Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Physical Review A

Quantum-mechanical approach to a free photon

Toshio Inagaki

Published Apr 1, 1994• 48 citations
Publisher

Abstract

The photon dynamics proposed by Cook [Phys. Rev. A 25, 2164 (1982); 26, 2754 (1982)] is reformulated in terms of conventional quantum mechanics. Here, the subject is confined to a photon in vacuum. The photon density D(x,t) and photon current density C(x,t) in the Cook theory are here regarded as the probability density and probability current density of a single photon, respectively. A wave function \ensuremath{\Psi} for a photon is defined by a 6\ifmmode\times\else\texttimes\fi{}1 column matrix ${(}_{\mathrm{\ensuremath{\varphi}}}^{\mathrm{\ensuremath{\psi}}}$) where \ensuremath{\psi} and \ensuremath{\varphi} correspond to the photon vector fields of the Cook theory. The Schr\"odinger equation for \ensuremath{\Psi} is easily obtained from equations of photon fields \ensuremath{\psi} and \ensuremath{\varphi} and the Hamiltonian form is determined as the time-evolution operator. By this reformulation, negative-frequency solutions were shown not to be physical and need not be considered for a physical photon. This explains why, in the Cook theory, they are dismissed from plane-wave expansions of photon fields \ensuremath{\psi} and \ensuremath{\varphi}.

DOI
10.1103/physreva.49.2839

Related papers

On the quantum-mechanics of a single photon

Michael K.‐H. Kiessling, Shadi Tahvildar-Zadeh

It is shown that a Dirac(-type) equation for a rank-two bi-spinor field ψph on Minkowski (configuration) spacetime furnishes a Lorentz-covariant quantum-mechanical wave equation in position-space representation for a single free photon. This equation does not encounter any of the roadblocks that have obstructed previous attempts (by various authors) to formulate a quantum-mechanical photon wave equation. In particular, it implies that the photon wave function ψph yields conserved non-negative Born-rule-type quantum probabilities and that its probability current density four-vector transforms properly under Lorentz transformations. Moreover, the eigenvalues of the pertinent photon Dirac Hamiltonian and the vector eigenvalues of the photon momentum operator yield the familiar Einstein relations E = ℏω and p = ℏk, respectively. Furthermore, these spin-1 wave modes are automatically transversal without the need of an additional constraint on the initial data. Some comments on other proposals to set up a photon wave equation are supplied as well.

arXiv:1801.00268•Journal of Mathematical Physics•Nov 1, 2018•37 citations

Physical meaning of the photon wave function

Toshio Inagaki

Recently, the author [Phys. Rev. A 49, 2839 (1994)] proposed a quantum-mechanical theory of a photon, in which negative energy states can be dismissed from physical photon states without causing any difficulties. In this Brief Report the physical meaning of a photon wave function is investigated more precisely. The interpretation of a photon wave function as a probability amplitude is guaranteed by requiring the coarse-grained condition that the linear dimensions of a volume or a surface concerned in the configuration space are large compared to the photon wavelengths. The calculation made here is essentially based on that presented by Mandel [Phys. Rev. 144, 1071 (1966)] in his analysis of the photon number operator. But our calculation includes that of Mandel as the zeroth-order approximation. As a result, it is shown that although the position operator ◯ in the ordinary quantum mechanics be considered a correct photon operator strictly cannot, it becomes meaningful to some extent under the coarse-grained condition. In connection with ◯, we also examine the velocity operator $[\mathbf{◯},\ifmmode \hat{H}\else \^{H}\fi{}]/i\ensuremath{\Elzxh}$ ($\ifmmode \hat{H}\else \^{H}\fi{}$ is the photon Hamiltonian), and show that the probability current density is definable under the coarse-grained condition.

