Behind the Technology: Unique Nature of the Quantum World
By E. Su USTUN
Quantum computers are big, expensive and incredibly fast machines that are governed by the laws and principles of quantum mechanics. As known, quantum mechanics took us to a different world on the earth by introducing unique features and behaviors of sub-atomic particles. According to the scientific experiments done, the sub-atomic particles had quite different natures and they were performing in a way that was impossible according to classical physics. Indeed, because of this reason Einstein, one of the most famous geniuses of the 20th Century was claiming that the theory of quantum mechanics was incomplete and the supporters of it were missing something, which was probably a constant value . To learn more about that EPR (Einstein, Podolsky, Rosen) argument you can visit directly https://plato.stanford.edu/entries/qt-epr/.
“God does not play dice.” -Albert Einstein
“Einstein, stop telling God what to do.” – Niels Bohr5th Solvay Conference
Before we move on to this game changer technology and its current and/or future implications, first we need to understand the underlying mechanism. As mentioned above, this technology is established on the base of the quantum physics and there is no way to truly comprehend this technolog without having an idea with the fundamentals of the quantum mechanics. Therefore, although quantum physics includes greater complexity and advanced level of maths, algebra, and physics, just to provide a general understanding and prepare a background for the following episodes, the unique features, in a nutshell are as follows:
Superposition: According to classical physics and Boolean Algebra, for a statement, or an event there can be only possiblities which are true or false. Again, the same also applies to the binary language of the classical computing. Today the language of the classical computing is composed of 2 digits: 0 and 1. On the other hand, in the quantum world, there is actually one more possible scenario: both at the same time.
That phenomenon is called as “superposition” and illustrated with a thought experiment named Schrödinger’s Cat. According to that experiment a cat is put to a box in which there will be quite little amount of radioactive substance that can be decayed within 1 hour and do nothing to the cat or poison it within that hour. Erwin Schrödinger concludes as after that one hour, until we look at the box and observe the situation both possibilities will exist at the same time. So, both a dead and an alive cat! Nonetheless, the observation will destroy the quantum nature of it and there will be only one result: either dead or alive.
Entanglement: As a term coined by Schrödinger1, entanglement indicates a relationship- indeed a matching correlation– between quantum states. In other words, in a quantum state, a particle cannot be deemed independent from others that it correlated with. In such a case, when a particle is for example, spinning upwards; the correlated one must then spin dawnwards even if they are separated via infra-red technology or so. More surprisingly, whenever an external intervention changes the direction of the spinning movement; the correlated particle also changes its direction simultaneously. As generally accepted, nothing can travel faster than speed of light; it is the highest speed in the universe. So, how can that be possible? Because of this strange situation Einstein called this phenomenon as “Spooky Action at a Distance2.” But that was not a spooky action; rather that was only unique nature of the quantum world. That was Entanglement! When these particles entangled each other they keep their communication regardless of the distance or external interventions.
Uncertainty: Often known as Heisenberg’s Uncertainty Principle, this phenomenon tells an impossibility to know a particle’s momentum and location with a total accuracy at the same time. To explain this impossibility better, we should firstly introduce another mesmerizing feature of quantum mechanics: Dual characteristics of the Light. According to numerous experiments done, the light exhibits characteristics of both a wave and a particle3. So the light is “both a particle that behaves like a wave and a wave behaves like a particle.” To know more about this you can visit https://wtamu.edu/~cbaird/sq/2013/01/16/is-light-a-particle-or-a-wave/
If we get back to the subject, Heisenberg claimed that the impossibility of measuring the momentum and the location at the same time is resulted from this dual nature of the light. De Broglie’s wave-length formula (λ) is equals to planck’s constant (h) divided by momentum (p). Hence, to find the momentum of the photon we need to consider it as a “wave” -since we need the wavelength in the formula- but this time, we cannot find the location. Because by acknowledging its wave nature, as a wave it must be then in everywhere
But if we considerit as a particle and decide to find the location. But in this case, we need to calculate the location twice and divide the difference to time. Nevertheless, once we observe the particle, this observation will turn into a intervention to this particle because to measure we are sending photons and the momentum of the photon is also added to the particle and it prevents an accurate result. In brief, when we learn one of this information accurately we can only predict a possibility for the another.
