Clebsch–Gordan coefficients

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In physics, the Clebsch–Gordan (CG) coefficients are numbers that arise in angular momentum coupling in quantum mechanics. They appear as the expansion coefficients of total angular momentum eigenstates in an uncoupled tensor product basis.

In more mathematical terms, the CG coefficients are used in representation theory, particularly of compact Lie groups, to perform the explicit direct sum decomposition of the tensor product of two irreducible representations into irreducible representations, in cases where the numbers and types of irreducible components are already known abstractly. The name derives from the German mathematicians Alfred Clebsch and Paul Gordan, who encountered an equivalent problem in invariant theory.

From a vector calculus perspective, the CG coefficients associated with the SO(3) group can be defined simply as integrals of spherical harmonics. The addition of spins in quantum-mechanical terms can be read directly from this approach. The formulas below use Dirac's bra–ket notation.

From the formal definition of angular momentum, recursion relations for the Clebsch–Gordan coefficients can be found. To find numerical values for the coefficients a phase convention must be adopted. In this article, the Condon–Shortley phase convention is chosen.

Angular momentum operators[edit]

Angular momentum operators are self-adjoint operators jx, jy, and jz that satisfy the commutation relations


\begin{align}
  &[\mathrm j_k, \mathrm j_l]
    \equiv \mathrm j_k \mathrm j_l - \mathrm j_l \mathrm j_k
    = i \hbar \sum_m \varepsilon_{k, l, m} \mathrm j_m
    & k, l, m &\in \{\mathrm x, \mathrm y, \mathrm z\}
\end{align}

where εk l m is the Levi-Civita symbol. Together the three operators define a vector operator (also known as a spherical vector):


\mathbf j = (\mathrm j_{\mathrm x}, \mathrm j_{\mathrm y}, \mathrm j_{\mathrm z})

By developing this concept further, one can define another operator j2 as the inner product of j with itself:


\mathbf j^2 = \mathrm j_{\mathrm x}^2 + \mathrm j_{\mathrm y}^2 + \mathrm j_{\mathrm z}^2
.

This is an example of a Casimir operator.

One can also define raising (j+) and lowering (j) operators (the so-called ladder operators):


\mathrm j_\pm = \mathrm j_{\mathrm x} \pm \mathrm j_{\mathrm y} i
.

Angular momentum states[edit]

It can be shown from the above definitions that j2 commutes with jx, jy, and jz:


\begin{align}
  &[\mathbf j^2, \mathrm j_k] = 0 & k &\in \{\mathrm x, \mathrm y, \mathrm z\}
\end{align}

When two Hermitian operators commute, a common set of eigenfunctions exists. Conventionally j2 and jz are chosen. From the commutation relations the possible eigenvalues can be found. These states are denoted |j m where j is the angular momentum quantum number and m is the angular momentum projection onto the z-axis. They satisfy the following eigenvalue equations:


\begin{align}
  \mathbf j^2 |j \, m\rangle &= \hbar^2 j (j + 1) |j \, m\rangle & j &\in \{0, \tfrac{1}{2}, 1, \tfrac{3}{2}, \ldots\} \\
  \mathrm j_{\mathrm z} |j \, m\rangle &= \hbar m |j \, m\rangle & m &\in \{-j, -j + 1, \ldots, j\}.
\end{align}

The raising and lowering operators can be used to alter the value of m:


  \mathrm j_\pm |j \, m\rangle = \hbar C_\pm(j, m) |j \, (m \pm 1)\rangle

where the ladder coefficient is given by:


  C_\pm(j, m) = \sqrt{j (j + 1) - m (m \pm 1)} = \sqrt{(j \mp m)(j \pm m + 1)}.

 

 

 

 

(1)

In principle, one may also introduce a (possibly complex) phase factor in the definition of C_\pm(j, m). The choice made in this article is in agreement with the Condon–Shortley phase convention. The angular momentum states are orthogonal (because their eigenvalues with respect to a Hermitian operator are distinct) and are assumed to be normalized:


  \langle j_1 \, m_1 | j_2 \, m_2 \rangle = \delta_{j_1, j_2} \delta_{m_1, m_2}.

