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Quantum Light Sources

A single photon, the “elementary particle” of light, is considered as one of the most suitable candidates for the transmission of information in future information networks that take advantage the laws of quantum physics. A wide range of applications can be possible, from long-distance Quantum Key Distribution through optical fibre networks to logic operations at the nanoscale on a semiconductor chip.

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Our mission is to develop novel structures for the integration of quantum light sources in future quantum photonic integrated circuits. We demonstrated the world’s first device able to generate and transmit a stream of single photons along a semiconductor chip. The generation of single photons on a chip can be achieved by the use of semiconductor quantum dots, objects with dimensions of a few nanometres. We can easily excite one electron to a high energy level inside the quantum dot, which emits one single photon every time it relaxes to the lower energy level. Since there is no preference to the direction they are emitted, photons tend to escape from the chip via random pathways. In order to collect them efficiently and route them along the chip, we have developed light-guiding structures making use of state-of-the-art nano-fabrication technology.

Electromagnetic waves can be confined and transmitted by waveguides. On a semiconductor chip, we can achieve light confinement in regions surrounded by a “photonic crystal”, an array of holes that create a “forbidden” region for light at certain frequencies to enter. A thin material strip sandwiched between two photonic crystal regions can act as a waveguide structure. By placing a quantum dot in the waveguide region, we can efficiently generate and transmit single photons along the chip. Such a device contains most of the essential tools – generation and transmission of single photons – to perform quantum logic operations on a semiconductor chip in a scalable way.

The interference of two photons on a beam splitter lies in the heart of most protocols for quantum logic operations using linear optics. For successful operation, the interfering photons need to be “indistinguishable” in the frequency, polarisation and bandwidth degrees of freedom. Photons generated by off-resonant excitation schemes in quantum dots suffer from poor coherence properties, which have a detrimental effect on photon indistinguishability. We demonstrated for the first time the emission of highly-indistinguishable photons along the chip by using optical excitation resonant with the quantum dot transition. Under such scheme, widely known as Resonance Fluorescence, perfect photon indistinguishabilities can be achieved that can lead to on-chip quantum logic operations with high fidelity.

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