Test of completeness of quantum root-mean-square errors

July 1, 2021 Published by

According to the uncertainty principle, which is a corner stone of quantum mechanics, it is impossible to measure both position and momentum of an elementary particle (e.g. electron as in Heisenberg’s original argument) with arbitrary accuracy or in other word with zero error. One of the major problems in quantum physics has been to generalize the classical root-mean-square error to quantum measurements to obtain an error measure satisfying both soundness (to vanish for any accurate measurements) and completeness (to vanish only for accurate measurements). S. Sponar et al, npj Quantum Inf. 7, 106 (2021) (28 June 2021)


Direct experimental test of commutation relation via imaginary weak value

June 25, 2021 Published by

The canonical commutation relation is a tenet of quantum theory and Heisenberg’s uncertainty relation is a direct consequence of it. However, a genuine direct experimental test has not been performed up to Date. The reason for this is that the product of two non-commuting observables, as occurring in the commutation relation, is in general non-Hermitian and therefore cannot be measured by a usual strong measurement. We have overcome this hurdle by measuring the imaginary part of the weak value of a suited path-qubit in a neutron interferometer experiment, directly verifying the canonical commutation relation. R. Wagner et al, Physical Review Research  3, 023243 (2021) (24 June 2021)

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Experimental test of entropic noise-disturbance uncertainty relations for three-outcome qubit measurements

June 5, 2021 Published by

We experimentally tested a tight information-theoretic measurement uncertainty relation, in terms of a proposed three-outcome POVM using neutron spin-1/2 qubits. The obtained results of the noise-disturbance trade-off relation for three-outcome POVM outperform prior results for projective measurements. S. Sponar et al, Physical Review Research  3, 023175 (2021) (5 June 2021)


Twisting neutral particles with electric fields

February 8, 2021 Published by

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)

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Tight State-Independent Preparation Uncertainty Relations for Qubits

September 30, 2020 Published by

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)


Spin-Rotation Coupling observed in Neutron Interferometry

February 14, 2020 Published by

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)


Additive-Manufactured and Topology-Optimized Permanent-Magnet Spin Rotator for Neutron Interferometry

July 15, 2019 Published by

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)

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Development and perfomance of a miniaturised spin rotator suitable for neutron interferometer experiments

March 6, 2019 Published by

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)


Experimental Test of Information-Theoretic Measurement Uncertainty Relation for general Qubit Observables

January 31, 2019 Published by

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)


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) (10 May 2018)