Table of Contents
- Introduction
- What Is Quantum Dynamics?
- Schrödinger Equation and Time Evolution
- Unitary Evolution and the Role of the Hamiltonian
- Schrödinger, Heisenberg, and Interaction Pictures
- Evolution of Wavefunctions
- Evolution of Operators
- Energy Eigenstates and Stationary States
- Superposition and Phase Evolution
- Probability Conservation and Unitarity
- Quantum Tunneling Dynamics
- Quantum Harmonic Oscillator Dynamics
- Quantum Spin Dynamics
- Entanglement and Nonlocal Dynamics
- Decoherence and Open System Dynamics
- Quantum Control and Coherent Manipulation
- Quantum Dynamics in Field Theory
- Conclusion
1. Introduction
Quantum dynamics is the study of how quantum systems evolve over time. It governs everything from electron transitions and atomic motion to quantum computing operations and particle interactions in quantum field theory. The dynamics are driven by the system’s Hamiltonian and manifest as changes in wavefunctions or operators, depending on the chosen picture.
2. What Is Quantum Dynamics?
Quantum dynamics answers the question: How does a quantum state evolve in time?
It provides a framework for:
- Predicting probabilities of measurement outcomes
- Modeling quantum interference, entanglement, and coherence
- Describing atomic and molecular motion
- Simulating quantum circuits and algorithms
3. Schrödinger Equation and Time Evolution
The fundamental equation governing dynamics is the time-dependent Schrödinger equation (TDSE):
\[
i\hbar \frac{\partial}{\partial t} |\psi(t)\rangle = \hat{H}(t) |\psi(t)\rangle
\]
This first-order linear differential equation determines the deterministic, unitary evolution of quantum states.
4. Unitary Evolution and the Role of the Hamiltonian
The Hamiltonian \( \hat{H} \) encapsulates the energy and interactions of the system and acts as the generator of time translations. Time evolution is described by a unitary operator:
\[
|\psi(t)\rangle = \hat{U}(t, t_0) |\psi(t_0)\rangle
\]
For time-independent \( \hat{H} \):
\[
\hat{U}(t) = e^{-i\hat{H}t/\hbar}
\]
5. Schrödinger, Heisenberg, and Interaction Pictures
| Picture | States | Operators |
|---|---|---|
| Schrödinger | Time-dependent | Fixed |
| Heisenberg | Fixed | Time-dependent |
| Interaction | Both evolve (split roles) | Both evolve (split roles) |
All pictures are equivalent in terms of physical predictions.
6. Evolution of Wavefunctions
In position representation:
\[
\psi(x, t) = \langle x | \psi(t) \rangle
\]
Evolves under:
\[
i\hbar \frac{\partial \psi(x,t)}{\partial t} = \left( -\frac{\hbar^2}{2m} \frac{\partial^2}{\partial x^2} + V(x) \right)\psi(x,t)
\]
This partial differential equation governs particle motion and tunneling.
7. Evolution of Operators
In the Heisenberg picture:
\[
\hat{A}_H(t) = e^{i\hat{H}t/\hbar} \hat{A} e^{-i\hat{H}t/\hbar}
\]
With dynamics governed by:
\[
\frac{d\hat{A}}{dt} = \frac{i}{\hbar}[\hat{H}, \hat{A}]
\]
This provides insights into symmetries and conservation laws.
8. Energy Eigenstates and Stationary States
If \( \hat{H} |\phi_n\rangle = E_n |\phi_n\rangle \), then:
\[
|\psi(t)\rangle = e^{-iE_n t/\hbar} |\phi_n\rangle
\]
These states evolve only by a global phase, leading to stationary probability distributions.
9. Superposition and Phase Evolution
For a superposition:
\[
|\psi(t)\rangle = c_1 e^{-iE_1 t/\hbar} |\phi_1\rangle + c_2 e^{-iE_2 t/\hbar} |\phi_2\rangle
\]
Relative phase \( (E_1 – E_2)t \) causes interference and oscillations, crucial in quantum computation and control.
10. Probability Conservation and Unitarity
The norm of the quantum state is preserved:
\[
\langle \psi(t) | \psi(t) \rangle = \text{constant}
\]
Implying that total probability remains 1. This is ensured by the unitarity of \( \hat{U}(t) \).
11. Quantum Tunneling Dynamics
Quantum particles can tunnel through potential barriers:
- Described by time-dependent wave packets
- Non-zero probability of finding the particle in classically forbidden regions
- Key in nuclear decay, semiconductors, and scanning tunneling microscopes
12. Quantum Harmonic Oscillator Dynamics
In energy eigenstates:
\[
E_n = \hbar \omega (n + \frac{1}{2})
\]
Wavefunctions oscillate with time via phase:
\[
\psi_n(x, t) = \psi_n(x) e^{-iE_n t/\hbar}
\]
Superpositions exhibit quantum beats and coherent dynamics.
13. Quantum Spin Dynamics
For a spin-1/2 system in a magnetic field:
\[
\hat{H} = -\gamma \vec{B} \cdot \vec{\sigma}
\]
Results in Rabi oscillations and spin precession — foundational in NMR and quantum control.
14. Entanglement and Nonlocal Dynamics
Entangled systems exhibit nonlocal correlations that evolve coherently:
- Bell states remain entangled over time
- Evolution can entangle or disentangle systems
- Important in teleportation, quantum cryptography, and quantum algorithms
15. Decoherence and Open System Dynamics
In real systems:
- Interaction with environment causes decoherence
- Evolution described by master equations and density matrices
- Leads to classical-like behavior without measurement
16. Quantum Control and Coherent Manipulation
Quantum dynamics enables:
- Precise control over qubit evolution
- Pulse sequences and gates in quantum computers
- Coherent population transfer in atomic systems (STIRAP, Rabi pulses)
17. Quantum Dynamics in Field Theory
Quantum fields evolve using:
\[
\frac{d\hat{\phi}(\vec{x}, t)}{dt} = \frac{i}{\hbar}[\hat{H}, \hat{\phi}(\vec{x}, t)]
\]
Field theory dynamics involve creation and annihilation operators, scattering amplitudes, and Feynman diagrams.
18. Conclusion
Quantum dynamics governs the evolution of everything from atomic particles to quantum computers. Whether through wavefunctions or operator equations, understanding these dynamics is essential for interpreting, predicting, and manipulating the behavior of quantum systems. It forms the beating heart of quantum mechanics, computation, and modern physics.
