News
January 16, 2026
Published by Stephan Sponar
In its original formulation, Heisenberg’s uncertainty principle describes a trade-off relation between the error of a quantum measurement and the thereby induced disturbance on the measured object. However, this relation is not valid in general. An alternative universally valid relation was derived by Ozawa in 2003, defining error and disturbance in a general concept, experimentally accessible via a tomographic method. Later, it was shown by Hall that these errors correspond to the statistical deviation between a physical property and its estimate. Recently, it was discovered that these errors can be observed experimentally when weak values are determined through a procedure named “feedback compensation”. Here, we apply this procedure for the complete experimental characterization of the error-disturbance relation between a which-way observable in an interferometer and another observable associated with the output of the interferometer, confirming the theoretically predicted relation. As expected for pure states, the uncertainty is tightly fulfilled. A. Asadian et al. Phys. Rev. Res. 8, L012011 (2026) (26. Dec 2025)
January 7, 2026
Published by Stephan Sponar
In its original formulation, Heisenberg’s uncertainty principle describes a trade-off relation between the error of a quantum measurement and the thereby induced disturbance on the measured object. However, this relation is not valid in general. An alternative universally valid relation was derived by Ozawa in 2003, defining error and disturbance in a general concept, experimentally accessible via a tomographic method. Later, it was shown by Hall that these errors correspond to the statistical deviation between a physical property and its estimate. Recently, it was discovered that these errors can be observed experimentally when weak values are determined through a procedure named “feedback compensation”. Here, we apply this procedure for the complete experimental characterization of the error-disturbance relation between a which-way observable in an interferometer and another observable associated with the output of the interferometer, confirming the theoretically predicted relation. As expected for pure states, the uncertainty is tightly fulfilled. A. Dvorak et al. Phys. Rev. Res. 7, 043334 (2025) (26. Dec 2025)
February 7, 2025
Published by Stephan Sponar
The Sagnac effect is seen in an interferometer, when two beams of light are sent around a closed path in opposite directions. If the path is rotated, the beams become out of phase when they reunite back at the start. If the interferometer is large enough, the rotation of the Earth can be enough to shift the phases of the two beams. The effect has also been seen for particles, including electrons and neutrons. The phase shift for neutrons is proportional to the frequency of the rotation and the orbital angular momentum (OAM) of the neutron. In our work used this phase shift to work backwards and measure the OAM of the neutron using the Sagnac effect, and the rotation of the Earth. On the Larmor instrument at ISIS, we used a spin echo interferometer to produce entangled neutrons, with any path dependent phase shift detected in changes to the neutron spins. Finally, we were able to measure the OAM to within 5% of its theoretical value. N. Geerits et al. Physical Review Research 7, 013046 (2025)
(13. Jan 2025)
June 25, 2024
Published by Stephan Sponar
The question whether measurable quantities of a quantum object have definite values prior to the actual measurement is a fundamental issue ever since quantum theory has been introduced in the early 20th century. Leggett-Garg inequalities (LGIs) study temporal correlations of a single system, which are derived under the assumption of so called macro-realism. There it is assumed, that any (macroscopic) object, which may occur in two (or more) macroscopically distinct states, is at any given time in a definite one of those states. The predictions of quantum mechanics stand in stark contradiction with these realistic theories, which manifest in violations of LGIs. We report on an experiment that demonstrates the violation of an LGI with neutrons, where the final measured value of the Leggett–Garg correlator K = 1.120 ± 0.026, obtained in a neutron interferometric experiment, is clearly above the limit K = 1 predicted by macro-realistic theories. E. Kreuzgruber et al. Phys. Rev. Lett. 132, 260201 (2024) (24. Jun 2024)
August 10, 2023
Published by Stephan Sponar
Neutron Orbital Angular Momentum (OAM) is an additional quantum mechanical degree of freedom, useful in quantum information, and may provide more complete information on the neutron scattering amplitude of nuclei. Various methods for producing OAM in neutrons have been discussed. In this work we generalize magnetic methods which employ coherent averaging and apply this to neutron interferometry. Two aluminium prisms are inserted into a nested loop interferometer to generate a phase vortex lattice with significant extrinsic OAM, 〈Lz〉≈0.35, on a length scale of≈220μm, transverse to the propagation direction. N. Geerits et al. Commun. Phys. 6, 209 (2023) (10. Aug 2023)
April 28, 2022
Published by Stephan Sponar
In our latest work we experimentally investigate the possibility that an individual neutron moving through a two-path interferometer may actually be physically distributed between the two paths. For this purpose, it is important to distinguish between the probability of finding the complete particle in one of the paths and the distribution of an individual particle over both paths. The results show that individual particles experience a specific fraction of the magnetic field applied in one of the paths, indicating that a fraction or even a multiple of the particle was present in the path before the interference of the two paths was registered, verified by the recently introduced method of feedback compensation. H. Lemmel et al. Phs. Rev. Research 4, 023075 (2022) (28. April 2022)
July 1, 2021
Published by Stephan Sponar
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)
June 25, 2021
Published by Stephan Sponar
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)
June 5, 2021
Published by Stephan Sponar
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)
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)