The general case in which the electric field rotates in the x–y plane and has variable magnitude is called elliptical polarization. The state vector is given by | ψ ⟩   = d e f   ( ψ x ψ y ) = ( cos ⁡ θ exp ⁡ ( i α x ) sin ⁡ θ exp ⁡ ( i α y ) ) . {\displaystyle |\psi \rangle \ {\stackrel {\mathrm {def} }{=}}\ {\begin{pmatrix}\psi _{x}\\\psi _{y}\end{pmatrix}}={\begin{pmatrix}\cos \theta \exp \left(i\alpha _{x}\right)\\\sin \theta \exp \left(i\alpha _{y}\right)\end{pmatrix}}.}

I = U ^ † U ^ ≈ ( I − i H ^ † ) ( I + i H ^ ) ≈ I − i H ^ † + i H ^ . {\displaystyle I={\hat {U}}^{\dagger }{\hat {U}}\approx \left(I-i{\hat {H}}^{\dagger }\right)\left(I+i{\hat {H}}\right)\approx I-i{\hat {H}}^{\dagger }+i{\hat {H}}.}

Einstein's conclusion from early experiments on the photoelectric effect is that electromagnetic radiation is composed of irreducible packets of energy, known as photons. The energy of each packet is related to the angular frequency of the wave by the relation ϵ = ℏ ω {\displaystyle \epsilon =\hbar \omega } where ℏ {\displaystyle \hbar } is an experimentally determined quantity known as the reduced Planck constant. If there are N {\displaystyle N} photons in a box of volume V {\displaystyle V} , the energy in the electromagnetic field is N ℏ ω {\displaystyle N\hbar \omega } and the energy density is N ℏ ω V {\displaystyle {N\hbar \omega \over V}}

The infinitesimal transition of the polarization state is | ψ ′ ⟩ − | ψ ⟩ = i H ^ | ψ ⟩ . {\displaystyle |\psi '\rangle -|\psi \rangle =i{\hat {H}}|\psi \rangle .}

Polarisationmeaning in Physics

In this ideal case, all the energy impinging on the crystal emerges from the crystal. An operator U with the property that U ^ † U ^ = I , {\displaystyle {\hat {U}}^{\dagger }{\hat {U}}=I,} where I is the identity operator and U is called a unitary operator. The unitary property is necessary to ensure energy conservation in state transformations.

A linear filter transmits one component of a plane wave and absorbs the perpendicular component. In that case, if the filter is polarized in the x direction, the fraction of energy passing through the filter is f x = ψ x ∗ ψ x = cos 2 ⁡ θ . {\displaystyle f_{x}=\psi _{x}^{*}\psi _{x}=\cos ^{2}\theta .\,}

Similarly for the spin angular momentum L = 1 ω E c ( | ψ R | 2 − | ψ L | 2 ) = N ℏ V ( | ψ R | 2 − | ψ L | 2 ) {\displaystyle {\mathcal {L}}={\frac {1}{\omega }}{\mathcal {E}}_{c}\left(\vert \psi _{\rm {R}}\vert ^{2}-\vert \psi _{\rm {L}}\vert ^{2}\right)={\frac {N\hbar }{V}}\left(\vert \psi _{\rm {R}}\vert ^{2}-\vert \psi _{\rm {L}}\vert ^{2}\right)} where E c {\displaystyle {\mathcal {E}}_{c}} is field strength. This implies that the spin angular momentum of the photon is l z = ℏ ( | ψ R | 2 − | ψ L | 2 ) . {\displaystyle l_{z}=\hbar \left(\vert \psi _{\rm {R}}\vert ^{2}-\vert \psi _{\rm {L}}\vert ^{2}\right).} the quantum interpretation of this expression is that the photon has a probability of ∣ ψ R ∣ 2 {\displaystyle \mid \psi _{\rm {R}}\mid ^{2}} of having a spin angular momentum of ℏ {\displaystyle \hbar } and a probability of ∣ ψ L ∣ 2 {\displaystyle \mid \psi _{\rm {L}}\mid ^{2}} of having a spin angular momentum of − ℏ {\displaystyle -\hbar } . We can therefore think of the spin angular momentum of the photon being quantized as well as the energy. The angular momentum of classical light has been verified.[2] A photon that is linearly polarized (plane polarized) is in a superposition of equal amounts of the left-handed and right-handed states.

