"Ask Me Anything” and They Did! Light in a Twist with Miles Padgett, University of Glasgow

"Ask Me Anything” and They Did! Light in a Twist with Miles Padgett, University of Glasgow

By Miles Padgett

About Me – Miles Padgett, Optical Scientist
The energy carried by light is fundamental to life on our planet. But as well as energy, light beams carry a momentum. So if I shine a laser pointer at you, in addition to making you slightly hotter you’d feel a small force pushing you away. This force is roughly equal to the weight of a single biological cell. Although not much use for moving you, this force is used within microscopes to move single cells without touching them – a technique called optical tweezers. All we have to do is shine the laser into a microscope, and when we move the laser the cells follow.

But light has other properties too. Beyond the energy and push of light my own team is interested in light’s twist. In optical tweezers we use this twist to rotate microscopic objects. Making cells dizzy might be fun, but perhaps not much use!

Light comprises millions and millions of individual photons, each of which can be encoded with information. The twist of light has been recognized for 100 years but until recently people thought that this was only clockwise or anti-clockwise – only suitable for transmitting 1s and 0s. However, in the last few years we’ve realized that the twist can be changed in size as well as direction.

Each individual photon can now carry much more information. So not only does the push and twist of light manipulate microscopic objects – it also holds the key to new communication systems.

Question: Hi Miles, last year I completed my undergraduate project in University of Glasgow optics department, but have since moved into teaching. I'm very interested in science outreach in schools, so my question is: What do you think is the best way to relate the high -level concepts you study to concepts which might be studied in a classroom, something that high school pupils can understand? Furthermore, is there any of this kind of outreach work that you yourself carry out in your capacity as Kelvin chair of natural philosophy? Thanks. (JohnRCC)

MP: As part of the QuantIC centre in Quantum Imaging here in Glasgow we have a very active outreach programme. Indeed I have participated myself in number of teachers workshops both trying to articulate the "spookiness" of quantum science but also so the practical applications to which it can be put.

Question: Do you think there’s a fundamental limit for the data capacity of communication systems or will we keep finding new ways to increase it? (lackalil)

MP: That's a really good question. I was part of a research programme a few years ago that sought to answer the question "how much information on a photon?" The answer was a lot more the one bit!
Wavelength division multiplexing transformed optical communications. But my understanding is that rather than being a victory for physics it was really a victory for engineering (okay i realise two aren't really different -- but you know what i mean). People knew already that light had different wavelengths that could be separated (e.g. with a prism!) the challenge was to do this in a compact and reliable way that could be reconfigured, miniaturised and made compatible with fibres.
In essence that is were we are now with spatial modes. The physics says it works, but the technology still needs developing.

Question: Dr. Padgett, I know that orbital angular momentum can be used to increase the information capacity of optical networks, but by how much? Factors of 2, 10, or more? What are the practical issues that will limit that increase? (MDDConsulting)

MP: I think that at the most basic level one needs to realise that there are many many properties of a light beam onto which one can encode information, intensity, polarisation, wavelength, direction etc.
Ideally whatever you use to encode you want to be easy to set up, switch between and measure! And you would like these measurements to be non-ambiguous - i.e. you want the states to be orthogonal.
One other option for encoding information is ≈beam shape, ie optical transverse modes and that is where OAM comes in.
The Laguerre-Gaussian modes that carry OAM are one suitable set of modes that can be used. But there are other modal sets too, e.g. Hermite Gaussian.
At a fundamental level all these model sets are equivalent -- but some modes fit the rest of the optical system better. Since most optical systems have round aperture (fibres too) the circular modes are perhaps the most obvious to use. The LAguerre-Gaussian modes are hence an obvious choice.
The amount of extra information you can get simply depends on how many modes you want to use. But as the number of modes goes up you need bigger and bigger lenses etc -- so the lunch is not quite free!
My group demonstrated OAM based information transfer as long ago as 2004. But to be honest I was quite skeptical it could ever be made to work really well -- I was wrong! In 2012 I reviewed a paper by Alan Willner and his group - it was fantastic.
He had made a system that really seemed to work -- since then he has gone on to publish more and more impressive results. Thank you Alan :)

Miles John Padgett, FRS FRSE is Professor of Optics in the School of Physics and Astronomy at the University of Glasgow. He has held the Kelvin Chair of Natural Philosophy since 2011 and has been Vice Principal for Research at Glasgow since 2014.

Posted: 7 December 2016 by Miles Padgett | with 0 comments