Note that an italicized j denotes an integer or a half-integer that labels the angular momentum of the system (e.g. j = 1/2 for an electron and j = 1 for a photon). On the other hand, the roman jx, jy, jz, j+, j, and j2 denote operators.

Tensor product space[edit]

Let V1 be the (2 j1 + 1)-dimensional vector space spanned by the states


\begin{align}
  &|j_1 \, m_1\rangle & m_1 &\in \{-j_1, -j_1 + 1, \ldots, j_1\}
\end{align}
,

and V2 the (2 j2 + 1)-dimensional vector space spanned by the states


\begin{align}
  &|j_2 \, m_2\rangle & m_2 &\in \{-j_2, -j_2 + 1, \ldots, j_2\}
\end{align}
.

The tensor product of these spaces, V3V1V2, has a (2 j1 + 1) (2 j2 + 1)-dimensional uncoupled basis


  |j_1 \, m_1 \, j_2 \, m_2\rangle \equiv |j_1 \, m_1\rangle \otimes |j_2 \, m_2\rangle
  \quad (m_1 \in \{-j_1, -j_1 + 1, \ldots, j_1\})
  \quad (m_2 \in \{-j_2, -j_2 + 1, \ldots, j_2\}
.

Angular momentum operators are defined to act on states in V3 in the following manner:


  (\mathbf j \otimes 1) |j_1 \, m_1 \, j_2 \, m_2\rangle \equiv \mathbf j |j_1 \, m_1\rangle \otimes |j_2 \, m_2\rangle

and


  (1 \otimes \mathrm \mathbf j) |j_1 \, m_1 \, j_2 \, m_2\rangle \equiv |j_1 \, m_1\rangle \otimes \mathbf j |j_2 \, m_2\rangle

where 1 denotes the identity operator.

The total[1] angular momentum operators are defined by


  \mathbf J \equiv \mathbf j \otimes 1 + 1 \otimes \mathbf j

The total angular momentum operators can be shown to satisfy the same kind of commutation relations:


  [\mathrm J_k, \mathrm J_l] = i \hbar \varepsilon_{k, l, m} \mathrm J_m
,

where k, l, m ∈ {x, y, z}. Hence, a set of coupled eigenstates exist for the total angular momentum operator as well:


\begin{align}
  \mathbf{J}^2 |[j_1 \, j_2] \, J \, M\rangle &= \hbar^2 J (J + 1) |[j_1 \, j_2] \, J \, M\rangle \\
  \mathrm{J}_z |[j_1 \, j_2] \, J \, M\rangle &= \hbar M |[j_1 \, j_2] \, J \, M\rangle
\end{align}

for M ∈ {−J, −J + 1, …, J}. Note that it is common to omit the [j1 j2] part.

One can derive the triangular condition that the total angular momentum quantum number J must satisfy:


  |j_1 - j_2| \leq J \leq j_1 + j_2
.

The total number of total angular momentum eigenstates is necessarily equal to the dimension of V3:


  \sum_{J = |j_1 - j_2|}^{j_1 + j_2} (2 J + 1) = (2 j_1 + 1) (2 j_2 + 1)
.

The total angular momentum states form an orthonormal basis of V3:


  \langle J_1 \, M_1 | J_2 \, M_2 \rangle = \delta_{J_1, J_2} \delta_{M_1, M_2}
.

Formal definition of Clebsch–Gordan coefficients[edit]

The coupled states can be expanded via the completeness relation (resolution of identity) in the uncoupled basis


  |[j_1 \, j_2] \, J \, M\rangle =
    \sum_{m_1 = -j_1}^{j_1} \sum_{m_2 = -j_2}^{j_2}
    |j_1 \, m_1 \, j_2 \, m_2\rangle \langle j_1 \, m_1 \, j_2 \, m_2 | J \, M\rangle

 

 

 

 

(2)

The expansion coefficients


\langle j_1 \, m_1 \, j_2 \, m_2 | J \, M \rangle

are the Clebsch–Gordan coefficients. Note that some authors write them in a different order such as j1 j2; m1 m2|J M.

