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

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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.