Delayed Choice or Quantum Eraser: Scientists who wanted to aswer the question that what happens if we use entangled photons in Young’s double slit experiment -See Footnote 3-? As previously mentioned, until a photon is observed it was in a superposition state and it is carrying the nature of both a wave and a particle at the same time. On the other hand, the observation was an intervention to the photon and thus with an observation, its wave nature was collapsing5.
In 1999, Kim et. al, decided to make an experiment and they set up a contraption in which a photon is divided into two by passing into a crystal and these two photons, afterwards become entangled with each other. One of these photons was going to a screen and the other one was going to either to the Detector A or the Detector B4. Scientists wanted to measure the photon which is directly going to the screen my measuring the entangled photon. Though there was no direct observation, or in quantum language intervention, to the photon that is going to the screen, observing the other entangled photon was directly and immediately affecting the former and its wave function collapsed just by observing the lother one. In other words, somehow the information of being ‘observed’ is gained by the entangled pair.
Further, scientists carried the detectors a little bit further and to provide the formers arrival sooner than the latter’s arrival to the detecter. To put it differently, they wanted to prevent this ‘information sharing’ between the entangled pairs by observing the latter after the entangled pair arrived to the screen. The result was even “spookier” than the entanglement! The information is carried back the time. Increadibly, the former knew(!) that the entangled pair (and indirectly itself) will be observed and it thus it stopped behaving as a wave.
To unveil the mystery and find an explanation scientists put additional detectors that one of them did not provide information to observers since it erased the information about photons by quantum eraser and others provided information regarding the photons (again by observation and measurement) Photons behaved as a particle when they went to detectors that were providing information; however, they kept behaving as a wave as they were passing through the detector that was erasing the information. What is striking here is, there is no way for photons to acces the information if and so which detector was erasing the information. At this point one more thing shall be emphasised, erasing the information is done after the photons were already arrived.
Quantum Tunneling: is a phenomenon that is derived from the photon’s ability to behave as a wave. As you may remember from the entanglement part, photons were in a superposition and they were behaving both as a wave and as a particle as long as they are not observed. Because of their wave functions in such a circumstance, again without an observation and external intervention, they spread all over and thus they are in everywhere. That is the underlying logic behind the quantum tunneling. In quantum tunneling, due to its wave function a photon can change its location instantly –even faster than the speed of light– by disappearing in its pre-location and reappearing in its post-location.
To make it more understandable this event is generally illustrated with a basketball that bounces at different points instantly. However, the most important point here is that photons keep their “bouncing” movement steadily regardless of all environmental and external effects. For example, even if twe put an energy barrier to that quantum state photons keep bouncing without losing momentum despite energy barriers. In brief, photons change their locations by disappearing and reappearing in a quantum state without getting affected by the gravity of any other energy or any kind of barriers. To learn more about quantum tunneling you can also visit: https://www.scientificamerican.com/article/quantum-tunneling-is-not-instantaneous-physicists-show/
That is the end of the Episode I of the Quantum Technologies series. With these fundamentals we, hopefully, established an understanding that will help us to analyze quantum technologies better and more comprehensively. So, it is time to pave the bricks one by one! Next we will move introducing quantum technologies and their current and future use cases.
PS: for the followers who do know Turkish language and want to understand the fundamentals in the simplest way I highly recommend the Quantum series of Bebar bilim both in its blog and (especially) in the youtube channel with the links: https://bebarbilim.net/category/uzay-ve-teknoloji/ and https://www.youtube.com/embed/videoseries?list=PLdbeYAM5mGiQH4UQZGJjghqpx5rrOuuuw
1 Schrödinger, E., 1935. “Discussion of Probability Relations Between Separated Systems,” Proceedings of the Cambridge Philosophical Society, 31: 555–563; 32 (1936): 446–451.
2 Einstein, A., Podolsky, B. & Rosen, N. Phys. Rev. 47, 777 (1935)
3 Young’s Double Slit Experiment: https://www.britannica.com/science/light/Youngs-double-slit-experiment
4 Y-H Kim, R. Yu, S. P. Kulik and Y. H. Shih, “ A delayed Choice Quantum Eraser”, (1999)
5 H. Fearn, A Delayed Choice Quantum Eraser Explained by the Transactional Interpretation of Quantum Mechanics