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Qubit

In this entry you will find information about what a qubit is. Start from the section that feels easier or closer to you (play, discover, or learn), and then explore the others to see how your understanding shifts and expands!

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Initially launched on 27 July 2021, the upgraded, mobile Q|Cards> game will soon be available for download at play stores as a fully digital and multiplayer online version of the Q|Cards> tabletop game.

Start playing Q|Cards> on your mobile device or on your tablet! The app uses IBM's quantum computing library Qiskit to simulate the quantum circuits of 2 to 5 qubits you will be composing in the gameplay!

Credits:
Viktor Minin (Code, Game Design)
Sonya Minina (Graphics, Game Design)
Boris Sokolov (Code, Physics)
Caterina Foti (Physics)
Guillermo García-Pérez (Game Concept, Game Design, Physics)
Matteo Rossi (Code, Physics)
Sabrina Maniscalco (Production, Physics)

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The simplest possible quantum systems are those whose Hilbert space has dimension equal to 2, that is tantamount to saying that its basis contains two vectors, representing two states of the system.

The simplest possible quantum systems are those whose Hilbert space has dimension equal to 2, that is tantamount to saying that its basis contains two vectors, representing two states of the system. Such two-state (or two-level) systems are called qubits (short version for quantum bit) and play an essential role in quantum information; not by chance, they represent the quantum version of the classical bit, that is, the fundamental unit of classical information and computation. An example is the spin, an intrinsic property of particles that, for an electron, can only present two values, $+\hbar/2$ or $-\hbar/2$, along any direction of the physical space, either $x$, $y$ or $z$. In this case, the two states forming the so-called computational basis of a qubit can be taken to be the spin up and spin down states along, e.g., the $z$ direction. Another typical example is the polarisation of photons, which can be, e.g., vertical or horizontal.

Quantum computers rely on the quantum behaviour of qubits to solve computational problems more efficiently than conventional computers. By efficiently, we mean that they can find solutions in fewer steps. To understand the quantum properties of qubits exploited by quantum computers, explore the entries on quantum superposition, wave-like behaviour, and entanglement, where they are explained in terms of spins.

Learn

In this link you’ll find a description of the qubit — the simplest possible quantum system — in terms of vectors on a Hilbert space. In addition, you will learn how to geometrically represent the state of the qubit on the surface of the so-called Bloch sphere.

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