Completed Projects

In this project, we developed deterministic and bright sources of polarization-entangled photon pairs by combining the advantages of In(Ga)As quantum dots (QDs) grown on (111) oriented GaAs substrates, in-situ cathodoluminescence lithography (CLL) of monolithically integrated microlenses, and resonant excitation schemes. These quantum light sources are key elements of advanced quantum communication schemes, such as the quantum repeater, which are based on entanglement distribution. In our concept, single (111) QDs in the active region of the sources enable the generation of polarization-entangled photon-pairs via the biexciton (XX) - exciton (X) radiative cascade because of the specific electronic states of these QDs with diminishing fine-structure splitting.

Partners: Prof. Dr. V. Haisler, ISP Novosibirsk

Funded by: German Research Foundation, Grant-No. RE 2974/12-1 and Russian Foundation for Basic Research

This project focused on the research and development of highly energy efficient and highly temperature stable vertical-cavity surface-emitting lasers (VCSELs) and nanometer-scale VCSELs for near-term and future applications in data communications, illumination, and sensing. Furthermore, we aimed to develop VCSELs with the highest possible modulation bandwidth and VCSELs that are suitable for integration onto silicon materials and circuits to enable future silicon photonics systems. We also developed nanolasers with the minimum possible volume and thus the highest possible bandwidth density.

Homepage of CRC 787

Principal investigators: Prof. James Lott, TU Berlin and Prof. Stephan Reitzenstein, TU Berlin

Funded by: German Research Foundation within the Collaborative Research Center 787 "Semiconductor Nanophotonics: Materials, Models, Devices"

In this project, we controlled photonic nanostructures by external feedback, optical injection and synchronization. This allowed us to study nonlinear dynamics in quantum systems and to externally manipulate and stabilize light-matter interaction in the regime of quantum electrodynamics (cQED). Our work would have important impact at an interdisciplinary level on the development of nonlinear dynamical systems towards the quantum limit and the understanding of fundamental light-matter interaction in the presence of time delayed single-photon feedback. Moreover, it opened up new perspectives for realizing secure data communication, ultra-fast on-chip random number generation, optical interconnects in next generation computer technology and for boosting the emerging field of QIT.

Funded by: European Research Council, ERC Consolidator Grant-No.: 615613

The project was part of the 2nd 'Poland - Berlin' call "Photonic Components and Systems for Production and Measurement in the fields of Communication, Medicine, Lighting and Security".

The project's ultimate goal was to develop a practical and efficient single-photon source with an optically excited semiconductor quantum dot as an emitter, which provides single photons on demand and is suitable for local-area secure data transfer (e.g., quantum key distribution - QKD) in the 2^nd telecommunication window (1.3 µm range).

Partners: JCMwave GmbH, Berlin, Germany,  PicoQuant GmbH, Berlin, Germany, Wroclaw University of Technology, Wroclaw, Poland, Marie Curie-Skłodowska University, Lublin, Poland, P.H. ELMAT Sp. z. o.o., Rzeszow, Poland

Funded by: European Regional Development Fund (EFRE) of the European Union in the framework of the programme to promote research, innovation and technologies (Pro FIT).

In this project we developed integrated sources of single and entangled photon pairs for applications in the field of quantum communication. Within an on-chip approach, electrically pumped whispering gallery mode microlasers resonantly excite single quantum dots embedded in adjacent micropillar cavities and Bragg-reflection waveguides. By this we achieved a new level of integration without the need of external light sources. Besides technological challenges, there exist a number of exciting physical questions such as an in-depth understanding of the non-linear processes involved in the generation of entangled photon pairs, which was be tackled by our project. Our work has set the ground for a quantum optics platform that could revolutionize the way we conduct quantum optics experiments and may in the long run become a new quantum technology.

