5.1 Quantization of the electromagnetic field

Quantizing the electromagnetic field is a game-changer in quantum optics. It treats light as a quantum system, explaining phenomena like spontaneous emission and the photoelectric effect. This approach is crucial for understanding light-matter interactions at the quantum level.

The quantized field is described as a collection of harmonic oscillators, with each mode representing a photon. This concept forms the foundation for quantum technologies and provides a framework for studying non-classical states of light.

Field Quantization in Quantum Optics

Quantization of the Electromagnetic Field

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• Field quantization treats the electromagnetic field as a quantum system
  • Field is described by quantum operators acting on quantum states
• Quantization of the electromagnetic field is necessary to explain phenomena that cannot be adequately described by classical electromagnetic theory
  • Spontaneous emission
  • Photoelectric effect
  • Lamb shift
• In the quantum description, the energy of the electromagnetic field is quantized
  • Each mode of the field has energy levels that are integer multiples of the photon energy
• The quantized electromagnetic field consists of a collection of harmonic oscillators, one for each mode of the field
  • Excitations of these oscillators correspond to photons
• The quantum nature of the electromagnetic field becomes important when dealing with systems at the microscopic scale ($atoms and molecules$)
  • Discrete nature of energy exchange between matter and radiation is significant

Importance of Field Quantization

• Field quantization is essential for understanding light-matter interactions at the quantum level
  • Describes the absorption and emission of photons by atoms and molecules
  • Explains the origin of spontaneous emission, where an excited atom emits a photon without external stimulation
• Quantization of the electromagnetic field is necessary for the development of quantum technologies
  • Quantum computing ($[superconducting qubits](https://www.fiveableKeyTerm:superconducting_qubits)$)
  • Quantum communication ($[quantum key distribution](https://www.fiveableKeyTerm:Quantum_Key_Distribution)$)
  • Quantum sensing ($[quantum metrology](https://www.fiveableKeyTerm:quantum_metrology)$)
• Field quantization provides a framework for studying non-classical states of light
  • Squeezed states
  • Entangled states
  • Single-photon states
• Quantization of the electromagnetic field is a fundamental concept in quantum electrodynamics (QED)
  • QED is the quantum field theory that describes the interactions between charged particles and photons
  • QED has been tested to unprecedented accuracy and is one of the most successful theories in physics

Hamiltonian for Quantized Electromagnetic Field

Derivation of the Hamiltonian

• The Hamiltonian for the quantized electromagnetic field is derived by applying the canonical quantization procedure to the classical Hamiltonian
• The classical Hamiltonian for the electromagnetic field is expressed in terms of the vector potential $A(r, t)$ and its conjugate momentum $\Pi(r, t)$
  • $A(r, t)$ and $\Pi(r, t)$ are related to the electric and magnetic fields
• The vector potential and its conjugate momentum are expanded in terms of a complete set of orthonormal mode functions
  • Mode functions satisfy the appropriate boundary conditions and the wave equation
• The coefficients in the expansion of $A(r, t)$ and $\Pi(r, t)$ are promoted to quantum operators
  • These operators satisfy the canonical commutation relations

Hamiltonian Expression and Interpretation

• The resulting Hamiltonian for the quantized electromagnetic field is a sum of independent harmonic oscillator Hamiltonians, one for each mode of the field
  • Creation ($a^†$) and annihilation ($a$) operators act on the Fock states of the field
• The Hamiltonian for the quantized electromagnetic field is expressed as: $H = \sum_{\mathbf{k},\lambda} \hbar\omega_{\mathbf{k}} (a_{\mathbf{k},\lambda}^{\dagger} a_{\mathbf{k},\lambda} + \frac{1}{2})$
  • $\mathbf{k}$ is the wave vector, $\lambda$ is the polarization, and $\omega_{\mathbf{k}}$ is the angular frequency of the mode
• Each term in the Hamiltonian represents the energy of a single mode of the field
  • The energy is the sum of the photon energies ($\hbar\omega_{\mathbf{k}}$) for each excitation of the mode
  • The ground state energy ($\frac{1}{2}\hbar\omega_{\mathbf{k}}$) is the zero-point energy of the harmonic oscillator
• The Hamiltonian describes the energy of the free electromagnetic field
  • Interactions between the field and matter can be introduced through additional terms in the Hamiltonian