Physical Review A•Mar 1, 1998•10 citations

5 The Mechanics of Photons

L.D. Faddeev, L.A. Khalfin, I.V. Komarov

We propose to treat Maxwell equations as a wave equation of motion for a photon (a quantum of light) and to develop the photon mechanics on this basis. Consider vectors of electric and magnetic fields E and H as a single object (a bivector) F , which corresponds to the function ψ in the wave theory of the electron. Multiplying the vacuum Maxwell equations − rotH+ 1 c ∂E ∂t = 0, rot E + 1 c ∂H ∂t = 0 (1) by hc 2πi , it is possible to rewrite them in the form HF + h 2πi ∂F ∂t = 0, (2) where H is the operator H = hc 2πi { 0 − rot rot 0 } ; (3) its meaning follows from the comparison of equation (2) with (1). Equation (2) can be interpreted as a wave equation for a photon.

CRC Press eBooks•May 21, 2004•0 citations

The Schrödinger description of a single photon

Gavriil Shchedrin

The formulation of the wave function of a single photon has always been in a con ict with the spatial localization of a photon. Lack of the correct treatment for the probability and current density of a single photon resisted self-consistent treatment of the wave function of a single photon. Here we proceed with the construction of the wave function for a single photon that allows us to introduce probability and current density of a single photon. The wave function of a single photon is constructed from the matrix elements of the electric and magnetic elds that couple vacuum state with the Fock state occupied by a single photon. Further we show that the spin of a photon, being projected on the direction of the photon ight propagation, de nes the time evolution of the photon wave function. As the result it can be identi ed with the Hamiltonian of a single photon that establishes Schrodinger equation for a single photon. The Schrodinger description naturally leads to the current and the probability density for a single photon that satis es the continuity equation. The Schrodinger formulation of the Maxwell equations provides a clear physical meaning for the spin of a photon in a similar way to the Dirac equation provides a natural explanation for the spin of an electron.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Oct 1, 2013•0 citations

Quantum mechanics of a photon

Hassan Babaei, Ali Mostafazadeh

A first-quantized free photon is a complex massless vector field A=(Aμ) whose field strength satisfies Maxwell’s equations in vacuum. We construct the Hilbert space H of the photon by endowing the vector space of the fields A in the temporal-Coulomb gauge with a positive-definite and relativistically invariant inner product. We give an explicit expression for this inner product, identify the Hamiltonian for the photon with the generator of time translations in H, determine the operators representing the momentum and the helicity of the photon, and introduce a chirality operator whose eigenfunctions correspond to fields having a definite sign of energy. We also construct a position operator for the photon whose components commute with each other and with the chirality and helicity operators. This allows for the construction of the localized states of the photon with a definite sign of energy and helicity. We derive an explicit formula for the latter and compute the corresponding electric and magnetic fields. These turn out to diverge not just at the point where the photon is localized but on a plane containing this point. We identify the axis normal to this plane with an associated symmetry axis and show that each choice of this axis specifies a particular position operator, a corresponding position basis, and a position representation of the quantum mechanics of a photon. In particular, we examine the position wave functions determined by such a position basis, elucidate their relationship with the Riemann-Silberstein and Landau-Peierls wave functions, and give an explicit formula for the probability density of the spatial localization of the photon.

arXiv:1608.06479•Journal of Mathematical Physics•Aug 1, 2017•25 citations

Quantum mechanical density matrix

J. W. Halleý

For systems which obey quantum mechanics, the formulation of the problem of treating large numbers of particles is, of course, somewhat different than it is for classical systems. The microscopic description of the system is provided by a wave function which (in the absence of spin) is a function of the classical coordinates { q i }. The mathematical model is provided by a Hamiltonian operator H which is often obtained from the corresponding classical Hamiltonian by the replacement p i → ( ħ /i)(∂/∂ q i ). In other cases the form of the Hamiltonian operator is simply postulated. The microscopic dynamics are provided by the Schrödinger equation i ħ (∂Ψ/∂ t ) = H Ψ which requires as initial condition the knowledge of the wave function Ψ({ q i }, t ) at some initial time t 0 . (Boundary conditions on Ψ({ q i }, t ) must be specified as part of the description of the model as well.) The results of experiments in quantum mechanics are characterized by operators, usually obtained, like the Hamiltonian, from their classical forms and termed observables. Operators associated with observables must be Hermitian. In general, the various operators corresponding to observables do not commute with one another. It is possible to find sets of commuting operators whose mutual eigenstates span the Hilbert space in which the wave function is confined by the Schrödinger equation and the boundary conditions. A set of such (time independent) eigenstates, termed ψ ν ( q ), is a basis for the Hilbert space.