Partners: Prof. G. Weihs, Universität Innsbruck, Dr. C. Schneider, Universität Würzburg

Funded by: German Research Foundation, Grant-No.: Re2974/9-1

The main objective of this project was to implement Reservoir Computing (RC, a neuro-inspired information processing scheme) in an optical network of nano-structures. Its realization requires spectrally tailored quantum dot micropillar arrays (QDMPA) and diffractive coupling to establish all-optical networks including hundreds of such emitters. Our underlying interdisciplinary approach combines three recent concepts by bridging nanostructures to a macroscopic complex system which is utilized for powerful computation. Namely, these concepts are RC as the functional concept, QDMPAs as the hardware platform, and diffractive coupling schemes for scalable optical networks to implement the complex neuro-inspired systems, capable of ultra-high speed information processing. It represents a unique opportunity to integrate these three concepts into a fully functional computing system with great potential in terms of performance, speed, compactness, energy-efficiency and future extensions to quantum machine learning.

For more information: http://neuroqnet.com/

Partner: Dr. Daniel Brunner, Department of Optics, FEMTO-ST, Besançon, France

Funded by: Volkswagen Stiftung

The goal of this project was the realization and investigation of single-emitter lasers and coupled laser arrays on the basis of semiconductor quantum dots. The regime of single-emitter lasing fundamentally differs from that of conventional lasers and offers novel and intriguing physical concepts and applications. Such lasers are driven by quantum- and correlation effects that are, currently, not completely understood. In addition to their theoretical description and conceptual understanding, the technological realization and experimental characterization provide demanding challenges. By combining microscopical modelling, quantum-optical spectroscopy and nanoscale sample preparation, the project aimed at providing a conclusive understanding of single-emitter lasers.

Partners: Prof. M. Kamp, Universität Würzburg, Dr. C. Gies, Universität Bremen

Funded by: German Research Foundation, Grant-No.: Re2974/10-1

References: M. Lermer, N. Gregersen, M. Lorke, E. Schild, P. Gold, J. Mork, C. Schneider, A. Forchel, S. Reitzenstein, S. Höfling, and M. Kamp. Appl. Phys. Lett. 102, 052114 (2013); C. Gies, M. Florian, P. Gartner, and F. Jahnke. Modelling single quantum dots in microcavities. In F. Jahnke, editor, Quantum Optics With Semiconductor Nanostructures. Woodhead Publishing Limited (2012); C. Gies, J. Wiersig, M. Lorke, and F. Jahnke. Phys. Rev. A 75, 013803 (2007).

Within this project we developed and study advanced GaN-based quantum devices. We demonstrated novel concepts to manipulate the emission process of GaN quantum dots and to exploit non-linear emission processes for the generation of quantum light. This included single photons with externally adjustable emission energy and entangled photon pairs. For this purpose we developed nanocavity systems in the GaN material system which allowed us to control and enhance the spontaneous emission of single quantum dots by cavity quantum-electrodynamics effects. As a central goal of the project we exploited the Purcell effect in photonic crystal cavities to enhance the probability of two-photon emission processes which is negligible for bare quantum dots but can be boosted in the presence of an optical resonance of a nanocavity structure. Cavity enhanced two-photon processes was also be exploited for frequency up- and down-conversion of light at the single-photon level. Moreover, we did for the first time embed single quantum dots into piezo-controlled GaN nanocavities to tune their excitonic emission energies via external strain fields.

Our approach allowed us to realize compact quantum systems for the triggered emission of single photons and entangled photon pairs with adjustable energy via two photon emission processes. Here, the GaN material system is of particular interest because of large exciton binding energies and large band offsets which enable operation at elevated temperatures up to 300 K. Most importantly, GaN QDs exhibit intrinsically a parity breaking which greatly enhances two-photon processes. The results of our project would have impact on the emerging quantum information technology, which relies crucially on the availability of advanced quantum light sources.

Partners: Prof. N. Grandjean, Ecole polytechnique fédérale de Lausanne (EPFL), Prof. A. Hoffmann, Institute of Solid State Physics, TU Berlin, Dr. A. Schliwa, Institute of Solid State Physics, TU Berlin

Funded by: German Research Foundation, Grant-No.: Re2974/8-1

References: G. Callsen, A. Carmele, G. Hönig, C. Kindel, J. Brunnmeier, M. R. Wagner, E. Stock, J. S. Reparaz, A. Schliwa, S. Reitzenstein, A. Knorr, A. Hoffmann, S. Kako, and Y. Arakawa, Phys. Rev. B 87, 245314 (2013). N. Vico Triviño, U. Dharanipathy, J.-F. Carlin, Z. Diao, R. Houdre, and N. Grandjean, Appl. Phys. Lett. 102, 081120 (2013).