Mode Functions in Quantization

Role of Mode Functions

• Mode functions are a complete set of orthonormal functions that satisfy the boundary conditions and the wave equation for the electromagnetic field
• The choice of mode functions depends on the geometry and boundary conditions of the system
  • Free space ($plane waves$)
  • Cavity ($standing waves$)
  • Waveguide ($guided modes$)
• In free space, plane waves are the most commonly used mode functions
  • Characterized by their wave vector $\mathbf{k}$ and polarization $\lambda$
• In a cavity, the mode functions are standing waves that satisfy the boundary conditions imposed by the cavity walls
  • Characterized by discrete wave vectors and polarizations
• The mode functions form a basis for the expansion of the vector potential and its conjugate momentum
  • Allows the classical field to be decomposed into a sum of independent harmonic oscillators

Quantization and Commutation Relations

• The coefficients in the expansion of $A(r, t)$ and $\Pi(r, t)$ in terms of the mode functions are the variables that are promoted to quantum operators during the quantization process
• The orthonormality of the mode functions ensures that the resulting quantum operators satisfy the canonical commutation relations
  • $[a_{\mathbf{k},\lambda}, a_{\mathbf{k}',\lambda'}^{\dagger}] = \delta_{\mathbf{k},\mathbf{k}'} \delta_{\lambda,\lambda'}$
  • $[a_{\mathbf{k},\lambda}, a_{\mathbf{k}',\lambda'}] = [a_{\mathbf{k},\lambda}^{\dagger}, a_{\mathbf{k}',\lambda'}^{\dagger}] = 0$
• The orthonormality of the mode functions also ensures that the Hamiltonian for the quantized field takes the form of a sum of independent harmonic oscillator Hamiltonians
• The choice of mode functions affects the form of the field operators and the Hamiltonian
  • Different mode functions lead to different representations of the quantized field
  • The physical predictions are independent of the choice of mode functions, as long as they form a complete basis

Physical Meaning of Field Operators

Field Operators and Quantum States

• The field operators, such as the vector potential operator $\hat{A}(r, t)$ and the electric field operator $\hat{E}(r, t)$, are quantum mechanical operators that act on the quantum states of the electromagnetic field
• The field operators are expressed in terms of the creation ($a^{\dagger}$) and annihilation ($a$) operators for each mode of the field
  • These are the fundamental operators in the quantized description of the electromagnetic field
• The creation operator $a^{\dagger}$ for a given mode creates a photon in that mode when applied to a quantum state
  • Increases the energy of the field by one photon energy $\hbar\omega$
• The annihilation operator $a$ for a given mode annihilates a photon in that mode when applied to a quantum state
  • Decreases the energy of the field by one photon energy $\hbar\omega$

Expectation Values and Commutation Relations

• The expectation values of the field operators, such as $\langle \hat{E}(r, t) \rangle$ and $\langle \hat{B}(r, t) \rangle$, correspond to the classical electric and magnetic fields, respectively, in the limit of large photon numbers
• The field operators satisfy the canonical commutation relations
  • $[\hat{A}_i(r, t), \hat{E}_j(r', t)] = i\hbar\delta_{ij}\delta(r - r')$
  • $[\hat{A}_i(r, t), \hat{A}_j(r', t)] = [\hat{E}_i(r, t), \hat{E}_j(r', t)] = 0$
• The commutation relations lead to the Heisenberg uncertainty principle for the electromagnetic field
  • Relates the uncertainties in the field amplitudes and phases
• The commutation relations between the field operators at different space-time points reflect the causality and locality of the electromagnetic field
  • Measurements of the field at space-like separated points do not influence each other
• The field operators provide a quantum mechanical description of the electromagnetic field
  • Allow for the calculation of observables and the study of quantum optical phenomena
  • Enable the description of non-classical states of light ($squeezed states, entangled states, single-photon states$)