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

What is a Photon?

A photon is a fundamental particle of light. It is the smallest unit of light that can exist, and it is the carrier of the electromagnetic force, which is one of the fundamental forces of nature.

Photons have no mass, and they travel at the speed of light (about 299,792,458 meters per second in a vacuum). They are often described as particles of light because they can be absorbed and emitted by matter, but they also exhibit wave-like properties, such as interference and diffraction.

Photons are produced when an atom or molecule changes energy states, for example when an electron in an atom jumps from a higher energy level to a lower energy level. This process releases a photon with energy equal to the difference between the two energy levels.

Photons can also be produced when charged particles, such as electrons, are accelerated or decelerated. This process, known as bremsstrahlung, produces photons with a continuous spectrum of energies.

Photons play a central role in many phenomena in physics and chemistry, including the emission and absorption of light, the operation of lasers and LEDS, and the interactions between atoms and molecules. They are also important in many technological applications, such as telecommunications, solar power, and imaging.

Photons in Quantum Computing

In quantum computing, photons are used to encode and transmit quantum information. Quantum information is a type of information that is stored and processed using the principles of quantum mechanics, which is a fundamental theory in physics that describes the behavior of matter and energy at the atomic and subatomic scale.

One of the key features of quantum information is that it can be stored and processed using quantum states, which are the fundamental units of quantum information. Quantum states can be represented using a variety of different physical systems, including photons.

In a quantum computing system that uses photons to encode quantum information, the state of a photon (also known as its polarization) is used to represent the quantum state. For example, a photon with vertical polarization might represent a “1” bit of quantum information, while a photon with horizontal polarization might represent a “0” bit.

Photons are used in quantum computing because they have several properties that make them well-suited for storing and transmitting quantum information. Photons are extremely small and lightweight, which makes them easy to manipulate and control. They also interact weakly with their environment, which makes them relatively resistant to noise and errors. Finally, photons can be easily generated, detected, and transmitted over long distances, which makes them useful for building large-scale quantum computing systems.

There are several different approaches to using photons in quantum computing, including using single photons to represent quantum bits (qubits) of information, using entangled photons to perform quantum teleportation, and using arrays of photonics elements to build scalable quantum computing architectures.