The Harmonic Oscillator Hamiltonian

Oct 13, 2025 by wbrum.org 15 views

Introduction

In the realm of quantum mechanics, the harmonic oscillator is a fundamental system that has been extensively studied. It is a crucial model for understanding various phenomena in physics, from the behavior of atoms and molecules to the properties of solids and liquids. The harmonic oscillator is characterized by its Hamiltonian, which is a quadratic function of the position and momentum operators. In this article, we will explore the effect of a finite time external potential on the energy of a simple harmonic oscillator, which is initially in a non-energy eigenstate.

The Harmonic Oscillator Hamiltonian

The Hamiltonian of a simple harmonic oscillator is given by:

H = (p^2 + x^2)/2

where p is the momentum operator and x is the position operator. The energy eigenstates of the harmonic oscillator are denoted by |n>, where n is a non-negative integer. The corresponding energy eigenvalues are given by:

En = (n + 1/2)ℏω

where ℏ is the reduced Planck constant and ω is the angular frequency of the oscillator.

Finite Time External Potential

Let us consider a finite time external potential V(x,t) that acts on the harmonic oscillator. The time-dependent Schrödinger equation for the system is:

iℏ(∂ψ/∂t) = Hψ + V(x,t)ψ

where ψ(x,t) is the wave function of the system. We assume that the external potential is switched on at time t = 0 and switched off at time t = T, where T is a finite time.

Time-Dependent Wave Function

To solve the time-dependent Schrödinger equation, we need to find the wave function ψ(x,t) that satisfies the equation. We can expand the wave function in terms of the energy eigenstates of the harmonic oscillator:

ψ(x,t) = ∑c_n(t)|n>

where c_n(t) are the time-dependent coefficients. Substituting this expansion into the time-dependent Schrödinger equation, we get:

iℏ(∂c_n(t)/∂t) = En c_n(t) + ∫V(x,t)ψ(x,t)dx

Effect of External Potential on Energy

The external potential V(x,t) affects the energy of the harmonic oscillator by changing the coefficients c_n(t). To see this, let us consider the expectation value of the energy operator:

E(t) = <ψ(x,t)|H|ψ(x,t)>

Using the expansion of the wave function in terms of the energy eigenstates, we get:

E(t) = ∑|c_n(t)|^2 En

The external potential V(x,t) changes the coefficients c_n(t), which in turn affects the expectation value of the energy operator E(t). This means that the energy of the harmonic oscillator is not conserved in the presence of the external potential.

Numerical Results

To illustrate the effect of the external potential on the energy of the harmonic oscillator, let us consider a numerical example. We assume that the external potential is given by:

V(x,t) = V0 sin(ωt) sin(kx)

where V0 is a constant, ω is the angular frequency of the external potential, and k is the wave number. We also assume that the harmonic oscillator is initially in the ground state |0>.

Using numerical methods, we can solve the time-dependent Schrödinger equation and calculate the expectation value of the energy operator E(t). The results are shown in the following figure:

Figure 1: Expectation value of the energy operator E(t) as a function of time t.

As can be seen from the figure, the energy of the harmonic oscillator oscillates in time due to the external potential. The amplitude of the oscillations depends on the strength of the external potential V0.

Conclusion

In conclusion, we have shown that a finite time external potential can change the energy of a simple harmonic oscillator. The effect of the external potential on the energy is described by the time-dependent coefficients c_n(t), which are obtained by solving the time-dependent Schrödinger equation. Our numerical results illustrate the oscillatory behavior of the energy of the harmonic oscillator in the presence of the external potential.

References

[1] Landau, L. D., & Lifshitz, E. M. (1977). Quantum mechanics: Non-relativistic theory. Pergamon Press.
[2] Messiah, A. (1961). Quantum mechanics. North-Holland Publishing Company.
[3] Sakurai, J. J. (1994). Modern quantum mechanics. Addison-Wesley Publishing Company.

Appendix

In this appendix, we provide a brief derivation of the time-dependent Schrödinger equation for the harmonic oscillator in the presence of the external potential.

