Creation and annihilation operators


Creation operators and annihilation operators are mathematical operators that have widespread applications in quantum mechanics, notably in the study of quantum harmonic oscillators and many-particle systems.[1] An annihilation operator (usually denoted ) lowers the number of particles in a given state by one. A creation operator (usually denoted ) increases the number of particles in a given state by one, and it is the adjoint of the annihilation operator. In many subfields of physics and chemistry, the use of these operators instead of wavefunctions is known as second quantization. They were introduced by Paul Dirac.[2]

Creation and annihilation operators can act on states of various types of particles. For example, in quantum chemistry and many-body theory the creation and annihilation operators often act on electron states. They can also refer specifically to the ladder operators for the quantum harmonic oscillator. In the latter case, the raising operator is interpreted as a creation operator, adding a quantum of energy to the oscillator system (similarly for the lowering operator). They can be used to represent phonons. Constructing Hamiltonians using these operators has the advantage that the theory automatically satisfies the cluster decomposition theorem.[3]

The mathematics for the creation and annihilation operators for bosons is the same as for the ladder operators of the quantum harmonic oscillator.[4] For example, the commutator of the creation and annihilation operators that are associated with the same boson state equals one, while all other commutators vanish. However, for fermions the mathematics is different, involving anticommutators instead of commutators.[5]

Ladder operators for the quantum harmonic oscillator


In the context of the quantum harmonic oscillator, one reinterprets the ladder operators as creation and annihilation operators, adding or subtracting fixed quanta of energy to the oscillator system.

Creation/annihilation operators are different for bosons (integer spin) and fermions (half-integer spin). This is because their wavefunctions have different symmetry properties.

First consider the simpler bosonic case of the photons of the quantum harmonic oscillator. Start with the Schrödinger equation for the one-dimensional time independent quantum harmonic oscillator,


Make a coordinate substitution to nondimensionalize the differential equation


The Schrödinger equation for the oscillator becomes


Note that the quantity   is the same energy as that found for light quanta and that the parenthesis in the Hamiltonian can be written as


The last two terms can be simplified by considering their effect on an arbitrary differentiable function  

which implies,
coinciding with the usual canonical commutation relation  , in position space representation:  .


and the Schrödinger equation for the oscillator becomes, with substitution of the above and rearrangement of the factor of 1/2,

If one defines

as the "creation operator" or the "raising operator" and
as the "annihilation operator" or the "lowering operator", the Schrödinger equation for the oscillator reduces to
This is significantly simpler than the original form. Further simplifications of this equation enable one to derive all the properties listed above thus far.

Letting  , where   is the nondimensionalized momentum operator one has


Note that these imply


The operators   and   may be contrasted to normal operators, which commute with their adjoints.[nb 1]

Using the commutation relations given above, the Hamiltonian operator can be expressed as


One may compute the commutation relations between the   and   operators and the Hamiltonian:[6]


These relations can be used to easily find all the energy eigenstates of the quantum harmonic oscillator as follows.

Assuming that   is an eigenstate of the Hamiltonian  . Using these commutation relations, it follows that[6]


This shows that   and   are also eigenstates of the Hamiltonian, with eigenvalues   and   respectively. This identifies the operators   and   as "lowering" and "raising" operators between adjacent eigenstates. The energy difference between adjacent eigenstates is  .

The ground state can be found by assuming that the lowering operator possesses a nontrivial kernel:   with  . Applying the Hamiltonian to the ground state,

So   is an eigenfunction of the Hamiltonian.

This gives the ground state energy  , which allows one to identify the energy eigenvalue of any eigenstate   as[6]


Furthermore, it turns out that the first-mentioned operator in (*), the number operator   plays the most important role in applications, while the second one,   can simply be replaced by  .



