure quantum communication across longer distances.

In the all-photonic quatum repeater scheme, 1) each repeater node prepares photons for the entanglement generation and loss-tolerant phototonic quibits. This state preparation is implemented by the sequential applications of linear optical elements, photon detectors and active feed forward techniques to single photons, which is accomplished in about 3 μs under the assumption of the use of a 150 μs active feedforward technique [Nature 445, 65 (2007)]. Then, 2) each repeater node sends photons to left-hand and right-hand sides, and an entanglement generation process is performed once photons meet. This process succeeds with a high probability, and, as a result, 3) quanum entanglement is established between the sender and the receiver.

The next generation of cryptography has the potential to transmit messages in total privacy, leveraging a built-in test against security breaches. But the quantum states used to encode these messages are delicate and challenging to send over long distances. For example, fibre optic channels transmitting photons would absorb and lose 90 per cent of those photons within 50 km.

Conventional communications networks like cell phone towers and fiber optic telephone cables simply amplify the weak signals as they travel through the system. The catch for quantum communications is that there are different rules which mean quantum information can’t be directly copied. Scientists have been trying to design practical workarounds to solve the problem.

A new design published in Nature Communications last month may provide an alternative. Hoi-Kwong Lo (University of Toronto), an associate fellow with CIFAR’s Quantum Information Science program, collaborated with lead author Koji Azuma and Kiyoshi Tamaki (both of NTT Basic Research Laboratories, Japan) on the work.

Previous designs for quantum repeaters rely on quantum memories that interface between matter and photons. Essentially, each repeater acts as a primitive quantum computer, capturing and then re-transmitting the photons in a kind of relay race. But these designs are difficult to implement, slow, and require cooling.

“Previously people were saying that the challenge is in making matter quantum memories — and then how could you interface between quantum memory and photons? But we are saying that there is an alternative,” Lo says.

Lo and his colleagues proposed replacing the interfaces with all-photon repeaters that use clusters of highly entangled quantum states. These cluster states create long-distance entanglements along the channel more efficiently, saving time and allowing the system to operate at room temperature.

The new design puts scientists one step closer to quantum encryption. Once the quantum states can be reliably transmitted over long distances, their fragile states become an advantage, easily collapsing and breaking the connection under the intrusion of an eavesdropper.

In principle, perfect security should be achievable through quantum encryption, but several loopholes remain before it can be implemented. Although scientists have gained success in closing the loopholes one at a time over the last few years, the challenge is closing them all simultaneously. Once this is accomplished and quantum repeaters become feasible, Lo says, “we will finally achieve ‘unconditional security’ – the holy grail of communication security.”