CMOS Compatible Single Photon Sources (CUSPIDOR)

The emerging quantum technologies will have a huge impact on society, disruptive changing the way information is processed and distributed. Quantum cryptography is a main concern for society, since by this technology any attack on the privacy of communication can be detected and, thus, misuse of private data impeded. For quantum cryptography, the efficient generation of single photon states is a vital task. Current approaches are bulky and expensive with low generation rates. To pave the way for widespread applications, the lessons learned from classical, Silicon- based information technology have to be recalled: devices need to be robust, highly integrated and cheap to be practically applicable. By far the densest and most robust integration of various functionalities is achievable with Si standard CMOS technology at extremely low costs. Thus, CUSPIDOR aims on developing a novel integrated quantum optical platform relying on a fully CMOS-compatible technology, which is able to provide sources of deterministic single photons. These photons will be generated at the telecommunications wavelengths, so that the existing elaborate telecommunication networks can be used for quantum communication. A newly developed type of silicon-germanium quantum dots (SiGe QDs) will be used as quantum emitter, showing telecom-wavelength emission up to and above room temperature. These QDs will be optimally and deterministically coupled with nano-photonic resonators for further enhancement of the single photon emission rate. This coupling will be achieved by site controlled QD growth in combination with precisely aligned, lithographically defined photonic crystal resonators, allowing upscaling and a straight forward implementation of areas of identical single photon sources. By implementing these sources in lateral p-i-n diodes, electrically triggered single photon emitter will be developed. In addition, the QDs will be used in CUSPIDOR to provide a strong optical nonlinearity for the realization of a single photon source via the implementation of an on-chip photon blockade. Such devices suppress photon transmission as long as a previous photon is in the device. Thus, a stream of single, temporally evenly separated photons leaves the device upon interaction with a laser beam. Quantum interference in coupled photonic crystal resonators increases the systems sensitivity providing a practical path to the first integrated photon blockade device- a holy grail of the Quantum photonics community, and provide opportunities for coherent quantum communication protocols not possible with a single quantum dot. The project will create a strong team of quantum photonics researchers proficient with material design and growth, advanced CMOS processes and nanophotonics design. A firm basis of design skills and fabrication expertise will be established that will provide a springboard for further innovation and the exploitation of quantum light sources. CUSPIDORS final target is a demonstrator for a compact, integrated, and flexible source of quantum states of light ready for prototyping.

The CUSPIDOR project was targeted towards the development of a new, CMOS compatible quantum optical platform suitable for the development of single photon sources emitting in the typical wave length range for optical data transmission. Such a platform could be directly linked to existing telecommunication networks, the utilization of which is regarded as prerequisite for building up quantum networks. For a future quantum optical platform, the required precision, scalability, costs and option for integrating control electronics can practically be established only under CMOS compatibility. Silicon-Germanium crystal sized only a few nano meter (SiGe quantum dots, QDs) were characterized with respect to their single photon emission properties. Such QDs develop under suitable growth conditions in a molecular beam epitaxy (MBE) reactor on predefined, periodically arranged positions. Within this project, these growth conditions were optimized with respect to large periodic distances between single QDs (100m x 100 m). Such large distances are important, since SiGe QDs are inherently inefficient photon emittes, that have to be integrated into photonic resonators of typically such dimensions for enhancing their emission efficiency. As layout of the photonic resonators we have chosen a structure for which ultra large Q-factors can be achieved by simple design rules, as has been shown in previous works (bichromatic layout). However, prior to CUSPIDOR, such resonators have never been fabricated and characterized with a single SiGe QD in their centers. With these bichromatic resonators and the optimized QD growth conditions for exactly one QD per resonator, world record Q-factors (Q ~ 100000) for resonators coupled to SiGe QDs could be achieved for the SOI platform. Because of these large Q-factors, the emission of a single SiGe QD could be clearly observed even at room temperature. The experimentally observed probability distribution of the photon emission time after excitation of the QD by a laser pulse indicates the emission of single photons for sufficiently low excitation energy. However, a strict proof for single photon emission could not be achieved within this project. Alternatively to SiGe QD based quantum optical sources, bichromatic resonators without QDs were optimized entangled photon pair sources. Via nonlinear response of Si to laser radiation, concentrated in the resonator, photon pairs of distinct wavelengths are generated. Compared to standard sources for photon pair generation, the resonator based sources are more efficient and have a smaller footprint. Optical chips combining input and output wave guides with bichromatic resonators were implemented. Only by this combination, the stability required for photon pair experiments could be achieved. The photon pairs generated in the resonators clearly show quantum optical behavior like entanglement, inconsistent with a classical description. Thus, these sources are suitable for integrated, quantum optical applications, like for example for quantum cryptography at telecommunication wavelengths.