Cambridge University Press eBooks•Nov 16, 2006•0 citations
The photon dynamics proposed by Cook [Phys. Rev. A 25, 2164 (1982); 26, 2754 (1982)] is reformulated in terms of conventional quantum mechanics. Here, the subject is confined to a photon in vacuum. The photon density D(x,t) and photon current density C(x,t) in the Cook theory are here regarded as the probability density and probability current density of a single photon, respectively. A wave function \ensuremath{\Psi} for a photon is defined by a 6\ifmmode\times\else\texttimes\fi{}1 column matrix (\ensuremathφ\ensuremathψ{(}_{\mathrm{\ensuremath{\varphi}}}^{\mathrm{\ensuremath{\psi}}}(\ensuremathφ\ensuremathψ​) where \ensuremath{\psi} and \ensuremath{\varphi} correspond to the photon vector fields of the Cook theory. The Schr"odinger equation for \ensuremath{\Psi} is easily obtained from equations of photon fields \ensuremath{\psi} and \ensuremath{\varphi} and the Hamiltonian form is determined as the time-evolution operator. By this reformulation, negative-frequency solutions were shown not to be physical and need not be considered for a physical photon. This explains why, in the Cook theory, they are dismissed from plane-wave expansions of photon fields \ensuremath{\psi} and \ensuremath{\varphi}.
It is shown that a Dirac(-type) equation for a rank-two bi-spinor field ψph on Minkowski (configuration) spacetime furnishes a Lorentz-covariant quantum-mechanical wave equation in position-space representation for a single free photon. This equation does not encounter any of the roadblocks that have obstructed previous attempts (by various authors) to formulate a quantum-mechanical photon wave equation. In particular, it implies that the photon wave function ψph yields conserved non-negative Born-rule-type quantum probabilities and that its probability current density four-vector transforms properly under Lorentz transformations. Moreover, the eigenvalues of the pertinent photon Dirac Hamiltonian and the vector eigenvalues of the photon momentum operator yield the familiar Einstein relations E = ℏω and p = ℏk, respectively. Furthermore, these spin-1 wave modes are automatically transversal without the need of an additional constraint on the initial data. Some comments on other proposals to set up a photon wave equation are supplied as well.
Recently, the author [Phys. Rev. A 49, 2839 (1994)] proposed a quantum-mechanical theory of a photon, in which negative energy states can be dismissed from physical photon states without causing any difficulties. In this Brief Report the physical meaning of a photon wave function is investigated more precisely. The interpretation of a photon wave function as a probability amplitude is guaranteed by requiring the coarse-grained condition that the linear dimensions of a volume or a surface concerned in the configuration space are large compared to the photon wavelengths. The calculation made here is essentially based on that presented by Mandel [Phys. Rev. 144, 1071 (1966)] in his analysis of the photon number operator. But our calculation includes that of Mandel as the zeroth-order approximation. As a result, it is shown that although the position operator ◯ in the ordinary quantum mechanics be considered a correct photon operator strictly cannot, it becomes meaningful to some extent under the coarse-grained condition. In connection with ◯, we also examine the velocity operator $[\mathbf{◯},\ifmmode \hat{H}\else \^{H}\fi{}]/i\ensuremath{\Elzxh}$ ($\ifmmode \hat{H}\else \^{H}\fi{}$ is the photon Hamiltonian), and show that the probability current density is definable under the coarse-grained condition.
We propose to treat Maxwell equations as a wave equation of motion for a photon (a quantum of light) and to develop the photon mechanics on this basis. Consider vectors of electric and magnetic fields E and H as a single object (a bivector) F , which corresponds to the function ψ in the wave theory of the electron. Multiplying the vacuum Maxwell equations − rotH+ 1 c ∂E ∂t = 0, rot E + 1 c ∂H ∂t = 0 (1) by hc 2πi , it is possible to rewrite them in the form HF + h 2πi ∂F ∂t = 0, (2) where H is the operator H = hc 2πi { 0 − rot rot 0 } ; (3) its meaning follows from the comparison of equation (2) with (1). Equation (2) can be interpreted as a wave equation for a photon.
