Harald Weinfurter

Experimental Quantum Physics

Ludwig-Maximilians-Universität München

Faculty of Physics

Schellingstr. 4

80799 Munich

Tel. +49 89 2180 2044

h.w[at]lmu.de

Group Webpage

Description

Research focus: quantum information, quantum many-body physics, quantum optics

Atom-Atom Entanglement

Entanglement between atomic systems and light is a promising approach for a quantum network of atomic quantum memories and photonic communication channels. In our project we work with single, optically trapped Rubidium atoms whose spin is entangled with the polarization of single photons. Two photons independently emitted from distant atoms are combined to create heralded entanglement between the atoms. These entangled atom pairs, together with fast and efficient state read-out are well suited for experiments on the foundations of quantum mechanics, e.g., for a test of Bell's inequality.


Quantum Cryptography

Weinfurer_QKD-setup

Quantum key distribution (QKD) allows two parties to exchange a secure key for cryptography using the quantum mechanical properties of light. We have achieved QKD over the record distance of 144 km, which is representative for a link to a low orbit satellite. In this context we also demonstrated that a key exchange with a fast moving device is possible by establishing a QKD link to an aircraft at a distance of 20 km to the ground station. Our current research focuses on the implementation of a compact transmitter suited for handheld user devices, where we plan to combine light from a laser diode array with laser written waveguides on a glass chip.


Solid state single photon sources

csm_Weinfurer_APL_cover_104_3_48d34d6078 csm_Weinfurer_APL_cover_104_3_48d34d6078

The development of reliable devices to generate single photons is crucial for future applications in applied physical and quantum information science, as well as for fundamental quantum optics experiments. In this context our group focuses on the efficient evanescent coupling of single photons radiated by a single defect center in diamond to photonic waveguide structures, e.g., tapered optical fibers and dielectric slot waveguides on a chip.

Publications

Cooperation and dependencies in multipartite systems

W. Kłobus, M. Miller, M. Pandit, R.Ganardi, L. Knips, J. Dziewior, J. Meinecke, H. Weinfurter, W. Laskowski, T. Paterek

NJP 23, 63057 (2021).

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We propose an information-theoretic quantifier for the advantage gained from cooperation that captures the degree of dependency between subsystems of a global system. The quantifier is distinct from measures of multipartite correlations despite sharing many properties with them. It is directly computable for classical as well as quantum systems and reduces to comparing the respective conditional mutual information between any two subsystems. Exemplarily we show the benefits of using the new quantifier for symmetric quantum secret sharing. We also prove an inequality characterizing the lack of monotonicity of conditional mutual information under local operations and provide intuitive understanding for it. This underlines the distinction between the multipartite dependence measure introduced here and multipartite correlations.

DOI: 10.1088/1367-2630/abfb89

Gaussian state entanglement witnessing through lossy compression

W. Kłobus, P. Cieśliński, L. Knips, P. Kurzyński, W. Laskowski

Physical Review A 103, 032412 (2021).

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We study the possibility of witnessing Gaussian entanglement between two continuous-variable systems with the help of two spatially separated qubits. Its key ingredient is a local lossy state transfer from the original systems onto local qubits. The qubits are initially in a pure product state, therefore by detecting entanglement between the qubits we witness entanglement between the two original systems.

DOI: 10.1103/PhysRevA.103.032412

Extending Quantum Links: Modules for Fiber- and Memory-Based Quantum Repeaters

P. van Loock, W. Alt, C. Becher, O. Benson, H. Boche, C. Deppe, J. Eschner, S. Höfling, D. Meschede, P. Michler, F. Schmidt, H. Weinfurter.

Advancing Quantum Technologies - Chances and Challenges Advanced Quantum Technologies, (2020).

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Elementary building blocks for quantum repeaters based on fiber channels and memory stations are analyzed. Implementations are considered for three different physical platforms, for which suitable components are available: quantum dots, trapped atoms and ions, and color centers in diamond. The performances of basic quantum repeater links for these platforms are evaluated and compared, both for present‐day, state‐of‐the‐art experimental parameters as well as for parameters that can in principle be reached in the future. The ultimate goal is to experimentally explore regimes at intermediate distances—up to a few 100 km—in which the repeater‐assisted secret key transmission rates exceed the maximal rate achievable via direct transmission. Two different protocols are considered, one of which is better adapted to the higher source clock rate and lower memory coherence time of the quantum dot platform, while the other circumvents the need of writing photonic quantum states into the memories in a heralded, nondestructive fashion. The elementary building blocks and protocols can be connected in a modular form to construct a quantum repeater system that is potentially scalable to large distances.

DOI: 10.1002/qute.201900141

Multipartite entanglement analysis from random correlations

L. Knips, J. Dziewior, W. Klobus, W. Laskowski, T. Paterek, P.J. Shadbolt, H. Weinfurter, J.D.A. Meinecke

NPJ Quantum Information 6 (1), 51 (2020).

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Quantum entanglement is usually revealed via a well aligned, carefully chosen set of measurements. Yet, under a number of experimental conditions, for example in communication within multiparty quantum networks, noise along the channels or fluctuating orientations of reference frames may ruin the quality of the distributed states. Here, we show that even for strong fluctuations one can still gain detailed information about the state and its entanglement using random measurements. Correlations between all or subsets of the measurement outcomes and especially their distributions provide information about the entanglement structure of a state. We analytically derive an entanglement criterion for two-qubit states and provide strong numerical evidence for witnessing genuine multipartite entanglement of three and four qubits. Our methods take the purity of the states into account and are based on only the second moments of measured correlations. Extended features of this theory are demonstrated experimentally with four photonic qubits. As long as the rate of entanglement generation is sufficiently high compared to the speed of the fluctuations, this method overcomes any type and strength of localized unitary noise.

DOI: 10.1038/s41534-020-0281-5

Long-Distance Distribution of Atom-Photon Entanglement at Telecom Wavelength

T. van Leent, M. Bock, R. Garthoff, K. Redeker, W. Zhang, T. Bauer, W. Rosenfeld, C. Becher, and H. Weinfurter.

Physical Review Letters 124, 010510 (2020).

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Entanglement between stationary quantum memories and photonic channels is the essential resource for future quantum networks. Together with entanglement distillation, it will enable efficient distribution of quantum states. We report on the generation and observation of entanglement between a 87Rb atom and a photon at telecom wavelength transmitted through up to 20 km of optical fiber. For this purpose, we use polarization-preserving quantum frequency conversion to transform the wavelength of a photon entangled with the atomic spin state from 780 nm to the telecom S band at 1522 nm. We achieve an unprecedented external device conversion efficiency of 57% and observe an entanglement fidelity between the atom and telecom photon of ?78.5±0.9% after transmission through 20 km of optical fiber, mainly limited by decoherence of the atomic state. This result is an important milestone on the road to distribute quantum information on a large scale.

DOI: 10.1103/PhysRevLett.124.010510

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