The fundamental difference between a classical bit and a quantum bit, often called a qubit, lies in the way they store and process information.
A classical bit is the basic unit of classical information and can exist in one of two possible states, commonly represented as 0 or 1. It can be physically realized using any two distinguishable states or symbols, such as the absence or presence of an electrical charge, the orientation of a magnetic field, or the presence or absence of a physical mark. Classical bits follow the laws of classical physics and can only be in one state at a time, representing a definite value of 0 or 1.
On the other hand, a qubit is the basic unit of quantum information and can exist in a superposition of states, which means it can be in multiple states simultaneously. This property arises from the principles of quantum mechanics. A qubit can be realized using various physical systems, such as the spin of an electron, the polarization of a photon, or the energy levels of an atom. Unlike classical bits, which can only be in a definite state, qubits can exist in a coherent superposition of 0 and 1. In other words, a qubit can represent a combination of both 0 and 1 at the same time, with certain probabilities associated with each state.
Another crucial aspect of qubits is their ability to exhibit quantum entanglement. Entanglement allows two or more qubits to be correlated in such a way that the state of one qubit depends on the state of the other, regardless of the physical distance between them. This property is not possible with classical bits, which are independent of each other.
The ability of qubits to be in superposition and to be entangled gives quantum computers their potential power. It enables them to perform certain computations much more efficiently than classical computers for specific tasks, such as factorization, optimization, and simulation of quantum systems. However, exploiting these quantum properties is challenging due to the susceptibility of qubits to environmental disturbances, a phenomenon known as quantum decoherence. Researchers are actively working on developing error-correcting techniques and robust qubit architectures to overcome these challenges and harness the power of quantum computing.