The project was dedicated to the fabrication and investigation of InGaAs quantum dots on GaAs (111) substrates. Theoretical investigations have shown that such quantum dots are promising candidates for the generation of entangled photon pairs for, e.g., quantum communication schemes. The project advanced the nontrivial growth of such quantum dots.

Partner: Prof. Dr. V. Haisler, Institute of Semiconductor Physics in Novosibirsk, Russia

Funded by: Federal Ministry of Education and Research, Grant-No.: 01DJ12097

References: A. Schliwa et al., “(111)‐Grown In(Ga)As/GaAs Quantum dots as ideal source of entangled photon pairs”, Phys. Rev. B 80, 161307(2009). E. Stock et al., „Single‐photon emission from InGaAs quantum dots grown on (111) GaAs”, Appl. Phys. Lett.96, 93112 (2010).

Finished: 30.6.2015

Microdisk lasers with a lateral emission-profile can be fabricated without cleaving the wafer in contrast to, e.g, edge-emitting lasers. Hence, they are promising candidates for on-chip quantum optics.

This projects aimed at the realization of quantum-dot microdisk lasers for electrical operation. The lasers had large Q-values and small mode volumes. Using a Limacon-shaped cross-section they were optimized to allow for lateral directional emission into a small angular range.

Partners: Prof. Dr. J. Wiersig, Institute of Theoretical Physics, Otto-von-Guericke University in Magdeburg, Germany, Prof. Dr. M. Kamp, Technische Physik, University of Würzburg, Germany

Funded by: German Research Foundation, Grant-No.: Re2974/2-1

Finished: 08.2014

Within this project we developed high-quality single-photon light-emitting devices with InGaAs quantum dots in the active layer. The electrically contacted devices allowed us to externally control the charge configuration in the quantum dots which were exploited in order to address and manipulate dark exciton states in the quantum dots. This allowed us to explore the potential of these states to act as qubits with long coherence times as building blocks for quantum information systems.

Partners: Prof. Dr. D. Gershoni, Technion in Haifa, Israel, Prof Dr. D. Bimberg, Institut für Festkörperphysik, Technische Universität Berlin, Germany

Funded by: German-Israeli-Foundation for Scientific Research and Development, Grant-No.: 1148-77.14/2011

References:M. Gschrey, M. Seifried, L. Krüger, R. Schmidt, J.-H. Schulze, T. Heindel, S. Burger, S. Rodt, F. Schmidt, A. Strittmatter, and S. Reitzenstein, "Enhanced photon-extraction efficiency from deterministic quantum-dot microlenses",  arXiv:1312.6298 (2013). T. Heindel, C. Schneider, M. Lermer, S. H. Kwon, T. Braun, S. Reitzenstein, S. Höfling, M. Kamp, and A. Forchel, "Electrically driven quantum dot-micropillar single photon source with 34% overall efficiency“, Appl. Phys. Lett. 96, 011107 (2010). E. Poem, Y. Kodriano, C. Tradonski, N. H. Lindner, B. D. Gerardot, P. M. Petroff and D. Geshoni, “Accessing the dark exciton with light.” Nature Phys. 6, 993 (2010). D. Bimberg, E. Stock, A. Lochmann, A. Schliwa, J. Töfflinger, W. Unrau, M. Munnix, S. Rodt, V. A. Haisler, A. I. Toropov, A. Bakarov, A. K. Kalagin, “Quantum Dots for Single- and Entangled-Photon Emitters”, IEEE Photonics J. 1, 58 (2009).

The project comprised experimental and theoretical investigations to gain insight into the non-linear and non-markovian dynamics of excitons and biexcitons in quantum dots. Systems with and without microresonators will be analyzed. Applying photocurrent spectroscopy and an advanced path-integral method we aimed at a comprehensive study of the coherent dynamics of excitonic complexes confined in single self-assembled quantum dots. The studies comprised also investigations on the coherent evolution of coupled quantum dot – microcavity systems via electrical readout.