Editorially, Laser Magic is packed with insight into optical disc invention and the resulting LaserDisc and DVD technologies that form the basis for both players and discs. I want to personally thank David Paul Gregg, recognized to be the “Father of the Optical Disc,” for graciously spending an afternoon with me covering the historical perspective and politics surrounding optical disc technology. And what a wonderful piece of work that David Robert Cellitti wrote on the history of the optical disc, entitled “World On A Silver Platter.” This article uncovers the politics of industrial competition aimed at the prize - “patent ownership,” and thus, royalties. Jim Taylor, our DVD Evangelist, deserves special recognition for putting together the most comprehensive “FAQ” article on DVD that I have ever read, and as well, Bob Niland for his superb “LaserDisc FAQ” article. Chris McGowan, one of the magazine’s contributing editors, wrote the excellent piece on “The Interactive Annotated Movie,” which covers the interactivity features of DVD and LaserDisc. And “Mr. Research,” our own Michael Coate, is to be commended for putting together “A Gallery Of Widescreen Special Editions And Boxed Sets” and for his extensive compilation research, which is published throughout Laser Magic. Also, my appreciation goes out to R. Michael Hayes for writing two very enlightening articles entitled ”Roadshow Movies” and “How To Read Motion Picture Credits.” Executive Publisher, Marlene Reber merits special recognition for her meticulous creation of our definitive Discographies. Lastly, Suzanne Hodges, our Film Review Editor, deserves your applause for her superb review work and Jim Bertz for his graphic design creativity, as well as the dedicated WSR staff.

This is not the only occasion[dubious – discuss] in which Maxwell's equations have forced a restructuring of Newtonian mechanics. Maxwell's equations are relativistically consistent. Special relativity resulted from attempts to make classical mechanics consistent with Maxwell's equations (see, for example, Moving magnet and conductor problem).

For a sinusoidal plane wave propagating along z {\displaystyle z} axis the orbital angular momentum density vanishes. The spin angular momentum density is in the z {\displaystyle z} direction and is given by L = | E | 2 8 π ω ( | ⟨ R | ψ ⟩ | 2 − | ⟨ L | ψ ⟩ | 2 ) = 1 ω E c ( | ψ R | 2 − | ψ L | 2 ) {\displaystyle {\mathcal {L}}={{\vert \mathbf {E} \vert ^{2}} \over {8\pi \omega }}\left(\left\vert \langle \mathrm {R} |\psi \rangle \right\vert ^{2}-\left\vert \langle \mathrm {L} |\psi \rangle \right\vert ^{2}\right)={\frac {1}{\omega }}{\mathcal {E}}_{c}\left(\vert \psi _{\rm {R}}\vert ^{2}-\vert \psi _{\rm {L}}\vert ^{2}\right)} where again the density is averaged over a wavelength.

This is a purely mathematical result. No reference has been made to any physical quantity or principle. It simply states that the uncertainty of one operator times the uncertainty of another operator has a lower bound.

The fraction of energy that emerges from the crystal is ⟨ ψ ′ | ψ ′ ⟩ = ⟨ ψ | U ^ † U ^ | ψ ⟩ = ⟨ ψ | ψ ⟩ = 1. {\displaystyle \langle \psi '|\psi '\rangle =\langle \psi |{\hat {U}}^{\dagger }{\hat {U}}|\psi \rangle =\langle \psi |\psi \rangle =1.}

Many of the implications of the mathematical machinery are easily verified experimentally. In fact, many of the experiments can be performed with polaroid sunglass lenses.

If the phase angles α x {\displaystyle \alpha _{x}} and α y {\displaystyle \alpha _{y}} differ by exactly π / 2 {\displaystyle \pi /2} and the x amplitude equals the y amplitude the wave is circularly polarized. The Jones vector then becomes | ψ ⟩ = 1 2 ( 1 ± i ) exp ⁡ ( i α x ) {\displaystyle |\psi \rangle ={\frac {1}{\sqrt {2}}}{\begin{pmatrix}1\\\pm i\end{pmatrix}}\exp \left(i\alpha _{x}\right)} where the plus sign indicates left circular polarization and the minus sign indicates right circular polarization. In the case of circular polarization, the electric field vector of constant magnitude rotates in the x–y plane.

