The title of this post is something very serious, even if it may appear crazy. It is indeed not that weird to discuss quantum computing and cats within the same post. This is what I will (try to) prove here.

Of course, by cats I refer to **Schrödinger’s cats**, and their common ground with **quantum computers** is simply what is called **quantum decoherence**.

_{[image credits: Wikimedia (public domain)]}

This post follows an interesting discussion we had the other day on the **steemstem chat** about the famous Schrödinger’s cat thought experiment.

Some mentioned that this ‘experiment’ was a paradox, something that I strongly disagree with.

With this post, I will extend my reasoning a little bit and provide more details, which will naturally bring us to the notion of quantum decoherence.

And for those interested by a ‘TL;DR’ (for once, I can start with the summary), quantum decoherence is what must be fought at all costs for making quantum computing available in everyday’s life.

Now you got the connection :)

_{ As usual, a stupid stuff is hidden in this post… ;) }

## QUANTUM SUPERPOSITION

One of the key principles behind quantum mechanics is the **superposition principle**. In a couple of words, this principle states that a quantum state can be seen as a superposition of different more basic components.

_{[image credits: Belsazar (CC BY-SA 3.0)]}

A simple example can be seen in the **qubit**. After all, I will briefly discuss quantum computing in this post so that we could start with a naive version of the qubit as an example.

For a classical computer, a bit has to be either 0 or 1. In term of quantum state, it is correspondingly either in a `| 0 >`

state or in a `| 1 >`

state.

A qubit consists in contrast in a **combination of these two basic ingredients**. It has hence at the same time a non-vanishing 1 and a non-vanishing 0 component.

The corresponding quantum state could be written as `α |0> + β |1>`

, with the α and β coefficients being potentially different. A qubit could hence be more 0-dominated or 1-dominated according to the values of our two α and β coefficients.

**At our macroscopic scale, there is nothing equivalent to such a superposition principle**, and it is fair to say that it hurts our naive, classical, vision of nature. This weirdness was highlighted by **Schrödinger** who thought about an imaginary experiment aiming to contest the validity of quantum superposition.

The idea was to design a shocking and non-reasonable situation, implying that the only reasonable conclusion was to reject **quantum superpotions**. Of course, Schrödinger failed ;)

## THE SCHRÖDINGER’S CAT EXPERIMENT

The heart of the Schrödinger’s cat experiment is to **mix a macroscopic system** (a cat) **with a microscopic system** (an atomic nucleus). I insist: this is a thought experiment. No cat has therefore ever been hurt by quantum mechanics.

_{[image credits: Robert Couse-Baker (CC BY 2.0)]}

We imagine that **a cat is sealed inside an opaque box together with a vial of poison whose opening would kill the animal**.

The experiment starts with a vial that is closed and a cat that is alive.

**The box however includes a mechanism that allows the vial to open**, the mechanism being piloted by **the decay of an atomic nucleus**.

This is where **nuclear physics** enters into the game. It tells us that **our nucleus has a given probability to decay which increases with time**.

Therefore, the nucleus can decay at any time, but the decay will always happen after a sufficiently long time. And at that exact moment… the vial opens and wooof the cat…

**The nucleus has to obey to quantum mechanics**, as it is a microscopic object. We can thus write a quantum state for it. This state is very similar to the qubit quantum state introduced above, but after replacing the 0’s and 1’s by a *decayed* and a *non-decayed* status.

The quantum state of the nucleus reads thus `α(t) |decayed> + β(t) |non-decayed>`

, with the α and β coefficients depending on the time *t*. Nuclear physics then tells us that after some time, α(t) is close to 1 and β(t) is close to 0.

## TOWARDS A QUANTUM CAT

And here comes Schrödinger’s provocation: **including a macroscopic object (the cat) in the quantum state**, `α(t) |decayed & dead> + β(t) |non-decayed & alive>`

to go back to the above notations.

_{[image credits: dingler1109 (CC BY 2.0)]}

In other words, **Schrödinger decided to apply the rules of the quantum world to the cat** whose destiny depends on the vial, and thus on the nucleus.

If the nucleus has decayed, then the cat is dead. However, if the nucleus is intact, then the cat is alive.

And here is the funny and weird situation: **the interpretation**.

According to quantum mechanics, **the cat is a zombie as neither alive nor dead as long as no one opens the box to check**.

That is the crucial point. Before a measurement aiming to check the health of the cat, quantum mechanics tells us that the cat is partly alive and partly dead. After such a measurement, of course the cat is either dead or alive…

Quantum mechanics predict that once a measurement is undertaken, either the nucleus has decayed and the cat is dead, or it hasn’t and the ca tis alive. **The zombie state of the cat is not observable**.

