Physics
Optical interferometers can probe an object at high resolution through its effect on their interference pattern. Neutrons also have an associated wavelength—the de Broglie wavelength— which is very much smaller than any optical wavelength and can in principle provide information on an atomic size scale. However, neutron interferometers are susceptible to low-frequency mechanical vibrations of the interferometer itself. Pushin et al. show that ideas based on error correction protocols used in quantum information science can be carried over to suppress such effects of environmental noise. They show that the formation of decoherence-free subspaces (DFSs), special states in which the system is manipulated in such a way as to protect it from the surrounding environment, can be formed by carefully designing the setup of the system. In this case, they use a figure-eight interferometer carved into a single ingot of silicon instead of the usual two-arm interferometer setup and show that a DFS formed at the center of the interferometer suppresses the effect of environmental noise.
Phys. Rev. Lett. 107 , 150401 (2011).
Physics
Optical interferometers can probe an object at high resolution through its effect on their interference pattern. Neutrons also have an associated wavelength—the de Broglie wavelength— which is very much smaller than any optical wavelength and can in principle provide information on an atomic size scale. However, neutron interferometers are susceptible to low-frequency mechanical vibrations of the interferometer itself. Pushin et al. show that ideas based on error correction protocols used in quantum information science can be carried over to suppress such effects of environmental noise. They show that the formation of decoherence-free subspaces (DFSs), special states in which the system is manipulated in such a way as to protect it from the surrounding environment, can be formed by carefully designing the setup of the system. In this case, they use a figure-eight interferometer carved into a single ingot of silicon instead of the usual two-arm interferometer setup and show that a DFS formed at the center of the interferometer suppresses the effect of environmental noise.
Phys. Rev. Lett. 107 , 150401 (2011).
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.
Perfect single-crystal neutron interferometers are adversely sensitive to environmental disturbances, particularly mechanical vibrations. The sensitivity to vibrations results from the slow velocity of thermal neutrons and the long measurement time that are encountered in a typical experiment. Consequently, to achieve a good interference solutions for reducing vibration other than those normally used in optical experiments must be explored. Here we introduce a geometry for a neutron interferometer that is less sensitive to low-frequency vibrations. This design may be compared with both dynamical decoupling methods and decoherence-free subspaces that are described in quantum information processing. By removing the need for bulky vibration isolation setups, this design will make it easier to adopt neutron interferometry to a wide range of applications and increase its sensitivity.
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.
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.
Matter-wave interferometry with solids is highly susceptible to minute fluctuations of environmental fields, including gravitational effects from distant sources. Hence, experiments require a degree of shielding that is extraordinarily challenging to achieve in realistic terrestrial or even space-based setups. Here, we design protocols that exploit the spatial correlations that are inherent in perturbations due to distant sources to reduce significantly their impact on the visibility of interference patterns. We show that interference patterns that are robust to such type of noise can be encoded in the joint probability distribution of two or more interferometers, provided that these are initialized in suitable states. We develop a general framework that makes use of N+1 interferometers, which may differ in their masses, to correct for environmental potential fields up to order N in their multipole expansion. Remarkably, our approach works for fields that fluctuate stochastically in any timescale and does not require the presence of quantum correlations among the different interferometers. Finally, we also show that the same ideas can be extended to the protection of entanglement between pairs of interferometers.
The quantum interference of de Broglie matter waves is probably one of the most startling and fundamental aspects of quantum mechanics. It continues to tax our imaginations and leads us to new experimental windows on nature. Quantum interference phenomena are vividly displayed in the wide assembly of neutron interferometry experiments, which have been carried out since the first demonstration of a perfect silicon crystal interferometer in 1974. Since the neutron experiences all four fundamental forces of nature (strong, weak, electromagnetic, and gravitational), interferometry with neutrons provides a fertile testing ground for theory and precision measurements. Many Gedanken experiments of quantum mechanics have become real due to neutron interferometry. Quantum mechanics is a part of physics where experiment and theory are inseparably intertwined. This general theme permeates the second edition of this book. It discusses more than 40 neutron interferometry experiments along with their theoretical motivations and explanations. The basic ideas and results of interference experiments related to coherence and decoherence of matter waves and certain post-selection variations, gravitationally induced quantum phase shifts, Berry's geometrical phases, spinor symmetry and spin superposition, and Bell's inequalities are all discussed and explained in this book. Both the scalar and vector Aharonov-Bohm topological interference effects and the neutron version of the Sagnac effect are presented in a self-contained and pedagogical way. Interferometry with perfect crystals, artificial lattices, and spin-echo systems are also topics of this book. It includes the theoretical underpinning as well as connections to other areas of experimental physics, such as quantum optics, nuclear physics, gravitation, and atom interferometry. The observed phase shifts due to the Earth's gravity and rotation indicate a close connection to relativity theory. Neutron interferometry can be considered as a central technique of quantum optics with massive particles. It has stimulated the development of interferometry with atoms, molecules and clusters. The book is written in a style that will be suitable at the senior undergraduate and beginning of graduate level. It will interest and excite many students and researchers in neutron, nuclear, quantum, gravitational, optical, and atomic physics. Lecturers teaching courses in modern physics and quantum mechanics will find a number of interesting and historic experiments they may want to include in their lectures.