自19世纪初以来，我们一直在使用计算机。目前，我们处于第四代计算机，其微处理器仅次于真空管，晶体管和集成电路。它们全部基于常规计算，该常规计算基于电路的经典现象，即在给定时间处于打开或关闭状态的电路处于单一状态。
当前正在开发第五代计算机，其中最流行的是量子计算或量子计算机。量子计算机的工作方式与传统计算机完全不同。量子计算机基于量子力学现象，即一次可能处于一种以上状态的现象。
常规计算与量子计算之间的区别：
| Conventional Computing | Quantum Computing |
|---|---|
| Conventional computing is based on the classical phenomenon of electrical circuits being in a single state at a given time, either on or off. | Quantum computing is based on the phenomenon of Quantum Mechanics, such as superposition and entanglement, the phenomenon where it is possible to be in more than one state at a time. |
| Information storage and manipulation is based on “bit”, which is based on voltage or charge; low is 0 and high is 1. | Information storage and manipulation is based on Quantum Bit or “qubit”, which is based on the spin of electron or polarization of a single photon. |
| The circuit behavior is governed by classical physics. | The circuit behavior is governed by quantum physics or quantum mechanics. |
| Conventional computing use binary codes i.e. bits 0 or 1 to represent information. | Quantum computing use Qubits i.e. 0, 1 and superposition state of both 0 and 1 to represent information. |
| CMOS transistors are the basic building blocks of conventional computers. | Superconducting Quantum Interference Device or SQUID or Quantum Transistors are the basic building blocks of quantum computers. |
| In conventional computers, data processing is done in Central Processing Unit or CPU, which consists of Arithmetic and Logic Unit (ALU), processor registers and a control unit. | In quantum computers, data processing is done in Quantum Processing Unit or QPU, which consists of a number of interconnected qubits. |