Nondestructive detection of photonic qubits

One of the biggest challenges in experimental quantum information is to keep the fragile superposition state of a qubit alive. Long lifetimes can be achieved for material qubit carriers as memories, at least in principle, but not for propagating photons that are rapidly lost by absorption, diffraction or scattering. The loss problem can be mitigated with a nondestructive photonic qubit detector that heralds the photon without destroying the encoded qubit. Such detector is envisioned to facilitate protocols where distributed tasks depend on the successful dissemination of photonic qubits, to improve loss-sensitive qubit measurements, and to enable certain quantum key distribution attacks. Here we demonstrate such a detector based on a single atom in two crossed fibre-based optical resonators, one for qubit-insensitive atom-photon coupling, the other for atomic-state detection. We achieve a nondestructive detection efficiency upon qubit survival of $(79\pm3)\,\%$, a photon survival probability of $(31\pm1)\,\%$, and preserve the qubit information with a fidelity of $(96.2\pm0.3)\,\%$. To illustrate the potential of our detector we show that it can provide, already with current parameters, an advantage for long-distance entanglement and quantum-state distribution, resource optimization via qubit amplification, and detection-loophole-free Bell tests.Comment: 27 pages, main text and methods, 4 main figures, 3 extended data figures, 1 extended data table, for supplementary information see journal referenc

A quantum network node with crossed optical fibre cavities

Quantum networks provide unique possibilities for resolving open questions on entanglement and promise innovative applications ranging from secure communication to scalable computation. While two quantum nodes coupled by a single channel are adequate for basic quantum communication tasks between two parties, fully functional large-scale quantum networks require a web-like architecture with multiply connected nodes. Efficient interfaces between network nodes and channels can be implemented with optical cavities. Using two optical fibre cavities coupled to one atom, we here realise a quantum network node that connects to two quantum channels. It functions as a passive, heralded and high-fidelity quantum memory that requires neither amplitude- and phase-critical control fields nor error-prone feedback loops. Our node is robust, fits naturally into larger fibre-based networks, can be scaled to more cavities, and thus provides clear perspectives for a quantum internet including qubit controlled quantum switches, routers, and repeaters.Comment: 12 pages, 8 figures (including Supplementary Information

Phenotypic and molecular characterization of colorectal cancer-derived circulating and disseminated tumor cells

Tumors shed large numbers of cells into the vasculature. It is supposed that these cells give rise to metastases that are the major cause of death from cancer. However, the isolation and characterization of the so called circulating tumor cells (CTCs) is still challenging since cells are rarely to find among millions of normal leukocytes. The objective of this thesis was to characterize colorectal cancer (CRC) - associated CTCs and disseminated tumor cells (DTCs) from bone marrow samples. We studied CTC incidence in blood samples of CRC patients with the help of the FDA-cleared CellSearchTM system. We found that the number of patients with CTCs and the amount of CTCs was significantly correlated with the stage of disease. In addition, a significant higher rate of patients with CTCs in tumor-draining venous blood compared to the central venous blood was found. Furthermore, genomic analyses of single CTCs have been performed. This comprised the evaluation of typical CRC-associated mutations such as point mutations in TP53, BRAF, KRAS as well as the detection of microsatellite instability (MSI). Additionally, some CTCs were used to study global chromosomal aberrations by comparative genomic hybridization technique. Results revealed a remarkable genomic heterogeneity among the CTCs of single patients. Moreover, we detected several cases of genomic disparity among CTCs and the prevailing clone of matched cancer tissue. To evaluate differential gene expression that might enable tumor cell dissemination, we studied CTCs that were obtained from patients’ blood samples and from an othotopic mouse model of metastatic CRC. Gene expression profiles of CTCs and cell samples from cancer tissue were compared. The down-regulation of cell adhesion molecules such as E-cadherin might be involved in tumor cell dissemination. However, a significant epithelial-mesenchymal transition (EMT) in CTCs could not be confirmed. DTCs isolated from human bone marrow samples seem to adopt a dedifferentiated phenotype and lack expression of CK20 and CK19. Absence of EGFR and the proliferation marker Ki67 confirmed previous reports of dormancy in DTCs. With the present work we were able to demonstrate that the molecular characterization of single CTCs is feasible. However, further studies are required to increase the knowledge about the molecular traits of CTCs which might help to improve therapy and prognosis in CRC

Untersuchung zur elektromagnetischen Strömungskontrolle mittels externer Magnetfelder bei der VGF-Kristallzüchtung

Die vorliegende Arbeit beinhaltet die Kontrolle und Beeinflussung der Schmelzströmung beim Vertical Gradient Freeze- (VGF-) Verfahren durch magnetische Wechselfelder, wobei speziell der Einfluss von rotierenden und wandernden Magnetfeldern auf den Stoff- und Wärmetransport untersucht wird. Das Potential der VGF-Magnetfeldzüchtung für die Reduktion der thermischen Spannungen bei der Züchtung von Ge- und GaAs-Kristallen wird demonstriert. Anhand von Experimenten zur Gasphasendotierung von Ge-Einkristallen mit Zink werden außerdem die Vorteile des Magnetfeldeinsatzes für eine effektive Durchmischung der Schmelze und für eine Reduzierung der radialen Dotierstoffsegregation gezeigt. Ein weiterer Schwerpunkt der Arbeit ist die Entwicklung von züchtungsrelevanten Modellanordnungen für systematische Strömungsexperimente mit niedrig schmelzenden Legierungen. Dadurch kann die gegenseitige Beeinflussung zwischen der magnetfeldinduzierten Strömung und der thermische Auftriebströmung beim VGF-Verfahren direkt untersucht werden