Saturday, November 3, 2012

Big Swirl

Superposition of +10 OAM plus -10 OAM

Quantum mechanics is usually perceived as the (somewhat bizarre) physics of the very small, while large objects obey the logic of common sense. One of the most exciting trends in modern physics is the attempt to create larger and larger objects that obey quantum rules and hence to quietly smuggle the weirdness of quantum reality into the realm of everyday life.

Recently Robert Fickler and his colleagues at the University Of Vienna have devised a clever experiment that expands the realm of quantum entanglement into the region of high quantum numbers by devising a scheme that entangles two photons with arbitrarily high values of "orbital angular momentum (OAM)"-- a quantity I will call "swirl".

Every photon has an intrinsic spin equal to one Planck unit. Most experiments in quantum entanglement operate by using special non-linear crystals to produce a pair of photons in which this spin degree of freedom is divided between the two photons in such a way that each photon is in an uncertain spin state but the spin of the larger two-photon state is quantum-determined. This leads to the unusual situation (characteristic of quantum entanglement) that no matter how distant the two photons are separated, they in some sense still form a single entity so that an action on one photon seems to instantly influence the properties of the other. One might imagine that this instant influence could be used to send faster-than-light signals and hence break the well-known Einstein speed limit but quantum theory possesses a subtle structure that allows Nature access to this superluminal channel while denying it to human beings.

Fickler and his friends start with the usual pair of photons entangled in spin-one space and by clever use of a Spatial Light Modulator (SLM) -- a liquid-crystal device not too different from the gadget that is producing this image on your flat screen -- they can add to each photon a lot of "swirl" beginning with 10 spin units (shown above) and working their way up to successfully entangling two photons each possessing 300 units of swirl. The amount of swirl they can add to the photons is limited not by the laws of physics but by the pixel density of their Spatial Light Modulator so with better technology much larger values of "swirl" than 300 can be entangled.

Using their current setup, Fickler and company conveniently produce not just a photon with a big swirl of 300 but a photon that exists in a superposition of 300 units of clockwise swirl plus 300 units of counterclockwise swirl -- a situation I have elsewhere called "Schrödinger's Carousel".  And this (300/-300) swirled photon is quantum-entangled with a similar high-swirl photon which can be located many meters away.

However clever this achievement, a swirl of only 300 units is still much too small to be perceived by human senses. But there is an analogous phenomenon created by lots of tiny aligned spins (much more than 300) called magnetism which might someday be coaxed to produce bizarre quantum phenomena perceivable by humans.

For me one of the most elegant experiments that connects the micro-world with the macro-world is the Einstein-de Haas Effect. If you suspend a magnet on a string and demagnetize it by heating, the magnet magically begins to rotate without the application of any force. This mysterious rotation is explained by the fact that magnetism is the result of an immense number of electrons whose spins (one unit each of AM) are all aligned in the same direction. When the magnet is heated, the direction of these spins is randomized. The overall rotation is still conserved however, and is transferred from the electrons to the crystal lattice and hence the whole magnet begins to spin.

Learning how to entangle big swirls is starting in Austria with pairs of photons but perhaps, using quantum entanglement, magnets, which are already remarkable things, may someday be transformed into macroscopic quantum objects that will behave in ways that seem truly miraculous.

On another note, I've just discovered how to add a "favicon" to the URL line of my blog, and after experimenting with many complex images, have decided to use this simple "white portal" on a purple background to symbolize my quantum tantric quest to discover radically new "doorways into Nature"

New doorway into Nature?

ADDENDUM: My younger brother Duke, an ex-Marine now living in Montana, suggested THIS as a candidate image for "doorway into Nature."

Brother Duke's old doorway into Nature

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