Pulsed Quantum Light
Ultrafast light pulses are currently subject to a flourishing field of research activities. They offer, for instance, the fascinating opportunity to study dynamic processes at very short timescales and promise the future multiplication of bit rates in optical communication systems. Over the last years, a highly sophisticated toolbox has been developed to realise ever shorter pulses, with specifically engineered temporal and spectral properties. On the other hand, making use of the quantum features of light has paved the way towards novel quantum communication and information applications.
In our group, we are interested in combining these two highly exciting fields of research and push forward into the upcoming field of ultrafast quantum optics. Although the theoretical foundations for this have already been laid by Roy Glauber in 1963 , until now the highly complex structure of ultrafast quantum pulses is still not fully understood.
Our goal is to achieve full control over this structure, theoretically as well as experimentally, to be able to generate any number of ultrafast quantum pulses with any temporal profile. With this competence, we will then be working towards new possibilities of information multiplexing. Using the generated quantum pulses as information carriers, novel quantum communication and information schemes come into reach, which could outrun existing schemes in terms of qubit rates and overall performance.
Theoretical description of pulse mode generation
A thorough theoretical description of ultrafast light pulses forms the key for all experiments and further applications of quantum light. For this purpose our research is concerned with developing theoretical models of the waveguided parametric down-conversion process, pumped by ultrafast light pules, which emit quantum light in multiple ultrafast pulse modes. Equipped with this theoretical framework we are not only able to precisely predict the properties of the generated states, but it also enables us to engineer ultrafast states for quantum information applications [2, 3] . It opens up new ways to create hyperentangled quantum states  and leads to the development of novel quantum light sources [2, 5, 6].
We are in a continuous process of refining our models and are adapting them to our ever advancing experimental investigations. Furthermore our theoretical framework leads to new insights into quantum physics in the ultrafast regime and fosters new development for practical quantum information processing.
Enhanced quantum communication by multimode coding
Our PDC sources creating ultrafast quantum states in multiple light pulses enable us to develop frequency multiplexed quantum communication protocols. In a collaboration with international partners we expanded the current continuous-variable quantum communication protocol based on EPR-states to include information coding on multiple ultrafast pulses. This protocol features a much more energy efficient information transmission than the current single-mode encodings and hence enables significantly higher quantum communication rates. Furthermore our encoding on multiple quantum light pulses also features an enhanced loss resilience in comparison to the standard protocol. 
Tailored Photon Pair Sources
One major aspect of our experimental work is the controlled generation of ultrafast quantum pulses. For this, we employ the process of ultrafast, waveguided parametric down-conversion (PDC). It has been shown earlier that PDC creates a vast multitude of orthogonal quantum pulse pairs . By careful engineering of the dispersion properties of the PDC crystal, it has been possible to reduce this number to one single pair of pulses, which made it possible to prepare pure, pulsed single-photon states .
In our group, we add two additional degrees of freedom to the PDC: Firstly, we employ nonlinear waveguides to further control the dispersion which governs the PDC process . Secondly, we make use of well-established pulse shaping techniques from classical ultrafast optics, to shape our ultrafast pump pulses. This will enable us to generate novel ultrafast quantum states, which feature a well-defined number of pulses in specific temporal profiles.
Current projects in this field include the setting up of PDC sources which generate those complex quantum states and, at the same time, are integrated with functional optical elements onto single samples. In addition, means of verifying the desired behaviour of the sources have to be developed and applied.
Quantum Pulse Gate and Quantum Pulse Shaper
The second important step towards full control over the complex structure of quantum pulses is the ability, to single out specific pulses from an ensemble while leaving the remaining structure untouched. This is a highly non-trivial task and existing filters cannot provide this behaviour .
We have transferred our knowledge on process engineering for PDC to ultrafast sum-frequency generation. Out came a device which actually features the desired behaviour, the Quantum Pulse Gate (QPG) [11, 12]. The idea behind the QPG is that the pump pulse and the quantum pulse travel at the same velocity through the nonlinear waveguide, which is a rare situation. Here, having at our disposal a formidable technology team, which provides us with special samples proves invaluable for the progress of the research.
Authors: Andreas Christ and Benjamin Brecht
- R. J. Glauber. Coherent and incoherent states of the radiation field. Phys. Rev., 131:2766-2788, 1963.
- Andreas Eckstein, Andreas Christ, Peter J. Mosley, and Christine Silberhorn. Highly efficient Single-Pass source of pulsed Single-Mode twin beams of light. Physical Review Letters, 106(1):013603, January 2011.
- Andreas Christ and Christine Silberhorn. Limits on the deterministic creation of pure single-photon states using parametric down-conversion. arXiv:1111.4095, November 2011.
- Peter J. Mosley, Andreas Christ, Andreas Eckstein, and Christine Silberhorn. Direct measurement of the Spatial-Spectral structure of waveguided parametric Down-Conversion. Physical Review Letters, 103(23):233901, December 2009.
- Andreas Christ, Kaisa Laiho, Andreas Eckstein, Thomas Lauckner, Peter J. Mosley, and Christine Silberhorn. Spatial modes in waveguided parametric down-conversion. Physical Review A, 80(3):033829, 2009.
- A. Christ, A. Eckstein, P. J. Mosley, and C. Silberhorn. Pure single photon generation by type-IPDC with backward-wave amplification. Optics Express, 17(5):3441-3446, March 2009.
- A. Christ, C. Lupo, and C. Silberhorn. Exponentially enhanced quantum communication rate by multiplexing continuous-variable teleportation. New J. Phys. 14 083007 (2012)
- C. K. Law, I. A. Walmsley, and J. H. Eberly. Continuous frequency entanglement: Effective finite hilbert space and entropy control. Phys. Rev. Lett., 84(23):5304-5307, 2000.
- Peter J. Mosley, Jeff S. Lundeen, Brian J. Smith, Piotr Wasylczyk, Alfred B. U'Ren, Christine Silberhorn, and Ian A. Walmsley. Heralded generation of ultrafast single photons in pure quantum states. Phys. Rev. Lett., 100(13):133601, 2008.
- A. M. Branczyk, T. C. Ralph, W. Helwig, and C. Silberhorn. Optimised generation of heralded fock states using parametric down conversion. New J. Phys., 12:063001, 2010.
- A. Eckstein, B. Brecht, and C. Silberhorn. A quantum pulse gate based on spectrally engineered sum frequency generation. Opt. Exp., 19(15):13770-13778, 2011.
- B. Brecht, A. Eckstein, A. Christ, H. Suche, and C. Silberhorn. From quantum pulse gate to quantum pulse shaper - engineered frequency conversion in nonlinear optical waveguides. New J. Phys., 13:065029, 2011.