All atomic nuclei are electrically charged, and many have spin causing them to behave like tiny magnets. If such a nucleus is placed in an external magnetic field, its magnetic moment will become aligned with that of the externally applied magnetic field. This phenomenon, nuclear magnetic resonance (NMR), is exploited in NMR spectroscopy and magnetic resonance imaging. The magnetised state of the nuclei may also be used to store information. Despite its utility, nuclear magnetism is weak and of short duration, limiting sensitivity and ‘memory’. With the support of the Marie Skłodowska-Curie Actions programme, the NuMagLongRx project developed computational tools for predicting the properties of novel long-lived nuclear spin states that will support development of methods to overcome both these hurdles.
Long-lived nuclear spin states provide new possibilities in nuclear magnetic resonance
Nuclear long-lived states were first described almost two decades ago by NuMagLongRx project coordinator Malcolm H. Levitt of the University of Southampton and colleagues. The scientists observed that the nuclear spin order in certain molecules is protected to a certain extent against some common relaxation mechanisms. It has an unusually long lifetime, far greater than the ordinary relaxation time constant (on average, about a second or less). Such states may be used to store spin order for a relatively long time, providing radically new experimental possibilities.
Supercomputers and quantum chemistry help predict long-lived nuclear spin states
Although long-lived nuclear spin states are potentially important for many applications, their existence and their lifetimes have proved hard to predict in detail. NuMagLongRx developed simulations and theory enabling these predictions under realistic conditions. The team combined supercomputers and molecular dynamics to simulate the motion of the molecules and quantum chemistry theory to predict how this motion influences the behaviour of the long-lived nuclear spin states. “When comparing the simulation results with experimental measurements, we realised that our existing theory did not fully explain the experimental observations. We discovered that a mechanism largely discounted by the research community is much more significant than expected. This is the rotational motion of the molecules in solution and the small magnetic fields it generates, which couple to the nuclei,” explains Levitt. When the team included this small term in their theoretical description, they significantly improved the agreement between simulation and experiment.
Record-breaking outcomes on nuclear long-lived states
“Our group has demonstrated that, in some cases, nuclear long-lived states may store information for over 1 hour in a room-temperature liquid in comparison to normal magnetic memory in the same substance under the same conditions,” states Levitt. NuMagLongRx outcomes mark the first time that the relaxation of long-lived nuclear spin states was successfully treated via a combination of molecular dynamics and computational chemistry, going beyond the current state of the art. Predicting the decay rate of nuclear magnetic information with good reliability for many systems, including metabolites in biofluids, will minimise the need for time consuming and expensive measurements in many cases. Levitt concludes: “Nuclear long-lived states are an excellent example of protected quantum states which are relevant to many other fields including quantum computing and quantum information processing.” NuMagLongRx has brought long-lived states closer to exploitation in NMR and its applications.
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