Creation and annihilation operators

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In physics, an annihilation operator is an operator that lowers the number of particles in a given state by one. A creation operator is an operator that increases the number of particles in a given state by one, and it is the adjoint of the annihilation operator. Depending on the context, the identity of the particles in question varies; for example, in quantum chemistry and many-body theory the creation and annihilation operators often act on electrons. Annihilation and creation operators 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. In many subfields of physics and chemistry, the use of these operators instead wavefunctions is known as second quantization.

The mathematics behind the creation and the annihilation operators is identical as the formulae for ladder operators that appear in the quantum harmonic oscillator. For example, the commutator of the annihilation and the creation operator associated with the same state equals one; all other commutators vanish.

While the concept of creation and annihilation operators is well defined for free field theories, in interacting QFTs, they can only be defined in the interaction picture, which does not exist according to Haag's theorem.

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[edit] Derivation of bosonic creation and annihilation operators

In the context of the quantum harmonic oscillator, we reinterpret 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.

Suppose the wavefunctions are dependent on N properties. Then

For bosons: ψ(1,2,3,4,...N) = ψ(2,1,3,4,...N)
For fermions: ψ(1,2,3,4,...N) = -ψ(2,1,3,4,...N)

For now let's just consider the case of bosons because fermions are more complicated.

Start with the Schrödinger equation for the one dimensional time independent quantum harmonic oscillator

\left(-\frac{\hbar^2}{2m} \frac{d^2}{d x^2} + \frac{1}{2}m \omega^2 x^2\right) \psi(x) = E \psi(x)

Make a coordinate substitution to nondimensionalize the differential equation

x \ \stackrel{\mathrm{def}}{=}\ \sqrt{ \frac{\hbar}{m \omega}} q.

and the Schrödinger equation for the oscillator becomes

\frac{\hbar \omega}{2} \left(-\frac{d^2}{d q^2} + q^2 \right) \psi(q) = E \psi(q).

Notice that the quantity ħω = hν is the same energy as that found for light quanta and that the parenthesis in the Hamiltonian can be written as

-\frac{d^2}{dq^2} + q^2 = \left[-\frac{d}{dq}+q \right] \left[\frac{d}{dq}+ q \right] + \frac {d}{dq}q - q \frac {d}{dq}

The last two terms in that equation form the commutator of q with its derivative. So let's calculate that commutator [ q, ∂/∂q ]

\left(q \frac{d}{dq}- \frac{d}{dq} q \right)f(q) = q \frac{df}{dq} - \frac{d}{dq}(q f(q)) = -f(q)

In other words [ q, d/dq ] = - 1 or [ d/dq, q ] = 1.

Therefore

\frac{1}{2} \hbar \omega \left( -\frac{d^2}{dq^2} + q^2 \right) = \hbar \omega \left[ \frac{-d/ dq + q}{\sqrt{2}}\right] \left[\frac{d / dq + q}{\sqrt{2}}\right] + \frac{1}{2} \hbar \omega

If we define

a^\dagger \ \stackrel{\mathrm{def}}{=}\ \frac{1}{\sqrt{2}} \left(-\frac{d}{dq} + q\right) as the "creation operator" or the "raising operator" and
a \ \stackrel{\mathrm{def}}{=}\ \frac{1}{\sqrt{2}} \left(+\frac{d}{dq} + q\right) as the "annihilation operator" or the "lowering operator"

the Hamiltonian becomes

H = \hbar \omega \left( a^\dagger a + \frac{1}{2} \right).

This Hamiltonian is significantly simpler than the original form. Further simplifications of this equation enables one to derive all the properties listed above thus far.

Letting p = - i d/dq, where p is the nondimensionalized momentum operator

a^\dagger = \frac{1}{\sqrt{2}}(q - i p)
a = \frac{1}{\sqrt{2}}(q + i p).

[edit] Matrix representation

The matrix counterparts of the creation and annihilation operators obtained from the quantum harmonic oscillator model are

a=\begin{pmatrix} 0 & \sqrt{1} & 0 & 0 & \dots & 0 & \dots \\ 0 & 0 & \sqrt{2} & 0 & \dots & 0 & \dots \\ 0 & 0 & 0 & \sqrt{3} & \dots & 0 & \dots \\ 0 & 0 & 0 & 0 & \ddots & \vdots & \dots \\ \vdots & \vdots & \vdots & \vdots & \ddots & \sqrt{n} & \dots \\ 0 & 0 & 0 & 0 & \dots & 0 & \ddots \\ \vdots & \vdots & \vdots & \vdots & \vdots & \vdots & \ddots \end{pmatrix}
a^{\dagger}=\left(\begin{array}{cccccc} 0 & 0 & 0 & \dots & \dots\\ \sqrt{1} & 0 & 0 & \dots & \dots\\ 0 & \sqrt{2} & 0 & \dots & \dots\\ 0 & 0 & \sqrt{3} & \dots & \dots\\ \vdots & \vdots & \vdots\\ 0 & 0 & 0 & \sqrt{n+1} & 0\dots\\ \vdots & \vdots & \vdots & \vdots & \vdots\end{array}\right)

Substituting backwards, the laddering operators are recovered. They can be obtained via the relationships a^\dagger_{ij} = \langle\psi_i | \hat{a}^\dagger | \psi_j\rangle and a_{ij} = \langle\psi_i | \hat{a} | \psi_j\rangle. The wavefunctions are those of the quantum harmonic oscillator, and are sometimes called the "number basis".