If unit vectors are defined such that | x ⟩   = d e f   ( 1 0 ) {\displaystyle |x\rangle \ {\stackrel {\mathrm {def} }{=}}\ {\begin{pmatrix}1\\0\end{pmatrix}}} and | y ⟩   = d e f   ( 0 1 ) {\displaystyle |y\rangle \ {\stackrel {\mathrm {def} }{=}}\ {\begin{pmatrix}0\\1\end{pmatrix}}} then the linearly polarized polarization state can be written in the "x–y basis" as | ψ ⟩ = cos ⁡ θ exp ⁡ ( i α ) | x ⟩ + sin ⁡ θ exp ⁡ ( i α ) | y ⟩ = ψ x | x ⟩ + ψ y | y ⟩ . {\displaystyle |\psi \rangle =\cos \theta \exp \left(i\alpha \right)|x\rangle +\sin \theta \exp \left(i\alpha \right)|y\rangle =\psi _{x}|x\rangle +\psi _{y}|y\rangle .}

For any legal[clarification needed] operators the following inequality, a consequence of the Cauchy–Schwarz inequality, is true. 1 4 | ⟨ ( A ^ B ^ − B ^ A ^ ) x | x ⟩ | 2 ≤ ‖ A ^ x ‖ 2 ‖ B ^ x ‖ 2 . {\displaystyle {\frac {1}{4}}\left|\langle ({\hat {A}}{\hat {B}}-{\hat {B}}{\hat {A}})x|x\rangle \right|^{2}\leq \left\|{\hat {A}}x\right\|^{2}\left\|{\hat {B}}x\right\|^{2}.}

Polarisation stateexample

The momentum density is given by the Poynting vector P = 1 4 π c E ( r , t ) × B ( r , t ) . {\displaystyle {\boldsymbol {\mathcal {P}}}={1 \over 4\pi c}\mathbf {E} (\mathbf {r} ,t)\times \mathbf {B} (\mathbf {r} ,t).}

If B A ψ and A B ψ are defined, then by subtracting the means and re-inserting in the above formula, we deduce Δ ψ A ^ Δ ψ B ^ ≥ 1 2 | ⟨ [ A ^ , B ^ ] ⟩ ψ | {\displaystyle \Delta _{\psi }{\hat {A}}\,\Delta _{\psi }{\hat {B}}\geq {\frac {1}{2}}\left|\left\langle \left[{\hat {A}},{\hat {B}}\right]\right\rangle _{\psi }\right|} where ⟨ X ^ ⟩ ψ = ⟨ ψ | X ^ | ψ ⟩ {\displaystyle \left\langle {\hat {X}}\right\rangle _{\psi }=\left\langle \psi \right|{\hat {X}}\left|\psi \right\rangle } is the operator mean of observable X in the system state ψ and Δ ψ X ^ = ⟨ X ^ 2 ⟩ ψ − ⟨ X ^ ⟩ ψ 2 . {\displaystyle \Delta _{\psi }{\hat {X}}={\sqrt {\langle {\hat {X}}^{2}\rangle _{\psi }-\langle {\hat {X}}\rangle _{\psi }^{2}}}.}

Stateof polarization of light

If only the traced out shape and the direction of the rotation of (x(t), y(t)) is considered when interpreting the polarization state, i.e. only M ( | ψ ⟩ ) = { ( x ( t ) , y ( t ) ) | ∀ t } {\displaystyle M(|\psi \rangle )=\left.\left\{{\Big (}x(t),\,y(t){\Big )}\,\right|\,\forall \,t\right\}} (where x(t) and y(t) are defined as above) and whether it is overall more right circularly or left circularly polarized (i.e. whether |ψR| > |ψL| or vice versa), it can be seen that the physical interpretation will be the same even if the state is multiplied by an arbitrary phase factor, since M ( e i α | ψ ⟩ ) = M ( | ψ ⟩ ) ,   α ∈ R {\displaystyle M(e^{i\alpha }|\psi \rangle )=M(|\psi \rangle ),\ \alpha \in \mathbb {R} } and the direction of rotation will remain the same. In other words, there is no physical difference between two polarization states | ψ ⟩ {\displaystyle |\psi \rangle } and e i α | ψ ⟩ {\displaystyle e^{i\alpha }|\psi \rangle } , between which only a phase factor differs.

Here [ A ^ , B ^ ]   = d e f   A ^ B ^ − B ^ A ^ {\displaystyle \left[{\hat {A}},{\hat {B}}\right]\ {\stackrel {\mathrm {def} }{=}}\ {\hat {A}}{\hat {B}}-{\hat {B}}{\hat {A}}} is called the commutator of A and B.