## SUMMARY: ALLEVIATING A NO-PARADOX AND QUANTUM COMPUTING

There is no paradox of the Schrödinger cat. Even if you can read the opposite in many places on the web (please do not trust anything you find on the Internet).

#### What one has here is **a paradoxal interpretation of a state that is not observable in an experiment that is useless as no one will challenge what will be found at the time of the measurements**.

Our macroscopic common sense tells us that the quantum state above-introduced, that we wrote `α(t) |decayed & dead> + β(t) |non-decayed & alive>`

, is non-realistic as zombie cats do not exist. This was also the point of Schrödinger, which thus challenges quantum superposition.

In fact, **writing a quantum state for a cat has no sense at all**, no matter it is dead, alive or zombie. The reason is that a cat has continuous exchanges with its environment, as most macroscopic systems. The ‘*cat system*’ is thus not an isolated system.

This means that in no time, its description through a quantum state is not valid anymore. This is what is called **quantum decoherence**. Decoherence appears at a pace proportional to the number of involved particles, and it has been observed and studied for instance in experiments with **simplified pseudo-Schrödinger’s cats made of light**.

For a cat, decoherence would be so fast that there is no way to observe any quantum effect. It is also the technological barrier to bypass for quantum computing.

**Decoherence can indeed reduce a qubit to a classical bit**, and makes us lose all the advantages of quantum computing. This is why quantum computers do not have at the moment a large number of qubits. It is damned hard to protect them from decoherence on a technological standpoint.

I hope that I have now convinced about this connection between cats and quantum computers :)

### STEEMSTEM

SteemSTEM is a community-driven project that now runs on Steem for more than 1.5 year. We seek to build a community of science lovers and to make the Steem blockchain a better place for Science Technology Engineering and Mathematics (STEM).

More information can be found on the **@steemstem** blog, on our **discord server** and in our last **project report**. Please also have a look on **this post** for what concerns the building of our community.

dexterdev (63)5 years agoMeanwhile, I am still struggling to cope with the Copenhagen interpretation. Why is pilot-wave theory not all considered seriously? :(

lemouth (74)5 years agoWell, it is actually considered as a topic of debate .

The pilot wave interpretation nevertheless does not contradict quantum mechanics, and does not bring anything new when comparisons with data are at stake. The "trajectories" of the particles in the pilot wave interpretation do not imply more information on what could be observed than with the usual quantum mechanics. Moreover, the pilot wave interpretation is as non-local as the Copenhagen interpretation.

In short: it is not wrong, but it cannot be tested (for now) and thus proved correct. It is also very non-intuitive and IMO more complicated.

dexterdev (63)5 years ago (edited)Maybe there are other points which I am unaware of. but Because of the deterministic nature, I feel pilot-wave theory more intuitive.

lemouth (74)5 years agoIf features a potential that does not vanish at infinity. This is highly non intuitive (and totally non-local) :)

dexterdev (63)5 years agoOh I never knew it before. But for a beginner, the unknowns in the pilot wave theory is in the initial conditions. The trajectory is completely deterministic, which is satisfying for me. But then I Don't know other issues with this theory.

lemouth (74)5 years agoIn any classical physics problem, any potential vanishes at infinity. This means particles cannot interact if they are infinitely far from each other.

The price to pay to get an interpretation close to classical mechanics is a bit high, IMO.

dexterdev (63)5 years agoI agree. So now both Copenhagen interpretation and pilot wave theory are non-intuitive for me. :(

dexterdev (63)5 years agoOh I never knew it before. But for a beginner, the unknowns in the pilot wave theory is in the initial conditions. The trajectory is completely deterministic, which is satisfying for me. But then I Don't know other issues with this theory.

lemouth (74)5 years agoDouble posting? :D

dexterdev (63)5 years agoShould I delete this? There was some connection issue.

star-vc (57)5 years agoHi @lemouth! Thank you for bringing up this post. It seems really interesting to me but at the same time few things were hard for me to understand since I am not an expert on Quantum Technology.