Applying the operator


  \mathrm J_{\mathrm z} = \mathrm j_{\mathrm z} \otimes 1 + 1 \otimes \mathrm j_{\mathrm z}

to both sides of the defining equation shows that the Clebsch–Gordan coefficients can only be nonzero when


  M = m_1 + m_2
.

Recursion relations[edit]

The recursion relations were discovered by physicist Giulio Racah from the Hebrew University of Jerusalem in 1941.

Applying the total angular momentum raising and lowering operators


  \mathrm J_\pm = \mathrm j_\pm \otimes 1 + 1 \otimes \mathrm j_\pm

to the left hand side of the defining equation gives


\begin{align}
  \mathrm J_\pm |[j_1 \, j_2] \, J \, M\rangle
    &= \hbar C_\pm(J, M) |[j_1 \, j_2] \, J \, (M \pm 1)\rangle \\
    &= \hbar C_\pm(J, M)
      \sum_{m_1, m_2}
        |j_1 \, m_1 \, j_2 \, m_2\rangle
        \langle j_1 \, m_1 \, j_2 \, m_2 | J \, (M \pm 1)\rangle
\end{align}

Applying the same operators to the right hand side gives


\begin{align}
  &
     \mathrm J_\pm
       \sum_{m_1, m_2}
         |j_1 \, m_1 \, j_2 \, m_2\rangle
         \langle j_1 \, m_1 \, j_2 \, m_2 | J \, M\rangle \\
  &= \hbar \sum_{m_1, m_2} \Bigl(
         C_\pm(j_1, m_1) |j_1 \, (m_1 \pm 1) \, j_2 \, m_2\rangle
       + C_\pm(j_2, m_2) |j_1 \, m_1 \, j_2 \, (m_2 \pm 1)\rangle
     \Bigr) \langle j_1 \, m_1 \, j_2 \, m_2 | J \, M\rangle \\
  &= \hbar \sum_{m_1, m_2} |j_1 \, m_1 \, j_2 \, m_2\rangle \Bigl(
         C_\pm(j_1, m_1 \mp 1) \langle j_1 \, (m_1 \mp 1) \, j_2 \, m_2 | J \, M\rangle
       + C_\pm(j_2, m_2 \mp 1) \langle j_1 \, m_1 \, j_2 \, (m_2 \mp 1) | J \, M\rangle
     \Bigr)
  \text{.}
\end{align}

where C± was defined in 1. Combining these results gives recursion relations for the Clebsch–Gordan coefficients:


  C_\pm(J, M) \langle j_1 \, m_1 \, j_2 \, m_2 | J \, (M \pm 1)\rangle
  = C_\pm(j_1, m_1 \mp 1) \langle j_1 \, (m_1 \mp 1) \, j_2 \, m_2 | J \, M\rangle
  + C_\pm(j_2, m_2 \mp 1) \langle j_1 \, m_1 \, j_2 \, (m_2 \mp 1) | J \, M\rangle
.

Taking the upper sign with the condition that M = J gives initial recursion relation:


  0 = C_+(j_1, m_1 - 1) \langle j_1 \, (m_1 - 1) \, j_2 \, m_2 | J \, J\rangle
    + C_+(j_2, m_2 - 1) \langle j_1 \, m_1 \, j_2 \, (m_2 - 1) | J \, J\rangle
.

In the Condon–Shortley phase convention, one adds the constraint that

\langle j_1 \, j_1 \, j_2 \, (J - j_1) | J \, J\rangle > 0

(and is therefore also real).

The Clebsch–Gordan coefficients j1 m1 j2 m2 | J J can then be found from these recursion relations. The normalization is fixed by the requirement that the sum of the squares, which equivalent to the requirement that the norm of the state |[j1 j2] J J must be one.

The lower sign in the recursion relation can be used to find all the Clebsch–Gordan coefficients with M = J − 1. Repeated use of that equation gives all coefficients.

This procedure to find the Clebsch–Gordan coefficients shows that they are all real in the Condon–Shortley phase convention.

