| Optical quantum information and my contributions.
In the early 20h century, the development of quantum mechanics coerced physicists into radically changing the concepts they used to describe the world. The new theory, though being incredibly successful, discomfited even its own founders and caused debates lasting until today. Albert Einstein, who was one of the first to apply and generalize Planck's quantum hypothesis, never liked the consequences following from quantum theory.
In the year 1935, together with Boris Podolsky and Nathan Rosen, he explicitly expressed his concerns [A. E., B. P., N. R., Physical Review 47, 777 (1935)] about a particular quantum phenomenon which was, in the same year, named "entanglement" by Erwin Schrödinger [E. S., Naturwissenschaften 23, 807 (1935)]. Entanglement, which challenges the intuition of physicists even, nowadays constitutes a quantum effect that can regularly occur if two or more bodies enter a situation in which they influence each other and separate again. Allegedly individual particles, once they are entangled, behave as a whole entity and have to be treated as such, even if they are spatially separated.
For a long time, entanglement was considered as nothing but a weird effect, useless apart from being a topic for philosophical discussions. It was not until the end of the 20th century that entanglement turned out to be not only the "great difference" between classical and quantum physics, but the essential ingredient of a new field of research -- quantum information [C. H. Bennett & D. P. DiVincenzo, Nature 404, 247 (2000)]. This branch of physics is a quantum theory of information that applies quantum effects, and in particular entanglement, to enhance conventional information processing. On a more fundamental level it is even a completion of classical information theory, as such including two complementary kinds of information: classical and quantum information. While the commonly known "bit" is a measure of classical information, the newly introduced "qubit" corresponds to the quantum mechanical counterpart [B. Schumacher, Phys. Rev. A 51, 2738 (1995)].
The physical implementation of single qubits as two-level quantum systems, as well as the quantitative study of their entanglement and their interaction with classical information, are essential prerequisites to harness the power of quantum mechanics for applications in quantum information science. Modern physical realizations of qubits cover a full range of different physical systems, such as individual trapped atoms, semiconductor quantum dots and single photons. Each of these has its own advantages within the two main branches of quantum information science: quantum computation and quantum communication. The former deals with the processing of quantum information, for which it was shown that it solves certain computational tasks more efficiently than classical computers. The latter studies quantum protocols for information transfer for which it opens up new possibilities, such as for example the unconditionally secure key exchange in cryptographic applications [N. Gisin, G. G. Ribordy, W. Tittel, H. Zbinden, Rev. Mod. Phys. 74, 145 (2002)]. Both fields are linked and attended by the study of the entanglement of qubits. The investigation of entanglement in the context of information theory leads thereby in turn to a deeper understanding of quantum theory itself.
My scientific work concentrated on different aspects of photonic quantum information. In the photonic domain, qubits are often encoded in the polarization states of single photons propagating in well-defined spatial modes. Single photons offer the advantage of being the fastest carriers of information while coupling extremely low to the environment and thus suffering from only very low decoherence.
Today's experiments on photonic entanglement consist of three main building blocks: a photon source, an interferometric linear optics network by which the photons are processed, and a so-called conditional detection of the photons at the output of the network.
As far as it concerns the source, over the last years the process of spontaneous parametric down conversion (SPDC) has become the standard tool to obtain polarization entangled photon states of two, four (very recently even of six) polarization qubits. In this process a non-linear crystal is pumped by strong laser radiation. Due to the non-linearity of the medium, photons of the pump beam can undergo a conversion into two, four, (or six etc.) light quanta of lower frequency. Conservation laws, imposing constraints on the conversion process, lead to the entanglement between different degrees of freedom of the photons in the down conversion emission. The entangled states emitted by the source can be directly used for the implementation of diverse quantum information tasks, or alternatively, can be subjected to further processing in optical networks. The latter are interferometric setups consisting of linear optical elements such as beam splitters, phase-shifters, etc. These networks are particularly relevant with regard to more than two photons, as in this case they allow the observation and study of various forms of multi-qubit entanglement. Moreover, they can be used to implement probabilistic logic gates which are required for the processing of quantum information. Experiments on photonic quantum information rely, furthermore, on the technique of so-called "conditional detection". This involves the detection of the down conversion photons by several Si-avalanche photo diodes (APDs) behind the optical network and the selection of events for which the photons leave the network in particular output modes. To this end the signals of the APDs are fed into an electronic multi-channel coincidence unit, which is able to register all possible instantaneous photon detections between the different channels.
The work during my Diploma focused on the development of a compact, robust and stable SPDC source of polarization entangled photon pairs. Former SPDC sources used large-frame ion ultra-violet (UV) pump lasers to obtain SPDC emission in the near-infrared (IR). Due to their high operating costs these lasers made the source devices not suitable for practical real-life applications far away from a laboratory environment. The aim of my studies was to replace the cumbersome and expensive lasers by, meanwhile commercially available, blue laser diodes. My colleagues and me developed one of the first UV laser diode based prototypes of an entangled photon source. Subsequent research in this direction is now about to pave the way towards every-day life applications of quantum communication [P. Trojek, H. Weinfurter, Appl. Phys. Lett. 92, 211103 (2008)]. We demonstrated the functionality of our source in several proof-of-principle experiments.
During my doctorate, I studied the entanglement of more than two qubits and how the various forms thereof can be employed for different applications in quantum information. In contrast to two photons, the generation of four photons requires on the one hand high pump field intensities, and on the other hand, for technical reasons, a kind of timing device. This leads to the usage of femto-second pulsed pump laser systems for the operation of the SPDC. My studies comprised the implementation of different interferometric setups, which, fed by the emission of the pulsed SPDC, lead to the observation of different four-photon polarization entangled states. These states were characterized and used for the demonstration of various quantum information protocols. For their characterization, in collaboration with my colleagues, I introduced new efficient tools that allow the investigation of the states with a minimal experimental measurement effort. The demonstration of some of the protocols first became possible by the newly introduced setups.
To put it short, the work I was doing forms a well-balanced mixture of theoretical scientific research on fundamental properties of quantum states and the experimental application of different techniques in the field of optics and photonics.