Whenever I feel like I understood about the Quantum Technology, new things sparks out which challenges my beliefs!! ;)

I have learnt many things here which was shocking for me seeing your dedication towards explaination such as

In fact, writing a quantum state for a cat has no sense at all, no matter it is dead, alive or zombie. The reason is that a cat has continuous exchanges with its environment, as most macroscopic systems. The ‘cat system’ is thus not an isolated system.' 😇

Hope one day I will be able to understand QM fully!! ;)

Cheers!! 😇

lemouth (74)5 years agoIn short: QM applies to the microscopic world and a cat is not part of this world. Therefore, applying QM to a cat makes no sense. I would be like trying to apply particle physics to genetics, somehow :)

star-vc (57)5 years agoThanks for this short explaination which is now clear to me! :)

lemouth (74)5 years agoYou are very welcome! :)

anevolvedmonkey (53)5 years agoI choose to believe in Many-worlds interpretation rather than the Copenhagen just because the cat doesn't have to die ; )

lemouth (74)5 years ago (edited)This topic is still a widely discussed one. There are pros and cons and no clear answer for now. I personally don't like it much for some of the usual reasons.

Moreover, there is an excellent agreement between all current experimental data and predictions made in the context of the Copenhagen interpretation. This is enough for me :)

anevolvedmonkey (53)5 years agoIndeed this is still a debated topic. Since they are just interpretations, I am open minded to both of them :)

lemouth (74)5 years agoWell, when calculations are at stake, the easiest is the best :)

lordneroo (73)5 years ago (edited)Another interesting read and I somehow managed to miss this.

I see what you did there! I wonder how many actually noticed.

Well, I don't trust anything I read on the internet, however I came across this pretty credible formula that stems from Schroedinger's equation for a single particle.

Never knew about quantum decoherence, sounds like an impossible obstacle in quantum computing indeed.

Have a good day :)

lemouth (74)5 years agoSome :)

A golden cat? mmmh... Maybe a maxwell daemon :D

An obstacle, but one can fight it. I have however not said this wold be easy. :)

lordneroo (73)5 years agoHehe! 'Maxwell's cat'! I will try to find another lamp for 'Schroedinger's demon' now :)

Have a good day!

lemouth (74)5 years agoThat is illuminative ^^

lordneroo (73)5 years ago (edited)Lol! :D

alexdory (59)5 years agoCats strike again!

Beautifully written and very interesting subject to me.

As a long time contributor to World Community Grid+BOINC(CPU and GPU mining since ~2001) , I really want to buy such a computer to do those computations. I know it's very expensive for the moment but I am still waiting for the price drop and then for someone to write the code for the specific apps we need, probably someone @Berkeley, the creators of BOINC.

"wooof the cat…" - not funny :)

And you have created your own paradox while debunking another paradox since you told us to not trust anything online - which is something that my mom tells me all the time.

About the decoherence: Are we sure that we can find computing mechanics to prevent the decoherence in molecule folding for example? Since I am guessing not everything can be written in code to prevent it.

All in all, this has made my morning, I had a bad week and once again I found fun online :D

Cheers!

lemouth (74)5 years agoCurrent quantum computers cannot handle a large number of qubits, and more work is therefore in order.

I provided information. It is up to anyone to think about it and to check it. I have argued everything, and everything can be tested. In this way, this is not that paradoxal, is it?

I don't know. This is not my field of expertise. Sorry.

I am glad to have at least made you smile :)

irelandscape (62)5 years agoMaybe one of the issues here is the definition of measurement.

Most people would put an anthropomorphic meaning to this word. But I guess the masurement doesn't have to be observed by a human being to be a measurement, right?

Most particles in the universe are not being observed by conscious beings but I guess that if such particle in a state of superposition interacts with macroscopic object then it will get a definite state. Is this the interpretation of physicist?

It's so mind bending...

lemouth (74)5 years agoOf course. The observer can be a human, a computer, a machine, etc.

Some physicists have actually claimed that a conscious being was needed. But the role of consciousness is still unclear (and subjects of debates). To answer the rest of the comment: In quantum mechanics, the superposition is always reduced to one of its component after a measurement. Until there, we just have a bunch of options with given probabilities.

terrylovejoy (62)5 years agoI admit I'm still grappling with the physics for quantum computers :(

But is what you're saying with quantum decoherence also the reason why the qubit state only has a very short lifetime in practical quantum computers? Therefore once you have programmed your qubits you need to get the result out fast before the states become damaged.

lemouth (74)5 years agoYou need to find a way to keep the qubit a qubit and not getting it reduced to a normal bit. This is the complex part as this means fighting decoherence.

greenrun (66)5 years agoI always like to read the arguments for and against the Schrödinger's cat paradox on the Internet!

I'm happy no cat has ever been harmed in the making of any quantum theories. Now I can log off and enjoy my sunny Sunday.

lemouth (74)5 years agoBut, as said in my post, this is not a paradox. Only the interpretation is paradoxal...