Partners: Prof. Dr. V. M. Axt, Institute of Theoretical Physics III, University of Bayreuth, Germany Prof. Dr. M. Kamp, Technische Physik, University of Würzburg, Germany

Funded by: German Research Foundation, Grant-No.: Re2974/5-1

References: P. Gold, M. Gschrey, C. Schneider, S. Höfling, A. Forchel, M. Kamp, S. Reitzenstein, „Single quantum dot photocurrent spectroscopy in the cavity quantum electrodynamics regime“, Phys. Rev. B 86, 161301(R) (2012). A. Vagov, M.D. Croitoru, M. Glässl, V.M. Axt and T. Kuhn, “Real-time path integrals for quantum dots: Quantum dissipative dynamics with superohmic environment coupling”, Phys. Rev. B 83, 094303 (2011).

The aim of the project was to accelerate the development and commercial uptake of Quantum Key Distribution (QKD) technologies by developing traceable measurement techniques, apparatus, and protocols that will underpin the characterisation and validation of the performance and security of such systems.

Website: http://empir.npl.co.uk/miqc2

Partners: see: http://empir.npl.co.uk/miqc2/partners

Funded by: The research within this EURAMET joint research project received funding from the European Union's Horizon 2020 Research and Innovation Programme and the EMPIR Participating States.

The quantum-dot microcavity system is driven under resonant conditions in the strong-coupling regime. The strictly-resonant excitation under provided new results on non-linear coupling processes, properties of the Jaynes-Cummings ladder, and the evolution of the Mollow-Triplet.

Partners: Prof. Dr. M. Kamp, Technische Physik, University of Würzburg, Germany, Prof. Dr. P. Michler, Institut für Halbleiteroptik und Funktionelle Grenzflächen, Universität Stuttgart

Funded by: German Research Foundation, Grant-No.: Re2974/3-1

References: S. Ates, S. M. Ulrich, A. Ulhaq, S. Reitzenstein, A. Löffler, S. Höfling, A. Forchel, P. Michler, "Non-resonant dot–cavity coupling and its potential for resonant single-quantum-dot spectroscopy", Nature Photonics 3, 724 (2009). S. Ates, S. M. Ulrich, S. Reitzenstein, A. Löffler, A. Forchel, P. Michler, "Post-Selected Indistinguishable Photons from the Resonance Fluorescence of a Single Quantum Dot in a Microcavity", Phys. Rev. Lett. 103, 167402 (2009).

Within the project single-photon sources based on epitaxial quantum dots were realized as "plug and play"-ready devices. The fabrication process was analyzed and tested with respect to its technical and economic feasibility. Special emphasis was placed on the site-controlled growth of the quantum dots.

The development of "plug and play"-ready single-photon sources focusses on:

• Self-aligned growth and integration of single quantum dots as emitters
• VCSEL technology compatible fabrication processes
• High single photon output flux
• Pure single photon emission
• Narrow emission linewidth
• Spectrally tunable emission of single photons
• Electrical triggering of single photon emission
• User friendly fiber outcoupling
• User friendly operation using Helium/Nitrogen free cooling

Partner: PD Dr. A. Strittmatter, Institut für Festkörperphysik, Technische Universität Berlin, Germany

Funded by: Federal Ministry of Education and Research, Grant-No.: 03V0630

Patent: U.S. Patent: „Optoelektronische Vorrichtung“, Publication number US20160320575 A1, Application number US 14/699,605

References: W. Unrau, D. Quandt, J.-H. Schulze, T. Heindel, T. D. Germann, O. Hitzemann, A. Strittmatter, S. Reitzenstein, Appl. Phys. Lett. 101, 211119 (2012). T. Heindel, C. Schneider, M. Lermer, S. H. Kwon, T. Braun, S. Reitzenstein, S. Höfling, M. Kamp, and A. Forchel, „Electrically driven quantum dot-micropillar single photon source with 34% overall efficiency“, Appl. Phys. Lett. 96, 011107 (2010).

Press release:  http://www.tu-berlin.de/?id=138056