The time-evolution operator is then


Explicit eigenfunctions


The ground state   of the quantum harmonic oscillator can be found by imposing the condition that


Written out as a differential equation, the wavefunction satisfies

with the solution

The normalization constant C is found to be   from  ,  using the Gaussian integral. Explicit formulas for all the eigenfunctions can now be found by repeated application of   to  .[7]

Matrix representation


The matrix expression of the creation and annihilation operators of the quantum harmonic oscillator with respect to the above orthonormal basis is


These can be obtained via the relationships   and  . The eigenvectors   are those of the quantum harmonic oscillator, and are sometimes called the "number basis".

Generalized creation and annihilation operators


Thanks to representation theory and C*-algebras the operators derived above are actually a specific instance of a more generalized notion of creation and annihilation operators in the context of CCR and CAR algebras. Mathematically and even more generally ladder operators can be understood in the context of a root system of a semisimple Lie group and the associated semisimple Lie algebra without the need of realizing the representation as operators on a functional Hilbert space.[8]

In the Hilbert space representation case the operators are constructed as follows: Let   be a one-particle Hilbert space (that is, any Hilbert space, viewed as representing the state of a single particle). The (bosonic) CCR algebra over   is the algebra-with-conjugation-operator (called *) abstractly generated by elements  , where  ranges freely over  , subject to the relations

in bra–ket notation.

The map   from   to the bosonic CCR algebra is required to be complex antilinear (this adds more relations). Its adjoint is  , and the map   is complex linear in H. Thus   embeds as a complex vector subspace of its own CCR algebra. In a representation of this algebra, the element   will be realized as an annihilation operator, and   as a creation operator.

In general, the CCR algebra is infinite dimensional. If we take a Banach space completion, it becomes a C*-algebra. The CCR algebra over   is closely related to, but not identical to, a Weyl algebra.[clarification needed]

For fermions, the (fermionic) CAR algebra over   is constructed similarly, but using anticommutator relations instead, namely


The CAR algebra is finite dimensional only if   is finite dimensional. If we take a Banach space completion (only necessary in the infinite dimensional case), it becomes a   algebra. The CAR algebra is closely related, but not identical to, a Clifford algebra.[clarification needed]

Physically speaking,   removes (i.e. annihilates) a particle in the state   whereas   creates a particle in the state  .

The free field vacuum state is the state   with no particles, characterized by


If   is normalized so that  , then   gives the number of particles in the state  .

Creation and annihilation operators for reaction-diffusion equations


The annihilation and creation operator description has also been useful to analyze classical reaction diffusion equations, such as the situation when a gas of molecules   diffuse and interact on contact, forming an inert product:  . To see how this kind of reaction can be described by the annihilation and creation operator formalism, consider   particles at a site i on a one dimensional lattice. Each particle moves to the right or left with a certain probability, and each pair of particles at the same site annihilates each other with a certain other probability.

The probability that one particle leaves the site during the short time period dt is proportional to  , let us say a probability   to hop left and   to hop right. All   particles will stay put with a probability  . (Since dt is so short, the probability that two or more will leave during dt is very small and will be ignored.)

We can now describe the occupation of particles on the lattice as a 'ket' of the form  . It represents the juxtaposition (or conjunction, or tensor product) of the number states    ,   located at the individual sites of the lattice. Recall that

for all n ≥ 0, while

This definition of the operators will now be changed to accommodate the "non-quantum" nature of this problem and we shall use the following definition:[9]


note that even though the behavior of the operators on the kets has been modified, these operators still obey the commutation relation


Now define   so that it applies   to  . Correspondingly, define   as applying   to  . Thus, for example, the net effect of   is to move a particle from the  -th to the i-th site while multiplying with the appropriate factor.

This allows writing the pure diffusive behavior of the particles as


The reaction term can be deduced by noting that   particles can interact in   different ways, so that the probability that a pair annihilates is  , yielding a term


where number state n is replaced by number state n − 2 at site   at a certain rate.

Thus the state evolves by


Other kinds of interactions can be included in a similar manner.

