Quantum mechanics is becoming more and more popular these days. All thanks to the advancement of quantum computers and the possibilities that may bring to the world once established. But how the classical counterpart differs from the quantum in terms of the basic unit of information?

The Bit

We know that in classical computing, the basic unit of information used is called a bit. It can be either a 0 or a 1. And everything we do on our modern-day laptops boils down to manipulating and transmitting bits back and forth with the help of tiny transistors.

To change the output of a bit, one of the logical gates is applied within a system with the help of transistors. On a very fundamental basis, that’s how the information flow in modern computers – constant move of ones and zeros.

The Qubit

However, the quantum bits, or qubits for short, are different and allow us to process the information in new and different ways. While the classical bit can be either 1 or 0, the quantum bit can be 1,0 and anything in-between thanks to the principle in quantum physics called superposition.

Physically speaking, one qubit is a system in quantum mechanics that has two states marked as |0⟩ and |1⟩. These are abstract vectors, written with bra–ket notation, In contrast to the classical case, in quantum mechanics, these two states interfere so that in general, the pure qubit state can be written as

Where the α and β are complex numbers and the following rule is applied

The possible states for a single qubit can be visualized using a Bloch sphere. Represented on such a sphere, a classical bit could only be at the top or at the bottom, where |0⟩ and |1⟩ are respectively. The rest of the surface of the sphere is inaccessible to a classical bit, but a pure qubit state can be represented by any point on the surface.

Image from wikipedia.org

Bloch sphere representation of a qubit.

Any point on the surface represents a pure state of a qubit, where any point inside represents a mixed state which we will discuss in another article.

The physical realizations of a two-state system can be different. Similar to a classical bit where the state of a transistor can be used to represent the state of the bit in the same computer, the quantum computer is likely to use various combinations of qubits in its design.

This could be an electron or a nucleus with spin 1/2 oriented in or against the direction of the magnetic field, an atom with two different energy states, or a photon with horizontal or vertical polarization. We are still in the early stages of development so new proposals are expected to emerge as we progress.

Another very important distinguishing feature between qubits and classical bits is that multiple qubits can exhibit quantum entanglement. In short, quantum entanglement is a property of 2+ qubits that allows a set of qubits to express higher correlation than is possible in classical systems. I will release another article soon, where I will go into detail about this strange phenomenon.

To wrap up, everything we mentioned here makes qubits fundamentally different and much more powerful than classical bits. Harnessing the powers of infinite superposition states, quantum interference and quantum entanglement are the key blocks for building a stable quantum computer.