Much of the mathematical apparatus of quantum mechanics appears in the classical description of a polarized sinusoidal electromagnetic wave. The Jones vector for a classical wave, for instance, is identical with the quantum polarization state vector for a photon. The right and left circular components of the Jones vector can be interpreted as probability amplitudes of spin states of the photon. Energy conservation requires that the states be transformed with a unitary operation. This implies that infinitesimal transformations are transformed with a Hermitian operator. These conclusions are a natural consequence of the structure of Maxwell's equations for classical waves.

When the state is written in spin notation, the spin operator can be written S ^ d → i ∂ ∂ θ {\displaystyle {\hat {S}}_{d}\rightarrow i{\partial \over \partial \theta }} S ^ d † → − i ∂ ∂ θ . {\displaystyle {\hat {S}}_{d}^{\dagger }\rightarrow -i{\partial \over \partial \theta }.}

Electromagnetic waves can have both orbital and spin angular momentum.[1] The total angular momentum density is L = r × P = 1 4 π c r × [ E ( r , t ) × B ( r , t ) ] . {\displaystyle {\boldsymbol {\mathcal {L}}}=\mathbf {r} \times {\boldsymbol {\mathcal {P}}}={1 \over 4\pi c}\mathbf {r} \times \left[\mathbf {E} (\mathbf {r} ,t)\times \mathbf {B} (\mathbf {r} ,t)\right].}

A birefringent crystal is a material that has an optic axis with the property that the light has a different index of refraction for light polarized parallel to the axis than it has for light polarized perpendicular to the axis. Light polarized parallel to the axis are called "extraordinary rays" or "extraordinary photons", while light polarized perpendicular to the axis are called "ordinary rays" or "ordinary photons". If a linearly polarized wave impinges on the crystal, the extraordinary component of the wave will emerge from the crystal with a different phase than the ordinary component. In mathematical language, if the incident wave is linearly polarized at an angle t h e t a {\displaystyle theta} with respect to the optic axis, the incident state vector can be written | ψ ⟩ = ( cos ⁡ θ sin ⁡ θ ) {\displaystyle |\psi \rangle ={\begin{pmatrix}\cos \theta \\\sin \theta \end{pmatrix}}} and the state vector for the emerging wave can be written | ψ ′ ⟩ = ( cos ⁡ θ exp ⁡ ( i α x ) sin ⁡ θ exp ⁡ ( i α y ) ) = ( exp ⁡ ( i α x ) 0 0 exp ⁡ ( i α y ) ) ( cos ⁡ θ sin ⁡ θ )   = d e f   U ^ | ψ ⟩ . {\displaystyle |\psi '\rangle ={\begin{pmatrix}\cos \theta \exp \left(i\alpha _{x}\right)\\\sin \theta \exp \left(i\alpha _{y}\right)\end{pmatrix}}={\begin{pmatrix}\exp \left(i\alpha _{x}\right)&0\\0&\exp \left(i\alpha _{y}\right)\end{pmatrix}}{\begin{pmatrix}\cos \theta \\\sin \theta \end{pmatrix}}\ {\stackrel {\mathrm {def} }{=}}\ {\hat {U}}|\psi \rangle .}

It can be seen that for a linearly polarized state, M will be a line in the xy plane, with length 2 and its middle in the origin, and whose slope equals to tan(θ). For a circularly polarized state, M will be a circle with radius 1/√2 and with the middle in the origin.

Quantum mechanics enters the picture when observed quantities are measured and found to be discrete rather than continuous. The allowed observable values are determined by the eigenvalues of the operators associated with the observable. In the case angular momentum, for instance, the allowed observable values are the eigenvalues of the spin operator.

What is polarization in Chemistry

The correspondence principle also determines the momentum and angular momentum of the photon. For momentum P z = N ℏ ω c V = N ℏ k z V {\displaystyle {\mathcal {P}}_{z}={N\hbar \omega \over cV}={N\hbar k_{z} \over V}} where k z {\displaystyle k_{z}} is the wave number. This implies that the momentum of a photon is p z = ℏ k z . {\displaystyle p_{z}=\hbar k_{z}.\,}

The spin angular momentum operator is l ^ z = ℏ S ^ d . {\displaystyle {\hat {l}}_{z}=\hbar {\hat {S}}_{d}.}

The fraction of energy in the x component of the plane wave is f x = | E | 2 cos 2 ⁡ θ | E | 2 = ψ x ∗ ψ x = cos 2 ⁡ θ {\displaystyle f_{x}={\frac {|\mathbf {E} |^{2}\cos ^{2}\theta }{\vert \mathbf {E} \vert ^{2}}}=\psi _{x}^{*}\psi _{x}=\cos ^{2}\theta } with a similar expression for the y component resulting in f y = sin 2 ⁡ θ {\displaystyle f_{y}=\sin ^{2}\theta } .

