Quantization of Maxwell Field
Canonical Quantization of Maxwell Field
The Hamiltonian of Maxwell Field is(in Feynman Gauge
):
Where
Proof
With Feynman Gauge, we have the canonical momentum as:One can rewrite them into the plane-wave expansion. In Schrodinger Picture:
Where
We can write the inverse transformation, which will represent
And their commutation relationship:
Proof
With that:With this result and the hold-order discussion in Classical theory, we can write the Hamiltonian as:
There is also a
One can also write these operators in Heisenberg Picture :
Proof
We can also check that:Extended Hilbert Space and Physical States
Why the extended Hilbert is needed
One can find that this quantization procedure leads to a Hamiltonian `without` lower bound. Different from the Dirac Field, using anti-commutation relationship has no help, because the negative sign of `scalar polarization` come from the classical theory. Moreover, in classical electromagnetic field theory, we know that `dof`of electromagnetic wave is 2. But here we have four. In classical we use gauge condition to eliminate them, but in quantum theory, for example, Lorentz gauge[Definition] : The Hilbert generated from the quantization procedure above are larger than what we need. And it is not positive definite. Physical subspace
is a subset of the Hilbert space, it is positive definite and describe the physical process can happen in the real world. States in Physical subspace
are physical states
, and the physical subspace is determined by the condition:
Which is the quantum version of Lorentz gauge
.
A stronger constraint(much more convenient for it is an eigen-equation) is
Proof
IfAnd this stronger condition leads to
This shows that in physical subspace
, the expectation of Hamiltonian is:
The right-hand-side is positive definite.
And we have the structure of physical subspace:
[Definition] : vacuum of the Maxwell Field
is the ground state of it. Denoting it as
Obviously,
[Theorem] : Any physical states can be expressed as:
Where only
transverse photons(polarizations) , and operator
Where
That’s why they(added states) are also called as states-with-zero-norm
.
Proof
To prove that we should check:[Theorem] : The states equivalence class
. The member of this equivalence class are related by gauge transformations. Explicitly speaking, one has:
Where
Proof
One has:With these theorems, we can use
[Definition] : 1-particle
state is defined as:
And the Lorentz covariance:
And the Lorentz covariant integral:
[Definition] : Consider the coordinate transformation(special Lorentz Transformation
)
The vacuum is unique, then:
Then one can check that the field operator will be:
Proof
The proof is similar to what we did before, just notingCausality of Maxwell Field
Our quantization will not violent to the causality demanded by special Relativistic Theory. That is to say:
[Theorem] : In Heisenberg Picture, for any two points
Proof
With the quantization thatPropagator of Maxwell Field
[Definition] : Feynman Propagator
of Maxwell Field is defined as:
And Maxwell Field is bosonic. One can compute that:
The calculation is similar to Scalar field, with
This form only work for Feynman Gauge
:
Discrete Symmetry of Maxwell Field
Spatial Inversion
We hope the Spatial Inversion
induces a unitary operator so as to
For Maxwell Field,
For creation/annihilation operators:
Charge Conjugation
Charge conjugation
for photon is:
Same to creation/annihilation operators:
Time Reversal
Time reversal
induces an anti-unitary operator. With conjugation operator
For helicity
, time reversal
inverse momentum and spin, so
Then we can prove that:
Do not sum for