Pedagoguery

Quantum mechanics is widely viewed as the physics of the very small: molecules, atoms, and subatomic particles. However, scientists are able to observe quantum behavior at macroscopic scales, demonstrating that quantum mechanics is all around us, it just escapes notice because of the way quantum systems interact with each other.

The classical example of quantum effects on the macro scale is Schrödinger's cat. This was a thought experiment that Erwin Schrödinger designed to demonstrate how weird quantum mechanics can be. In it, we imagine a cat in an opaque box. Also inside the box is a bottle of poison gas, which is linked to the decay of an atom of a radioactive element. Within a set time peeriod, there is a 50% chance that the atom decays, releasing the gas. So, according to quantum mechanics, after that set period of time, the cat is neither dead nor alive, but a superposition of both states – a nonsensical state. It is only the act of observation that “collapses the wave function”, forcing the cat to be in one state or another.

Another classical example of how quantum effects can be seen at larger scales is the phenomenon of entanglement. Entanglement is two particles or systems share a connection. Let's take for example, two electrons that have a net spin of zero. If electrons behaved classically, then you could set up one to spin clockwise on a vertical axis and the other counterclockwise on the same axis. If measured along the axis, you would see one spinning clockwise and the other counterclockwise. If measured perpendicular to the axis, you would detect no spin at all. Quantum mechanically, however, they behave very differently. If you measure the spin of one, it will be spinning either clockwise or counterclockwise at random, and the other would be spinning the opposite way, if measured along the same axis. However, it does not matter what axis you start with, so long as both electrons are measured along that axis – one will always be clockwise and the other counterclockwise. It also does not matter how far apart the electrons are. They could be at opposite ends of the galaxy, and if you measured one, you would instantly know the spin of the other.

The common understanding of how these effects stay localized to the micro scale is that in a larger system, collisions with other particles end up changing the state of the particles in question. In effect, the information “leaks out”, a phenomenon called decoherence. But can decoherence be fought off in larger systems? It seems that it can.

The first case of this happening in experiments was in 2003 when Gabriel Aeppli of University College London tested the magnetic susceptibility of lithium fluoride salt. Magnetic susceptibility is the phenomenon of the atoms of a substance aligning themselves to an external magnetic field. They learned that this happened faster than could be explained by the forces the atoms exerted on each other. The only factor they could use to explain the quickness of the action was entanglement. To avoid thermal effects, the experiment was conducted at extremely low temperature – milliKelvins. Since then, however, other groups have been successful in observing entanglement in a number of different systems, including copper carboxylate at room temperature and higher.

We have also learned that biological systems use quantum effects like entanglement. For example, European robins migrate every year from Scandinavia to equatorial Africa and back. It was theorized that they make use of the earth's magnetic field, but when migrating birds were placed in an artificial magnetic field with reversed polarity, it showed no effect. Further experiments determined that the birds could detect the inclination of the magnetic field with respect to the surface of the earth, and that is all they needed to help them navigate. However, when the birds were blindfolded, they lost this ability. The likely exploitation is entanglement. There is apparently a molecule in the birds' eyes that contains a pair of entangled electrons with a net zero spin. When a photon hits this molecule, the energy causes the electrons to separate, where they can interact with the ambient magnetic field in a way that changes the chemistry of the molecule. This change in chemistry is translated into nerve impulses that the bird uses to decipher the magnetic field.

Plants also use entanglement during photosynthesis. Light causes electrons to be ejected from certain molecules in plant cells, and those electrons have to make their way to the reaction center to deposit their energy. However, this process happens with astonishingly high efficiency – far more so than can be explained classically. In a quantum world, however, electrons can take all possible paths. Electromagnetic fields within the plant cell can cancel some of the paths and reinforce others, which can reduce the chance that the electron will take a wasteful detour.

Scientists have not yest found out how these natural systems maintain entanglement over such relatively long timescales and in such large systems, but figuring that out could be the key to harnessing some of those effects ourselves in ways such as quantum computing.

Next time, the volcanoes of the solar system.