Grovers Algorithm — Programming on Quantum Computers — Coding with Qiskit S2E3

In this episode, Jin explains how some quantum algorithms can outperform their classical counterpart and shows us how to implement Grover's Algorithm.

Video Production by:
Paul Searle, Clinton Herrick & David Rodriguez

Writing by:
Olivia Lanes, Jin-Sung Kim, Abe Asfaw & Leron Gil

[tomwong]

Great video! A couple of minor corrections:

1) At 6:08, the (oracle + reflection) were together referred to as "Grover's diffusion operator." Together, they are actually called "Grover's iterate." The reflection by itself is Grover's diffusion operator.

2) At 12:42, the reflection should be 2|sXs| - I. That is, the factor of 2 is missing.

Looking forward to where this series is going!

[danielwang]

Say it with me everyone, "Thank you Qiskit!!!"

[k_pragya]

Aaamazzinnngg...!!! Jin came back in the same way he vanished out in last week's video. Yayy... Team Qiskit never fails to add the fun spice in the learning recipe!! Thanks Jin for the helpful knowledge!!

[michaeljburt]

This is absolutely mind bending. I am a continual studier of theoretical physics, but seeing it applied so rapidly and so readily is jarring. Wow.

[b.ambrozio]

I understood how the classical computation solved the search example over the 10 elements array, and I (think I) also understood Grover's algorithm. However, I didn't understand how Grover's algorithm would solve the problem given in the begin (find the index of the array that has the number 7). Would be nice seeing Grover's algorithm implementation actually solving the problem just solved by the classical computation.

[МаринаЛисниченко-оъ]

What is going on from the 14:00? Why do we use again Hadamard after oracle in reflection? What I see in the 2 and 3 rows of the "reflection" is a repetition of the oracle. And after that we use CZ - why?

[smcg]

That was so cool. I’m new to quantum algorithms. This has been really helpful. The videos have a clear direction, are easy to understand and well filmed. Well done everyone! 😊

[jordanchuri]

Great video. One of the better descriptions of an oracle that I have seen/read for Grover's algorithm.

[namanjain]

Great explanation! A query tho: In this case, you just hardcoded the oracle for | 11 >. So, you were looking for the item at index 3 (if actually searching through a list). Doesn't that mean you already know where the item is ?!!. How could the oracle be implemented more generally?

[adhishagammanpila]

Great explanation, I am curious why didn't the example of finding 7 from the array was not done using using the Quantum Algorithm ?

[juanete]

Hello.
I don't understand. You were looking for the number "7", in binary 111, which is at index 9 (1001 in binary), but you got 11.
I guess you just mixed the original problem with a simpler one.
You would need to change the oracle, how would it be?
And how is the initialization done if we want to use mylist as the input instead of random qubits?

[SankalpSanand]

2 things for some people confused with the benefit of this and why are we using CZ gate:
1. This can be used to quickly find whether a particular element exists in a list of elements
2. We design an oracle so that it can flip the state, e.g if we needed to find state |01> the oracle would've been, circuit.x(0) then circuit.cz(0, 1).

[vasu_h]

If the oracle already knows which vector to negate the phase of, doesnt it already know that the answer is in fact what we are looking for ?

[iliqnew]

Amazing! A question tho: how does the quantum version of this algorithm refers to the problem in the beginning? Or how can we pass the function through the circuit in order to get the index of the winning number in the list? I just still can't see how the two problems are the same. I've learned the operators used in these circuits but please somebody HELP! Quantum computers are so interesting to me but I still can't "translate" the boring for-loops to a quantum circuit and I really want to

[noahchou]

Thanks for the wonder video. However, i have difficulties in understanding the implementation of the reflection. 14:08 you mentioned takes all superposition state back to all zero state. If the state is all zero state, then the control z can not add the negative phase right? (Do you mean all one state)? Even if this is true, how does this implementation means the reflection operator 2 |s> <s| - I ? Thanks if there is some explanation for my confusion.

[matthewwongsakdi]

Can someone please explain when the |11> winning state was defined during the coding process?

[abhirammodak]

I have a basic doubt - if the oracle can single out the entry we are looking for, why is the subsequent circuit required?

[morikaghayan]

Very informative video! Just one question, why was only 1 shot needed if there were four possible strings, shoudn't it have used two calls to the oracle?

[thomasmonti]

Nice 1903/83 its very clear now thank for all your support...

[adityaprakash]

Thank you. 😁Eagerly waiting for next video