Quantum 101 Episode 4: Superposition Explained | Schrödinger's Cat

If you've heard anything about quantum mechanics, you've probably heard of Schrödinger's cat. It's a famous thought experiment (*not a real experiment*) in which a cat is said to be both alive and dead at the same time.

It's in a quantum superposition. But what does that really mean? This video discusses what quantum superposition really is. And if this bizarre thought experiment relates to the real world at all. The answer is yes… and no.

Join Katie Mack, Perimeter Institute’s Hawking Chair in Cosmology and Science Communication, over 10 short forays into the weird, wonderful world of quantum science. Episodes are published weekly, subscribe to our channel so you don’t miss an update.

Follow our host "AstroKatie" Katie Mack:

Follow Perimeter:

[wmstuckey]

I like that Katie mentions the quantum bit of information (qubit). What people fail to mention when talking about quantum superposition using Schrodinger's Cat is the difference between a classical bit of information (like a computer bit being on or off) and a qubit. Both bits produce one of two outcomes when queried (measured), but a classical bit has only one measurement possible while a qubit can be measured in many different ways (infinitely many, actually), each with two possible outcomes.

For example, when you pass an electron through an inhomogeneous magnetic field, the electron is either deflected towards the North magnetic pole ("up") or towards the South magnetic pole ("down"). You can orient the N-S magnetic field in any direction you like and the electrons will still give one of those two outcomes, so electron spin is a qubit with two outcomes of spin "up" and spin "down" relative the the N-S magnetic field. Now suppose you pass electrons through a N-S magnetic field oriented vertically and then send those that deflected "up" (literally up in this case) to a N-S magnetic field oriented horizontally. What do you expect to find?

Well since the electrons have vertical spin up and spin is a vector (picture an arrow pointing upward here), then you probably expect the electron to pass straight through the horizontal magnetic field, i.e., they won't be deflected left or right at all ("up" or "down" relative to the horizontal N-S field). That's because the electron's spin vector (arrow) points up which means it doesn't point side-to-side (left of right) at all, so your horizontal spin measurement of a vertical spin up electron should seemingly yield a result of zero horizontal spin. But, what you find instead is that 50% of the vertical spin up electrons are deflected left ("up" towards North pole) and 50% are deflected right ("down" towards South pole). True, 50% left plus 50% right *averages* to zero, but that's not what you expect from the measurement of a vector quantity in ordinary classical mechanics. [Aside: Quantum mechanics gives the classically expected results on average over the discrete or quantum measurement outcomes.] This is quantum superposition, a vertical spin up electron is a quantum superposition of 50% horizontal spin left and 50% horizontal spin right and we write that as |V+> = |H+> + |H-> (divided by root 2 for normalization, but I don't need that to make my point).

The point here is the horizontal spin measurement of the quantum state |V+> produces each of its two "up"-"down" (left-right) results in 50-50 fashion. This is exactly what you hear people say about Schrodinger's Cat, i.e., you open the box and find the cat is awake with 50% probability or find the cat is asleep with 50% probability. With that information alone, Schrodinger's Cat could be a classical bit or a qubit. If Schrodinger's Cat is a qubit, then there must be a measurement of the cat-box system like the vertical spin measurement of the state |V+> that produces |V+>, i.e., |H+> + |H->, with 100% certainty. We know the measurement "open the box" producing the "Asleep Cat"-"Awake Cat" results in 50-50 fashion is analogous to the horizontal spin measurement of |V+>, so what is the measurement of the cat-box system producing |Asleep Cat> + |Awake Cat> with 100% certainty in analogy with the vertical spin measurement of the state |V+> that produces |V+> with 100% certainty? And what does its outcome mean physically? If you can't articulate that measurement and outcome of the cat-box system, and every possible measurement between that measurement and the "open the box" measurement, then the cat-box system is just a classical bit ... like opening a box to find a ball or no ball. No quantum superposition there 🙂

To read more about the quantum information approach to superposition and entanglement for the "general reader, " see "Einstein's Entanglement: Bell Inequalities, Relativity, and the Qubit" due out in June 2024 with Oxford UP.

