{ "cells": [ { "cell_type": "code", "execution_count": 3, "metadata": { "ExecuteTime": { "end_time": "2021-09-27T19:23:32.835945Z", "start_time": "2021-09-27T19:23:31.674318Z" }, "lines_to_next_cell": 0, "tags": [ "remove-cell" ] }, "outputs": [], "source": [ "import numpy as np\n", "import matplotlib.pyplot as plt\n", "import warnings\n", "warnings.filterwarnings(\"ignore\")" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Lecture 3: Thermomechanical Noise: Brownian Motion of an Oscillator\n", "\n", "\n", "```{admonition} Expected prior knowledge\n", ":class: tip\n", "Before the start of this lecture, you should be able to:\n", "\n", "- Peform calculations with power spectral densities\n", "- Explain different types of fluctuating (stochastic) variables \n", "```\n", "\n", "```{admonition} Learning goals\n", ":class: important\n", "After this lecture you will be able to:\n", "\n", "- Calculate the power spectral density of the thermal noise spectrum of a harmonic oscillator\n", "- Predict how the power spectrum changes for lower or higher damping\n", "- Explain and draw a representation of thermal noise in an I-Q representation of signals\n", "```\n", "\n", "In principle, even when things are at rest (not moving), at finite temperature everything is actually moving a little tiny bit. For example, if I knock on this desk, I can make it vibrate, and you can hear a sound. But even if I didn't hit this desk, it is vibrating already by some small amount, due to its thermal energy producing Brownian motion. If we look at it, of couse it doesn't look like its moving, but in principle, if I looked very carefully, I would see it bouncing up and down.\n", "\n", "You might ask yourself: what does any of this have to do with quantum mechanics? Well, in quantum mechanics, it turns out that things are \"sort of\" still moving even at zero temperature, and if you look hard enough, you can (try) to see this quantum motion... but more on that later.\n", "\n", "### The damped harmonic oscillator \n", "\n", "But before we start, it will be useful to review the behavior of the damped harmonic oscillator (DHO). A DHO consists of a mass on a spring under the influence of friction. We will consider the DHO under the influence of an external driving force $F_e(t)$, giving \n", "\n", "$$\n", "F = ma = -kx - cv + F_e(t)\n", "$$\n", "\n", "[