"It's really rewarding when you discover something new and when what you have achieved is tangible."
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Angela Dudley
Generating a hologram of an object that does not physically exist and creating twisted laser beams sounds like science fiction, but not for CSIR National Laser Centre scientists!
One such person is CSIR laser scientist Angela Dudley. The 25-year-old is involved in mathematical optics research, where the mathematical basis and applications of modern optics are investigated. But what Dudley is specifically interested in, is 'twisted' laser beams that could ultimately result in ground-breaking implications for quantum cryptography studies. An optical vortex is a beam of light that moves in 'corkscrew-like' manner. On a flat surface, an optical vortex looks like a ring of light, with a dark hole in the centre. Apart from their interesting visual characteristics, these beams possess orbital angular momentum; much like our earth possesses orbital angular momentum, evident as it orbits the sun.
"I am working on digital holography, the technology of which is used to generate the vortex beams," says Dudley. "Previously, conventional holograms were created by illuminating an object with a laser beam and interfering the beam reflected off the object with a reference beam. This interference pattern was recorded onto photographic film and was then termed a hologram. By reversing the process and illuminating the hologram with the reference beam the initial object is reconstructed."
However, she says, "This process is all well and good, but what happens if we want to generate a hologram of an object that does not physically exist? This has given rise to the need for digital holograms. So instead of using the interference of light to determine the hologram, we mathematically calculate the interference pattern ourselves and write it to a liquid crystal display device called a spatial light modulator." A spatial light modulator usually modulates the intensity of the light beam, however, it is also possible to produce devices that modulate the phase of the beam; or both the intensity and the phase simultaneously. As such, it can be used as part of holographic display technology and form also a component in optical computing
The fact that vortex beams carry orbital angular momentum has lead to applications in both classical and quantum optics. Dudley says, "Previously in quantum communication systems, information was encoded by making use of the spin or polarisation states of photons (light). However, unlike spin angular momentum where one is restricted to only two possibilities: 'spin clockwise' and 'spin anti-clockwise'; orbital angular momentum possesses an infinite number of possible states." Quantum physics deals with atomic and subatomic systems.
Quantum cryptography makes use of quantum entanglement in the transmitting of information (a key) in quantum states. This communication system is able to detect 'eavesdropping'; it involves two communicating users who share information that can be used as a 'key' to encrypt and decrypt messages. "Encoding the alphabet is a good example to illustrate the benefit of using orbital angular momentum in quantum communication," she says. " Instead of using the spin states of many photons to represent the letter 'A' through an 'on-off' sequence similar to Morse code, a single orbital angular moment state can be used to represent the letter. The ambitious research endeavour includes project leaders Drs Andrew Forbes and Stef Roux; and another PhD student at the CSIR National Laser Centre, Attie Hendriks. The team is collaborating with quantum theorists at the University of KwaZulu-Natal.
Should this research prove successful, it will have ground-breaking implications for the study of quantum cryptography. Dudley says, "Even though this work is in the early stages and it will be developed in the coming years, accomplishing what we have set out to do will lead to advancements into secure and efficient quantum communication. Being able to transfer information securely is essential for military applications and even 'everyday' internet banking."
"I have worked for the CSIR for close to a year and a half now, and within this time I have travelled overseas twice," says Dudley. "I can't think of any other institution that provides its employees with so much exposure. Apart from being privileged to meet and work with researchers from overseas, I have also been exposed to the work of scientists outside of my research field within the CSIR." When asked what the future holds for Dudley, she says, "I would love to continue with research even after I receive my PhD; it's really rewarding when you discover something new and when what you have achieved is tangible."
Beams carrying orbital angular momentum were first discovered in 1992, however, research in this field is only starting to be explored in South Africa. Since then scientists have spent a lot of time developing this area of physics. "In the past few years, research groups have seen that this property holds the key to secure quantum communication and they have started to investigate it further. So what does this mean for us? Well, we just need to work quickly!"
Her team's innovative research has featured in Popular Mechanics and for the second year in a row, Dudley was recently awarded the best PhD poster at the Annual South African Institute of Physics Conference by the Laser, Optics and Spectroscopy Specialised Group.
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