SPIN SPACE - Spatially encoded telecoms and quantum technologies using spin-enabled all-optical switching

Our planet is criss-crossed with optical fibres that influence almost every aspect our lives in the 21st century. However, despite the great advances in optical fibre communications technologies that have occurred in the past 20 years, we have already almost run out of data capacity. With more of the world online, and the "Internet of Things" predicted to connect up to a trillion devices in the next 20 years, we need to find better ways of overcoming fundamental limits in how much data we can send. Also looming on the horizon are new technologies that may use optical fibre telecommunication networks, such as quantum optics technologies, sending, for example, completely secure data using single photons. However, sending many of these photons but keeping each one separate is a major challenge.

In answer to these new technologies, it has been suggested that sending information via a microstructured fibre may offer solutions to the challenges above. Microtructured fibres are rather like a stick of Brighton rock with a pattern running through. The simplest of these may be several cores running in parallel but optically isolated, whilst more complex designs involve controlled light leakage between the cores, or indeed a honeycomb structure with light travelling in the air. Recent ideas propose sending a pattern of light (either a light intensity pattern or a pattern of polarization) through the microstructured fibre with complex changes in the pattern containing encoded information.

While much work is presently being carried out on signal propagation in microstructured fibres, it is clear that to create the signal, a means of producing a spatial laser pattern is required that can switch pattern over GHz timescales. More importantly, to perform functions such as rerouting signals from one area of the array or changing the pattern one requires a device with an optical input and output to be fed into the later fibre network. Switching an array using electrical contacts is tricky - one needs to individually access many micron-sized areas at fast speeds. We propose that if small (few micrometer) lasers are fabricated into a small forest of pillars that emit individual points of light vertically, we can generate complex patterns easily. We use a semiconductor laser, where the spin, the electron's intrinsic magnet, interacts differently depending on the light polarization - in some cases the photon is absorbed, in other cases the spin is in the wrong direction and the light passes through. We will thus control light pulses to flip the spins and perform optical logic in spatial arrays of these devices. This will allow incoming signals to be switched and re-routed.

When the laser power is turned down, and very specific frequencies are used, we find that the light becomes intrinsically "grainy" - and turns into individual photons. We also know that the semiconductor can be prepared so that it behaves like a collection of atoms - at very specific wavelengths, the photon only "sees" one electron. Rather like an atom, the photon may be absorbed and an electron gains energy - however in our case it also interacts with the electron's spin. When the electron drops from its excited state and emits a photon, the photon has changed polarization. We can then filter out the outgoing photons from the incoming ones and use the scattered photons as a "single photon source" - where exactly one photon is produced per optical pulse. This source allows completely secure information to be sent, and is the starting point for photon quantum computing, where many of these individual photons are made to interact and encode information for a quantum computer.

Industry
This project provides a test-bed for two new technologies in optical fibres. Technologies based on multicore fibres, whether for standard telecoms or to build up a fledgling quantum secure network, are clearly disruptive technologies that, on a national scale, will be costly and require planning for the next 30 years. Nevertheless, with a ten-fold increase in internet traffic every 4 years, a capacity crunch is rapidly approaching: the network presently used will reach its fundamental (non-linear Shannon) limit by 2020-2025. A careful choice of next-generation technology will be necessary, and one that will have to be made in the next few years. This project will form just one part of this decision-making process, but a crucial one. Sources and switches are an important component in such a network, and thus exploration of the VCSEL source and switch solution provides one of the key components to test the viability of microstructured fibres.

The potential of our approach is that it not only opens routes to enhanced capacity, but also offers new ways of switching and routing using optics, thus enhancing network flexibility. This latter aspect of all optical processing could also lead to new applications in information security and processing. Thus the technology being explored in this project is at an early stage, but nevertheless it has the potential to impact the information and communications industries, from manufacturers of devices and subsystems right through to operators and users.

In addition, while the UK is currently investing heavily in future quantum information technologies, we believe that the only realistic solution for medium to long range quantum cryptography will be to use fibres and telecom wavelength photons. Moreover, the only commercially viable solution will be to either use an existing fibre network, or one in parallel and very similar to the existing one. If this technology is to be widely available in 20-30yrs time, planning of next-generation classical and quantum-ready networks will need to occur in parallel. This exploratory project does not address all the issues to be faced, but does take the first step of bringing two fields and technologies into a common platform. By promoting dialogue between fields and constructing a common technological language we also hope to promote a cultural change.

Policy
Closely linked to the industrial applications is the impact this project will have on government policy. Leaders will need to plan for future telecommunications infrastructure in order to support industrial development. We will feed into nationwide projects that have the strongest influence.

Inward Investment
By patenting our most novel and successful devices we will ensure that any innovation is retained within the UK. The UK is well-placed to become a future pioneer of the most advanced telecoms technology, for example by allowing the development of spatially encoded fibres and quantum communications networks.

People training
In this project we will train 4 individuals, and additional PhD students in practical solutions to a subject which spans industrial applications to fundamental physics. This is key for the future development of novel technologies. Such individuals may later go on to work in companies, bringing ideas from the latest in fundamental research, or may stay in academia where they will have a broad oversight into the issues and challenges when developing ideas from the lab bench to an industrial product. They may even go on into their own start-up companies ready to develop their new innovations themselves.