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Asking Neutrons where they have been – Past of a quantum particle

May 10, 2018 Published by

In our latest neutron optical experiment we investigate the paths taken by neutrons in a three – beam interferometer. In various beam-paths of the interferometer, the energy of the neutrons is partially shifted so that the faint traces are left along the beam-path. Which-path information is extracted from these faint traces with minimal-perturbations. Theory is derived by simply following the time evolution of the wave function of the neutrons, which clarifies the observation in the framework of standard quantum mechanics. Which-way information is derived from the intensity, sinusoidally oscillating in time at different frequencies, which is considered to result from the interfering cross terms between stationary main component and the energy-shifted which-way signals. Final results give experimental evidence that the partial wave functions of the neutrons in each beam path are superimposed and present in multiple locations in the interferometer. Hermann Geppert-Kleinrath, Tobias Denkmayr, Stephan Sponar, Hartmut Lemmel, Tobias Jenke, and Yuji Hasegawa, Phys. Rev. A  97, 052111 (2018)

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Confined Contextuality in Neutron Interferometry: Observing the Quantum Pigeonhole Effect

November 29, 2017 Published by

Contextuality is one of the most counterintuitive aspects of quantum mechanics. While previous contextuality experiments have only been able to demonstrate the global incompatibility between the predictions of quantum mechanics and non-contextual hidden variable theories , but failed to isolate precisely where that incompatibility occurs. We could show, for the first time, that quantum contextuality can be confined to specific observables, which also manifests in a violation of the pigeon hole principle. M. Waegell, T. Denkmayr, H. Geppert, D. Ebner, T. Jenke, Y. Hasegawa, S. Sponar, J. Dressel, J. Tollaksen, Physical Review A 96, 052131 (2017)

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Is Heisenberg’s error-disturbance uncertainty relation violated? Experimental study of competing approaches

September 4, 2017 Published by

Since the theoretical findings of Masanao Ozawa, namely a violating and thus a necessary reformulation of Heisenberg’s original error-disturbance uncertainty relation, this particular field has experienced increased attention. However, soon after publication of our experimental verification an alternative theory was presented by Paul Busch, and Pekka Lahti, and Reinhard F. Werner (BLW) which in contrast stated the validity of Heisenberg’s relation. We now carried out the first experimental comparison of these two competing approaches leading to a surprising result: Despite the strong controversy, in case of projectively measured qubit observables both approaches even lead to equal outcomes. S. Sponar and G. Sulyok, Physical Review A 96 022137 (2017)

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Experimental Demonstration of Direct Path State Characterization by strongly measuring Weak Values

January 7, 2017 Published by

We developed a measurement scheme, used in a matter-wave interferometric experiment, in which the neutron path system’s quantum state was characterized via direct measurements using both strong and weak interactions. Experimental evidence is given that strong interactions outperform weak ones for tomographic accuracy. Our results are not limited to neutron interferometry, but can be used in a wide range of quantum systems. T. Denkmayr et al., Physical Review Letters 118, 010402 (2017)

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Mixed State Uncertainty Relations

September 30, 2016 Published by

In the last decade, several universally valid forms of error-disturbance uncertainty relations were derived for completely general quantum measurements for arbitrary states.  An optimal form for spin measurements for some pure states was established recently. However, the bound in his inequality is not stringent for mixed states. Masanao Ozawa derived a new bound tight in the corresponding mixed state case, which was tested by our group. We experimentally observed the attainability of the new bound. B. Demirel et al., Physical Review Letters 117, 140402 (2016)

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Review paper published

March 11, 2016 Published by

The validity of quantum-mechanical predictions has been confirmed with a high degree of accuracy in a wide range of experiments. Although the statistics of the outcomes of a measuring apparatus have been studied intensively, little has been explored and is known regarding the accessibility of quantum dynamics. Sponar et al., Atoms 4, 11 (2016)

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Uncertainty Relation in Quantum Information Theory

July 13, 2015 Published by

Uncertainty relation in quantum information theory publish in PRL and relieved an Editor’s Suggestion! Information is a key quantity in science and plays a significant role in many economic sectors such as communication technologies, cryptography, or data storage. In quantum communication and information technology the transfer and encryption of information is studied. Sulyok et al., Phys. Rev. Lett115, 030401 (2015)

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Experimental Demonstration of a generalized Error-Disturbance Uncertainty Relation

January 16, 2012 Published by

Heisenberg’s uncertainty principle. is certainly one of the most famous foundations of quantum physics. According to this principle, not all properties of a quantum particle are determined with arbitrary accuracy. In the early days of quantum theory, this has often been justified by the notion that every measurement inevitably recoils the quantum particle, which disturbs the results of any further measurements. This, however, turns out to be an oversimplification. In our neutron polarimetric experiment different sources of quantum uncertainty could now be distinguished, validating theoretical results of an error-disturbance uncertainty relation proposed by Masanao Ozawa. Y. Hasegawa et al., Nature Physics 8, 185-189 (2012)