# News

February 8, 2021
Published by Stephan Sponar
In our paper, twisting neutral particles with electric fields, we demonstrate that a polarized neutron impinging on a longitudinally oriented electric field can obtain a unit of orbital angular momentum entangled to its spin degree of freedom. The coupling strength is proportional to the electric field strength and the transverse wavevector, which depends on the beam divergence. To obtain maximally entangled states the neutron must experience a voltage drop of roughly 10^8 V rad. While such fields are not feasible in laboratory environment they do appear naturally between the lattice planes of perfect crystals. Niels Geerits and Stephan Sponar, *Physical Review A * **103**, 022205 (2021) (8 February 2021)

September 30, 2020
Published by Stephan Sponar
In our latest neutron polarimetric experiment we study tight state-independent preparation uncertainty relations for quits. The tight relations are based on the relationship between the expectation values of Pauli spin observables and two standard measures of uncertainty, namely standard deviations and Shannon entropies. We present a neutron polarimetric test of the tight state-independent preparation uncertainty relations for orthogonal, as well as non-orthogonal, Pauli spin observables. The final results, obtained with pure and mixed spin states, reproduce the theoretical predictions evidently for arbitrary initial states of variable degree of polarization. S. Sponar, A Danner, K Obigane, S Hack, and Y Hasegawa, *Physical Review A * **102**, 042204 (2020) (6 October 2020)

February 14, 2020
Published by Stephan Sponar
In our latest neutron interferometric experiment the effect of spin–rotation coupling, also referred to as Sagnac-Mashhoon phase was measured with neutrons. The coupling of spin with the angular velocity of a rotating magnetic field is an extension of the well-known Sagnac effect, including the intrinsic spin of a quantum mechanical particle when replacing the orbital angular momentum, by the total angular momentum. The principle of the experiment is the following: in one path of the interferometer the neutron’s spin is manipulated by a rotating magnetic field to undergo a cyclic path. As the orientations before and after the manipulation are the same, only a phase difference between the sunbeams is induced. This phase difference results in a shift of the observed interference fringes which turns out to depend solely on the frequency of the rotation of the magnetic field. A. Danner, B. Demirel, W. Kersten, H. Lemmel, R. Wagner, S. Sponar, and Y. Hasegawa, *npj Quantum Information * **6**, 23 (2020) (14 February 2020)

July 15, 2019
Published by Stephan Sponar
Topology-optimized 3D-printed magnets are interesting for Larmor spin-rotators in neutron optics in general, and in neutron interferometry in particular. Using 3D-printed magnets instead of magnetic coils avoids heat dissipation, which is the the main cause of loss in fringe visibility, due to temperature gradients in the interferometer. This study applies the technique to implement an arbitrary neutronic phase gate, for rotations of up to 4π of the neutron’s spinor wave function in one arm of the interferometer. This is achieved by varying the distance between the 3D-printed magnets, while maintaining homogeneity of the magnetic action over the neutron beam’s profile. W. Kersten, L. Brandl, R. Wagner, C. Huber, F. Bruckner, Y. Hasegawa, D. Suess, and S. Sponar, Additive-Manufactured and Topology-Optimized Permanent-Magnet Spin Rotator for Neutron Interferometry, *Physical Review Applied* **12**, 014023 (2019). (12 July 2019)

March 6, 2019
Published by Stephan Sponar
A rotating field generator is designed for an interferometer experiment. Its magnetic field is simulated, an according coil mount as well as a water cooler are fabricated and the device is successfully tested in a polarimeter experiment. The field simulations show a sufficient field homogeneity. Although the field transition is initially indicated as too long, the experimental results accord qualitatively with the assumption of a sudden field transition. The presented rotating field generator is suited for the proposed experiment by Mashhoon et al. investigating spin-rotation coupling in neutron interferometry.
Armin Danner, Bülent Demirel, Stephan Sponar and Yuji Hasegawa, * J. Phys. Commun. * **3**, 035001 (2019). (6 March 2019)

January 31, 2019
Published by Stephan Sponar
In our latest neutron optical experiment we test the joint-measurement noise trade-off for variouse Pauli observables by implementing four-outcome positive-operator valued measure POVMs. The noise associated to the measurement of an observable is defined via conditional Shannon entropies and a tradeoff relation between the noises for two arbitrary spin observables is demonstrated. The optimal bound of this tradeoff is experimentally obtained for various non-commuting spin observables. For some of these observables this lower bound can be reached with projective measurements, but we observe that, in other cases, the tradeoff is only saturated by general quantum measurements (i.e., positive-operator valued measures), as predicted theoretically. Bülent Demirel, Stephan Sponar, Alastair A. Abbott, Cyril Branciard, and Yuji Hasegawa, *New J. Phys ***21**,013038 (2019) (31 January 2019)

May 10, 2018
Published by Stephan Sponar
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) (10 May 2018)

November 29, 2017
Published by Stephan Sponar
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) (27 November 2017)

September 4, 2017
Published by Stephan Sponar
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) (28 August 2017)

January 7, 2017
Published by Stephan Sponar
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) (6 January 2017)