EncryptionQuantum encryption for wiretap-proof communication a step closer
Polarized light, in which all the light waves oscillate on the same plane, forms the foundation for technology such as LCD displays in computers and TV sets, and advanced quantum encryption. There are two ways to create polarized light, but each has its problems: filtering normal unpolarized to block unwanted light waves (but here, half of the light emitted, and thereby an equal amount of energy, are lost), or using light which is polarized at the source (but here, polarization is either too weak or hard to control). Now there is a better way: By emitting photons from a quantum dot at the top of a micropyramid, researchers are creating a polarized light source with a high degree of linear polarization, on average 84 percent. As the quantum dots can also emit one photon at a time, this is promising technology for quantum encryption, a growing technology for wiretap-proof communication.
Quantum encryption draws ever closer to real-world application // Source: networkmagazine.com.tw
By emitting photons from a quantum dot at the top of a micropyramid, researchers at Linköping University in Sweden are creating a polarized light source for such things as energy-saving computer screens and wiretap-proof communications. A Linköping University release reports that polarized light, in which all the light waves oscillate on the same plane, forms the foundation for technology such as LCD displays in computers and TV sets, and advanced quantum encryption. Normally, polarized light is created by normal unpolarized light passing through a filter that blocks the unwanted light waves. At least half of the light emitted, and thereby an equal amount of energy, are lost in the process.
A better method is to emit light which is polarized right at the source. This can be achieved with quantum dots — crystals of semiconductive material so small that they produce quantum mechanical phenomena. Until now, however, researchers have only achieved polarization which is either entirely too weak or hard to control.
The release notes that a semiconductive materials research group led by Professor Per Olof Holtz is now presenting an alternative method in which asymmetrical quantum dots of a nitride material with indium are formed at the top of microscopic six-sided pyramids. With these, the researchers have succeeded in creating light with a high degree of linear polarization, on average 84 percent. The results are being published in the Nature periodical Light: Science & Applications.
“We’re demonstrating a new way to generate polarized light directly, with a predetermined polarization vector and with a degree of polarization substantially higher than with the methods previously launched,” Professor Holtz says.
In experiments, quantum dots were used which emit violet light with a wavelength of 415 nm, but the photons can in principle take on any color at all within the visible spectrum through varying the amount of the metal indium.
“Our theoretical calculations point to the fact that an increased amount of indium in the quantum dots further improves the degree of polarization,” says reader Fredrik Karlsson, one of the authors of the article.
The micropyramid is constructed through crystalline growth, atom layer by atom layer, of the semiconductive material gallium nitride. A couple of nanothin layers in which the metal indium is also included are laid on top of this. From the asymmetrical quantum dot thus formed at the top, light particles are emitted with a well-defined wavelength.
The results of the research are opening up possibilities, for example for more energy-effective polarized light-emitting diodes in the light source for LCD screens. As the quantum dots can also emit one photon at a time, this is very promising technology for quantum encryption, a growing technology for wiretap-proof communication.
— Read more in A. Lundskog et al., “Direct generation of linearly polarized photon emission with designated orientations from site-controlled InGaN quantum dots,” Light: Science & Applications 3 (31 January 2014): e139 (doi:10.1038/lsa.2014.20)
