photons (original) (raw)

Definition: quanta of light energy

Alternative term: light quanta

Categories: general optics, quantum photonics, physical foundations

Related: quantum opticsquantum photonicsphoton countingsingle-photon sourcesspontaneous emissionstimulated emissionnonclassical lightphononsExplaining the Nature of Photons to Lay PersonsLength of a Photon

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Contents

What is a Photon?

Some Key Properties of Photons

Energy Quantization

Wave-particle Duality

Zero Photon Mass

Angular Momentum

Boson Nature

Entangled States

Photons in Laser Physics

Photons in Quantum Technology

Frequently Asked Questions

Summary:

This article provides a comprehensive introduction to the concept of the photon. It explains that a photon is a discrete quantum of electromagnetic energy, described in modern quantum optics as an elementary excitation of the electromagnetic field.

Key properties are detailed, including energy quantization, the famous wave-particle duality, zero rest mass, angular momentum, and their nature as bosons. The text also clarifies the concept of entangled photons.

Furthermore, the article discusses the role of photons in laser physics, particularly for understanding stimulated emission and laser noise, and explores their crucial applications in modern quantum technologies like quantum communication, computing, and sensing.

(This summary was generated with AI based on the article content and has been reviewed by the article’s author.)

What is a Photon?

When a weak light beam hits a sensitive photodetector, energy is found to be delivered in the form of discrete quanta, rather than continuously. This can be interpreted such that the light beam consists of small bunches of energy, called photons or light quanta (German 'Lichtquanten' = portions of light). The photon energy is ($h \nu = h c / \lambda$), i.e. the product of Planck's constant ($h$) and the optical frequency ($\nu$), and is also related to the vacuum wavelength ($\lambda$).

The idea that light consists of such energy bunches had already been used early in the 20th century by Max Planck in the context of thermal radiation, and by Albert Einstein when investigating the photoelectric effect. The term photon, however, was coined only in 1926 by the physical chemist Gilbert N. Lewis [1].

Although a 'naïve' interpretation of photons as particles of light gives a useful picture for the intuitive understanding of many quantum phenomena, it can be seriously misleading to apply it without understanding its limitations. A consistent and very powerful, but certainly not simple description of the nature of light is achieved by modern quantum optics. Here, photons are seen as the elementary excitations of the electromagnetic quantum field. This theory attributes fairly surprising properties to photons, which cannot be reconciled either with a simple particle picture or with a pure wave picture, but accurately match a wide range of observations.

Some Key Properties of Photons

Energy Quantization

When light is absorbed by atoms or other particles, only amounts of energy which are integer multiples of the photon energy ($h \nu$) can be transferred to or from the light field. Similarly, light emission at a given wavelength is limited to such quanta. This can be easily interpreted as absorption or emission of some number of photons, and would thus so far be compatible with a simple particle picture of photons. Such processes are possible only if the involved atoms, ions or molecules are able to accept such amounts of energy, i.e., only if they have quantum-mechanical energy levels with an energy difference corresponding to the photon energy, or in some cases some integer multiple of it (→ two-photon absorption). A pure wave picture could explain these energy constraints as resonance effects, but cannot explain the quantization of exchanged energy.

Note that fractions of photon energies can be transferred in some kinds of interactions of light with matter, such as Raman scattering or Compton scattering, but in such cases the residual energy stays in an outgoing photon.

The energy quantization is also apparent in the interaction with sensitive photodetectors which can register single photon absorption events. This finds applications in various areas of science and technology.

Wave-particle Duality

The propagation of light (e.g. in free space or in a waveguide) is essentially that of a wave field. The quantum-mechanical field amplitude arising at some point in space and time is the superposition of contributions which correspond to different possible paths for light. These contributions can constructively or destructively interfere with each other, and this is the basis of the well-known optical interference effects. The likelihood or rate of photon detection at a certain location is proportional to the modulus squared of the quantum-mechanical field amplitude, and may thus be suppressed if different field contributions cancel each other, leading to a weak overall field. A pure particle picture is hard to reconcile with such phenomena.

For example, in the classical double-slit experiment an ordinary particle would have to go through one of the two slits, and the other slit would be irrelevant; it could not be explained why particles can reach certain locations behind the double slit only when one of the slits is blocked, but not when both are open (destructive interference).

Zero Photon Mass

Photons have zero mass, and therefore cannot be brought to rest. There are phenomena of “slow light”, but these occur only for light in media, where the electromagnetic field strongly interacts with matter.

Angular Momentum

Photons can carry angular momentum of two different forms: spin angular momentum corresponding to helicity ($\pm\hbar$) and also (in suitably structured beams) as an orbital angular momentum ($l \: \hbar$).

Boson Nature

Due to their boson nature, photons obey Bose–Einstein statistics (in contrast to Fermi–Dirac statistics for electrons). Multiple photons “prefer” to populate the same mode of the radiation field. This can be seen e.g. in the process of stimulated emission (and is thus also very important for lasers), also in the energy spectrum of thermally excited radiation (black body radiation) and in the Hong–Ou–Mandel effect, a quantum interference effect explained in the article on interference.

Entangled States

Photons can occur in entangled states, where certain properties (e.g. polarization) are correlated between different photons, even though these properties acquire definite values only when a measurement is performed. As measurements on the different photons can occur at different places, this seemed to imply the possibility of superluminal transmission of information (Einstein–Podolsky–Rosen paradox), but a closer inspection shows that in reality this is not the case.