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The energy per unit volume in classical electromagnetic fields is (cgs units) and also Planck units: E c = 1 8 π [ E 2 ( r , t ) + B 2 ( r , t ) ] . {\displaystyle {\mathcal {E}}_{c}={\frac {1}{8\pi }}\left[\mathbf {E} ^{2}(\mathbf {r} ,t)+\mathbf {B} ^{2}(\mathbf {r} ,t)\right].}

The spin of the photon is defined as the coefficient of ℏ {\displaystyle \hbar } in the spin angular momentum calculation. A photon has spin 1 if it is in the | R ⟩ {\displaystyle |R\rangle } state and −1 if it is in the | L ⟩ {\displaystyle |L\rangle } state. The spin operator is defined as the outer product S ^   = d e f   | R ⟩ ⟨ R | − | L ⟩ ⟨ L | = ( 0 − i i 0 ) . {\displaystyle {\hat {S}}\ {\stackrel {\mathrm {def} }{=}}\ |\mathrm {R} \rangle \langle \mathrm {R} |-|\mathrm {L} \rangle \langle \mathrm {L} |={\begin{pmatrix}0&-i\\i&0\end{pmatrix}}.}

Welcome to Laser Magic™ 1998, “The Ultimate Widescreen Video Disc Guide!” This massive piece has been a work in progress for over two years. It was envisioned to serve as an authoritative and essential home theatre resource focused on DVD and LaserDisc optical storage mediums. Laser Magic has two components to its contents: 1) articles which cover the history of optical disc invention and the particulars of the DVD and Laser Disc formats, and 2) an evaluation of the end product experience described through review synopses, with picture and sound quality scores and picture format and soundtrack data indices cross referenced to previous issues of Widescreen Review. The latter component includes synopsis reviews of 1,982 LaserDisc titles reviewed since Issue 1, which was first published in November 1992, and 291 DVD titles reviewed since September’s 1997 Issue 25 through June 1998’s Issue 28. For those readers not familiar with Widescreen Review magazine, movies on disc are evaluated and scored according to picture and sound quality presentation. The value range is “1” BAD to “5” EXCELLENT. A FiveFive score represents reference quality for the particular time frame of release. Thus, a title that received a P5 and S5 score in 1992 may be considered less than reference quality by 1998 disc standards. One of the data indices, entitled “Gary’s Best DVDs And Laser Discs,” is a record of every title that has ever received a FiveFive picture and sound score, referenced to the particular issue in Widescreen Review in which the title was reviewed. As our scope is “the movie experience” presented in its original intended “widescreen theatrical format,” as viewed on widescreen display devices, only those titles released on DVD or LaserDisc in a widescreen aspect ratio are reviewed. For the complete review, readers are directed to the original magazine issue. For a master index to all the 2,273 titles reviewed since 1992, we direct our readers to the DVDiscography™ and LaserDiscography™ in Laser Magic. As a further resource to our readers, we have researched and compiled a comprehensive “Widescreen Filmography” of every known widescreen formatted film from 1926 through July 1998, and cross referenced each title to available discs and reviews. We also have included our “Widescreen DVD Releases” DVDiscograpy identifying 291 DVDs reviewed out of 566 released and/or announced by publishers and studios/distributors. Additionally, Laser Magic contains a “Digital Sound Filmography,” which lists every movie released in North America with a digital soundtrack by format - Cinema Digital Sound, Dolby® Digital, DTS® Digital Sound, and Sony Dynamic Digital Sound® - through July 1998. In recognition of the contribution to disc presentation quality that the THX® division of Lucasfilm has made, we present at the back end of the “THX Digital Mastering Program” article, a current through July 1998 index of THX-certified titles released on LaserDisc, DVD, and VHS, cross referenced to reviews in Widescreen Review. The article, which originally appeared in Issue 5 and was written by S. Ross Hering and Susan Griffin, has been updated for this special edition by Karen Fromel and J.C. Mitchell.

The connection with quantum mechanics is made through the identification of a minimum packet size, called a photon, for energy in the electromagnetic field. The identification is based on the theories of Planck and the interpretation of those theories by Einstein. The correspondence principle then allows the identification of momentum and angular momentum (called spin), as well as energy, with the photon.

