Ramsey interference with single photons

Interferometry using discrete energy levels in nuclear, atomic or molecular systems is the foundation for a wide range of physical phenomena and enables powerful techniques such as nuclear magnetic resonance, electron spin resonance, Ramsey-based spectroscopy and laser/maser technology. It also plays a unique role in quantum information processing as qubits are realized as energy superposition states of single quantum systems. Here, we demonstrate quantum interference of different energy states of single quanta of light in full analogy to energy levels of atoms or nuclear spins and implement a Ramsey interferometer with single photons. We experimentally generate energy superposition states of a single photon and manipulate them with unitary transformations to realize arbitrary projective measurements, which allows for the realization a high-visibility single-photon Ramsey interferometer. Our approach opens the path for frequency-encoded photonic qubits in quantum information processing and quantum communication.Comment: 16 page

Experiments on quantum frequency conversion of photons

Koherent Photonen zwischen verschiedenen Zuständen zu konvertieren eröffnet faszinierende neue Möglichkeiten und Anwendungen in Quantenoptik-Experimenten. In dieser Arbeit werden drei Experimente zu dieser Thematik vorgestellt. Das erste Experiment demonstriert Quanten-Frequenzkonversion von polarisationsverschränkten Photonen. Kohärente Frequenzkonversion einzelner Photonen bietet eine elegante Lösung für den oft schwierigen Kompromiss die optimale Wellenlänge zu finden: beispielsweise bezüglich der optimalen Übertragung und Speicherung in Quantennetzwerken. In unseren Experimenten verifizieren wir die erfolgreiche Konversion der Verschränkung durch die Verletzung einer Clause-Horne-Shimony-Holt (CHSH) Bell-Ungleichung. Ausserdem wird der fast optimale Verschränkungstransfer genau durch Quantenzustands - und Quantenprozess - Tomografie charakterisiert. Unsere Implementation ist robust und flexibel und dadurch ein geeigneter Baustein für zukünftige Quantentechnologien. Der zweite Teil der Arbeit widmet sich der Vorstellung eines neuen deterministischen Schemas zur photonischen Quanteninformationsverarbeitung. Während einzelnen Photonen viele Vorteile für Quanteninformationstechnologien aufweisen, sind die bisher ungelösten Herausforderungen deterministische Einzelphotonen-Quellen, deterministische Photon-Photon Wechselwirkungen sowie nahe 100% effiziente Detektion. All diese können mit einem einzigen, vielseitigen Prozess gelöst werden -- einem Vier-Wellen-Mischungs-Prozess der hier als Spezialfall des generellerem Schemas der "Kohärenten Photonen-Konversion" vorgeschlagen wird. Dies kann viele, wertvolle Werkzeuge für die Quantenverarbeitung bereitstellen, angefangen von skalierbaren Multi-Photonen-Quellen, über die Implementation von deterministischen Verschränkungsgattern bis hin zur Ermöglichung hocheffizienter Einzelphotonen-Detektion. Beachtlicherweise würde sogar skalierbares optisches Quantencomputing ermöglicht. In unserem Experiment demonstrieren wir mit Hilfe von Photonischen-Kristall-Fasern einen Vier-Farben nichtlinearen Prozess, der für die "Kohärente Photonen-Konversion" geeignet wäre. Dafür weisen wir die Erzeugung korrelierter Photonenpaare bei den vorhergesagten Wellenlängen nach und bestimmen quantitativ die lineare Verstärkung der Wechselwirkung abhängig von der benutzen Pumpleistung. Wir erörtern weiterhin, wie man mit Hilfe derzeitiger Technologie in das angestrebte Regime der deterministischen Wechselwirkung gelangen könnte. Ausserdem könnte das vorgeschlagene Prinzip auch in anderen physikalischen Systemen, wie opto-mechanischen, elektromechanischen oder supraleitenden Systemen eingesetzt werden, welche auch zum Teil sehr hohe intrinsiche Nichtlinearitäten aufweisen. Im dritten Experiment wird die Erzeugung und der Nachweis von diskreten Farb-verschränkten Zuständen beschrieben. Wir erzeugen diese Zustände durch die Übertragung von Polarisationsverschränkung von nicht-degenerierten Photonenpaaren auf den Farb-Freiheitsgrad. Wir weisen dann streng die Verschränkung nach und quantifizieren den Grad der Verschränkung mithilfe der Rekonstruktion einer reduzierten Dichtematrix. Unsere Technik kann generalisiert werden um den Transfer von Polarisationsverschränkung auf andere photonische Freiheitsgrade zu ermöglichen wie zum Beispiel den Bahndrehimpuls der räumlichen Moden einzelner Photonen.Coherently converting photons between different states offers intriguing new possibilities and applications in quantum optical experiments. In this thesis three experiments on this theme are presented. The first experiment demonstrates the quantum frequency conversion of polarization entangled photons. Coherent frequency conversion of single photons offers an elegant solution for the often difficult trade-off of choosing the optimal photon wavelength, e.g. regarding optimal transmission and storage of photons in quantum memory based quantum networks. In our experiments, we verify the successful entanglement conversion by violating a Clauser-Horne-Shimony-Holt (CHSH) Bell inequality and fully characterised our close to unity fidelity entanglement transfer using quantum state- and process tomography. Our implementation is robust and flexible, making it a practical building block for future quantum technologies. The second part of the thesis introduces a deterministic scheme for photonic quantum information processing. While single photons offer many advantages for quantum information technologies, key unresolved challenges are scalable on-demand single photon sources; deterministic two-photon interactions; and near 100%-efficient detection. All these can be solved with a single versatile process – a novel four-wave mixing process that we introduce here as a special case of the more general scheme of coherent photon conversion (CPC). It can provide valuable photonic quantum processing tools, from scal- ably creating single- and multi-photon states to implementing deterministic entangling gates and high-efficiency detection. Notably, this would enable scalable photonic quantum computing. Using photonic crystal fibres, we experimentally demonstrate a nonlinear process suited for coherent photon conversion. We observe correlated photon-pair production at the predicted wavelengths and experimentally characterise the enhancement of the interaction strength by varying the pump power. We further explain how current technology can provide a feasible path towards deterministic operation. Our scheme could also be implemented in opto-mechanical or superconducting systems which can exhibit extremely strong intrinsic nonlinearities. The third experiment demonstrates the creation and verification of discrete color entanglement. We experimentally create high-quality, discretely color- entangled states by transferring polarization entanglement of non-degenerate photons onto the color. We then unambiguously verify and quantify the amount of entanglement by reconstructing a restricted density matrix. Our technique can be generalized to transfer polarization entanglement for example onto orbital angular momentum

