Advances in artificial intelligence and virtual reality are increasing the amount of data we need to process, while the limits of integrated circuits are approaching. Consequently, quantum computers, which use quantum computational principles instead of transistor gates, are emerging as a viable alternative. What exactly is quantum computing, and what can it do to make it a viable solution? A reporter with no prior scientific background delves into the topic, from quantum computing to the current hot topic.


Copenhagen interpretation
1. The state of a particle is determined by its wave function. The square of the wave function represents the probability density for the measured value.
2. All physical quantities are meaningful only when observable. The physical quantities possessed by physical objects are not objective values unrelated to observation, but rather values influenced by the observational process.
3. It is impossible to measure physical quantities that are related to each other simultaneously and accurately according to the uncertainty principle proposed by Heisenberg.
4. Particles such as electrons have complementary particle and wave properties.
5. Quantum jumps are possible. In quantum physics, allowed states can only possess discrete, specific physical quantities. Therefore, to change from one state to another, one must disappear from one state and simultaneously reappear in the other.


Quantum mechanics, the quintessence of scientists
As previously discussed, quantum mechanics was sparked by Max Planck's quantization hypothesis while solving the blackbody radiation problem. In the early 20th century, in physics, where it was thought there was nothing left to discover, quantum mechanics, a branch of classical mechanics, emerged. Quantum mechanics wasn't created by a single, brilliant scientist. Of course, the theory of relativity was also created by a single, brilliant scientist. The foundation of quantum mechanics was established through the overlapping research of numerous scientists. Even the research results of scientists who are negative about quantum mechanics.

The quantum mechanics framework established in the 1920s was successful in understanding, predicting, and applying microscopic phenomena. However, the mathematical results of quantum mechanics were far from the innate intuition of humans. Even eminent physicists like Albert Einstein found this difficult to accept. Consequently, various interpretations of quantum mechanics emerged to explain the mathematical results.

The Copenhagen interpretation refers to the interpretation centered around Niels Bohr, Werner Heisenberg, and Max Born. The Copenhagen interpretation is currently the mainstream interpretation of quantum mechanics. However, it is not absolute. Other interpretations exist, such as the many-worlds interpretation proposed by Hugh Everett in 1957. The many-worlds interpretation will be discussed in a future article.
Bohr became famous for his research on the quantum nature of atoms. In 1918, the Danish government approved the establishment of Bohr's Institute for Theoretical Physics. Bohr established the institute in Copenhagen. Physicists from around the world flocked to the Copenhagen Institute, where they stayed for several years to conduct research.

Copenhagen immediately became a center for quantum mechanics research, where a group of scientists led by Heisenberg later developed the Copenhagen interpretation.


The big picture, the principle of complementarity, and the principle of uncertainty
The Copenhagen interpretation is based on Bohr's complementarity principle and Heisenberg's uncertainty principle. First, let's look at what the two principles are.

The principle of complementarity states that although particles that make up atoms possess two completely different properties, such as wave and particle properties, both properties are necessary to completely describe phenomena related to the particles that make up atoms. Light exhibits wave properties in experiments such as interference and diffraction, and particle properties in experiments with the photoelectric effect. However, both properties do not appear simultaneously in a single experiment. It was confirmed that particles such as electrons and protons also have the same properties. Bohr summarized this duality of light and particles as the principle of complementarity.

The uncertainty principle states that a certain degree of uncertainty always exists between the observer and the observed. Rather than being a theory of a specific individual, it is a fundamental premise of quantum theory, and was formulated by Heisenberg based on the research of Bohr, Cramer, Slater, and others.

Bohr and Heisenberg began working together on quantum mechanics in Copenhagen around 1927. By studying the frequencies at which light is emitted under various conditions, they generalized the conditions for the quantization of photon energy, which had been previously postulated by Planck, Einstein, and Bohr himself.
The two men moved beyond the classical mechanics perspective that viewed physical objects as either particles or waves, and proposed that particles could be both waves and particles. Bohr's new theories were based on numerous experiments of the time and the observation that matter exhibited duality between waves and particles. And then Heisenberg announced the uncertainty principle, which states that both the position and momentum of a particle cannot be measured accurately.

The Copenhagen interpretation of quantum mechanics, pioneered by Bohr and Heisenberg, concludes that human "observation" of an event "changes the reality" of that event. The core of the Copenhagen interpretation is that it is unnecessary to assume that the value of a physical quantity exists prior to the act of measurement. Conversely, in classical mechanics, physical quantities expressed in formulas exist independently of human measurement. In other words, according to the Copenhagen interpretation, quantum mechanics requires consideration of both the observer and the object, not just one.

