Mapping of size distributions of a mouse’s gray matter by quantum-controlled proton MRI. (l) Brain proton MRI; (c) mean cellular size; (r) distribution peak
The smallest devices for storing information are qubits – quantum bits. Qubits are all around us – they are in the nuclei of atoms, for example. The trick is to learn to read and use the information they contain.
One technology that reads the information in quantum systems is magnetic resonance (MR). MR – including the familiar MRI scan – exploits the quantum nature of the protons found, for example, in the water molecules in our bodies. These protons have a property known as “spin”: states of rotation that are measured in one of two discrete values, often referred to as “up” or “down.” Spin states endow the protons in a molecule with magnetism, transforming them into “quantum compass needles” that “point” along one of two orientations. These micro-magnets are what enable magnetic resonance: Protons, placed in certain conditions in an external magnetic field, will rotate like a top, emitting very weak – but observable – radio waves. MR equipment detects the distinctive radio waves emitted from the protons in such molecules as water, using their locations to build an image.
Worlds of information, in addition to the spatial location of protons, could be extracted from MRI if the protons’ emitting states could be efficiently preserved. Unfortunately, the quantum states of these micro-magnet protons are exquisitely delicate; the information they record is rapidly destroyed by interference from the tiny magnetic fluctuations that occur spontaneously in their surroundings.
How can one protect these delicate systems, on the one hand, and get them to reveal the subtle information they carry about their environment, on the other? Prof. Lucio Frydman, together with visiting scientist Dr. Gonzalo Alvarez and postdoctoral fellow Dr. Noam Shemesh, all in the Institute’s Chemical Physics Department, illustrated a new approach to answering this question, showing that some forms of interference can help, rather than hurt, in preserving this information. Their research, as reported in Physical Review Letters, reveals a new family of MR-based methods that may be used, among other things, to probe the size and shape of small pores, cells or tiny nanostructures.
The new methods operate by exposing nuclear spins to one kind of environmental interference while protecting them from all the others. The interference chosen by Frydman and his group comes from fluctuating magnetic fields associated with the random, Brownian motions that arise from regular quantum particle collisions. This manipulation turns the nuclei into “spies” that, during their random walks, can “scout out” the confines of a cell or the boundaries of their local microstructures. Combined with the spatial imaging ability of a standard MRI, the result is a completely new way to measure microscopic structural architectures at extremely high resolution. As an added plus, the technique is noninvasive: Shemesh, Alvarez and Frydman demonstrated its ability to perform complex biological imaging – in this case, “virtual histologies” – mapping cell-size distribution in whole brains.
Possible additional applications of this method range from solid-state physics to materials sciences and, of course, to the life sciences – where they could, for example, measure the changes in size and shape that the nerve cells’ axons experience due to neural stimulation or maturation. It could also lead to the development of new contrast mechanisms for medical MRI scans, and to better diagnostic methods for observing physiological changes in diseased tissues or those caused by neurodegeneration.
Prof. Lucio Frydman’s research is supported by the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research; the Helen and Martin Kimmel Award for Innovative Investigation; the Helen and Martin Kimmel Institute for Magnetic Resonance Research, which he heads; the Adelis Foundation; the Mary Ralph Designated Philanthropic Fund of the Jewish Community Endowment Fund; Gary and Katy Leff, Calabasas, CA; Paul and Tina Gardner, Austin TX; and the European Research Council.