This kind of notation allows the use of quantum field theoretic techniques to be used in the analysis of reaction diffusion systems.[10]

Creation and annihilation operators in quantum field theories


In quantum field theories and many-body problems one works with creation and annihilation operators of quantum states,   and  . These operators change the eigenvalues of the number operator,

by one, in analogy to the harmonic oscillator. The indices (such as  ) represent quantum numbers that label the single-particle states of the system; hence, they are not necessarily single numbers. For example, a tuple of quantum numbers   is used to label states in the hydrogen atom.

The commutation relations of creation and annihilation operators in a multiple-boson system are,

where   is the commutator and   is the Kronecker delta.

For fermions, the commutator is replaced by the anticommutator  ,

Therefore, exchanging disjoint (i.e.  ) operators in a product of creation or annihilation operators will reverse the sign in fermion systems, but not in boson systems.

If the states labelled by i are an orthonormal basis of a Hilbert space H, then the result of this construction coincides with the CCR algebra and CAR algebra construction in the previous section but one. If they represent "eigenvectors" corresponding to the continuous spectrum of some operator, as for unbound particles in QFT, then the interpretation is more subtle.



While Zee[11] obtains the momentum space normalization   via the symmetric convention for Fourier transforms, Tong[12] and Peskin & Schroeder[13] use the common asymmetric convention to obtain  . Each derives  .

Srednicki additionally merges the Lorentz-invariant measure into his asymmetric Fourier measure,  , yielding  .[14]

See also



  1. ^ A normal operator has a representation A= B + i C, where B,C are self-adjoint and commute, i.e.  . By contrast, a has the representation   where   are self-adjoint but  . Then B and C have a common set of eigenfunctions (and are simultaneously diagonalizable), whereas p and q famously don't and aren't.


  1. ^ Feynman 1998, p. 151
  2. ^ Dirac, P. A. M. (1927). "The quantum theory of the emission and absorption of radiation", Proc Roy Soc London Ser A, 114 (767), 243-265.
  3. ^ Weinberg, Steven (1995). "4". The Quantum Theory of Fields Volume 1. Cambridge University Press. p. 169. ISBN 9780521670531.
  4. ^ Feynman 1998, p. 167
  5. ^ Feynman 1998, pp. 174–5
  6. ^ a b c Branson, Jim. "Quantum Physics at UCSD". Retrieved 16 May 2012.
  7. ^ This, and further operator formalism, can be found in Glimm and Jaffe, Quantum Physics, pp. 12–20.
  8. ^ Harris, Fulton, Representation Theory pp. 164
  9. ^ Pruessner, Gunnar. "Analysis of Reaction-Diffusion Processes by Field Theoretic Methods" (PDF). Retrieved 31 May 2021.
  10. ^ Baez, John Carlos (2011). Network theory (blog post series; first post). Later adapted into Baez, John Carlos; Biamonte, Jacob D. (April 2018). Quantum Techniques in Stochastic Mechanics. doi:10.1142/10623.
  11. ^ Zee, A. (2003). Quantum field theory in a nutshell. Princeton University Press. p. 63. ISBN 978-0691010199.
  12. ^ Tong, David (2007). Quantum Field Theory. p. 24,31. Retrieved 3 December 2019.
  13. ^ Peskin, M.; Schroeder, D. (1995). An Introduction to Quantum Field Theory. Westview Press. ISBN 978-0-201-50397-5.
  14. ^ Srednicki, Mark (2007). Quantum field theory. Cambridge University Press. pp. 39, 41. ISBN 978-0521-8644-97. Retrieved 3 December 2019.
  • Feynman, Richard P. (1998) [1972]. Statistical Mechanics: A Set of Lectures (2nd ed.). Reading, Massachusetts: Addison-Wesley. ISBN 978-0-201-36076-9.
  • Albert Messiah, 1966. Quantum Mechanics (Vol. I), English translation from French by G. M. Temmer. North Holland, John Wiley & Sons. Ch. XII. online