For now... ;)

greenrun (66)5 years agoFor now? Oh, that was me being too quick to jump to conclusions :)

lemouth (74)5 years agoJust finishing preparing a box here ;)

alexander.alexis (63)5 years agoThere are some pop media sciency figures that sometimes suggest that there is something logically contradictory about quantum physics. I've yet to encounter a case where quantum physics is indeed contradictory.

An example of contradiction they sometimes give is of a particle being at two places at once. Well what's contradictory about that? The only way to make it contradictory is to add the statement that "if x is at point A, then it cannot at the same time be at point B", which will of course make the statement self-contradictory.

The only true contradiction is to say that something both is and is not the case at the same time, for example "a particle is and is not in two places at the same time". If a physicist ever tried to claim something like that, none of his equations would make sense.

So I don't worry about contradictions. It's usually non-philosophers who haven't studied logic who think there's something contradictory about the quantum world.

Non-intuitive, yes, but that's different.

lemouth (74)5 years agoThe double slit experiment is very funny with this respect. You may want to have a look to this old post of mine.

Another point is quantum mechanics is that it tells you that the position is not uniquely defined. Everything is probabilistic which is one of the complications in its understanding. Another complication is that a particle is a wave. That does not help.

In short, the quantum world is tough, but exciting!

k4r1nn (66)5 years agoInteresting reading. One single course about quantum physics I took a few years ago was useful to give me some basic understanding but I know it is nearly not enough to add any of my opinions except to applaud your knowledge. Especially when you connect it with computing science.

lemouth (74)5 years agoThanks for your nice comment! But please don't be shy and don't hesitate to comment. I am always happy to discuss ^^

theabsolute (65)5 years agoI think the different interpretations of QM are interestestimg (pilot wave, Everett's many world's, wavefunction collapse, etc.)

I majored in physics, but I still enjoy learning more and more about it as well as abstract math. Learning is a lifelong process. There are a lot of great videos on YouTube as well.

lemouth (74)5 years agoThe different interpretation are interesting, and each has pros and cons. This is still a hot topic 60 years later.

Note that many of these interpretations do not add anything as soon as we confront the predictions to experimental data. I have for instance in mind pilot waves. This is why the simplest Copenhagen interpretation is commonly admitted: there is an amazing agreement with data and I insist, it is simple.

alexdory (59)5 years agoOur community is open and we are friendly, I used to be shy but it helps nobody.

On the other hand asking questions is a commendable thing since it shows that you want to improve yourself, an action that can only mean good for you :P

lemouth (74)5 years agoI 100% agree with you!

rudyardcatling (64)5 years agoluring me in with cats right before i'm off to sleep tsk ... cheeky

... i'll leave the tab open for reading tonight but i thought in case you didnt know this might interest you btw ::

https://bhi.fas.harvard.edu/news/bhi-paper-competition

lemouth (74)5 years agoI didn't know about this black hole contest. It is a really nice initiative. I hope they will get nice essays from all over the world :)

I am waiting for another comment from you for tomorrow then ;)

theabsolute (65)5 years agoYou say to not trust anything on the internet, yet I read this on the internet. That is a paradox in itself.

lemouth (74)5 years agoSomehow. By not trusting, I was implicitly saying that one should double-check, get alternative versions, investigate the various authors, etc... Of course, in physics, it may be easier to do than for more controversial fields :)

dimelopons (36)5 years agoThat's the key part, its abstract but at the same time concrete enough to actually be able to fact check. Unlike almost every project in the cryptospace rn hahha

lemouth (74)5 years agoThis is what is good with science. In principle, everything can be verified. I said "in principle" because unfortunately, not everything is open access and unfortunately again, some people cannot access their favorite theory to be falsified. Those are of course two counterexamples among others ;)

But yes, everything I have said has been argued and is in addition open to debate. I don't ask anyone to 'trust' me. I think I have given enough information so that anyone can get his/her own idea and verify anything he/she would be confused with.

ceruleanblue (56)5 years agoThis was quite an informative post, and I'm glad to see some quantum discussion on Steemit! I have been following IBMs movements for a little while now, and I have so many hopes for these projects. As an encryption enthusiast, Quantum is life-changing.

lemouth (74)5 years agoThis came fro ma lively discussion on the steemstem discord. I don't know whether you are there already, but maybe will you be interested in joining?

ceruleanblue (56)5 years agoYes, I have been hanging around there a bit, and interesting I didn't realize this was steemstem's kinda thing, was briefly thinking of doing some QISkit tutorials for utopian, though perhaps they would be better directed towards the stem community :)

lemouth (74)5 years agoYou can always use the steemstem tag. Maybe our curators will see it. Getting utopian upvotes does not prevent us from upvoting too ;)

ceruleanblue (56)5 years agoHaha good to know, thank you! :) I think that decides it, as I have been interested in doing this for a while now.

lemouth (74)5 years agoJust do something you have fun in doing. That is my only recommendation :)

justtryme90 (71)5 years agoQuantum superpositions are an LED desk lamp?

lemouth (74)5 years agoIt looked like a quantum superpotion. I may be too imaginative... ;)

justtryme90 (71)5 years agoCreativity is important as a scientist.

lemouth (74)5 years agoI agree. And here come the superpotions :D

samminator (75)5 years agoI heard about the Schrödinger cat for the first time when you made a comment on my post; I think it was last week. And I humorously said "quantum cat". I wasn't wrong after all :).

