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Imaging and Applied Optics

Structured Waves: From Matter to Light

Dmitry A. Pushin, Dusan Sarenac, J. Nsofini, Ian Hincks, P. Miguel, M. G. Huber, B. Heacock, M. Arif, Charles W. Clark, David G. Cory

Published Jan 1, 2018• 1 citation
Publisher

Abstract

Neutrons are important probes of mater and quantum physics. They are particularly powerful at characterizing magnetic structures. In order to extend the applications of neutron physics as a quantum probes/sensors we have developed methods based on Quantum Information Processing for preparing structured waves such as orbital and spin-orbit states of neutrons. Such states may be prepared for the beam as a whole, or with reference to the coherence length of the neutron wavepacket, which is much smaller than the width of useful beams. I will describe experiments to prepare and characterize neutron beams with specific orbital and spin-orbit structure based on neutron interferometry and will show how we can apply these methods to other type of probes such as light.

DOI
10.1364/lsc.2018.sw2h.3

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Neutrons are important probes of mater and quantum physics. They are particularly powerful at characterizing magnetic structures. In order to extend the applications of neutron physics as a quantum probes/sensors we have developed methods based on Quantum Information Processing for preparing structured waves such as orbital and spin-orbit states of neutrons. Such states may be prepared for the beam as a whole, or with reference to the coherence length of the neutron wavepacket, which is much smaller than the width of useful beams. I will describe experiments to prepare and characterize neutron beams with specific orbital and spin-orbit structure based on neutron interferometry and will show how we can apply these methods to other type of probes such as light.
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).
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.
Neutrons are proper tools for testing basic laws of quantum physics since they are massive and can be handled and measured with high efficiency. Suitable postselection experiments demonstrate coherence features of sub-ensembles even when the whole ensemble seems to have lost its coherence. All experiments have the capacity to explain more details by additional pre- and post-selection methods. It will be shown that specific losses are unavoidable in any interaction. Coherence and decoherence are intrinsic quantum effects and can shed light on the measurement problem. Quantum contextuality is a consequence of the entanglement of different degrees of freedom. This makes quantum phenomena more strongly correlated than classical ones. Most experiments have been performed with perfect neutron interferometers and some others by using ultracold neutrons and spin-echo systems. An event by event based interpretation can also be brought into agreement with the experimental results. In many cases parasitic beams carry the same information as the main beam and this relates such measurements to "weak" measurements. The coupling and entanglement of various parameter spaces guide us to a more elaborate discussion of quantum effects.
Optics and interferometry with matter waves is the art of coherently manipulating the translational motion of particles like neutrons, atoms and molecules. Coherent atom optics is an extension of techniques that were developed for manipulating \emph{internal} quantum states. Applying these ideas to translational motion required the development of techniques to localize atoms and transfer population coherently between distant localities. In this view position and momentum are (continuouse) quantum mechanical degree of freedom analogous to discrete internal quantum states. In our contribution we start with an introduction into matter-wave optics in section 1, discuss coherent atom optics and atom interferometry techniques for molecular beams in section 2 and for trapped atoms in section 3. In section 4 we then describe tools and experiments that allow us to probe the evolution of quantum states of many-body systems by atom interference.
Perfect crystal neutron interferometry has provided the basis for the realization of many basic experiments for testing quantum mechanics with massive particles. The following quantum effects have been verified: 4π-symmetry of spinors, spin-superposition, gravity-induced quantum interference effects, the Aharonov-Casher effect, the magnetic Josephson effects, the scalar Aharonov-Bohm effect etc. More recently, steps towards neutron quantum optics have been undertaken. Measurements have dealt with the coherence properties of the neutron beam, the photon exchange between the neutron and various electromagnetic fields, the particle number-phase uncertainty, squeezing phenomena, and the Wigner-functional behavior of a neutron system. These achievements opened the field of direct Fourier spectroscopy which brings a direct access to the space and time dependent correlation functions which describe condensed matter. Various perspectives for further experiments in the field of fundamental and condensed matter physics are discussed.