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Quantum Information Enabled Neutron Interferometry

J. Nsofini

Published Jun 12, 2017• 0 citations
OpenAlex

Abstract

Neutron interferometry with its ability to encode and extract information provides a test \nbed for quantum mechanics and precise measurement of physical quantities of significant \nimportance in physics. However, this significant key investigative technique is weakened \nby its very fragile nature; neutron interferometry has proven to be highly sensitive to \nenvironmental noise. \n \nOnce brought to its optimal state, the use of neutron interferometry enabled with \nquantum information, represents a milestone. Indeed a realization of high-quality neutron \ninterferometry could pave a way to probe materials research such as probing properties \nwater in proteins and topological materials. Thus understanding and solving the sensitivity \nof neutron interferometers to noise is a key step toward possible applications. This is the \ncore of the work done in this Thesis. \n \nWe incorporated two theoretical techniques developed for quantum information sciences \ninto the construction of a new polarized neutron interferometry beam line: The technique \nof quantum error correction and the technique of open quantum system. One focus of this \nwork is to report on the design, the construction as well as the characteristic features of \nthis beamline. \n \nThis thesis involves experimental data, showing how the neutron beam intensity at the \nexit of a three-blade neutron interferometer can be controlled by the interferometer blades \nthickness. Secondly, is also presents an alternative and simplified quantum information \napproach to dynamical diffraction, based on repeated application of a coherent beam- \nsplitting unitary at coarse-grained lattice sites. Demanding translational invariance added \nto a computationally tractable number of sites in the coarse-graining reproduced many \nresults typical of standard dynamical diffraction theory and experiments. Building on that, \na proposal for a new five-blade neutron interferometer is presented and its robustness to \nnoise, resulting from dynamical diffraction together with low-frequency external mechanical \nvibration is discussed. \n \nSteps ahead in our work, neutron interferometry may be improved as well as adapted to \nmore applications by incorporating the spin and orbital degrees of freedom to a path-based \nivinterferometer. In this concern, we propose a method to prepare the spin-orbit state by \npassing a polarized neutron beam through a quadrupole magnetic field. Initially designed \nfor a beam the size of a coherent length, we extend this method to work for spatially \ndisplaced beams by using linear magnetic gradients.

