Going through The IBM Quantum Challenge

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Igor Rodrigues

Fun Exercises

This last week has been fun and exciting. After seeing many people take on IBM’s Quantum Challenge (An online event that the company organized to celebrate the approaching anniversary of its Quantum Experience) I decided to give it a twist and try to speed-run the 4 exercises on an improvised Youtube Livestream. Little did I know what I was about to experience.

It was a tough lesson of how rusty my classical programming skills have gotten. The first exercise was a fairly simple but fun warm-up to quantum circuits and how to create well known states. Exercise 2 was probably more of a tour around some error correction features from qiskit and my speed-run ended with exercise 3 which required almost no quantum skills but expected you to remember the difference between comparing a classical integer and a string ?

It was definitely fun!

An Intuitive Struggle

Exercise 4 was another beast.

What I liked about the exercise is that it felt more like a puzzle. Following my intuition I tried to approach the problem by reverse-engineering the given unitary gate, disentangling and rotating back and forth.

The inspiration of this idea comes from quantum tomography where you basically measure a circuit from different perspectives in order to try and triangulate what state the system is in. In a similar fashion I created a unitary operation given the 16×16 matrix and applied it to 3 circuits with 3 different initial states. After that, it is as simple as trying to find a sequence of operations that take you back to the initial state for all 3 circuits. Then you just need to reverse the circuit and Voilà! This of course involved a lot of guessing and playing by hand and for more complicated unitary matrices it might just be too tedious! You do not get an optimal circuit right away but it worked for the challenge!

I did learn a lot while flexing my quantum muscles. Specially, I did realize that my intuition around IBM’s U3 gate is far from complete. It truly resonates with this other post of mine.

A Great Achievement

All in all, it was a great achievement from both an educational and community perspective. Kudos to everyone in IBM who made this all possible. At the time of writing more than 4 Billion circuits have been run during the challenge days. Truly inspiring to see so many people engaged with it!