It is very important to remember that Schrödinger’s cat thought experiment was proposed as an absurd extrapolation of the Copenhagen Interpretation. Although we are ignorant about the boundary between quantum and classical systems, quantum physics has very little to do with cats or any other macroscopic system. For all of the reasons that we have discussed so far, real experiments in quantum physics are conducted with very simple systems. In fact, most experiments are conducted with photons using instruments that are just more sensitive versions of the components we have been using in the experiments.
Real experiments in quantum physics follow the same basic steps as Schrödinger’s cat thought experiment. First you need to set up the particle (e.g., cat, photon, electron, etc.) in a superposition of states. This process is called preparation of quantum state. The quantum state is then allowed to evolve, after which a measurement is made to force the system into a certain state (collapsing the wavefunction, if you adhere to the Copenhagen Interpretation). The process is commonly repeated over and over again to measure the probabilities of the various outcomes.
The following experiment is not too exciting, but shows all of these basic steps in the simplest possible way. Figure 121a shows the system we used to detect single photons (Figure 33), to which we have added two pieces of polarizing film. Now, remember that the previous polarization of a photon exiting a polarizer is not important, but its polarization is instead reset to the polarizer’s angle (Figure 121b). Therefore, the first polarizer causes all photons entering the system to be polarized at 45°. In quantum physics experiments, this process is thought of as preparing the photon’s quantum state to a 50%/50% superposition of 0° and 90° polarizations. The photon is then allowed to spend some time in this superposition of quantum states until its quantum state collapses when a measurement is made by the analyzing polarizer and PMT. The analyzer is simply a second piece of polarizing film placed within a mount that allows it to rotate (we used a Thorlabs CLR1 SM1-compatible rotatable mount). If a photon exits the analyzing polarizer, it is detected by the PMT (of course, taking into account the absorption of the bandpass filters and quantum efficiency of the PMT).
Figure 121 A basic quantum physics experiment consists of preparing a quantum state, allowing the state to evolve, and then conducting a measurement to collapse the system into a state that can be detected. (a) In this simple setup, the polarization of photons is set to 45°, which is a 50%/50% superposition of polarization states at 0° and 90°. Photons spend some time as they traverse the flight tube in a superposition of quantum states until their polarization collapses when they reach the measurement instrument comprising the “analyzer” (another piece of polarizing film) and PMT. (b) The important thing to remember is that a polarizer doesn’t simply allow a photon to pass or not pass, but rather sets the polarization of photons that manage to pass through the polarizer. (c) Looking at the process one photon at a time, the first polarizer prepares the photon’s quantum state, while the second polarizer (the analyzer), along with the detector, conducts the measurement. The process is commonly repeated many times to determine the probability of occurrence for each possible outcome of the experiment.
What is important to understand is that, from the second polarizer’s point of view, the photon has two possible polarization states: (1) the photon has the same polarization as the analyzer and can thus go through, or (2) the photon’s polarization is orthogonal to that of the polarizer and should be absorbed. The angle between the first and second polarizer only changes the probability of occurrence of each possible outcome. Once detected by the PMT, the collapse finalizes, and we either detect or don’t detect the presence of a photon. Do you see how this experiment is analogous to the Schrödinger’s Cat gedankenexperiment (thought experiment)?
The whole field of experimental quantum physics, as well as all of the modern technologies of quantum teleportation, quantum cryptography, and others follow the basic protocol of preparing a quantum state, allowing the state to evolve, and finally performing a measurement. Of course, what differentiates each experiment is how the quantum state is prepared, as well as the things that you can do to a particle in superposition of states without causing collapse of its wave function.
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