Quantum Information Group, Telecom Wavelength Single Photon Source
Fibre optic cables form the backbones of our existing communications networks. As applications of quantum information develop, the need for distributed quantum systems is clear. Optical losses in fibre are small in the telecom wavelength bands around 1.31 µm and 1.55 µm (the wavelength of light is related to its colour however these wavelengths are in the infra-red region of the spectrum and so not seen by the human eye).
One of the key building blocks for quantum networks is the single photon source. This is a device capable of controlled generation of light on the finest scale possible: the emission of a single photon. We have chosen to use the emission from single quantum dots since it is possible to tune their emission wavelength and they can be easily embedded into robust electrical devices.
To obtain long wavelength emission we wanted to produce large quantum dots and maintain a low dot density. This is traditionally difficult since during the growth process high densities are normally reached before large dots begin to form. We have used a subtlety of the self-assembly process to achieve a low density of large dots while ensuring the other dots remain small. A spectral filter removes light from the small dots and leaves only the emission from a single large dot such as the one shown at the centre of this atomic force microscope image. Such a device will only emit one photon in response to an excitation pulse and can generate single photons on-demand.
Constructing a pillar microcavity around the quantum dot helps couple the photons to a single mode fibre. With such a structure we have performed correlation measurements ~1.3 µm to prove that multiphoton pulses from the source are strongly suppressed. We divide the light from the source onto two single photon detectors and look at the time delays between the detection of photons. Since a single photon is the smallest unit of light it must go either to one detector or the other: it cannot split and generate a signal in both detectors. A single photon source will therefore not generate counts in both detectors at the same time, as seen by a lack of coincidences at zero delay.
This is the first time that strong suppression of multiphoton emission has been observed from a quantum dot at telecom wavelengths. The source is now being optimised and is expected to find its first application in our prototype quantum cryptography system.
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