The formulation of the wave function of a single photon has always been in a con ict with the spatial localization of a photon. Lack of the correct treatment for the probability and current density of a single photon resisted self-consistent treatment of the wave function of a single photon. Here we proceed with the construction of the wave function for a single photon that allows us to introduce probability and current density of a single photon. The wave function of a single photon is constructed from the matrix elements of the electric and magnetic elds that couple vacuum state with the Fock state occupied by a single photon. Further we show that the spin of a photon, being projected on the direction of the photon ight propagation, de nes the time evolution of the photon wave function. As the result it can be identi ed with the Hamiltonian of a single photon that establishes Schrodinger equation for a single photon. The Schrodinger description naturally leads to the current and the probability density for a single photon that satis es the continuity equation. The Schrodinger formulation of the Maxwell equations provides a clear physical meaning for the spin of a photon in a similar way to the Dirac equation provides a natural explanation for the spin of an electron.
A first-quantized free photon is a complex massless vector field A=(Aμ) whose field strength satisfies Maxwell’s equations in vacuum. We construct the Hilbert space H of the photon by endowing the vector space of the fields A in the temporal-Coulomb gauge with a positive-definite and relativistically invariant inner product. We give an explicit expression for this inner product, identify the Hamiltonian for the photon with the generator of time translations in H, determine the operators representing the momentum and the helicity of the photon, and introduce a chirality operator whose eigenfunctions correspond to fields having a definite sign of energy. We also construct a position operator for the photon whose components commute with each other and with the chirality and helicity operators. This allows for the construction of the localized states of the photon with a definite sign of energy and helicity. We derive an explicit formula for the latter and compute the corresponding electric and magnetic fields. These turn out to diverge not just at the point where the photon is localized but on a plane containing this point. We identify the axis normal to this plane with an associated symmetry axis and show that each choice of this axis specifies a particular position operator, a corresponding position basis, and a position representation of the quantum mechanics of a photon. In particular, we examine the position wave functions determined by such a position basis, elucidate their relationship with the Riemann-Silberstein and Landau-Peierls wave functions, and give an explicit formula for the probability density of the spatial localization of the photon.
For systems which obey quantum mechanics, the formulation of the problem of treating large numbers of particles is, of course, somewhat different than it is for classical systems. The microscopic description of the system is provided by a wave function which (in the absence of spin) is a function of the classical coordinates { q i }. The mathematical model is provided by a Hamiltonian operator H which is often obtained from the corresponding classical Hamiltonian by the replacement p i → ( ħ /i)(∂/∂ q i ). In other cases the form of the Hamiltonian operator is simply postulated. The microscopic dynamics are provided by the Schrödinger equation i ħ (∂Ψ/∂ t ) = H Ψ which requires as initial condition the knowledge of the wave function Ψ({ q i }, t ) at some initial time t 0 . (Boundary conditions on Ψ({ q i }, t ) must be specified as part of the description of the model as well.) The results of experiments in quantum mechanics are characterized by operators, usually obtained, like the Hamiltonian, from their classical forms and termed observables. Operators associated with observables must be Hermitian. In general, the various operators corresponding to observables do not commute with one another. It is possible to find sets of commuting operators whose mutual eigenstates span the Hilbert space in which the wave function is confined by the Schrödinger equation and the boundary conditions. A set of such (time independent) eigenstates, termed ψ ν ( q ), is a basis for the Hilbert space.