Quantum mechanics provoked resistance from many physicists. At the Fifth and Sixth Solvay Conferences held in Brussels in 1927 and 1930, Bohr presented his interpretation of quantum mechanics, based on the principle of complementarity, to the physicists of the time. This sparked a debate between Bohr and Einstein, which solidified the foundations of quantum mechanics.


Building a New Foundation with Philosophy
Bohr and Heisenberg were dedicated to laying the philosophical foundation for expanding our intuition about quantum mechanics.

According to Heisenberg's uncertainty principle, we must use a microscope with a short wavelength to accurately determine the position of an electron. However, due to the Compton effect, we inevitably obtain an inaccurate value for the electron's momentum. In other words, position and momentum are in an uncertain relationship within a very small range.

Meanwhile, Bohr, who had been preoccupied with the philosophical foundations of quantum mechanics since losing to Einstein in the debate over the existence of light quanta, also reached a similar view. We must always describe the microscopic world of atomic phenomena based on the terms and concepts derived from the macroscopic world. Therefore, our terminology for describing the microscopic world is limited. A term that is consistent is constrained by the complementary relationship between definability and observability.
For example, when microscopic phenomena, such as the interaction between light and matter, are defined by observational propositions of macroscopic systems, such as particles or waves, certain limitations are imposed. This principle of complementarity enabled Bohr to escape the wave-particle duality dilemma.


Einstein was not satisfied at all
Einstein could not accept the indeterministic nature of quantum mechanics, as represented by the two principles of Bohr and Heisenberg. Einstein was not alone in his criticism of the indeterminism of quantum mechanics. Max von Laue, Erwin Schrödinger, and Planck were also critical of the Copenhagen interpretation. Einstein, as the most renowned and widely known scientist among them, particularly emphasized his position. And Einstein continued to argue with Bohr endlessly, not believing in quantum mechanics until his death.

In 1935, Einstein, along with Boris Podolsky and Nathan Rosen, made a sharp critique of quantum mechanics.
The three, who are abbreviated to EPR by taking only the first letters of their surnames, assumed that a theory is complete if it satisfies the following conditions: "Every element of physical reality must have a counterpart within a physical theory. If, without disturbing a system in any way, the value of a physical quantity can be predicted accurately, that is, with a probability equal to 1, then there exists a physical reality corresponding to this physical quantity." After examining quantum mechanical descriptions based on this criterion of completeness, EPR concluded that the quantum mechanical description of physical reality given by the wave function is not complete.

This is the EPR paradox, which states that when there are two quantum entangled particles, there must be some real element that quantum mechanics cannot explain, as long as we assume the criterion of reality proposed by EPR and the special theory of relativity.

In the 1950s, David Bohm proposed the deterministic hidden variable theory, which attempted to revive Einstein's causal position. In 1964, John Bell proposed the so-called 'Bell's inequality', which can be used to verify quantum mechanical problems through experiments.

Let's assume EPR's locality assumption—that is, that a measurement of a physical quantity on one side always produces a reliably opposite value for the same quantity on another side. Considering these three physical quantities, we showed that the common-sense inequality that simply states that physical quantities are correlated does not hold in quantum mechanical calculations. The generalization of this is Bell's inequality.

Since the 1970s, several experiments have been conducted to verify this Bell's inequality. The result? Bohr's victory. All quantum mechanical experiments failed to satisfy Bell's inequality.


The cat may be alive or dead.
In 1935, Schrödinger also strongly criticized the Copenhagen interpretation, which posits that an observer's act of measurement influences the object. The thought experiment he proposed at the time would later become known as "Schrödinger's Cat." Schrödinger was deeply displeased with the Copenhagen interpretation, which relies on the uncertainty principle. Therefore, to expose the irrationality of the Copenhagen interpretation, he extended events in the microscopic world to those in the macroscopic world.
A cat is trapped in a box. The box is connected to a machine containing a radioactive nucleus and a canister of poison gas. At the start of the experiment, the probability of the nucleus decaying within one hour is adjusted to 50%. If the nucleus collapses, poison gas will be released and the cat will die.”

In this situation, Schrödinger criticized the expression of the wave function as a combination of the cat's alive and dead states, saying that a "cat that is both dead and alive" does not actually exist.

"A cat must be either alive or dead. Since a cat cannot be both alive and dead, quantum mechanics is incomplete and unrealistic."

The Copenhagen interpretation concludes that a cat is a superposition of being alive and dead, and that its being alive or dead is determined by the observation.

Ironically, Schrödinger's cat has become a classic quantum mechanics experiment, a metaphor so catchy that everyone except cat owners likes to mention it.

We've explored how quantum mechanics came to be through the Copenhagen interpretation and its criticisms. In the next article, we'll explore the many-worlds interpretation.