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Atomic Spins Evade Heisenberg Uncertainty Principle

posted by on Monday April 03 2017, @03:51PM   
from the the-universe-is-stranger-than-you-imagine dept.

martyb writes:

You may (or may not) have heard about the Heisenberg Uncertainty Principle:

In quantum mechanics, the uncertainty principle, also known as Heisenberg's uncertainty principle, is any of a variety of mathematical inequalities asserting a fundamental limit to the precision with which certain pairs of physical properties of a particle, known as complementary variables, such as position x and momentum p, can be known.

According to a report in ChemistryWorld, a new technique allows atomic spin properties to be measured simultaneously with greater accuracy — Atomic Spins Evade Heisenberg Uncertainty Principle:

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.

Is this the harbinger of finer-grained and/or quicker MRIs?

References: G Colangelo et al, Nature, 2017, DOI: 10.1038/nature21434

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Original Submission

• No, not quite No, not quite (Score: 2, Informative) by Anonymous Coward on Monday April 03 2017, @04:01PM (1 child)

  by Anonymous Coward on Monday April 03 2017, @04:01PM (#488244)

  This isn't evading the Heisenberg Uncertainty Principle.

  The journal has a decent Editor's Summary for this paper:

  Many quantum systems that are currently used to enhance metrological precision obey the regular Heisenberg uncertainty relations that apply to conjugate variables such as position and momentum. These systems can be 'squeezed' to reduce the uncertainty of one variable at the expense of greater uncertainty in another, and thereby to surpass the limits set by classical physics in metrology. However, spin systems and pseudo-spin systems obey different uncertainty relations because of their underlying symmetries. On the basis of these relations, the authors demonstrate simultaneous measurement of spin amplitude and spin angle beyond classical limits. This approach has potential applications in spin-based sensors and could increase the sensitivity for several applications, such as magnetic resonance measurements, in which spin relaxation rates could be correlated with precession frequency with higher precision than is currently possible.

  Also, from the paper's Intro section (my emphasis added):

  For simple harmonic oscillator systems, it is well known that quantum measurement back-action couples angle and amplitude, or equivalently the quadratures X and P, as required to preserve the Heisenberg uncertainty relation δXδP≥1/2 (we take ħ=1 throughout). This limits angle tracking to the standard quantum limit25 (SQL), with uncertainty δψ∼ N−1/2, where N is the mean number of excitations (here and throughout, ‘∼’ indicates equality to within a factor of order unity). In contrast, uncertainty principles do not prevent spin systems from being tracked beyond the SQL. As the spin components Fy and Fz precess about the x axis, they are governed by the Robertson (not Heisenberg) uncertainty relation

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• Re:No, not quite (Score: 3, Interesting) by FatPhil on Monday April 03 2017, @08:34PM

  by FatPhil (863) <reversethis-{if.fdsa} {ta} {tnelyos-cp}> on Monday April 03 2017, @08:34PM (#488354) Homepage

  Not just that:
  
  > 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.
  
  HUP applies where it applies for purely mathematical rules of definition, not *because* measurement disturbs the system. Even if there was a measurement that didn't disturb the system, the HUP would still apply. The above is often given as an easy to understand reason, but it's just another one of those lies that are told to simplify things to people who don't want to look at the equations. The waveform simply has many (possible) positions and many (possible) frequencies (which determine the momentum), you cannot accurately define one without making the other less well defined.
  
  This seems to be a reasonable write-up: http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec14.html

  --
  Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves

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