01 – Quantum Principles: Superposition

As this blog series progresses there will be many occasions where we take quantum phenomena to their theoretical and logical extremes. So to start off, I want to outline some of the key quantum principles and mechanisms that will be essential to truly grasping the following articles. First; quantum superposition.

Most people will mention Schrödinger’s cat when asked about quantum weirdness. This classic thought experiment remains incredibly relevant as we develop quantum technologies and traverse larger distances through the cosmos.

For those that don’t know it, the 1935 thought experiment went something like this: A cat is placed in a box and the box is sealed. The box contains a radioisotope which will decay (release radiation) at some point in the next given period of time (according to the Half-Life of the isotope), subsequently releasing a poisonous gas in the box and killing the cat. Upon sealing the cat in the box, you have no way of knowing if the isotope has decayed and therefore no way of knowing if the cat is alive or dead. This is the idea of superposition – that there are two possible states which are both currently equally as likely and therefore can be described as both being true and both being false – the cat is alive and dead.

If you are sceptical that this has any real bearing on the world around us, and instead is more likely to demonstrate a quirk of the mind, you wouldn’t be wrong to think so. The intriguing thing is that this superposition has since been demonstrated at a fundamental level, raising enormous questions about the metaphysical nature of the universe.

Before we get to that, there are some discrepancies with Schrödinger’s game – not least the cat’s own perspective. It is intuitively presumed that there must be some objective truth to the inside of the box, regardless of observation. It can be rather unsettling to think the experiment represents something actually true about the universe, rather than just a limit of our perception. This is all valid reasoning, until we remove the complex variables of the billions of particles involved in the original and get down to the quantum level of individual electrons.

In the double-slit experiment – a reproducible, falsifiable experiment – it was shown that even individual electrons can act not like the particles they were once thought to be, but instead like a wave. To discover this, individual electrons were fired toward a detector wall, passing through a middle wall with two vertical, adjacent slits in it. If the electron were a particle, it would have either hit the gap between the slits, or travel through one or the other and reach the detector wall, hitting it in a pattern that resembles two extended versions of the slits in the middle wall – think of throwing painted tennis balls through a door, you would make a pretty narrow strip on the wall beyond the door.

What actually happens is closer to ripples in water, there develops a pattern on the detector wall with multiple strips which dissipate in strength as they get farther from the centre. Remember, this is just one electron. The calculations for this suggested some spooky answers; the electron was apparently going through both slits simultaneously, and neither, and just one or the other. So, baffled, they set about investigating this wave-like behaviour.

A measuring device was put at the slits to determine which the electron went through. When they did this and repeated the experiment, they found the result on the detector wall changed, a pattern of two extended strips developed. The electron was now behaving like a particle.

This is now known as the observer effect, and has been demonstrated experimentally countless times, even with study participants simply averting and directing their attention away and toward the on-going double-slit set up.

So, there is something very real about the original thought experiment. Using the knowledge of this mechanism, as well as newly developed tests for this electron self-interference, researchers are devising practical applications for the phenomenon of superposition. In the near future, we hope to have working descriptions of the natural occurence of superposition, how this manifests to observers and the upper size limit of quantum weirdness. So, look out for upcoming nth degree articles on the quantum internet, macroentanglement, conscious quantum systems, the cosmic web, quantum time and many more.