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Quantum mechanics and optics, two fields that seem quite different, in principle rather than in practice. Quantum mechanics is the study of the smallest building blocks that make up individual particles and matter. It is a field which has inspired numerous scientific breakthroughs in our understanding of how light, matter and energy interact with each other. On the other hand though quantum optics is a slightly smaller topic where we study the properties of light such as its propagation phases or interference effects by using wave-particle duality principle. What if we used the smallest building block of all, an atom, to design something that combines two quantum mechanics with optical properties? This is exactly what researchers at the University of St Andrews (Scotland) and the Queensland University of Technology (Australia) have done by creating a single atom which switches light on and off with single-atom precision. Quantum phases are defined as different states of matter which can be explained by their spatial correlations. For example, total internal reflection (TIR) is one example of a phase transition which requires that the spatial correlation between the intensity of an electromagnetic wave and its electric or magnetic field. In fact this is exactly what a phase switch is, which can be described as a physical system that selects between different spatial correlations. In the 1980s it was established by Hans Briegel and H.D. Kimble that it is possible to use single atoms to create a phase switch in a light wave. The design of such switches involves multiple beam splitters, atom-sized mirrors and the basic idea involves switching light from one path to another based upon the atom in question changing its state from solid to gaseous or vice versa. The difficulty however with using a single atom for this is that at room temperature "a gas atom might absorb or emit only dozens of photons before undergoing irreversible thermal decoherence. This represents far too low of a photon flux to be useful for quantum computation." Therefore, in order to overcome this issue researchers have used a special material known as an atom chip. An atom chip is a material with specially designed potential wells which can trap at least one atom of the desired type. The idea behind an atom chip is that the atoms are cooled down to near absolute zero temperature where it becomes superconducting thus creating an extremely small resistance which allows the sampling of single-atom quantum behavior on demand through electric control. By introducing doping molecules into the superconducting material, the research team is able to control this change of state and thereby create a phase switch by adjusting the potential well. The collaboration between researchers from St Andrews and QUT is led by Dr Tim Brown and involves work with Professor Tian-Ming Gao and Professor Hazem Al-Omari who both work in the Department of Physics and Astronomy at St Andrews. Quantum Optics Lab is a group led by Yong Qin (PhD student) at QUT's Quantum Optics Laboratory. "We are now able to make a single atom switch which can couple light of laser frequency with an oscillation for more than 100 ns" says Yong Qin. cfa1e77820
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