Quantum mechanics: from theory to reality

Physicists from the Massachusetts Institute of Technology (MIT) have succeeded in reproducing one of the most important results in quantum mechanics, known as the “Elitzur-Vaidman bomb tester.” It’s amazing that they achieved this in a primitive system, which means they saw a quantum effect in a system from everyday physics. This achievement raises the question: What really separates the quantum from the classical?

In 1993, Israeli physicists Avshalom Elitzur of the Weizmann Institute of Science and Lev Vaidman of Tel Aviv University proposed the following thought experiment: Imagine you have a bomb with “ultra-sensitive trigger” that explodes on contact with a single particle. As long as the bomb is stored safely, nothing can affect the wave, which allows us to control the timing of its explosion. However, the bombs may malfunction, appearing like any normal bomb but failing to detonate when the detonator is fired.

Let’s say we found an old bomb in storage and we want to know if it works. In everyday physics, the only way to test it is to try to break it: if it works, it will explode; if it doesn’t work well, it will always be there. The problem with such testing is that it destroys active bombs, preventing their future use. We need a method that allows to “try both and not install” the detonator.

The “yes and no” option will sound familiar to anyone interested in quantum mechanics. Quantum particles can exist in a mixed state of many possible properties, such as not being close to the bomb but also not too far away from it, still in a space that combines the probabilities of being in these states. This situation is called superposition. As long as the particle is not measured, which means that there is no external object that can see and try to find its location, it will always be in this mixed state, between activating and not activating the bomb. Once we observe and measure its location, it will “collapse,” cease to exist in a mixed state and appear only in one place, just like anything else in classical physics. This means that just looking at a particle affects its state. It should be noted that you cannot choose where the particle will fall, but only cause it to fall.

Elitzur and Vaidman developed a method of using a quantum particle to test a bomb without destroying it. They proposed taking a photon—a particle of light—and passing it through a beam. The path of the photon would lie between the two possible paths, which means that the photon would be in a mixed state of the two possible paths. One path goes through the bomb, and the other does not touch the bomb. After that, the trajectories are diverted to meet another piece of the beam, directed backwards from the previous one, and the photon’s trajectory is measured at the end.

Since the measurement is done at the end, by the time the particle reaches the second division, it is in a mixed state and does not “select” a particular path. Only once a measurement is made does the photon fall into one particular path. It can fall on the road with a bomb, causing an explosion, thus showing that we have a live bomb, which is already dead. However, it is also possible that it will fall on the way without the bomb. In this case, the presence of the bomb will leave residual effects in the photon region, and we will be able to detect them.

The bomb experiment demonstrates the application of the principles of quantum mechanics: superposition and measurement. Another important principle of quantum mechanics is wave-particle duality, which means that the mixed state of a quantum particle also exhibits wave-like properties. Because of this, passing a particle of light through a beam of light causes wave behavior, such as interference.

When a photon is transmitted through a beam – an optical device – it behaves like a light wave. If one wave arrives, it will split between two paths. If two waves arrive, they will be combined into one wave. This also applies to the photon-if it reaches the full space without superposition, its state will change and merge. If it reaches mixed status, it will become full status.

After splitting, the photon is in a high state, meaning that there is a fifty percent chance that it will pass through the path of the bomb, and a fifty percent chance that it will pass in another way. Mirrors are installed to direct the photon to one of the splitters. If the bomb is a dud, the photon split up, didn’t encounter anything of interest in its path, and was recombined in a second split. Therefore, the photon is not in a mixed state, as the second detector transforms the mixed state into an absolute state. Therefore, if the bomb is a dud, the photon will continue to the detector D.

If the bomb is live, there are two possibilities after the measurement. In the first case, with a probability of fifty percent, a photon will pass through the bomb, causing it to explode, and the information of the bomb will fail. This is an interesting little possibility. In the second case, with the same probability of fifty percent, the photon will not pass through the bomb and will reach the second part of the separation, where it will be separated. Now there is a fifty percent chance that we will be able to measure a photon in detector C. If we find a photon there, we will know for sure that the bomb is alive, even though it was not hit by a photon.

The method of Elitzur and Vaidman allows the identification of live bombs in a quarter of the cases—compared to zero cases in classical physics. With further development, this success rate can be greatly improved.

This concept is important, as it is one of the basic examples of tasks that can be achieved with quantum measures that are not possible with traditional methods alone. Highlights the fundamental differences between classical mechanics and quantum mechanics: superposition and the effect of dimensions in quantum processes. Although the experiment itself has no practical application, it improved the understanding that quantum systems are very different from classical systems. This understanding underpins many current quantum technologies, including quantum computing.

However, researchers have now been able to replicate this effect in the laboratory using only ancient particles. To achieve this they used the strong relationship between particles and waves in quantum mechanics, and the fact that the function that describes the superposition of a particle behaves like a wave. In their experiment, a drop of oil floating in liquid silicone represented a particle in a bomb test. A wave in liquid silicone represents a wave field in the particle and the tracks it leaves on an unpredicted path, bombarded by interference.

The researchers’ success rate in identifying “bombs” – represented by small blocks in a pool of liquid – is consistent with the expectations of the Elitzur-Vaidman experiment. Instead of using a quantum particle, which combines the properties of a particle and a wave, the researchers used a classical particle and an ordinary wave in a liquid, to achieve the same results.

An important achievement by researchers is the use of simulating the quantum world to identify a bomb with ancient steps – which seems to be an impossible task. Imitation of nature is an important and valuable tool in all areas of science, and is used, for example, when simulating the structure of the human brain through machine learning methods or reconstructing the characteristics of light receptors in the wings of a butterfly to produce solar cells. Here, there is a new application of this principle, and it is interesting to guess what other quantum systems can be imagined using the old steps.

On the other hand, it is important to remember the main difference between quantum and classical systems. A quantum experiment uses a single particle – a quantum particle that combines particles with wave properties. In classic games, two separate parts are used—one part and one wave. Furthermore, if we wish to perform further experiments involving many quantum particles, we will need a wave composed of many particles. This wave would be so complex that a system with only a few tens of particles would already be too complex to reproduce.