Explicit expression[edit]

For an explicit expression of the Clebsch–Gordan coefficients and tables with numerical values, see table of Clebsch–Gordan coefficients.

Orthogonality relations[edit]

These are most clearly written down by introducing the alternative notation


  \langle J \, M | j_1 \, m_1 \, j_2 \, m_2 \rangle \equiv \langle j_1 \, m_1 \, j_2 \, m_2 | J \, M \rangle

The first orthogonality relation is


    \sum_{J = |j_1 - j_2|}^{j_1 + j_2} \sum_{M = -J}^J
      \langle j_1 \, m_1 \, j_2 \, m_2 | J \, M \rangle
      \langle J \, M | j_1 \, m_1' \, j_2 \, m_2' \rangle
  = \langle j_1 \, m_1 \, j_2 \, m_2 | j_1 \, m_1' \, j_2 \, m_2' \rangle
  = \delta_{m_1, m_1'} \delta_{m_2, m_2'}

(derived from the fact that 1 ≡ ∑x |x⟩ ⟨x|) and the second one is


    \sum_{m_1, m_2}
      \langle J \, M | j_1 \, m_1 \, j_2 \, m_2 \rangle
      \langle j_1 \, m_1 \, j_2 \, m_2 | J' \, M' \rangle
  = \langle J \, M | J' \, M' \rangle
  = \delta_{J, J'} \delta_{M, M'}
.

Special cases[edit]

For J = 0 the Clebsch–Gordan coefficients are given by


    \langle j_1 \, m_1 \, j_2 \, m_2 | 0 \, 0 \rangle
  = \delta_{j_1, j_2} \delta_{m_1, -m_2} \frac{(-1)^{j_1 - m_1}}{\sqrt{2 j_2 + 1}}
.

For J = j1 + j2 and M = J we have


 \langle j_1 \, j_1 \, j_2 \, j_2 | (j_1 + j_2) \, (j_1 + j_2) \rangle = 1
.

For j1 = j2 = J / 2 and m1 = −m2 we have


    \langle j_1 \, m_1 \, j_1 \, (-m_1) | (2 j_1) \, 0 \rangle
  = \frac{(2 j_1)!^2}{(j_1 - m_1)! (j_1 + m_1)! \sqrt{(4 j_1)!}}
.

For j1 = j2 = m1 = −m2 we have


    \langle j_1 \, j_1 \, j_1 \, (-j_1) | J \, 0 \rangle
  = (2 j_1)! \sqrt{\frac{2 J + 1}{(J + 2 j_1 + 1)! (2 j_1 - J)!}}.

For j2 = 1, m2 = 0 we have


\begin{align}
  \langle j_1 \, m \, 1 \, 0 | (j_1 + 1) \, m \rangle &= \sqrt{\frac{(j_1 - m + 1) (j_1 + m + 1)}{(2 j_1 + 1) (j_1 + 1)}} \\
  \langle j_1 \, m \, 1 \, 0 |  j_1      \, m \rangle &= \frac{m}{\sqrt{j_1 (j_1 + 1)}} \\
  \langle j_1 \, m \, 1 \, 0 | (j_1 - 1) \, m \rangle &= -\sqrt{\frac{(j_1 - m) (j_1 + m)}{j_1 (2 j_1 + 1)}}
\end{align}

Symmetry properties[edit]


\begin{align}
\langle j_1 \, m_1 \, j_2 \, m_2 | J \, M \rangle
  &= (-1)^{j_1 + j_2 - J} \langle j_1 \, (-m_1) \, j_2 \, (-m_2) | J \, (-M)\rangle \\
  &= (-1)^{j_1 + j_2 - J} \langle j_2 \, m_2 \, j_1 \, m_1 | J \, M \rangle \\
  &= (-1)^{j_1 - m_1} \sqrt{\frac{2 J + 1}{2 j_2 + 1}} \langle j_1 \, m_1 \, J \, (-M)| j_2 \, (-m_2) \rangle \\
  &= (-1)^{j_2 + m_2} \sqrt{\frac{2 J + 1}{2 j_1 + 1}} \langle J \, (-M) \, j_2 \, m_2| j_1 \, (-m_1) \rangle \\
  &= (-1)^{j_1 - m_1} \sqrt{\frac{2 J + 1}{2 j_2 + 1}} \langle J \, M \, j_1 \, (-m_1) | j_2 \, m_2 \rangle \\
  &= (-1)^{j_2 + m_2} \sqrt{\frac{2 J + 1}{2 j_1 + 1}} \langle j_2 \, (-m_2) \, J \, M | j_1 \, m_1 \rangle
\end{align}

