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Cambridge Scientists Create Huge Quantum Particles

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  • by students (763488) on Tuesday January 10, 2012 @01:18AM (#38647580) Homepage Journal

    Does anybody here understand what these scientists have supposedly achieved?

    This is in my area of research, and I read and understood the abstract. It does not seem like something that should be posted on Slashdot.

    In this case, quantum fluid means a fluid that is cold enough, dense enough, and made of low enough mass particles that it has some quantum mechanical properties (interference is an example in the abstract).

    Making a bigger quantum fluid is not really a challenge - you just need a bigger refrigerator and a bigger tank of helium. In this case, they made a bigger quantum fluid of a very specialized type.

    But isn't the whole point to quantum science that observation collapses a state into one thing or the other?

    No. That is just one small part of quantum mechanics.

  • by Baron Eekman (713784) on Tuesday January 10, 2012 @05:10AM (#38648686)

    The expression "quantum fluid" can be misleading.

    What they did here is make a system of coherent "polaritons" just as laser light is a bunch of coherent photons/light waves. As mentioned in the article abstract, a polariton is some combination of a photon (light particle) and an exciton. In turn, an exciton is a bound state of an electron and a hole from the semiconductor. (A hole is the 'vacant' positive charge created when an electron is removed, and may for all practical purposes be regarded as an anti-electron within the semiconductor.) If I understand correctly, the novelty in this work is not making the polariton condensate but the visualization of it. In that sense, the summary if way off.

    This is surely not easy to grasp for the layman. What does this imply? As parent mentioned, making coherent quantum states or matter is a standard affair by now, and research focuses on extending our capabilities on all levels. It is necessary for our understanding of the fundamentals of quantum mechanics and how many particles conspire to make laboratory but also everyday matter. The practical possibilities for making devices out of "quantum fluids" is severely limited, since you almost always need extremely low temperatures to produce them. Only superconductors come close.

  • by Anonymous Coward on Tuesday January 10, 2012 @09:00AM (#38649674)

    One simple obvious thing I haven't figured out, is if super-cooled Helium qualifies as a Bose-Einstein condensate, and if not why not? Everything I see on this refers to BEC's being created in 1995, but I equally see the properties of supercooled helium as being due to Bose-Einstein Statistics, even to a point of noting that Helium IV creates a superfluid faster than (at higher temperature) because Helium IV is naturally a boson, while Helium III is only a boson in pairs.

    Do superfluids qualify as a condensate, could they qualify if they were cooled further, and what are the actual differences between a Helium IV superfluid and a condensate?

    (Yes I work in BEC)

    Helium 4 becomes superfluid at low temperatures because there is Bose-Einstein condensation present in the Helium, but it's only a small fraction of the total sample (at very low temperatures the condensate fraction is ~ 10%, while the superfluid fraction is in ~ 100%). The actual situation in this system is massively complicated by the fact that Helium is a liquid in this regime.

    Einstein's prediction of condensation was for an ideal gas (i.e., no interactions between the atoms). The novelty here is that the transition to a different phase of matter is therefore driven by the quantum statistics, rather than interactions between the atoms (as in every other phase transition).

    The next-most-simple scenario is a near-ideal gas, i.e., a bunch of atoms which scatter off one another (in one-on-one collisions only), and which don't scatter very hard. This system will also undergo condensation, though of course some pedants will argue that it isn't the Bose-Einstein transition anymore because interactions are present. It's very closely related though.

    This near-ideal-gas system provided (in ~ 1950s ) a theoretical basis for qualitatively understanding superfluid Helium, despite the fact that it's a terrible approximation, as liquid Helium is just that: Liquid, and so the atoms are interacting with one another like crazy. The condensates realized in the late nineties are formed in gaseous samples, where the near-ideal-gas type of model can be quantitatively accurate, and virtually 100% pure condensates can be formed in this way. One upshot of this is that you can observe a lot of the phenomenology that is associated with all BEC in principle, but which in practice would be impossible to observe in a system like liquid Helium.

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