What Are Quanta?

Navigating quantum physics is quite the conundrum. Perhaps the first thing that comes to mind is Schrödinger’s cat – or maybe even entanglement. The most confusing part of it, other than perhaps the math, is its implications. The thought of many worlds and half–dead, half-alive cats [2, 3] is quite a scary notion, but it can be understood.

Let’s not [1] get ahead of ourselves, and first discuss the central figure of quantum mechanics: quanta. Quanta are discrete packets of energy, or better put, small blocks of energy that all real matter is made of. Quanta’s defining property is wave-particle duality – that is – it behaves both like a wave and a particle [5]. When we look at small particles, we start to see them behave less like round balls, but rather as waves. Experiments that use diffraction grating, where particles traverse through many thin slits, show that light, classically considered a wave, can interfere with itself [5]. It doesn’t even need to be light; massive particles like C60 can replicate the effect just fine [6]. It’s like walking through multiple doors at once – it doesn’t seem to make sense. However, there is a resolution to this paradox: wave functions. Wave functions are mathematical descriptors of the small particles, or rather, quanta. Using wave functions, we can describe the quanta as traveling through multiple slits at the same time [5].

This mathematical solution creates far more questions than it answers, like how do quanta even physically achieve such an effect? When we shine a light through a diffraction grating and place a detector to check which slit it travels through, light travels straight and produces no interference [8], making the quanta behave like a particle [5]. This is called wave function collapse, where rather than going through all possible slits, it goes through one; the act of observing which slit it goes through causes the collapse itself [8]. So, when light isn’t being measured, no collapse happens, and it goes through all paths simultaneously [5]. Wave functions and their collapse aren’t magic, as we know that they’ll always lead to a result [7, 8] (more formally, eigenvectors representing wave functions in Hilbert spaces are elements of unitary groups, but that’s too much jargon).

So, the answer to the question previously posed is we don’t know. Quantum mechanics itself never describes what wave functions are or why they collapse – it merely states that they do [2, 3]. Perhaps the most well-known thought experiment, Schrödinger’s cat, is an example of this. A living cat is put in a box with some poison or radioactive chemical and is locked in so no one may observe. Whether the cat is alive or not depends on whether or not an atom inside the box decays. When the box is finally open, there is a 50% chance of the cat being dead, and a 50% chance of it being alive [1]. The confusion comes from inside: is the cat alive or dead? While we can replicate the experiment [1], reading the answer proves to be impossible without modifying and therefore destroying the experiment. So, we must propose some conjectures. The Copenhagen interpretation simply accepts the phenomena literally; that is, the cat is both alive and dead while in the box: a “simple” explanation [2]. The multi-worlds theory, or the Everett interpretation [3], states the universe is split into two, one where the cat is alive and one where it is dead, and the opening merely chooses one of these two universes [3].

While we don’t fully understand what wave functions represent, they still do have a major use, namely, quantum computing [10, 11]. Using quantum computers we can generate truly random numbers and analyze them at a much quicker rate than we can normally [10], thanks to the inherent randomness of wave functions. This lets us create more secure applications, as well as break them, a major application and concern for cybersecurity [10]. Quantum computing isn’t merely limited to the computing sector, however, as we see major applications in simulating medicine and chemical reactions, thanks to quantum computers’ ability to simulate large numbers of possibilities and therefore account for more atoms [10, 11]. Furthermore, the technology has uses in finance. We can use algorithms that are highly efficient, especially on quantum computers, to better predict the safety and return value of investments [10]. While quantum computing has remained mainly theoretical and been too noisy, meaning having a lot of randomness in its results, it has recently started to gain commercial viability thanks to error-correction innovations and more efficient algorithms [11].

Both propositions sound preposterous and mind-bending, but this is part of what makes quantum mechanics a subject worth studying. Because of developments in quantum mechanics and superposition, we can create quantum computers [10], which rely on superposition, to solve many practical problems in finance, medicine, and many other fields. So, while we may not know the answer, we can still apply quantum mechanics for the benefit of humankind.

References

1. Romero-Isart, O., Juan, M. L., Quidant, R., & Cirac, J. I. (2010). Toward quantum superposition of living organisms. New Journal of Physics, 12(3), 033015.

2. Heisenberg, W., & Bohr, N. (1958). Copenhagen interpretation. Physics and philosophy.

3. Stein, H. (1984). The Everett interpretation of quantum mechanics: Many worlds or none?. Noûs, 635-652

4. An introduction to Hilbert spaces - carmamaths.org. (2008). Retrieved March 6, 2023, from https://carmamaths.org/brailey/Lecture_Notes/NOTES-Intro%20to%20Hilbert%20Spaces.pdf

5. Dimitrova, T. L., & Weis, A. (2008). The wave-particle duality of light: A demonstration experiment. American Journal of Physics, 76(2), 137-142.

6. Arndt, M., Nairz, O., Vos-Andreae, J., Keller, C., Van der Zouw, G., & Zeilinger, A. (1999). Wave–particle duality of C60 molecules. nature, 401(6754), 680-682.

7. Stone, M. H. (1932). On One-Parameter Unitary Groups in Hilbert Space. Annals of Mathematics, 33(3), 643–648. https://doi.org/10.2307/1968538

8. Bassi, A., Lochan, K., Satin, S., Singh, T. P., & Ulbricht, H. (2013). Models of wave-function collapse, underlying theories, and experimental tests. Reviews of Modern Physics, 85(2), 471.

9. Linear algebra in Hilbert space - reed college. (n.d.). Retrieved March 6, 2023, from https://www.reed.edu/physics/courses/P342.S10/Physics342/page1/files/Lecture.16.pdf

10. Bova, F., Goldfarb, A., & Melko, R. G. (2021). Commercial applications of quantum computing. EPJ quantum technology, 8(1), 2.

11. Shang, H., Fan, Y., Shen, L. et al. Towards practical and massively parallel quantum computing emulation for quantum chemistry. npj Quantum Inf 9, 33 (2023). https://doi.org/10.1038/s41534-023-00696-7