A convenient way to derive these relations is by converting the Clebsch–Gordan coefficients to Wigner 3-j symbols using 3. The symmetry properties of Wigner 3-j symbols are much simpler. Care is needed when simplifying phase factors, because the quantum numbers can be integer or half integer, e.g., (−1)2j is equal to 1 for integer j and equal to −1 for half-integer j. The following relations, however, are valid in either case:


  (-1)^{4 j} = (-1)^{2 (j - m)} = 1

and for j1, j2, and J appearing in the same Clebsch–Gordan coefficient:


  (-1)^{2 (j_1 + j_2 + J)} = (-1)^{2 (m_1 + m_2 + M)} = 1
.

Relation to Wigner 3-j symbols[edit]

Clebsch–Gordan coefficients are related to Wigner 3-j symbols which have more convenient symmetry relations.


  \langle j_1 \, m_1 \, j_2 \, m_2 | J \, M \rangle
  = (-1)^{j_1 - j_2 + M} \sqrt{2 J + 1}
    \begin{pmatrix}
      j_1 & j_2 &  J \\
      m_1 & m_2 & -M
    \end{pmatrix}

 

 

 

 

(3)

Relation to Wigner D-matrices[edit]


\begin{align}
  &\int_0^{2 \pi} d \alpha \int_0^\pi \sin \beta \, d\beta \int_0^{2 \pi} d \gamma \,
  D^J_{M, K}(\alpha, \beta, \gamma)^*
  D^{j_1}_{m_1, k_1}(\alpha, \beta, \gamma)
  D^{j_2}_{m_2, k_2}(\alpha, \beta, \gamma) \\
  &= \frac{8 \pi^2}{2 J + 1}
    \langle j_1 \, m_1 \, j_2 \, m_2 | J \, M \rangle
    \langle j_1 \, k_1 \, j_2 \, k_2 | J \, K \rangle
\end{align}

Relation to spherical harmonics[edit]

In the case where integers are involved, the coefficients can be related to integrals of spherical harmonics:


  \int_{4 \pi} Y_{\ell_1}^{m_1}{}^*(\Omega) Y_{\ell_2}^{m_2}{}^*(\Omega) Y_L^M (\Omega) \, d \Omega
  = \sqrt{\frac{(2 \ell_1 + 1) (2 \ell_2 + 1)}{4 \pi (2 L + 1)}}
    \langle \ell_1 \, 0 \, \ell_2 \, 0 | L \, 0 \rangle
    \langle \ell_1 \, m_1 \, \ell_2 \, m_2 | L \, M \rangle

It follows from this and orthonormality of the spherical harmonics that CG coefficients are in fact the expansion coefficients of a product of two spherical harmonics in terms a single spherical harmonic:


  Y_{\ell_1}^{m_1}(\Omega) Y_{\ell_2}^{m_2}(\Omega)
  = \sum_{L, M}
    \sqrt{\frac{(2 \ell_1 + 1) (2 \ell_2 + 1)}{4 \pi (2 L + 1)}}
    \langle \ell_1 \, 0 \, \ell_2 \, 0 | L \, 0 \rangle
    \langle \ell_1 \, m_1 \, \ell_2 \, m_2 | L \, M \rangle
    Y_L^M (\Omega)

Other Properties[edit]

\sum_m (-1)^{j - m} \langle j \, m \, j \, (-m) | J \, 0 \rangle = \delta_{J, 0} \sqrt{2 j + 1}

SU(N) Clebsch–Gordan coefficients[edit]

For arbitrary groups and their representations, Clebsch–Gordan coefficients are not known in general. However, algorithms to produce Clebsch–Gordan coefficients for the special unitary group are known.[2] [3] In particular, SU(3) Clebsch-Gordan coefficients have been computed and tabulated because of their utility in characterizing hadronic decays, where a flavor-SU(3) symmetry exists that relates the up, down, and strange quarks.[4][5] A web interface for tabulating SU(N) Clebsch–Gordan coefficients is readily available.