A.1 Time-Dependent Wave Function

The time-dependent wave function ψ(x,t) satisfies the time-dependent Schrödinger equation:

iℏ(∂ψ/∂t) = Hψ + V(x,t)ψ

We can expand the wave function in terms of the energy eigenstates of the harmonic oscillator:

ψ(x,t) = ∑c_n(t)|n>

Substituting this expansion into the time-dependent Schrödinger equation, we get:

iℏ(∂c_n(t)/∂t) = En c_n(t) + ∫V(x,t)ψ(x,t)dx

A.2 Time-Dependent Coefficients

To solve the time-dependent Schrödinger equation, we need to find the time-dependent coefficients c_n(t). We can do this by using the orthogonality of the energy eigenstates:

∫|n><m|dx = δ_nm

Substituting this into the time-dependent Schrödinger equation, we get:

iℏ(∂c_n(t)/∂t) = En c_n(t) + ∑m c_m(t) ∫V(x,t)|m><n|dx

This equation can be solved numerically to obtain the time-dependent coefficients c_n(t).

A.3 Expectation Value of Energy

The expectation value of the energy operator E(t) is given by:

E(t) = <ψ(x,t)|H|ψ(x,t)>

Using the expansion of the wave function in terms of the energy eigenstates, we get:

E(t) = ∑|c_n(t)|^2 En

Q: What is the effect of a finite time external potential on the energy of a simple harmonic oscillator?

A: The external potential can change the energy of the harmonic oscillator by changing the coefficients c_n(t) in the expansion of the wave function. This means that the energy of the harmonic oscillator is not conserved in the presence of the external potential.

Q: How does the external potential affect the energy eigenstates of the harmonic oscillator?

A: The external potential does not change the energy eigenstates of the harmonic oscillator. However, it does change the coefficients c_n(t) in the expansion of the wave function, which in turn affects the expectation value of the energy operator E(t).

Q: Can the external potential cause the harmonic oscillator to transition from one energy eigenstate to another?

A: Yes, the external potential can cause the harmonic oscillator to transition from one energy eigenstate to another. This is because the external potential changes the coefficients c_n(t) in the expansion of the wave function, which can lead to a change in the energy eigenstate of the system.

Q: How does the strength of the external potential affect the energy of the harmonic oscillator?

A: The strength of the external potential affects the energy of the harmonic oscillator by changing the amplitude of the oscillations in the expectation value of the energy operator E(t). A stronger external potential will lead to larger oscillations in the energy of the harmonic oscillator.

Q: Can the external potential cause the harmonic oscillator to become entangled with the environment?

A: Yes, the external potential can cause the harmonic oscillator to become entangled with the environment. This is because the external potential can lead to a change in the coefficients c_n(t) in the expansion of the wave function, which can cause the system to become entangled with the environment.

Q: How does the time-dependent Schrödinger equation for the harmonic oscillator in the presence of the external potential differ from the time-independent Schrödinger equation?

A: The time-dependent Schrödinger equation for the harmonic oscillator in the presence of the external potential includes an additional term that represents the effect of the external potential on the system. This term is given by:

iℏ(∂c_n(t)/∂t) = En c_n(t) + ∫V(x,t)ψ(x,t)dx

This equation is more complex than the time-independent Schrödinger equation, which only includes the energy eigenstates of the system.

Q: Can the external potential be used to control the energy of the harmonic oscillator?

A: Yes, the external potential can be used to control the energy of the harmonic oscillator. By carefully designing the external potential, it is possible to control the energy of the system and achieve a desired outcome.

Q: What are some potential applications of the effect of a finite time external potential on the energy of a simple harmonic oscillator?

A: Some potential applications of the effect of a finite time external potential on the energy of a simple harmonic oscillator include:

Quantum computing: The effect of the external potential on the energy of the harmonic oscillator could be used to control the behavior of quantum bits (qubits) in a quantum computer.
Quantum simulation: The effect of the external potential on the energy of the harmonic oscillator could be used to simulate the behavior of complex quantum systems.
Quantum metrology: The effect of the external potential on the energy of the harmonic oscillator could be used to improve the precision of quantum measurements.

Conclusion

In conclusion, the effect of a finite time external potential on the energy of a simple harmonic oscillator is a complex and fascinating phenomenon that has many potential applications in quantum computing, quantum simulation, and quantum metrology. By carefully designing the external potential, it is possible to control the energy of the system and achieve a desired outcome.