# Hermitian Hamiltonians: The Foundation of Quantum Mechanics

## What are Hermitian Hamiltonians?

Hermitian Hamiltonians are a fundamental concept in quantum mechanics, serving as the mathematical foundation for describing the energy and time evolution of quantum systems. They are named after the mathematician Charles Hermite and are a specific type of operator that satisfies certain mathematical properties.

## The Mathematical Properties of Hermitian Hamiltonians

For a Hamiltonian operator H to be considered Hermitian, it must satisfy the following properties:
• Self-adjointness: The adjoint of the Hamiltonian, denoted as H†, is equal to the Hamiltonian itself. In other words, H = H†. This property ensures that the Hamiltonian is a linear, self-adjoint operator.
• Real eigenvalues: The eigenvalues of a Hermitian Hamiltonian are always real. This property is crucial because the eigenvalues of the Hamiltonian represent the possible energy levels of the quantum system.
• Orthogonal eigenvectors: The eigenvectors of a Hermitian Hamiltonian corresponding to different eigenvalues are orthogonal to each other. This property allows for the construction of a complete orthonormal basis set, which is essential for describing the state of a quantum system.

## The Physical Significance of Hermitian Hamiltonians

Hermitian Hamiltonians play a central role in quantum mechanics due to their physical significance:

### Energy Conservation

The Hermitian property of the Hamiltonian ensures that the total energy of a quantum system is conserved. This is a direct consequence of the real eigenvalues of the Hamiltonian, which represent the possible energy levels of the system. The conservation of energy is a fundamental principle in physics and is essential for the consistency and stability of quantum systems.

### Unitary Time Evolution

The time evolution of a quantum system is described by the Schrödinger equation, which involves the Hamiltonian operator. The Hermitian property of the Hamiltonian guarantees that the time evolution operator is unitary, preserving the normalization and orthogonality of the quantum states. This unitary time evolution ensures that the probabilities of measuring different outcomes remain consistent over time.

### Observable Quantities

In quantum mechanics, observable quantities, such as position, momentum, and energy, are represented by Hermitian operators. The Hermitian property ensures that the expectation values of these observables are real, which is consistent with the outcomes of physical measurements. Moreover, the eigenstates of Hermitian operators form a complete basis set, allowing for the description of any quantum state as a linear combination of these eigenstates.

## Examples of Hermitian Hamiltonians

Hermitian Hamiltonians are ubiquitous in quantum mechanics and are used to describe a wide range of physical systems:

### Harmonic Oscillator

The Hamiltonian for a quantum harmonic oscillator, which describes the energy of a particle in a quadratic potential, is Hermitian. The eigenstates of this Hamiltonian are the well-known harmonic oscillator wavefunctions, and the eigenvalues represent the quantized energy levels of the system.

### Hydrogen Atom

The Hamiltonian for a hydrogen atom, which describes the energy of an electron in the Coulomb potential of a proton, is also Hermitian. The eigenstates of this Hamiltonian are the atomic orbitals, and the eigenvalues correspond to the discrete energy levels of the hydrogen atom.

### Spin Systems

The Hamiltonians describing spin systems, such as the Pauli matrices for a single spin-1/2 particle or the Heisenberg Hamiltonian for interacting spins, are Hermitian. These Hamiltonians capture the magnetic properties and interactions of spin systems, which are fundamental to many areas of physics, including quantum information and condensed matter physics.

## Non-Hermitian Hamiltonians and PT-Symmetry

While Hermitian Hamiltonians are the standard in quantum mechanics, there has been growing interest in non-Hermitian Hamiltonians, particularly those with PT-symmetry (parity-time symmetry). PT-symmetric Hamiltonians, which are invariant under the combined operation of parity and time reversal, can exhibit real eigenvalues despite being non-Hermitian. These systems have been studied in the context of optical lattices, where they can lead to novel phenomena such as unidirectional invisibility and exceptional points.

## Conclusion

Hermitian Hamiltonians are a cornerstone of quantum mechanics, providing the mathematical framework for describing the energy and time evolution of quantum systems. Their unique properties, such as real eigenvalues and orthogonal eigenvectors, ensure the conservation of energy, unitary time evolution, and the existence of observable quantities. Understanding Hermitian Hamiltonians is essential for anyone studying quantum mechanics and its applications in various fields, from atomic and molecular physics to condensed matter and quantum information science.