Student Guide

Quantum Circuits

How does a quantum computer work?

This resource was originally published in PhysicsQuest 2021: Introduction to the World of Quantum.

How does a quantum computer work?

  • Quantum state card pieces from the link "game" in Overview
    Intro

    Thousands of scientists around the world are working hard to develop quantum computers – machines that can solve certain problems that are far beyond the reach of the world’s best supercomputers. Famous examples include code breaking and simulating complex molecules. Unlike ordinary computers, these machines exploit the laws of quantum mechanics. Like ordinary computers, the way a quantum computer works can be understood in terms of a few simple logic operations that act on bits of information. By combining many such operations together to form logic circuits, one can solve interesting problems. This document contains a detailed explanation of pictorial rules that can be used to design and analyze quantum logic circuits, along with a few practice problems to help students become familiar with the basic rules of quantum logic.

    After reading the introduction, what is your essential question or objective for this activity?

    Before the experiment

    What do you know about how computers work? How do you think quantum computers are the same? Different? Use what you learned in the other activities in this setup to help you come up with some guesses.

    • Think about this for yourself.

    • Discuss with your group.

    • Draw group models on the white board.

    • Discuss with the class.

    Setting up

    Rules of the game: Use this guide to help you set up the game.

    • Separate the playing cards into four piles: initial states, secret states, two-qubit gates, and one-qubit gates. Shuffle each pile and lay the piles face-down on a flat surface.

    • One player deals 4 two-qubit gate cards and 4 one-qubit gate cards to each player face-down so that the players cannot see each other’s cards. The dealer also selects an initial state card and displays it face-up so that all players can see the initial state.

    • Each player then randomly draws one secret state card, being careful to not let the other players see what their secret state is.

    • One of the players who did not deal (this player can be chosen randomly) begins by laying down a gate card of their choice face-up, underneath the initial state. The players then take turns, each time either laying down one gate card from their hand or drawing a new card from either gate card pile if they prefer.

    • The game concludes when a player is able to, on their turn, complete a circuit that transforms the initial state into their secret state. This could happen either before or after they put down a gate card. After the circuit is complete, the player then lays their secret state card at the bottom of the circuit, and the other players must confirm it is correct. If no players can complete a circuit that produces their secret state even after all cards are played, then it is a draw.

    During the experiment

    Play the game according to the quantum circuits card game.

    Conclusion

    • Answer the following questions and discuss with your class:

      1. How classical logic works.
      2. How quantum logic works.
      3. How to build and analyze quantum circuits.

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