Real-Time Imaging of Quantum Entanglement

Quantum Entanglement is widely regarded as one of the most prominent features of quantum mechanics and quantum information science. Although, photonic entanglement is routinely studied in many experiments nowadays, its signature has been out of the grasp for real-time imaging. Here we show that modern technology, namely triggered intensified charge coupled device (ICCD) cameras are fast and sensitive enough to image in real-time the effect of the measurement of one photon on its entangled partner. To quantitatively verify the non-classicality of the measurements we determine the detected photon number and error margin from the registered intensity image within a certain region. Additionally, the use of the ICCD camera allows us to demonstrate the high flexibility of the setup in creating any desired spatial-mode entanglement, which suggests as well that visual imaging in quantum optics not only provides a better intuitive understanding of entanglement but will improve applications of quantum science.Comment: Two supplementary movies available at the data conservancy projec

Frequency Multiplexing for Quasi-Deterministic Heralded Single-Photon Sources

Single-photon sources based on optical parametric processes have been used extensively for quantum information applications due to their flexibility, room-temperature operation and potential for photonic integration. However, the intrinsically probabilistic nature of these sources is a major limitation for realizing large-scale quantum networks. Active feedforward switching of photons from multiple probabilistic sources is a promising approach that can be used to build a deterministic source. However, previous implementations of this approach that utilize spatial and/or temporal multiplexing suffer from rapidly increasing switching losses when scaled to a large number of modes. Here, we break this limitation via frequency multiplexing in which the switching losses remain fixed irrespective of the number of modes. We use the third-order nonlinear process of Bragg scattering four-wave mixing as an efficient ultra-low noise frequency switch and demonstrate multiplexing of three frequency modes. We achieve a record generation rate of $4.6\times10^4$ multiplexed photons per second with an ultra-low $g^{2}(0)$ = 0.07, indicating high single-photon purity. Our scalable, all-fiber multiplexing system has a total loss of just 1.3 dB independent of the number of multiplexed modes, such that the 4.8 dB enhancement from multiplexing three frequency modes markedly overcomes switching loss. Our approach offers a highly promising path to creating a deterministic photon source that can be integrated on a chip-based platform.Comment: 28 pages, 9 figures. Comments welcom