Now I have the privilege of reading about it in details. You don't know how happy I am right now.

lemouth (74)5 years agoYou were indeed not wrong... You still need to search for the bra and ket thing. :)

samminator (75)5 years agoI did sir. I used some materials online. And I stumbled on this youtube video:

https://m.youtube.com/watch?v=LWF9-FLbKz0

I never knew they are actually called Dirac notation. The more I learn, the more I discover I know very little.

I think I should research more.

Thanks for the boost

lemouth (74)5 years agoThis is indeed it :)

flyyingkiwi (55)5 years agolemouth (74)5 years agoYou are very welcome. I really thought such a post was in order, especially after the lively discussion we had on the chat, and I then used a fraction of my week-end rest time to write it... :)

osariemen (55)5 years ago (edited)Now we are placing animals (cat to be specific) and quantum computers

Ans is no but I have an insight and it's a nice have a knowledge like this....

`Please is there anymore to quantum computers?`

lemouth (74)5 years agoI beg your pardon? I didn't fully get your comment (and therefore can't understand it). Do you mind clarifying wha you have in mind? Thanks in advance!

osariemen (55)5 years agoThe first part of the comment is the fact that am surprised about the article

The quote is the question you asked at the end... Which my answer is No but I have the knowledge of it, what is left for me is to do my work assignment by reading more

I hope am clear now.... Thanks

lemouth (74)5 years agoOk I see. In fact, decoherence is the common grounds. It explains what is going on with the Schrödinger's cat, and it is the reason why quantum computing is tough.

gentleshaid (74)5 years agoCan we use a plant in place of the cat in a real-life experiment? oh no! Plants are also living and someone will get hurt. Treat plants kindly =)

lemouth (74)5 years agoStill too macroscopic. This won't change a thing :)

scambuster (44)5 years agoI'm waiting for my cat to wake from her nap . We will discuss this theory at length . My results will be posted .

lemouth (74)5 years agoDo you need a box for her? :D

quiplet (44)5 years agoThis may be controversial to state, but isn't Haejin's

wavetheory analysis originally based in quantum mechanics?lemouth (74)5 years agoI didn't read his theory and I cannot therefore comment it. But wave theory is old of several centuries. I am wondering how anyone could claim having something new here...

shauntaemonte (41)5 years agoCan I say I loved the first graphic and pretend I read and understood the article? I'm sure it was very interesting to those whose brains could endure it. :)

lemouth (74)5 years agoPlease try! I am here to help in case something is obscure. I made it easy to follow, or at least I tried :)

shauntaemonte (41)5 years agoOk, I will. I'll comment again when I have. :)

lemouth (74)5 years agoCool thanks! :)

gmzct (13)5 years agoMy cat :)

lemouth (74)5 years agoIt does not look very quantum, to be honest. Have you tried to put it in a box? ;)

gmzct (13)5 years agono :)@lemouth

lemouth (74)5 years agoYou should. Imagine for one second the beauty behind a quantum cat ;)

alexs1320 (71)5 years agoThe Nightstand Lamp? :)

lemouth (74)5 years agoThe superpotion! :)

andreina89 (47)5 years agoVery interesting post I love cats greetings

lemouth (74)5 years agoBut what about quantum cats? :D

bachuslib (54)5 years agoIn this example, the cat showed decoherence from his form and strong entanglement with a dog. Not sure we can blame poison vial for that. Maybe it was due to staring in a rapidly changing colors of a LED Potion Desk Lamp which gave him seizures.

lemouth (74)5 years agoArf.... why haven't I thought about that... I however thought about the 'superpotion' principle ;)

yann85 (52)5 years agoDTC ? Dans ton chat pardi! ;-)

lemouth (74)5 years agoOooouh ca fait longtemps ca :D

tsoldovieri (62)5 years agoExcellent @lemouth. Thanks for sharing this interesting post about Quantum Mechanics. It was a pleasure to read it. Regards.

lemouth (74)5 years agoThanks for passing by!