Quantum Measurements: What Are They?

We are now ready to tackle quantum measurements, an integral part of quantum information science and quantum computing. A quantum measurement occurs when a qubit interacts with a measurement device designed to give you information about the quantum state of the qubit. Ideally, the measurement device is equipped with signals such as light bulbs or sound generators or electrical outputs that tell you which output channel has the outgoing qubit. For most purposes in QIS and QC, we don’t need to worry about the details of the measurement devices, but for those who want to build quantum computers they are a big deal.

Let’s start with a concrete example: linearly polarized light and a polarizing beam splitter (PBS), as shown in the following figure: