Quantum Information Group, Entangled Photon Pair Source
Entangled photons pairs have interrelated properties, such that measuring one photon affects the other photon instantly, even if it is hundreds of kilometres away. This strange phenomenon of quantum physics was described by Einstein as 'Spooky action at a distance', and has a number of very exciting applications.
In quantum communication, entangled photons can be used to create a secret key, shared between two users. Measurement of a photon by the first user yields a random result, but entanglement changes the properties of the second photon, such that the second user's measurement is the same. This kind of system is compatible with quantum repeaters, so keys may be formed over longer distances than for single photon systems.
The inter-related properties of photon pairs increases the resolution in quantum imaging. This might be especially useful when imaging sensitive biological systems.
Quantum computing takes advantage of a quantum mechanical phenomenon known as superposition, where the input to the quantum computer can be all the possible alternatives simultaneously. This imparts a massive amount of parallel processing power for certain problems that would usually be solved by the sequential trial of possible solutions, for example database searching. A source of entangled photons is required to realise an optical quantum computer.
At Toshiba, we are developing a source for entangled photons based on a single quantum dot. A dot emits two photons per excitation cycle, which typically are not entangled due to structural asymmetry of self-assembled quantum dots. However, we have discovered that by careful control of the growth process, dots with optimum symmetry can be realised. Our measurements have shown that such dots emit light with the characteristic properties of entangled photon pairs. Furthermore, unlike most other entangled photon sources, dots emit no more than one photon pair per cycle, which is essential for some applications, such as realising a scalable optical quantum computer. We believe that with further development, our device could be implemented by a simple semiconductor structure, similar to a conventional LED (light-emitting diode), which could bring entanglement, and it's applications, to mainstream technology.
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