The eigenvectors of the differential spin operator are exp ⁡ ( i α x − i s θ ) | s ⟩ . {\displaystyle \exp \left(i\alpha _{x}-is\theta \right)|s\rangle .}

To get an understanding of what a polarization state looks like, one can observe the orbit that is made if the polarization state is multiplied by a phase factor of e i ω t {\displaystyle e^{i\omega t}} and then having the real parts of its components interpreted as x and y coordinates respectively. That is: ( x ( t ) y ( t ) ) = ( ℜ ( e i ω t ψ x ) ℜ ( e i ω t ψ y ) ) = ℜ [ e i ω t ( ψ x ψ y ) ] = ℜ ( e i ω t | ψ ⟩ ) . {\displaystyle {\begin{pmatrix}x(t)\\y(t)\end{pmatrix}}={\begin{pmatrix}\Re (e^{i\omega t}\psi _{x})\\\Re (e^{i\omega t}\psi _{y})\end{pmatrix}}=\Re \left[e^{i\omega t}{\begin{pmatrix}\psi _{x}\\\psi _{y}\end{pmatrix}}\right]=\Re \left(e^{i\omega t}|\psi \rangle \right).}

We can write the circularly polarized states as | s ⟩ {\displaystyle |s\rangle } where s = 1 for | R ⟩ {\displaystyle |\mathrm {R} \rangle } and s = −1 for | L ⟩ {\displaystyle |\mathrm {L} \rangle } . An arbitrary state can be written | ψ ⟩ = ∑ s = − 1 , 1 a s exp ⁡ ( i α x − i s θ ) | s ⟩ {\displaystyle |\psi \rangle =\sum _{s=-1,1}a_{s}\exp \left(i\alpha _{x}-is\theta \right)|s\rangle } where α 1 {\displaystyle \alpha _{1}} and α − 1 {\displaystyle \alpha _{-1}} are phase angles, θ is the angle by which the frame of reference is rotated, and ∑ s = − 1 , 1 | a s | 2 = 1. {\displaystyle \sum _{s=-1,1}\vert a_{s}\vert ^{2}=1.}

The treatment to this point has been classical. It is a testament, however, to the generality of Maxwell's equations for electrodynamics that the treatment can be made quantum mechanical with only a reinterpretation of classical quantities. The reinterpretation is based on the theories of Max Planck and the interpretation by Albert Einstein of those theories and of other experiments.[citation needed]

The connection to physics can be made if we identify the operators with physical operators such as the angular momentum and the polarization angle. We have then Δ ψ L ^ z Δ ψ θ ≥ ℏ 2 , {\displaystyle \Delta _{\psi }{\hat {L}}_{z}\,\Delta _{\psi }{\theta }\geq {\frac {\hbar }{2}},} which means that angular momentum and the polarization angle cannot be measured simultaneously with infinite accuracy. (The polarization angle can be measured by checking whether the photon can pass through a polarizing filter oriented at a particular angle, or a polarizing beam splitter. This results in a yes/no answer that, if the photon was plane-polarized at some other angle, depends on the difference between the two angles.)

The wave is linearly polarized (or plane polarized) when the phase angles α x , α y {\displaystyle \alpha _{x}\,,\;\alpha _{y}} are equal, α x = α y   = d e f   α . {\displaystyle \alpha _{x}=\alpha _{y}\ {\stackrel {\mathrm {def} }{=}}\ \alpha .}

The photon energy can be related to classical fields through the correspondence principle that states that for a large number of photons, the quantum and classical treatments must agree. Thus, for very large N {\displaystyle N} , the quantum energy density must be the same as the classical energy density N ℏ ω V = E c = | E | 2 8 π . {\displaystyle {N\hbar \omega \over V}={\mathcal {E}}_{c}={\frac {\vert \mathbf {E} \vert ^{2}}{8\pi }}.}

Circularpolarisation state

Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. An individual photon can be described as having right or left circular polarization, or a superposition of the two. Equivalently, a photon can be described as having horizontal or vertical linear polarization, or a superposition of the two.

While the initial state was linearly polarized, the final state is elliptically polarized. The birefringent crystal alters the character of the polarization.

Polarizedstateof neuron

The description of photon polarization contains many of the physical concepts and much of the mathematical machinery of more involved quantum descriptions, such as the quantum mechanics of an electron in a potential well. Polarization is an example of a qubit degree of freedom, which forms a fundamental basis for an understanding of more complicated quantum phenomena. Much of the mathematical machinery of quantum mechanics, such as state vectors, probability amplitudes, unitary operators, and Hermitian operators, emerge naturally from the classical Maxwell's equations in the description. The quantum polarization state vector for the photon, for instance, is identical with the Jones vector, usually used to describe the polarization of a classical wave. Unitary operators emerge from the classical requirement of the conservation of energy of a classical wave propagating through lossless media that alter the polarization state of the wave. Hermitian operators then follow for infinitesimal transformations of a classical polarization state.

