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How are measurements performed in quantum mechanics?

As an experimentalist, I can prepare quantum states and observe definite outcomes in my lab. But the transition from a probabilistic wavefunction to a concrete data point on my detector feels philosophically and technically loaded. I'm asking about the practical bridge: what are we doing physically when we measure a spin or a photon's polarization?

 

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By Keerthi Gupta Answered 1 year ago

In the lab, we never "see" a wavefunction. A measurement is a specific, irreversible physical interaction that amplifies a quantum property into a classical record. For instance, to measure a photon's polarization, I send it through a birefringent crystal that routes it based on that property to one of two distinct, macroscopic detectors. The "collapse" is our updating of knowledge upon seeing which detector clicks. From a practical standpoint, decoherence the entanglement of the quantum system with the vast number of degrees of freedom in the detector and environment is what makes this record definite and irreversible. We then read that classical environment.

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