Quantum measurements are processes in quantum mechanics that extract classical information from quantum systems. They differ fundamentally from classical measurements in several key ways: 1. **Wavefunction Collapse**: - Upon measurement, a quantum system's wavefunction collapses to one of the eigenstates of the observable being measured. This process is probabilistic, with the [[probability]] of each outcome given by the squared amplitude of the wavefunction component corresponding to that eigenstate [[1](https://www.quera.com/glossary/measurement)]. 2. **Quantum vs. Classical Measurement**: - Unlike classical measurements, which are generally non-invasive, quantum measurements inherently disturb the system. This disturbance is due to the wavefunction collapse and is a central feature of quantum mechanics [[4](https://eitca.org/quantum-information/eitc-qi-qif-quantum-information-fundamentals/quantum-information-properties/quantum-measurement/examination-review-quantum-measurement/what-is-the-purpose-of-a-measurement-in-quantum-information/)]. - In classical physics, measurements can be made with minimal impact on the system being measured, whereas quantum measurements alter the state of the system [[6](https://www.statepress.com/article/2024/03/asu-thinking-beyond-webinar-quantum-measurement)]. 3. **Measurement Operators**: - Quantum measurements are described mathematically by operators. These measurement operators, or observables, have eigenvalues corresponding to the possible outcomes of the measurement. The system's state post-measurement is one of the operator's eigenstates [[3](https://www.studysmarter.co.uk/explanations/physics/quantum-physics/quantum-measurement/)]. 4. **Information Extraction**: - The primary purpose of a quantum measurement is to extract classical information from a quantum system. However, the act of measurement changes the state of the system, which is a stark contrast to classical measurement practices [[1](https://www.quera.com/glossary/measurement)]. 5. **Quantum Measurement Theory**: - Quantum measurement theory provides a framework for understanding how measurements affect quantum systems. This includes the probabilistic nature of outcomes and the concept of wavefunction collapse [[3](https://www.studysmarter.co.uk/explanations/physics/quantum-physics/quantum-measurement/)]. ### Summary Quantum measurements are a unique aspect of quantum mechanics, involving wavefunction collapse, probabilistic outcomes, and system disturbance. These measurements are essential for extracting information from quantum systems but fundamentally alter the state of those systems, differing significantly from classical measurement techniques. [The importance of quantum measurements ](http://www.qi.damtp.cam.ac.uk/files/QIC-3.pdf)