Are the 'double-slit' and 'quantum eraser' experiments of quantum mechanics possible with just everyday items?

A popular science magazine featured an article suggesting that we can perform the 'double-slit experiment' and 'quantum eraser experiment' of quantum mechanics using only everyday tools. In reality, those experiments are merely experiments in classical electromagnetism rather than quantum mechanics. However, the article was received favorably for making the 'wonders of quantum mechanics' feel accessible to readers, and this misunderstanding has since spread.

In what the article calls a 'double-slit experiment,' you prepare a laser pointer, a wire, and a piece of cardboard to serve as a screen. When you arrange them as shown in Figure 1 and turn on the laser pointer, interference fringes appear on the cardboard.

The article claims that this is a quantum mechanical double-slit experiment, stating that 'a single photon passed through either the left or right side of the wire, and then interference fringes were formed on the screen.'

However, in reality, this cannot be considered a quantum mechanical experiment involving a single photon. Compared to a quantum beam where photons fly sparsely, the light emitted from a commercially available laser pointer is high-intensity classical laser light, which can be understood using only classical electromagnetism without the need for quantum mechanics. Therefore, it is incorrect to call the experiment in Figure 1 a 'quantum mechanical double-slit experiment.'

The article also introduces the 'quantum eraser experiment' as something that can be easily done using only familiar items. For example, if you arrange a polarizing plate as shown in Figure 2, the component of the electromagnetic wave with an electric field oscillating in the horizontal plane can pass through, and if you arrange it as shown in Figure 3, the component with an electric field oscillating in the vertical plane can pass through.

Then, as shown in Figure 4, if you overlap two polarizing plates so that their directions are orthogonal, the light is blocked by the second polarizing plate.

Based on these properties of polarizing plates, in the 'quantum eraser experiment,' you first arrange a laser pointer, two polarizing plates with orthogonal polarization directions, and a wire as shown in Figure 5. Then, the laser light interference fringes that should appear on the screen disappear.

The article explains the reason for this as follows: 'For photons that passed through the polarizing plate on the right side relative to the direction of the laser light, the photon was observed to have horizontal plane oscillation, and for photons that passed through the polarizing plate on the left, the photon was observed to have vertical plane oscillation; in each case, the wave function collapsed, preventing interference on the screen.' In other words, it claims that the interference fringes disappeared because the polarization, which is the spin degree of freedom of the photon, observed the 'position of the photon'—that is, whether it was on the left or right side of the wire.

However, this experiment using a commercially available laser pointer is not an experiment in the quantum realm. It is a phenomenon that can be explained entirely by classical electromagnetism. Furthermore, classical light also ceases to interfere if its polarization states are different. It is not a particularly mysterious result.

A 'quantum eraser' is said to be something that erases the record of position information of a photon that has passed through a double slit. In this experiment, you can prepare one more polarizing plate and have it act as a 'quantum eraser.' As shown in Figure 6, if you insert that third polarizing plate into the beam line at an oblique angle, interference fringes appear on the screen.

The article interprets this phenomenon as: 'The information regarding polarization stored at the first slit was erased by the inserted polarizing plate, allowing the photon to regain its ability to interfere.' However, this 'quantum eraser experiment' cannot be viewed as a pure quantum mechanical experiment either. This is because the laser intensity is high, and the laser light behaves as a classical electromagnetic wave. Moreover, the phenomenon in Figure 6 also occurs in classical electromagnetism.

In other words, the 'double-slit experiment' and 'quantum eraser' introduced are basically experiments within the scope of classical electromagnetism. For this reason, some experimentalists point out that it is more appropriate to consider the quantum eraser experiment using a commercially available laser pointer as a 'classical simulator that allows for a simulated experience of quantum phenomena.'

If you want to perform a real double-slit experiment or quantum eraser experiment, you are required to set the laser intensity extremely low so that individual photons fly sparsely. Only then can it be considered that a single photon is passing through the wire double slit or the polarizing plate. And it is difficult to perform such an experiment with a commercially available laser pointer.

Furthermore, some people misunderstood the principle of this quantum eraser and claimed that interference could be erased by using the spatial orbital angular momentum of photons instead of polarization, which is the spin angular momentum of photons. That person thought that if interference fringes disappear because photon position information is recorded in the photon's spin angular momentum degree of freedom, then interference fringes would similarly disappear if photon position information were recorded in the photon's orbital angular momentum degree of freedom.

However, this is a major error. The reason the interference fringes disappeared when position information was stored in polarization is simply because quantum entanglement occurred between the position degree of freedom and the polarization degree of freedom, causing decoherence in the position degree of freedom. For quantum entanglement or decoherence to occur, two different degrees of freedom are required, but in the case of the orbital angular momentum degree of freedom, it is exactly the same as the photon's position degree of freedom, which is recorded together in the spatial wavefunction part. Therefore, neither quantum entanglement nor the decoherence that erases interference fringes can occur in the first place.

In fact, it is well known that normal interference fringes appear in spatial wavefunctions that can be written as a superposition of spherical harmonics, which are eigenstates of orbital angular momentum. For example, as shown in Figure 7, even if an experiment is conducted where the superposition of orbital angular momentum eigenstates is changed on the left and right sides of a wire, the interference fringes on the screen do not disappear.

Therefore, a 'quantum eraser experiment' using orbital angular momentum is nonsense from the very starting point of the idea.

In summary, the 'double-slit experiment' and 'quantum eraser experiment' using commercially available laser pointers may look like quantum phenomena at first glance, but they can actually be fully explained by classical electromagnetism alone. Therefore, they do not constitute pure quantum mechanics experiments. I believe it is important for even general science magazines to avoid incorrect interpretations of quantum mechanics.