[edit] Mathematical details

The operators derived above are actually a specific instance of a more generalized class of creation and annihilation operators. The more abstract form of the operators satisfy the properties below.

Let H be the one-particle Hilbert space. To get the bosonic CCR algebra, look at the algebra generated by a(f) for any f in H. The operator a(f) is called an annihilation operator and the map a(.) is antilinear. Its adjoint is a†(f) which is linear in H.

For a boson,

[a(f),a(g)]=[a^\dagger(f),a^\dagger(g)]=0
[a(f),a^\dagger(g)]=\langle f|g \rangle,

where we are using bra-ket notation.

For a fermion, the anticommutators are

{a(f),a(g)} = {a * (f),a * (g)} = 0
\{a(f),a*(g)\}=\langle f|g \rangle.

A CAR algebra.

Physically speaking, a(f) removes (i.e. annihilates) a particle in the state |f> wheareas a†(f) creates a particle in the state |f>.

The free field vacuum state is the state with no particles. In other words,

a(f)|0\rangle=0

where |0> is the vacuum state.

If |f> is normalized so that <f|f>=1, then a†(f) a(f) gives the number of particles in the state |f>.

Note that the creation and annihilation operators are "generalized complex conjugates" of each other. Usually, the notation is chosen in such a way that the a†(f) is the creation operator, and a(f) is the annihilation operator. The † reminds us that something "extra" is being added to the system. The topic can be misleadingly confusing if this is not done.

[edit] 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 sitation when a gas of molecules A diffuse and interact on contact, forming an inert product: A+A\rightarrow\varnothing . To see how this kind of reaction can be described by the annihilation and creation operator formalism consider ni particles at a site i on a 1-d lattice. Each particle diffuses independently, so that the probability that one of them leaves the site for short times dt is proportional to nidt, say αnidt to hop left and αnidt to hop left.All n particles will stay put with a probability 1 − 2αnidt.

We can now describe the occupation of particles on the lattice as a `ket' of the form |n_{1},n_{2}...\rangle. A slight modification of the annihilation and creation operators is needed so that a|n\rangle=n|n-1\rangle and a^{+}|n\rangle=|n+1\rangle . This modification preserves the commutation relation [a,a + ] = 1, but allows us to write the pure diffusive behaviour of the particles as

\partial_{t}|\psi\rangle=-\alpha\sum(2a_{i}^{+}a_{i}-a_{i-1}^{+}a_{i}-a_{i+1}^{+}a_{i})|\psi\rangle=-\alpha\sum(a_{i}^{+}-a_{i-1}^{+})(a_{i}-a_{i-1})|\psi\rangle

The reaction term can be deduced by noting than n particles can interact n(n − 1) different ways, so that the probability that a pair annihilate is λn(n − 1)dt and the probability that no pair annihilates is 1 − λn(n − 1)dt leaving us with a term

\lambda\sum(a_{i}a_{i}-a_{i}^{+}a_{i}^{+}a_{i}a_{i})

yielding

\partial_{t}|\psi\rangle=-\alpha\sum(a_{i}^{+}-a_{i-1}^{+})(a_{i}-a_{i-1})|\psi\rangle+\lambda\sum(a_{i}^{2}-a_{i}^{+2}a_{i}^{2})|\psi\rangle

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.

[edit] Notational caveats and considerations

In quantum mechanics, Dirac bra-ket notation is often used. However, there is some ambiguity in this notation, particularly when there is the need to differentiate between these things:

  • The lowest energy state
  • The zero state
  • The vacuum state
  • The zero ket

Often, these are all interchangeably notated as |0>, or even | >. As a result, it is necessary to read carefully, and consider the context in which the notation is used.

For example, in the quantum harmonic oscillator, the ground state has the property that when the annihilation operator b is applied to it, it satisfies b|0> = 0| > = 0

The intermediate step is rarely indicated as it is considered necessary only when more conceptual/mathematical rigour is needed.

In this example, the lowest energy state is denoted as |0>. It is labeled as the "zero state", but it is important to emphasize that any state can be labeled as the "zero" state. The zero state is often used as a reference state to other quantum states. Therefore, the |0> state need not be the state with the absolutely lowest energy. In the case of the harmonic oscillator, it is due to the particulars of the mathematics that the ground state is chosen to be |0>. The vacuum state is the state where no quanta is available to be extracted. This special null state is denoted by | >. This vacuum state is also known as the "zero ket" because there are zero particles in the state. Unfortunately, the lowest energy state |0> is also known as the "zero ket" for the different reason that the state is labeled as "zero". Care must be taken that the four concepts listed above are not mixed together.

Sometimes, the terms "null state" and "empty state" are used interchangeably for |> and |0>. The meaning for this usage is again dependent on the context.

[edit] The vacuum state

The vacuum state is a conceptual state which has no particles. The state is usually denoted as |0>, not the "empty ket" | >. Interestingly enough, no actual function actually represents the |0> state, but for notational purposes, we define the vacuum state as being normalized such that <0|0> = 1 and that |0> is orthogonal to all other states of the form |N>, where N is any indexing of quantum states for a particular system.


[edit] See also

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