The eigenvectors of the spin operator are | R ⟩ {\displaystyle |\mathrm {R} \rangle } and | L ⟩ {\displaystyle |\mathrm {L} \rangle } with eigenvalues 1 and −1, respectively.

There are two ways in which probability can be applied to the behavior of photons; probability can be used to calculate the probable number of photons in a particular state, or probability can be used to calculate the likelihood of a single photon to be in a particular state. The former interpretation violates energy conservation. The latter interpretation is the viable, if nonintuitive, option. Dirac explains this in the context of the double-slit experiment:

Thus, energy conservation requires that infinitesimal transformations of a polarization state occur through the action of a Hermitian operator.

We can see that 1 = | ψ R | 2 + | ψ L | 2 . {\displaystyle 1=|\psi _{\rm {R}}|^{2}+|\psi _{\rm {L}}|^{2}.}

An ideal birefringent crystal transforms the polarization state of an electromagnetic wave without loss of wave energy. Birefringent crystals therefore provide an ideal test bed for examining the conservative transformation of polarization states. Even though this treatment is still purely classical, standard quantum tools such as unitary and Hermitian operators that evolve the state in time naturally emerge.

s-polarization vs p polarization

For a plane wave, this becomes: E c = ∣ E ∣ 2 8 π {\displaystyle {\mathcal {E}}_{c}={\frac {\mid \mathbf {E} \mid ^{2}}{8\pi }}} where the energy has been averaged over a wavelength of the wave.

The initial polarization state is transformed into the final state with the operator U. The dual of the final state is given by ⟨ ψ ′ | = ⟨ ψ | U ^ † {\displaystyle \langle \psi '|=\langle \psi |{\hat {U}}^{\dagger }} where U † {\displaystyle U^{\dagger }} is the adjoint of U, the complex conjugate transpose of the matrix.

This represents a wave with phase α {\displaystyle \alpha } polarized at an angle θ {\displaystyle \theta } with respect to the x axis. In this case the Jones vector | ψ ⟩ = ( cos ⁡ θ exp ⁡ ( i α x ) sin ⁡ θ exp ⁡ ( i α y ) ) {\displaystyle |\psi \rangle ={\begin{pmatrix}\cos \theta \exp \left(i\alpha _{x}\right)\\\sin \theta \exp \left(i\alpha _{y}\right)\end{pmatrix}}} can be written with a single phase: | ψ ⟩ = ( cos ⁡ θ sin ⁡ θ ) exp ⁡ ( i α ) . {\displaystyle |\psi \rangle ={\begin{pmatrix}\cos \theta \\\sin \theta \end{pmatrix}}\exp \left(i\alpha \right).}

To see this note S ^ d exp ⁡ ( i α x − i s θ ) | s ⟩ → i ∂ ∂ θ exp ⁡ ( i α x − i s θ ) | s ⟩ = s [ exp ⁡ ( i α x − i s θ ) | s ⟩ ] . {\displaystyle {\hat {S}}_{d}\exp \left(i\alpha _{x}-is\theta \right)|s\rangle \rightarrow i{\partial \over \partial \theta }\exp \left(i\alpha _{x}-is\theta \right)|s\rangle =s\left[\exp \left(i\alpha _{x}-is\theta \right)|s\rangle \right].}

For a sinusoidal plane wave traveling in the z direction, the momentum is in the z direction and is related to the energy density: P z c = E c . {\displaystyle {\mathcal {P}}_{z}c={\mathcal {E}}_{c}.}

These concepts have emerged naturally from Maxwell's equations and Planck's and Einstein's theories. They have been found to be true for many other physical systems. In fact, the typical program is to assume the concepts of this section and then to infer the unknown dynamics of a physical system. This was done, for instance, with the dynamics of electrons. In that case, working back from the principles in this section, the quantum dynamics of particles were inferred, leading to Schrödinger's equation, a departure from Newtonian mechanics. The solution of this equation for atoms led to the explanation of the Balmer series for atomic spectra and consequently formed a basis for all of atomic physics and chemistry.

