Preface – This post is part of the Quantum Computing series.

Table of Contents

## Introduction

In the previous module, we discussed classical computing in detail. In this module, we will discuss the key differences between classical and quantum computing. This is perhaps the last module before we dive deep into the nitty-gritty details of quantum computing. So, without further ado let’s get started, shall we?

## Classical Computing

Once again, Classical or Modern Computers are made up of fundamental units called “bits”. A single bit can take a binary state i.e., 0 or 1, high or low. A bit can be in a single state at a time. It can either be 0 or 1, either at high or low voltage, and nothing in between.

## Quantum Computing

Quantum computers on the other hand, as we discussed before are fundamentally different from Classical Computers. Fundamental units of Quantum Computers are called Quantum bits or “Qubits”. Unlike “bits”, “qubits” can be in multiple states at the same time. But before talking about the nature of qubits, let’s talk about what exactly they are made of.

Transistors that make up classical processors are basically silicone (NPN/PNP) semiconductors. The transistor is designed to represent the high or low voltage; the principle as we discussed above, on which modern computing is tethered. An interesting challenge presented itself when physicists began thinking about what should be used to represent a Qubit.

It was quite obvious that it should be a Quantum material, but basically, everything is a Quantum Material when it is cooled below a certain threshold temperature. Photons, electrons, superconducting materials, etc behave Quantum Mechanically. So, where to start?

Well, the good news is there are several different effective Quantum Computers that use different materials as qubits. Their effectiveness and efficiency vary according to various circumstances. We will discuss this further in the upcoming modules. Just remember that there is not a single material like the transistor which resides in different Quantum processors. Different groups and companies have different materials representing Qubits and which method is more effective than another is a much-debated topic in the Quantum Computing Community.

For now, just keep in mind that a qubit can either be in state 1, state 0, or a combination of states 1 and 0. And this is because our Qubits are “Quantum Mechanical”.

## Difference between Classical Computing and Quantum Computing

Okay, enough with the jargon. Let’s summarize some points we discussed until now for some much-needed clarity.

Classical Computing |
Quantum Computing |

A classical computer is fundamentally made up of binary digits or “bits”. | A Quantum Computer is fundamentally made up of “Qubits” or Quantum Bits |

A bit can either be in state 0 or state 1 at a given instance. | A Qubit can either be in state 0, state 1, or a combination of state 1 and state 0 at a given instance. |

Bits are made up of transistors in a classical processor. | Qubits are made up of “Quantum” materials like ions, photons, or superconducting materials. |

Classical computers are relatively less error-prone than quantum computers | Quantum Computers are a lot more error-prone than classical computers |

Transistors do not need sub-zero temperatures to operate. | Qubits require sub-zero temperatures to preserve their Quantum nature. |

Classical computers cannot run algorithms like optimization problems, machine learning models, etc. Very efficiently in most cases. | Quantum computers can theoretically run algorithms like optimization problems, machine learning models, etc. much more efficiently than Classical Computers. |

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