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Atomic Spins Evade Heisenberg Uncertainty Principle

New measurements revise the limits of quantum fuzziness

By Tim Wogan, ChemistryWorld on March 31, 2017

Credit: Richard Kail Getty Images

   Many seemingly unrelated scientific techniques, from NMR spectroscopy to medical MRI and timekeeping using atomic clocks, rely on measuring atomic spin – the way an atom’s nucleus and electrons rotate around each other. The limit on how accurate these measurements can be is set by the inherent fuzziness of quantum mechanics. However, physicists in Spain have demonstrated that this limit is much less severe than previously believed, measuring two crucial quantities simultaneously with unprecedented precision.

   Central to the limits of quantum mechanics is the Heisenberg uncertainty principle, which states that it is not possible to know a particle’s position and momentum with absolute accuracy, and the more precisely you measure one quantity, the less you know about the other. This is because to measure its position you have to disturb its momentum by hitting it with another particle and observing how the momentum of this second particle changes. A similar principle applies to measuring a particle’s spin angular momentum, which involves observing how the polarisation of incident light is changed by the interaction with the particle – every measurement disturbs the atom’s spin slightly. To infer the spin precession rate, you need to measure the spin angle, as well as its overall amplitude, repeatedly. However, every measurement disturbs the spin slightly, creating a minimum possible uncertainty.

The alternative approach suggested by Morgan Mitchell’s group at the Institute of Photonic Sciences in Barcelona, could circumvent this problem. The spin angle, they say, is in fact two angles: the azimuthal angle (like longitude on the Earth’s surface) and the polar angle (like latitude). To measure the precession rate, you need only the azimuthal angle. Therefore, by loading as much uncertainty as possible into the polar angle, you can measure the two quantities you need – the azimuthal angle and amplitude of the spin – and therefore measure the spin precession rate much more accurately than previously thought possible. ‘There are experiments that people are doing now that people expect to be limited by the Heisenberg uncertainty principle which in fact are not,’ says Mitchell.

Actually achieving this in practice, however, proved extremely difficult. The team cooled down a cloud of atoms to a few microkelvin, applied a magnetic field to produce spin motion and illuminated the cloud with a laser to measure the orientation of the atomic spins. ‘Not all the technologies we used for the experiment existed when we started,’ says Giorgio Colangelo, another member of the research team. ‘We had to design and develop a particular detector that was fast enough and with very low noise. We also had to improve a lot the way we were preparing the atoms and find a way to efficiently use all the dynamic range we had in the detector.’ The researchers hope that atomic timekeeping and nitrogen-vacancy magnetometry, which uses the precession of nitrogen defects in diamonds to measure magnetic fields, may benefit from the techniques unveiled here in the next few years. ‘We really hope that, in the long term, magnetic resonance techniques such as NMR and MRI may benefit, but right now they are limited by some other effects,’ says Colangelo.

Eugene Polzik of the University of Copenhagen in Denmark is impressed: ‘It sets a new and clever way of measuring certain magnetic field disturbances using an ensemble of quantum spins,’ he says. ‘It would be easy for me to look at this and say “Oh, yes, right: it doesn’t contradict quantum mechanics,” but to figure out how to achieve this, to understand how relevant it is and under what circumstances it is relevant – this is an excellent and elegant development.’

References

G Colangelo et al, Nature, 2017, DOI: 10.1038/nature21434
This article is reproduced with permission from Chemistry World. The article was first published on March 30, 2017.

Quantum Microscope Spies on Chemical Reactions in Real Time

Diamond-based imaging system uses magnetic resonance of electrons to detect charged atoms By Sara Reardon, Nature magazine on March 7, 2017

Researchers say their new, non-invasive imaging technique could theoretically be used to image the interior of a living cell.  Credit: John Bavosi Getty Images

   A quantum microscope that uses a sensor built from diamonds could allow researchers to study such nanoscale mysteries as how DNA folds in a cell, why drugs work or how bacteria metabolize metals. Crucially, the microscope can image individual ions in a solution and reveal biochemical reactions as they occur—without interfering in the process. The team behind the system described the results in a February 14 preprint on the arXiv server.
   Researchers have long wanted an imaging system for molecular structures that works like hospital magnetic resonance imaging (MRI) machines, which reveal structures inside the human body without harming them. The idea behind a quantum MRI—which images at the quantum level using electron spins—is to do the same for chemical reactions including those involving metal ions. Current magnetic resonance techniques can only reveal structures measuring 10 micrometres or more, and the only way to detect metal ions inside a cell is to add reactive chemicals or freeze the cell so it can be imaged under powerful microscopes—procedures that kill the cell.
   A hospital MRI machine works by placing a patient inside a magnetic field, such that protons in the body's atoms align with the machine's magnet. The machine then sends radio pulses through the body area being imaged, which knocks the protons out of alignment. When these pulses are switched off, the protons realign and emit electromagnetic waves at a particular frequency. If the frequency emitted by the body's tissues matches that of sensors in the machine, the two frequencies will resonate like guitar strings tuned to the same note. The machine uses this resonance to reconstruct an image of the body.
A team led by physicists Lloyd Hollenberg and David Simpson at the University of Melbourne, Australia, wanted to use this technique to detect metal ions in cells. Some metal ions can be harmful to cells, whereas others are necessary for biochemical reactions, such as those involved in metabolism. The catch is that an MRI sensor needs to be about the same size as the item being imaged, which is currently impossible when trying to look at a single atom.

Flawed diamonds

   To make their quantum MRI microscope, the researchers used 2-millimetre-wide diamonds that contained atomic-sized flaws in their crystal structure. These flaws are sensitive to changes in magnetic fields and can be 'tuned' to resonate with the spin of the molecule or ion that is being detected. When the diamond's flaws are illuminated with a green laser, the diamond fluoresces red, and the brightness of that fluorescence depends on the strength and direction of an applied magnetic field.
   Hollenberg, Simpson and their colleagues used a diamond that had an array of flaws in specific locations just below its surface and placed it at the end of a microscope next to a sample. The researchers tuned the defects to a frequency that resonated with the spin of an ionized form of copper that is missing two electrons (Cu2+). By touching the diamond probe to the surface of a sample containing copper ions, the resonance between the two stimulated fluorescence in the diamond flaws. The researchers used a computer program to examine the colour coming off the diamond flaws and to reconstruct an image of the sample, revealing the precise location of each copper ion. Next, the researchers flooded the sample with an acid that adds an electron to Cu2+, turning it into Cu+. As they added the acid, they imaged the sample and watched the Cu2+ spin pattern disappear. The pattern then reappeared over the course of an hour as the sample was oxidized to Cu2+ on exposure to air. Such a method could one day allow researchers to watch biochemical reactions as they occur in cells.
   Because the method is non-invasive, it could theoretically be used to image the interior of living cells—something that Simpson and Hollenberg's team is working towards. The main obstacle is that the diamond probe needs to be physically close to the sample to produce a signal. But the team says that the current method will still be useful for understanding drug mechanisms and investigating proteins found on the cell membrane. The researchers are also trying to adapt the system so it can detect different metals, including iron.
   Friedemann Reinhard, a physicist at the Technical University of Munich in Germany, praises the work. “The innovations here bring it a lot closer to the application,” he says. His group is also working with diamond microscopy, creating a system that could image molecules in 3D.
He adds that although the new technique still needs improvements, such as the ability to find copper ions in low-concentration solutions, it is “definitely a great step ahead.”

This article is reproduced with permission and was first published on March 6, 2017.

SOURCE :

https://www.scientificamerican.com/article/quantum-microscope-spies-on-chemical-reactions-in-real-time/