Quantum Measurements

Abstract

1 Basic Ideas 3 1.1 Measurement in Quantum Mechanics . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Widely Prevalent View of Quantum Measurements (which is wrong!) . . . . 4 1.3 Incomplete State Information and Density Matrices . . . . . . . . . . . . . . 5 1.3.1 Density Matrices and Operators . . . . . . . . . . . . . . . . . . . . . 5 1.3.2 Reduced Density Operators . . . . . . . . . . . . . . . . . . . . . . . 7 1.3.3 Time Evolution of Density Matrices . . . . . . . . . . . . . . . . . . . 9 1.4 Unpredictability of Measurement Results and Apparatus States . . . . . . . 9