How to explain Coherent Superposition to a 5 year old

This is how i will explain to my pears!
Coherent superposition refers to a fundamental concept in quantum mechanics 
where a quantum system exists in a combination of multiple states simultaneously. These states are said to be "coherent" because the phases between the different states are well-defined and consistent, meaning they can interfere with each other in a predictable way. This is different from classical states, where a system is in only one definite state at a time.
For example, in a coherent superposition, an electron could be in two different locations at once, and the probability of observing the electron at one location or the other is determined by the interference of the quantum states.

Child:
Rahul! what is Coherent Superposition?

Me:
Imagine you have a ball, and instead of it being in one place, it’s like magic — the ball is in two places at the same time! That’s what happens in a special kind of magic called “coherent superposition.” The ball can be in both spots at once, and only when you look at it, you see it in one spot. It’s like playing hide and seek with a magical ball!

Child:
Ok are you saying? The ball is outside the house and also ball is inside the house!
So at a time, i will only be able to see in one place, either inside the house or outside the house but actually it is in both places?
And if i went outside the house, to see the ball and as i am watching it, my friend will not be able to see that ball inside the house or will he also be able to see the ball?

Me:
Yes, that’s a great way to think about it! If the magical ball is in both places — inside and outside the house — when *you* go outside and look at it, you’ll see it only outside. But because you looked at it, the magic changes, and now the ball is only in one place. So, if your friend looks inside the house at the same time, he won’t see the ball there anymore. The ball was in both places *before* anyone looked, but once someone sees it, it “chooses” just one place!

Child:
Is it same as Schrödinger’s Cat?

Me:
Yes, that’s very similar to Schrödinger’s cat! In that thought experiment, there’s a cat in a box that could be both alive *and* dead at the same time (in a superposition of states) until someone opens the box and looks. Once you check, the cat is either definitely alive or definitely dead — just like how the ball is in both places until someone sees it.

Schrödinger’s cat helps illustrate the strange nature of quantum mechanics, where things can be in multiple states at once until they’re observed.

Child:
And suppose me and my friend both in call with each other and same time we open our eyes to see the ball? who will be able to see the ball, me outside the house? or my friend inside the house?

Me:
Great question! In the strange world of quantum mechanics, if you and your friend both open your eyes at *exactly* the same time to look at the ball — one outside the house and one inside — it’s like the universe has to “decide” where the ball will be. Once the ball is seen, it picks just one place, and both of you will agree on where it ends up.

So, if you see the ball outside, your friend will *not* see it inside, and if your friend sees it inside, you won’t see it outside. It doesn’t matter who looks first — what matters is that once one of you sees it, the ball “decides” where it is, and both of you will see it in the same place. It’s as if the ball can no longer be in two places at once after it’s observed!

Now Child becomes greedy:
Can you corelate this to qubit? how a “if” condition will be cheked to compare two numbers, in 1 qubit, 2 qubit and 4 qubit Quantum computer?

Me:
Let’s break it down step by step and connect this to qubits and how a quantum computer might check conditions like an “if” statement to compare two numbers.

What is a Qubit?
- A qubit is like a regular bit in classical computing, but instead of being just 0 or 1, it can be in a superposition of both 0 and 1 at the same time (like your ball being inside and outside the house).
- When measured, the qubit “chooses” a definite state (0 or 1), just like the ball picking one place when you and your friend look at it.

Using Qubits for Comparison:

In a classical computer, you might write an “if” statement like this:

if (a > b):
 # Do something

In quantum computing, this comparison works a bit differently because qubits can represent multiple possibilities at once. Let’s walk through how a comparison might look in different qubit setups:

1 Qubit Computer:
With a 1-qubit quantum computer, it’s very limited because you only have a single qubit that can represent both 0 and 1 in superposition. However, you can only compare very simple things, like a single-bit number comparison (0 or 1). Here’s how the logic would go:

- If the qubit represents `0`, maybe that means `a <= b`.
- If the qubit represents `1`, that means `a > b`.

But because it’s in superposition, it can be in both states at once, and only when you measure it will you “know” the result. It’s like saying, “I don’t know yet, but once I measure it, I’ll know which condition is true.”

2 Qubit Computer:
With 2 qubits, the possibilities expand because 2 qubits can represent four states at the same time (00, 01, 10, and 11). Now, you can represent more complex numbers or conditions. For example:
- You could encode two 1-bit numbers (let’s say `a` and `b`) into the qubits.
- The qubits would be in a superposition of all possible outcomes (00, 01, 10, 11).

To check if `a > b`, a quantum computer could manipulate the qubits to compare the numbers in superposition, and when you measure it, you’ll collapse the state and see the outcome.

For instance:
- `00` could mean `a == b`
- `01` could mean `a < b`
- `10` could mean `a > b`

But until you measure, the qubits can be in a combination of all these states.

4 Qubit Computer:
Now with 4 qubits, things become even more powerful, as 4 qubits can represent 16 different states (from 0000 to 1111). This allows you to encode more complex numbers or conditions. For example, you can compare two 2-bit numbers (`a` and `b`):

- The quantum computer could evaluate all possible combinations of `a` and `b` simultaneously.
- You could have superpositions representing different outcomes:
— `0000`: `a == b`
— `0100`: `a < b`
— `1000`: `a > b`
— And so on for all 16 possible states.

A quantum computer could process all these possibilities at once and, when you measure the qubits, return the result of whether `a > b` based on which state it collapses into.

Example:
Imagine comparing `a = 2` and `b = 3` using a 2-qubit quantum computer:
- You can encode `a = 10` and `b = 11` (binary representations of 2 and 3) into the qubits.
- The quantum computer will run the comparison algorithm, and the qubits will be in a superposition of all possible outcomes.
- When you measure, the qubits might collapse to `01`, which could mean `a < b`.

On a 4-qubit machine, you could compare larger numbers, like `a = 6` (110 in binary) and `b = 5` (101 in binary), encoding them into qubits and running a comparison in superposition.

Quantum Advantage:
In classical computing, you’d need to check one condition at a time (`if a > b`, then `if a < b`, etc.). But in quantum computing, thanks to superposition, all these conditions can be checked at the same time, and you get a result when you measure the qubits.

Quantum computers use a technique called quantum gates (similar to classical logic gates) to process and manipulate qubits for these comparisons. When a result is measured, the quantum state collapses, just like in our ball analogy.