The primary quality that makes quantum cryptography special, is the security provided by the use of qubits backed by Heisenberg's principle of uncertainty. Millions of these qubits are generated by the single photon generation sources, but a large number of bits are lost along the way, resulting in periodic loss of information.
Currently, the bit rate is very low, hovering around a few thousands of bits per second. This could be used in small-scale secure communications between two or more people, but for realtime communication, this could be inadequate or could be used as the starting phase for another cryptographic system.
To enhance the bit rate, technology developers will have to improve the sources of single and entangled photons, single photons on demand, and all other types of photon generators, which, in turn, can improve the applicability of quantum cryptography.
Since photons are the carriers of information, it is vital to have efficient and accurate single-photon detectors. However, current photodetectors have high recovery time of nearly a microsecond once they detect a photon and due to this, they cannot count more than a million photons per second.
In Germany, physicists have achieved remarkable control over single-photon creators, helping them emit the photons one at a time. The researchers also have total user control over emission time and pulse shape of each photon. If this device is operated without interruption and is limited only by the trapping time of the tightly trapped calcium ion, it can hold the ion for many hours.
Photodetectors, on the other hand, can improve their detection rate using lower loss fibres, which retain photons for longer periods without allowing them to fade away.
"The specific requirements of single photon detectors include operation at telecommunication wavelengths, low dark count, low afterpulse rate, high single-photon quantum efficiency and number resolving capability," says Frost & Sullivan research analyst, Haritha Ramachandran. "Semiconductor avalanche photodetectors (APDs) are the most promising single-photon detectors from both device performance and practicality points of view."
Physicists with Toshiba Research Europe and the University of Cambridge have developed a device that makes it easier to detect single photons with low dark count rates and high quantum efficiencies. They achieved this by passing resonant tunnel current through a double-barrier structure, as the current is sensitive to the capture of single photo-excited holes by an adjacent layer of quantum dots.
"This ability to detect single photons has proven itself to be a boon in terms of assisting researchers in varied diagnostic fields, such as medical imaging, chemical analysis and environmental monitoring," observes Ramachandran. "The device depends on a quantum dot, a tiny semiconductor island that owing to its essentially zero-dimensional physical extent, forces electrons to possess only certain discrete energies."
While the indium-gallium-arsenide/indium phosphide (InGaAs/InP) APDs are insufficient for quantum key distribution (QKD) applications, the silicon (Si) APD photon counters have exhibited far superior characteristics and are used for light detecting and ranging (LIDAR) and other military systems.
Security in communications took another leap forward with photon entanglement, a stunning discovery by the University of Geneva in collaboration with the Los Alamos National Laboratory, in the US. This technology promises to heavily influence the future of computers as the key to creating the quantum computer. By coding information in a large number of entangled photons, quantum computer will be able to provide superior data protection and enable calculations in 10 seconds that would otherwise take 10 years.
"Quantum cryptography, at present, is limited to point-to-point fibre-optic or line-of-sight laser communications, but general network security requires more conventional algorithms," notes Ramachandran. "Hence, both quantum and conventional cryptography are needed, as they complement each other and encourage research in other technologies to make systems more secure."
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