See also[edit]

References[edit]

  1. ^ The word "total" is often overloaded to mean several different things. In this article, "total angular momentum" refers to a generic sum of two angular momentum operators j1 and j2. It is not to be confused with the other common use of the term "total angular momentum" that refers specifically to the sum of orbital angular momentum and spin.
  2. ^ Alex, A.; M. Kalus; A. Huckleberry; J. von Delft (February 2011). "A numerical algorithm for the explicit calculation of SU(N) and SL(N,C) Clebsch–Gordan coefficients". J. Math. Phys. 82: 023507. arXiv:1009.0437. Bibcode:2011JMP....52b3507A. doi:10.1063/1.3521562. 
  3. ^ Kaplan, L. M.; Resnikoff, M. (1967). "Matrix products and explicit 3, 6, 9, and 12j coefficients of the regular representation of SU(n)". J. Math. Phys. 8: 2194. Bibcode:1967JMP.....8.2194K. doi:10.1063/1.1705141. 
  4. ^ de Swart, J. J. (1963). "The Octet model and its Clebsch-Gordan coefficients". Rev. Mod. Phys. 35: 916. Bibcode:1963RvMP...35..916D. doi:10.1103/RevModPhys.35.916. 
  5. ^ Kaeding, Thomas (1995). "Tables of SU(3) isoscalar factors". arXiv:nucl-th/9502037 [nucl-th]. 

External links[edit]

Further reading[edit]

  • Quantum mechanics, E. Zaarur, Y. Peleg, R. Pnini, Schaum's Easy Oulines Crash Course, McGraw Hill (USA), 2006, ISBN (10-)007-145533-7 ISBN (13-)978-007-145533-6
  • Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles (2nd Edition), R. Eisberg, R. Resnick, John Wiley & Sons, 1985, ISBN 978-0-471-87373-0
  • Quantum Mechanics, E. Abers, Pearson Ed., Addison Wesley, Prentice Hall Inc, 2004, ISBN 978-0-13-146100-0
  • Physics of Atoms and Molecules, B. H. Bransden, C. J. Joachain, Longman, 1983, ISBN 0-582-44401-2
  • The Cambridge Handbook of Physics Formulas, G. Woan, Cambridge University Press, 2010, ISBN 978-0-521-57507-2.
  • Encyclopaedia of Physics (2nd Edition), R. G. Lerner, G. L. Trigg, VHC publishers, 1991, ISBN (Verlagsgesellschaft) 3-527-26954-1, ISBN (VHC Inc.) 0-89573-752-3
  • McGraw Hill Encyclopaedia of Physics (2nd Edition), C. B. Parker, 1994, ISBN 0-07-051400-3
  • Biedenharn, L. C.; Louck, J. D. (1981). Angular Momentum in Quantum Physics. Reading, Massachusetts: Addison-Wesley. ISBN 0-201-13507-8. 
  • Brink, D. M.; Satchler, G. R. (1993). "Ch. 2". Angular Momentum (3rd ed.). Oxford: Clarendon Press. ISBN 0-19-851759-9. 
  • Condon, Edward U.; Shortley, G. H. (1970). "Ch. 3". The Theory of Atomic Spectra. Cambridge: Cambridge University Press. ISBN 0-521-09209-4. 
  • Edmonds, A. R. (1957). Angular Momentum in Quantum Mechanics. Princeton, New Jersey: Princeton University Press. ISBN 0-691-07912-9. 
  • Messiah, Albert (1981). "Ch. XIII". Quantum Mechanics (Volume II). New York: North Holland Publishing. ISBN 0-7204-0045-7. 
  • Zare, Richard N. (1988). "Ch. 2". Angular Momentum. New York: John Wiley & Sons. ISBN 0-471-85892-7.