If the crystal is very thin, the final state will be only slightly different from the initial state. The unitary operator will be close to the identity operator. We can define the operator H by U ^ ≈ I + i H ^ {\displaystyle {\hat {U}}\approx I+i{\hat {H}}} and the adjoint by U ^ † ≈ I − i H ^ † . {\displaystyle {\hat {U}}^{\dagger }\approx I-i{\hat {H}}^{\dagger }.}

The probability for a photon to be in a particular polarization state depends on the fields as calculated by the classical Maxwell's equations. The polarization state of the photon is proportional to the field. The probability itself is quadratic in the fields and consequently is also quadratic in the quantum state of polarization. In quantum mechanics, therefore, the state or probability amplitude contains the basic probability information. In general, the rules for combining probability amplitudes look very much like the classical rules for composition of probabilities: [The following quote is from Baym, Chapter 1][clarification needed]

The expected value of a spin measurement on a photon is then ⟨ ψ | S ^ | ψ ⟩ = | ψ R | 2 − | ψ L | 2 . {\displaystyle \langle \psi |{\hat {S}}|\psi \rangle =\vert \psi _{\rm {R}}\vert ^{2}-\vert \psi _{\rm {L}}\vert ^{2}.}

An operator S has been associated with an observable quantity, the spin angular momentum. The eigenvalues of the operator are the allowed observable values. This has been demonstrated for spin angular momentum, but it is in general true for any observable quantity.

Some time before the discovery of quantum mechanics people realized that the connection between light waves and photons must be of a statistical character. What they did not clearly realize, however, was that the wave function gives information about the probability of one photon being in a particular place and not the probable number of photons in that place. The importance of the distinction can be made clear in the following way. Suppose we have a beam of light consisting of a large number of photons split up into two components of equal intensity. On the assumption that the beam is connected with the probable number of photons in it, we should have half the total number going into each component. If the two components are now made to interfere, we should require a photon in one component to be able to interfere with one in the other. Sometimes these two photons would have to annihilate one another and other times they would have to produce four photons. This would contradict the conservation of energy. The new theory, which connects the wave function with probabilities for one photon gets over the difficulty by making each photon go partly into each of the two components. Each photon then interferes only with itself. Interference between two different photons never occurs.—Paul Dirac, The Principles of Quantum Mechanics, 1930, Chapter 1

Polarisation statemeaning

The fraction in both components is ψ x ∗ ψ x + ψ y ∗ ψ y = ⟨ ψ | ψ ⟩ = 1. {\displaystyle \psi _{x}^{*}\psi _{x}+\psi _{y}^{*}\psi _{y}=\langle \psi |\psi \rangle =1.}

The number of photons in the box is then N = V 8 π ℏ ω | E | 2 . {\displaystyle N={\frac {V}{8\pi \hbar \omega }}\vert \mathbf {E} \vert ^{2}.}

If unit vectors are defined such that | R ⟩   = d e f   1 2 ( 1 i ) {\displaystyle |\mathrm {R} \rangle \ {\stackrel {\mathrm {def} }{=}}\ {1 \over {\sqrt {2}}}{\begin{pmatrix}1\\i\end{pmatrix}}} and | L ⟩   = d e f   1 2 ( 1 − i ) {\displaystyle |\mathrm {L} \rangle \ {\stackrel {\mathrm {def} }{=}}\ {1 \over {\sqrt {2}}}{\begin{pmatrix}1\\-i\end{pmatrix}}} then an arbitrary polarization state can be written in the "R–L basis" as | ψ ⟩ = ψ R | R ⟩ + ψ L | L ⟩ {\displaystyle |\psi \rangle =\psi _{\rm {R}}|\mathrm {R} \rangle +\psi _{\rm {L}}|\mathrm {L} \rangle } where ψ R = ⟨ R | ψ ⟩ = 1 2 ( cos ⁡ θ exp ⁡ ( i α x ) − i sin ⁡ θ exp ⁡ ( i α y ) ) {\displaystyle \psi _{\rm {R}}=\langle \mathrm {R} |\psi \rangle ={\frac {1}{\sqrt {2}}}\left(\cos \theta \exp(i\alpha _{x})-i\sin \theta \exp(i\alpha _{y})\right)} and ψ L = ⟨ L | ψ ⟩ = 1 2 ( cos ⁡ θ exp ⁡ ( i α x ) + i sin ⁡ θ exp ⁡ ( i α y ) ) . {\displaystyle \psi _{\rm {L}}=\langle \mathrm {L} |\psi \rangle ={\frac {1}{\sqrt {2}}}\left(\cos \theta \exp(i\alpha _{x})+i\sin \theta \exp(i\alpha _{y})\right).}