[Deipnosophist_the_Gastronomer]

When did Schrödinger's cat go from being [alive / dead] to [asleep / awake]?

[paulbadertscher]

Katie Mack + Carlo Rovelli = the best chance a lay person will have at understanding this stuff.

[Mizraab]

That 'Well Yes...and No' is such a brilliant thing to add in the Superposition episode!

[Sam-sgmp]

The Observer or Drashta in Sanskrit is explained in volumes of Ashtavakra Mahageeta.
the concept of ‘Drashta’ or witness or observer in a very unique way.
Also Osho as explained in his discourse of Ashtavakra Maha Gita. In which he talks about enlightenment or state of mind i.e. the superposition and the realisation of enlightenment by being the Drashta or an observer is explained.

[Paul]

Actually the cat is not "both" asleep and awake, it is *neither* asleep nor awake because it is in a *superposition* of the two, until observation. (NB. in this formulation of the problem one also needs the assumption that the cat does not wake up on its own during the experiment.)

A good resolution to all this was recently given by Presentist Fragmentalism.

[cspeacock]

Peter Harness delves into this idea of superposition and quantum entanglement in his hit series "Constellation." It's a mind-bending show, available on AppleTV. It's worth a watch.

[arilatvala]

It is sad to note that good lecturing is spoiled with mixing overplay of music.

[lenkyl]

this series is fantastic! thank you dr mack for being so passionate about cosmology and being such a great communicator of it!

[priyakulkarni]

Your music in the video destroyed 😅😅😅😅😅 super position

[xanth]

once you've "observed" something and it's "picked" a state, does it go back to superposition when you stop observing it ? Where is it getting the energy needed to change states?

[ericadenison]

General Physics I: Classical Mechanics
General Physics II: Electromagnetism
Modern Physics III: Forget what you have learnt and start from square one! Hence I am here life long learning is fun!

[Geminiblack]

It appears and to my understanding I can tell when we see the world around us we are currently in superposition where there can be infinite universe and species in different universe but when we have the capability to focus on each of them then we will know there is an ultimate power where all these converges and settles into one entity.

[stevenverrall]

Great fun educational video that clearly makes its point!

[MrPedalpaddle]

Excellent! There clearest explanation of Super Position yet.

[biosphere]

Anyone who is offended or feelings are hurt over a thought experiment doesn't deserve quasi virtue signaling consideration.

[neIntangible]

I'd like to see if other experiments, similar to the DS, but done with different patterns and shapes, such as a double cross, or an ( X ), if you will, or other configurations, ( O )... have they been tried or are we ultimately limited to the interpretation of only the double slit ( II )... ?🤔

[funnyguy_]

guys this is basically a "do you eat twix on the left side or right side first" without opening the wrapper

[LcdDrmr]

So, "superpositions is just a way of talking about quantum uncertainty" means that there is no such "thing" as superposition, rather the behavior of a single particle appears to be wave-like, and therefore is a mystery when viewed from the standpoint of classical mechanics. Quantum behavior doesn't "make sense", so we make up a concept of particles being in lots of places at once (totally ignoring that the energy required to have the same particle be millions of particles at the same time violates conservation laws, as does "many worlds" interpretations) and call it superposition, something that can never be observed because observation causes all the particles but one to disappear. This seems to dress up something that isn't understood with magical thinking, the anathema of science. Superposition seems more a placeholder, like "dark matter", than an actual reality that can be demonstrated to exist.

[gdr]

At quantum scale, charged particles with certain additional energies exist as waves. In that state, phase of it changes very fast and hence only amplitude peak direction (phase)detectable by interacting with Photon of suitable energy resulting in non quantum state. This appears instantaneous ( simultaneous) when compared to classical interpretation. Charged particle with certain additional energy comparable to a spinning coin in the air. When coin falls on the surface, loses additional energy and takes stable state: head or tail but one state only.