Page 2: Make it happen!

Contents

Scully and Drühl's thought experiment is not as easy to realize as it is to draw. Even separating the γ and Φ photons is difficult to accomplish, and keeping exactly two atoms stationary in a slit for a very long measurement is not exactly trivial. How to get an electro-optical shutter between the atoms is another problem.

Therefore, the performance of an analog experiment by Yoon-Ho Kim, R. Yu, S. P. Kulik, Y. H. Shih and Marlan O. Scully – yes, the very Scully who was the main author of the first paper – was to wait until the end of the 1990s. The structure and measurement of the "which way" information were implemented completely differently from Scully and Drühl's original design, but the common principle is recognizable.

The basic idea here is based on using entangled pairs of photons that take on the roles of γ and Φ in the thought experiment. You don't need atoms with special energy levels to store whether a source has emitted a photon γ, but you measure the point of origin of the entangled photons and can thus assign them to a source (in this experiment, one of the slits of the good old double-slit experiment). The entangled photons can be generated relatively easily with the help of two passive optical components and then split apart.

Initially, a beta-barium borate crystal (BBO crystal) was used to generate pairs of entangled photons. When a laser beam hits such a crystal, two entangled photons are occasionally emitted, each of which has half the energy (and therefore wavelength) of the photons of the laser light. Entanglement of quantum particles means that they are in a common quantum state and are not independent of each other –properties of one particle are mirrored or replicated in the other particle. The entangled photons produced in the BBO crystal are always polarized at right angles to each other, which can be exploited to separate them from each other and treat them separately. In addition, their light waves are in phase, so they oscillate up and down synchronously.

The second component, a so-called Glan-Thompson prism, consists of two prisms of calcite crystal glued together. Such crystals have a specific preferred direction (optical axis). Light that does not fall along this axis is refracted to different degrees depending on the direction of polarization (birefringence) and produces two parallel beams of light with perpendicular polarization. The Glan-Thompson prism reflects one of the two polarized beams at the contact surface of the prisms like a mirror, while the other beam passes the contact surface straight ahead. The prism therefore deflects the two beams away from each other in different directions.

In the experiment by Kim et al., a BBO crystal is placed behind a double slit and illuminated with laser light from the other side of the double slit. A Glan-Thompson prism located behind the BBO crystal deflects such photons, which are created as entangled pairs with different polarization in the BBO crystal, in different directions. The diagram above shows that a light path is split in two starting from each of the slits. The photons deflected upwards (which in this experiment have the same polarization direction to each other and are therefore capable of interference, which is ensured by the geometry when passing through the Glan-Thompson prism) are focused via a lens and projected onto a movable detector D₀, which scans their superposition pattern. In the following, we refer to them as "signal photons".

The photons entangled with the signal photons are referred to as "partner photons". The light paths, which are deflected downwards due to their polarization, are further separated from each other via an ordinary prism and guided to four detectors (D₁, D₂, D₃ and D₄) via three beam splitters (BS_a, BS_b and BS_c) and two full mirrors (M_a and M_b). As can be seen from the coloring of the light paths in the diagram above, only D₁ and D₂ can be reached by photons from both slits, while D₃ can only be reached by photons from the upper slit and D₄ only by photons from the lower slit.

A coincidence counter connects all detectors and registers which detectors receive a photon if one arrives in D₀ at the same time. Or, to be more precise, only afterwards: in the Kim experiment setup, the detection in D_1-D4 only took place 8 nanoseconds after D₀ was triggered due to the 2.5 m longer light path. The path to the detectors D₁ to D₄ could be extended at will as long as the coincidence to an impact in D₀ can still be reconstructed (in the simplest case, the signal line from D₀ to the coincidence counter is simply extended by the corresponding amount).

What is the result of the experiment? What does the image captured by detector D₀ look like? If you only measure the photons at D₀ that are coincident with D₁, you get a clear interference image:

For coincidences of D₀ and D₂, an interference image is also obtained, but slightly shifted to the side:

And for coincidences of D₀ and D₃, which register such cases where the photon must have passed through the lower slit, the diffraction pattern of a single slit is obtained, on the other hand:

It is thus confirmed that signal photons whose origin has been measured do not form an interference pattern, whereas an interference pattern is formed for signal photons for which there is no information about the slit passed through. The beam splitter BS_c cancels out the "which-path" information of the entangled partner photons on the way to the detectors D₁ and D₂, similar to the case of the Mach-Zehnder interferometer.

To summarize: Only individual photons are detected by the detectors. Events are counted in which D₀ registers a signal photon whose partner photon created at the BBO crystal is registered by one of the other detectors. Photons will reach all 4 detectors D₁ to D₄ in random sequence. If you look at the registrations at D₀, which are in the context of detectors D₁ and D₂, which can be reached from both columns, you will see an interference pattern in D₀. This means that the signal photon has traveled the two paths emanating from the slits simultaneously in superposition. If one of the paths is blocked, as is the case for detectors D₃ and D₄, the interference pattern disappears.

In the experimental setup above, all detectors can be reached by the entangled partner photons at any time, and only chance decides which path a particular photon takes. The experiment could also be modified in such a way that instead of the beam splitters BS_a and BS_b an electro-optical component is used which either reflects the light paths to D₃ or D₄ or lets them pass through and superimposes them in D₁ and D₂, in which case it would be possible to control deliberately or programmatically whether or not an interference pattern arises in D₀.

Or has already been created, because the decision can still be made if the light path to the electro-optical component is sufficiently long, even if the associated signal photon has already reached D₀ and the superposition pattern is already in the box – or rather the photon's contribution to this pattern, which only results from many registered photons. One could even imagine postponing the measurement of the partner photons by such a long light path that all signal photons of D₀ have already been registered and the result is already known before one has decided what it should look like or have looked like.

We have heard of the "spooky action at a distance" between entangled particles that can influence each other over long distances without delay (albeit without the possibility of being able to transmit usable information instantaneously), but how is it possible to influence an event backwards in time – a process known as "retrocausality"?