Of course, quantum theory can be applied to any kind of electromagnetic wave phenomena, not only to visible light. However, quantum effects are not as important e.g. in the field of radio technology, as in optics and laser technology. This is because the photon energy of radio waves is very tiny compared with the thermal energy ($k_\textrm{B} T$) at room temperature, whereas the opposite is true for optical phenomena.

Photons in Laser Physics

Various phenomena in laser physics, such as stimulated emission as the basis for light amplification in laser gain media, are often explained based on photons. Nevertheless, much of laser physics can be described with purely classical pictures, not involving photons; for example, light amplification can be described with rate equation models involving excitation numbers and classical light wave amplitudes or intensities.

A stronger involvement of photons, however, is natural in the context of laser noise, or more generally noise in optics. For example, the high-frequency intensity noise of lasers is usually at the shot noise level, and its magnitude is compatible with a simple model of completely randomly arriving (uncorrelated) photons. Lower (sub-shot noise) intensity noise may be observed for squeezed states of light, as can be interpreted as photons becoming correlated, such that they arrive in a more regular fashion. The calculation of squeezing and many other phenomena is again usually based not on a simple particle picture, but either on full-blown quantum mechanics or on a simplified semiclassical model involving amplitude and phase fluctuations which have to obey certain rules.

Photons in Quantum Technology

Photons are the fundamental carriers of quantum information in optical quantum technologies. Their quantum properties — including superposition, entanglement and non-classical statistics — form the backbone of protocols and devices across quantum communication, quantum computing, quantum sensing and quantum metrology. Some examples:

• Quantum key distribution is an essential task in quantum communication, guaranteeing (under some assumptions) that remote parties share the same encryption keys while ensuring that no unauthorized party can get hold of them.
• Quantum computing can utilize different technologies, including some outside of photonics, but photons can serve as “flying” qubits (quantum bits) and can be quantum-manipulated in various ways. Photonic platforms operate with room-temperature optics and can support high-rate operations, although practical systems often rely on cryogenic detectors and latency-limited feed-forward.
• Quantum sensing utilizes quantum properties of photons to enhance sensitivity and precision beyond the standard quantum limit (quantum-enhanced metrology).
• Quantum random number generation exploits quantum properties for reliably generating true random numbers, as required for cryptographic security and simulations.

Quantum photonics has many devices with which photons can be generated, manipulated or detected; some examples:

• Single-photon sources generate photons either on demand or in “heralded” form.
• Photon pair sources generate photons with strong correlations.
• Manipulation of photons, i.e., of many of their quantum states, can often be done with simple optical elements. For example, qubits may be polarization-encoded and manipulated with polarizing optics.
• Single-photon detectors can register single photons, enabling photon counting, including coincidence measurements for correlation functions and histogram generation. Some detectors can resolve photon numbers e.g. of Fock states.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a photon?

A photon is a discrete quantum or 'bunch' of electromagnetic energy. In modern quantum optics, it is described as an elementary excitation of the electromagnetic quantum field, and light beams consist of streams of such photons.

How is the energy of a photon calculated?

The energy of a photon is given by the formula ($E = h \nu = h c / \lambda$), where ($h$) is Planck's constant, ($\nu$) is the optical frequency, ($c$) is the speed of light, and ($\lambda$) is the vacuum wavelength.

What is the wave-particle duality of photons?

Wave-particle duality describes how photons exhibit properties of both waves and particles. Their propagation and interference are wave-like, while their interaction with matter, such as in absorption or detection, occurs in discrete, particle-like energy quanta.

Do photons have mass?

No, photons have zero rest mass. Consequently, they always travel at the speed of light in a vacuum and therefore cannot be brought to rest.

Why are photons important for understanding lasers?

While many aspects of lasers can be described without quantum mechanics, photons are essential for understanding phenomena like stimulated emission, the fundamental process of light amplification. They are also central to describing laser noise, such as shot noise and the properties of squeezed states of light.

How are photons used in quantum technology?

In quantum technology, photons act as carriers of quantum information, known as qubits. Their quantum properties, like superposition and entanglement, are exploited for applications in quantum communication, quantum computing, and quantum sensing.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 36 suppliers for single-photon detectors. Among them:

⚙ hardware

SPAD 23 is a photon-counting array with 23 hexagonally packed single-photon avalanche diodes (SPADs) with best-in-class performance. The system software enables photon counting and time tagging and can be accessed through TCP/IP for easy integration into LabVIEW, MATLAB or Python. See our data sheet.

SPAD 512 is a camera integrating a 512×512 SPAD image sensor. Up to 100,000 fps in 1-bit mode enable high-speed imaging (photon-counting). Fine time gating enables the study of time-varying samples. See our data sheet.

SPAD Lambda is a linear detector with a 320×1 SPAD array. The detector is capable of both time gating and time stamping for the ultimate control over time-varying signals of interest. This arrangement is ideal for spectral detection applications. Thanks to microlenses and state-of-the-art production facilities, this detector offers high detection efficiency. See our data sheet.

⚙ hardware

The Hamamatsu Photonics MPPC (Multi-Pixel Photon Counter) is a device called SiPM, which is a photon counting device that is a multi-pixelized Geiger mode APD. While it is an optical semiconductor device, it has an excellent detection ability, so this device can be used in a variety of applications to detect very low-level light at the photon counting level.

Hamamatsu Photonics SPAD (Single Photon Avalanche Diode) is an element with a structure of a single pixel that combines a Geiger mode APD and a quenching resistor into one set. It is an optical semiconductor element that enables photon counting.

Bibliography

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