A double-loop perfect crystal neutron interferometer has been used for novel quantum state reconstruction experiments. For the measurement of combined spatial and momentum distributions a quadrature method has been used which permits the reconstruction of the related Wigner function. When neutrons pass through narrow slits the transverse momentum quantization causes a confinement-induced phase shift which has been measured by neutron inter-ferometry. The related wave fields for reflecting and absorbing walls have been calculated and mapped in form of wave carpets. It is of interest to note that just those neutrons contribute to the phase shift which classically do not touch the walls. In another set-up installed at the pulsed neutron spallation source ISIS neutrons can be trapped between perfect crystal plates for several seconds. This enables a multiple passage of the same neutron through a spin rotation unit, which provides the basis for new Zeno-effect experiments. Unavoidable quantum losses will be discussed and it will be shown how they become relevant for the quantum measuring process.
A double-loop perfect crystal neutron interferometer has been used for novel quantum state reconstruction experiments. For the measurement of combined spatial and momentum distributions a quadrature method has been used which permits the reconstruction of the related Wigner function. When neutrons pass through narrow slits the transverse momentum quantization causes a confinement-induced phase shift which has been measured by neutron inter-ferometry. The related wave fields for reflecting and absorbing walls have been calculated and mapped in form of wave carpets. It is of interest to note that just those neutrons contribute to the phase shift which classically do not touch the walls. In another set-up installed at the pulsed neutron spallation source ISIS neutrons can be trapped between perfect crystal plates for several seconds. This enables a multiple passage of the same neutron through a spin rotation unit, which provides the basis for new Zeno-effect experiments. Unavoidable quantum losses will be discussed and it will be shown how they become relevant for the quantum measuring process.
The curious dual nature of the neutron (sometimes a particle, sometimes a wave) is wonderfully manifested in the various non-local interference effects observed in neutron interferometry. Here we deal with unavoidable quantum losses as they appear in neutron phase-echo and spin rotation experiments. In all cases of an interaction, parasitic beams are produced which cannot be recombined with the original beam in an ideal way. This means that a complete reconstruction of the original state would, in principle, be impossible. Thus a kind of intrinsic irreversibility occurs, even when the original quantum state survives to a very high extent. Even small interaction potentials can have huge effects when they are applied periodically and resonance effects appear. This gives various constraints for repetitive measurements and prevents a complete freezing of a quantum state in Zeno-like experiments. Additionally, a spectral change occurs, due to the dispersive action of the interaction, which has to be taken into account when many repetitive measurements are considered. A dedicated proposal for a repetitive neutron spin rotation experiment within a perfect crystal resonator will be analysed in detail. Unavoidable losses due to quantum phenomena can be separated from losses caused by experimental imperfections.
Neutron phase-echo and spin rotation experiments have shown that in all cases of an interaction, parasitic beams are produced that cannot be recombined with the original beam in an ideal way. This means that a complete reconstruction of the original state would, in principle, be impossible. Thus, a kind of intrinsic irreversibility occurs, even when the original quantum state survives to a very high extent. When plane waves are used, completely reversible situations can be constructed in certain cases, but in any physical situation, wave packets have to be used which do not permit complete reversibility. Even small interaction potentials can have huge effects when they are applied periodically, and resonance effects appear. This gives various constraints for repetitive measurements and prevents a complete freezing of a quantum state in Zeno-like experiments. Additionally, a spectral change occurs, due to the dispersive action of the interaction, which has to be taken into account when many repetitive measurements are considered. A dedicated proposal for a repetitive neutron spin rotation experiment within a perfect crystal resonator will be analyzed in detail. Unavoidable losses due to quantum phenomena can be separated from losses caused by experimental imperfections.
Quantum optics experiments contribute to a deeper understanding of quantum theory. Neutron phase-echo and spin rotation experiments have shown that, in all cases of an interaction, parasitic beams are produced which cannot be recombined with the original beam in an ideal way. This means that a complete reconstruction of the original state would, in principle, be impossible. Thus a kind of intrinsic irreversibility occurs, even when the original quantum state survives to a very high extent. When plane waves are used, completely reversible situations can be constructed in certain cases, but in any physical situation, wave packets have to be used which do not permit complete reversibility. Even small interaction potentials can have huge effects when they are arranged periodically and resonance effects appear. This gives various constraints for repetitive measurements and prevents a complete freezing of a quantum state in Zeno-like experiments. Additionally, a spectral change occurs, due to the dispersive action of the interaction, which has to be taken into account when many repetitive measurements are considered. A dedicated proposal for a repetitive neutron spin rotation experiment within a perfect crystal resonator will be analyzed in detail. Unavoidable losses due to quantum phenomena can be separated from losses caused by experimental imperfections.
The neutron interferometer set-up S18 at the ILL reactor has been used for the first neutron quantum state reconstruction experiment. By a simultaneous measurement of the coherence function and of the modulated momentum distribution behind the interferometer, the Wigner function of various quantum states can be reconstructed. This is equivalent to knowledge of the particle wavefunction. Its spatial dimension follows from the measurement of the coherence function, which is the autocorrelation function of the wavefunction and the shape in momentum space follows from momentum post-selection experiments. Non-orthogonal traces through the ( x , k )-phase space have to be measured for an unambiguous reconstruction. Nearly classical (coherent) and non-classical states in the form of Schrödinger cat-like states have been identified. Comparisons of calculated and measured Wigner functions will be shown.
The transition between the quantum and classical world is a topical problem in quantum physics, which can be investigated by neutron interferometric methods. Here we discuss unavoidable quantum losses as they appear in neutron phase-echo and spin rotation experiments and we show how entanglement effects in a single-particle system demonstrate quantum contextuality, i.e. an entanglement between external and internal degrees of freedom in single-particle systems. This contextuality phenomenon also shows that a quantum system carries much more information than usually extracted. In all cases of an interaction, parasitic beams are produced which cannot be recombined completely with the original beam. This means that a complete reconstruction of the original state is, in principle, impossible which causes a kind of intrinsic irreversibility. Even small interaction potentials can have huge effects when they are applied in quantum Zeno-like experiments. The path towards advanced neutron quantum optics will be discussed.
Neutron interferometry provides a powerful tool to investigate particle and wave features in quantum physics. Single particle interference phenomena can be observed with neutrons and the entanglement of degrees of freedom, i.e., contextuality can be verified and used in further experiments. Entanglement of two photons, or atoms, is analogous to a double slit diffraction of a single photon, neutron or atom. Neutrons are proper tools for testing quantum mechanics because they are massive, they couple to electromagnetic fields due to their magnetic moment, they are subject to all basic interactions, and they are sensitive to topological effects, as well. The 4π-symmetry of spinor wave functions, the spin-superposition law and many topological phenomena can be made visible, thus showing interesting intrinsic features of quantum physics. Related experiments will be discussed. Deterministic and stochastic partial absorption experiments can be described by Bell-type inequalities. Neutron interferometry experiments based on post-selection methods renewed the discussion about quantum non-locality and the quantum measuring process. It has been shown that interference phenomena can be revived even when the overall interference pattern has lost its contrast. This indicates a persisting coupling in phase space even in cases of spatially separated Schrödinger cat-like situations. These states are extremely fragile and sensitive against any kind of fluctuations and other decoherence processes. More complete quantum experiments also show that a complete retrieval of quantum states behind an interaction volume becomes impossible in principle, but where and when a collapse of the wave-field occurs depends on the level of experiment.