Project abstract
Single photon emission, detection and manipulation will be explored on the basis of a novel semiconductor technology platform. By using semiconductor nanostructures like quantum dots and high-Q microcavity pillars or high-Q photonic crystal cavities, the interaction of individual photons with quantised electronic systems can be strongly enhanced to allow a full control of light generation, detection and manipulation at the single photon level. Within the framework of the project major building blocks of future single photon systems will be developed and explored. The consortium combines leading European research laboratories from universities, public research institutes and the industry with complementary expertise in nanostructure technology, optoelectronic devices and quantum physics. The project is expected to have a large impact on different areas of optical data communication (e.g. quantum cryptography) and on-chip data processing (like highly integrated single photon circuits). Several key devices, such as single photon emitters, single photon detectors as well as optical buffers, will be demonstrated.

    Project objectives
The ultimate miniaturisation level in optoelectronics will be achieved by approaching the quantum level of light and current, i.e., ultimately one deals with information transfer and processing based on single photons and single electrons. However, the strong reduction of the number of photons and electrons involved in such miniaturised systems results in more than a linear reduction of the intensity or current. Due to the quantum nature of photons and electrons the classical physical properties of light, matter and their interaction change drastically, e.g., in a coherent regime, energy can be loss-less transferred from electrical to optical power and vice versa. Very recently, two groups succeeded for the first time to demonstrate this fundamental coherent interaction of a single photon and a single exciton (i.e., a single isolated electron-hole pair state) in a solid-state system by using semiconductor quantum dots embedded in a high-Q microcavity [1, 2]. These experiments are very important as they open the possibility to realise optoelectronic devices based on cavity quantum electrodynamics using a scalable semiconductor technology platform with all the advantages of semiconductor manufacturability. Otherwise, one would be limited to hybrid systems with no or very limited scalability, like cold atoms in magnetic traps, which are widely used for basic experiments e.g. in quantum computing.
Some of these quantum effects (e.g., discretisation of radiation and charge, entanglement of single photon and electron states, creation of macroscopic quantum states by coherent coupling between photon and electron modes, long distance quantum teleportation via single photons) are very interesting for future applications in light generation, optical communication and data processing. This includes for example single photon sources for long distance and high-speed quantum cryptography or ultra-low power, high density photonic circuits based on single photon functionalities and optical high-speed buffering.
According to recent experimental breakthroughs [1, 2], two complementary roads will be followed to explore the interaction of single particles, i.e., photons and electrons, in semiconductor nanostructures and to realize semiconductor based single photon devices. Both are based on the same fabrication technologies and use self-assembled quantum dots embedded in a microcavity. However, each of them has specific advantages for different applications.

1. High-Q microcavity pillars with emission perpendicular to the substrate plane (Advantages: e.g., vertical access for excitation and photon emission, high direct fibre coupling efficiency, robust technology for single devices)
   
2. High-Q microcavities realised in photonic-crystal membranes with in-plane emission (Advantages: e.g., potential for large scale integration and ultimate miniaturization, more favourable for the realisation of optical buffers by EIT due to the possibility to use waveguides with a very low group velocity)
   

The goal of the proposed project is to establish a semiconductor technology platform, which enables reliable single photon experiments, to realise single photon devices and to test their potential for system applications. In particular, three major application areas will be addressed within the frame of the project:

1. Quantum key distribution (QKD)
2. Quantum metrology (QM)
3. Optical data processing (ODP)

Key devices for these applications in the framework of the project are:

1. Single photon source (SPS)
2. Single photon detector (SPD)
3. Optical buffers based on electromagnetic induced transparency (EIT)

The devices will be developed for operation in the wavelength range of 1 - 1.3 µm. While approach (A) will be used to realise single devices for QKD and QM, approach (B) will be explored for the realisation of integrated multi-functional devices on the single photon level for ODP applications.
       

[1]    J.P. Reithmaier et al., “Strong coupling in a single quantum dot-semiconductor microcavity system”, Nature 432, 197 (2004).
[2]    T. Yoshie et al., “Vacuum Rabi splitting with a single quantum dot in a photonic crystal nanocavity”, Nature 432, 200 (2004)..