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Neutron interferometry with its ability to encode and extract information provides a test \nbed for quantum mechanics and precise measurement of physical quantities of significant \nimportance in physics. However, this significant key investigative technique is weakened \nby its very fragile nature; neutron interferometry has proven to be highly sensitive to \nenvironmental noise. \n \nOnce brought to its optimal state, the use of neutron interferometry enabled with \nquantum information, represents a milestone. Indeed a realization of high-quality neutron \ninterferometry could pave a way to probe materials research such as probing properties \nwater in proteins and topological materials. Thus understanding and solving the sensitivity \nof neutron interferometers to noise is a key step toward possible applications. This is the \ncore of the work done in this Thesis. \n \nWe incorporated two theoretical techniques developed for quantum information sciences \ninto the construction of a new polarized neutron interferometry beam line: The technique \nof quantum error correction and the technique of open quantum system. One focus of this \nwork is to report on the design, the construction as well as the characteristic features of \nthis beamline. \n \nThis thesis involves experimental data, showing how the neutron beam intensity at the \nexit of a three-blade neutron interferometer can be controlled by the interferometer blades \nthickness. Secondly, is also presents an alternative and simplified quantum information \napproach to dynamical diffraction, based on repeated application of a coherent beam- \nsplitting unitary at coarse-grained lattice sites. Demanding translational invariance added \nto a computationally tractable number of sites in the coarse-graining reproduced many \nresults typical of standard dynamical diffraction theory and experiments. Building on that, \na proposal for a new five-blade neutron interferometer is presented and its robustness to \nnoise, resulting from dynamical diffraction together with low-frequency external mechanical \nvibration is discussed. \n \nSteps ahead in our work, neutron interferometry may be improved as well as adapted to \nmore applications by incorporating the spin and orbital degrees of freedom to a path-based \nivinterferometer. In this concern, we propose a method to prepare the spin-orbit state by \npassing a polarized neutron beam through a quadrupole magnetic field. Initially designed \nfor a beam the size of a coherent length, we extend this method to work for spatially \ndisplaced beams by using linear magnetic gradients.
Neutron interferometry has proved to be a very precise technique for measuring the quantum mechanical phase of a neutron caused by a potential energy difference between two spatially separated neutron paths inside interferometer. The path length inside the interferometer can be many centimeters (and many centimeters apart) making it very practical to study a variety of samples, fields, potentials, and other macroscopic medium and quantum effects. The precision of neutron interferometry comes at a cost; neutron interferometers are very susceptible to environmental noise that is typically mitigated with large, active isolated enclosures. With recent advances in quantum information processing especially quantum error correction (QEC) codes we were able to demonstrate a neutron interferometer that is insensitive to vibrational noise. A facility at NIST’s Center for Neutron Research (NCNR) has just been commissioned with higher neutron flux than the NCNR’s older interferometer setup. This new facility is based on QEC neutron interferometer, thus improving the accessibility of neutron interferometry to the greater scientific community and expanding its applications to quantum computing, gravity, and material research.
We provide a quantum information description of a proposed five-blade neutron interferometer geometry and show that it is robust against low-frequency mechanical vibrations and dephasing due to the dynamical phase. The extent to which the dynamical phase affects the contrast in a neutron interferometer is experimentally shown. In our model, we consider the coherent evolution of a neutron wavepacket in an interferometer crystal blade and simulate the effect of mechanical vibrations and momentum spread of the neutron through the interferometer. The standard three-blade neutron interferometer is shown to be immune to dynamical phase noise but prone to noise from mechanical vibrations, and the decoherence free subspace four-blade neutron interferometer is shown to be immune to mechanical vibration noise but prone to noise from the dynamical phase. Here, we propose a five-blade neutron interferometer and show that it is immune to both low-frequency mechanical vibration noise and dynamical phase noise.
Abstract Among the various areas of neutron optical research, neutron interferometry has provided some of the most challenging and spectacular developments. In most cases, separated coherent beams are used which are produced either by wavefront division (Young’s type) or by amplitude division (Mach− Zehnder type). These beams are subsequently coherently superposed after passing through regions of space where the neutron wavefunction is modified in phase and amplitude by various interactions: nuclear, magnetic, electromagnetic, gravitational, or by geometry and topology (as in Aharonov−Bohm-type experiments). Various types of neutron interferometers that have been tested are shown in Fig. 2.1. The first neutron interferometer was based upon wavefront division and biprism deflection, but it had several constraints for interferometric applications mainly due to the small beam separation (100 μm) (Maier-Leibnitz and Springer 1962). Nevertheless, this method gained further interest for diffraction experiments from macroscopic objects and for the detection of weak interaction effects where the long flight paths (10 m) enhance the sensitivity for the measurements of these effects (Gähler et al. 1980; Klein et al. 1981a). The perfect-crystal interferometer (Rauch et al. 1974) provides widely separated coherent beams and has become a standard method for advanced neutron optical investigations. The development of diffraction and phase grating interferometers extended the Mach-Zehnder interferometer method to very slow neutrons (loffe et al. 1985; Gruber et al. 1989). The superposition of spin-up and spin-down states in the longitudinal direction provides the basis for Larmor and Ramsey interferometry, where no lateral beam splitting is necessary. The well-known spin-echo systems (Mezei 1972) and zero-field spin-echo systems (Gähler and Golub 1987; Dubbers et al. 1989) are examples of such interferometers, which also play an important role in atom interferometry (Bord é 1989; Kasevich and Chu 1991; Riehle et al. 1991).
A decoherence-free subspace (DFS) is an important class of quantum-error-correcting (QEC) codes that have been proposed for fault-tolerant quantum computation. The applications of QEC techniques, however, are not limited to quantum-information processing (QIP). Here we demonstrate how QEC codes may be used to improve experimental designs of quantum devices to achieve noise suppression. In particular, neutron interferometry is used as a test bed to show the potential for adding quantum error correction to quantum measurements. We built a five-blade neutron interferometer that incorporates both a standard Mach-Zender configuration and a configuration based on a DFS. Experiments verify that the DFS interferometer is protected against low-frequency mechanical vibrations. We anticipate these improvements will increase the range of applications for matter-wave interferometry.
We investigate quantum coherences in the presence of noise by entangling the spin and path degrees of freedom of the output neutron beam from a noisy three-blade perfect crystal neutron interferometer. We find that in the presence of dephasing noise on the path degree of freedom the entanglement of the output state reduces to 0, however the quantum discord remains nonzero for all noise values. Hence even in the presence of strong phase noise nonclassical correlations persist between the spin and the path of the neutron beam. This indicates that measurements performed on the spin of the neutron beam will induce a disturbance on the path state. We calculate the effect of the spin measurement by observing the changes in the observed contrast of the interferometer for an output beam postselected on a given spin state. In doing so we demonstrate that these measurements allow us to implement a quantum eraser and a which-way measurement of the path taken by the neutron through the interferometer. While strong phase noise removes the quantum eraser, the spin-filtered which-way measurement is robust to phase noise. We experimentally demonstrate this disturbance by comparing the contrasts of the output beam with and without spin measurements of three neutron interferometers with varying noise strengths. This demonstrates that even in the presence of noise that suppresses path coherence and spin-path entanglement, a neutron interferometer still exhibits uniquely quantum behavior.
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.