All the information we have about a physical system is described by a mathematical object that we call a wave function: ?(x,t)
We can only know certain properties of a system. We can only get information from what we actually measure. It is not permissible to ask questions that do not correspond to an observation or measurement.
Randomness exists. The result of a measurement is random. We only have access to knowledge through probabilities. When making a measure- ment, the wave function ? collapses to the measured state
Between two measurements, the wave function ? changes in a way dictated by the Schrödinger equation:
The 2 fundamental principles of quantum physics are quantum entanglement and superposition
This is how you can solve the schrodinger's equation (simplified case of a two dimensional box)
A blackbody (which is by the way not black but glowing) would only absorb and emit but not reflect light. A blackbody is a theoritical object. Even tough, stars come pretty close to being backbodies, they are only theoretical.
Rayleigh tought that blackbody was made of vibrating particles which constantly emit light. They could then predict the color of the blackbody depending on its temperature. However, they quickly ran into an issue. Since the energy the blackbody absorbs heats it up, the frequency of the light (and emission spectrum) would increase too. This being unbounded, that would imply an infinite ultraviolet emission and an infinite heat capacity.
This a problem today known has the ultraviolet catastrophe.
Another scientist Max Planck was also studying radiation in a different context; he was trying to explain why heat always flows from a hot object to a cold object.
He unintentionally solved the ultraviolet catastrophe by considering that energy comes in discrete quantities (everything his quantized). The Energy (E) is equal to the factor of the Planck constant (h) and the frequency (v). The value of h is very accurately known as 6.626*E-34 (m²*kg)/s. I like to thing of this quantization of matter as instead of having a line to represent the energy level of an atom, we have a stair-like curve (the atom is either in a state or another by not in between) just as shown in fig1.
The double slit experiment
The double slit experiment performed by Thomas Young at the end of the 19th century consisted of a laser beam going through 2 slits at the same time, the results of the experiment being observed on a screen behind the slits.
The interference pattern is proof that the light behaves as a wave.
The wave-particle duality
light as a wave
This model of the light explains why the border of the shadows are a bit fuzzy and the colors of the rainbow. However,according to this theory, the higher the intensity is, the more electrons should be knocked down. It explains also part of the double slit experiment.
light as a particle
The photoelectric effect
The photoelectric effect
This conversation on the nature of light and other wavicle has lead to the most well-known fight in the history of modern physics between Bohr and Einstein:
The Bohr-Einstein controversy
This is a famous debate between Einstein and Bohr on quantum mechanics but also on the philosophy of science. During this debate Einstein is said to have asked Bohr: "God does not play dice", to which Bohr would have replied: "But who are you, Albert Einstein, to tell God what to do?"
This raised a more general question on the true meaning of science to what Einstein replied that "What we call science has the sole purpose of determining what is."
Heisenberg's uncertainty principle
One of the most fundamental of quantum mechanics was stated around 1927 by the german physicist Werner Heisenberg.
The uncertainty principle states that : "the more precisely the position of some particle is determined, the less precisely its momentum can be predicted from initial conditions, and vice versa."
In his book A Brief History of Time, Stephen Hawking explains the uncertainty principle. In order to measure the position and velocity of a particle, it must be illuminated with short-wavelength light so that the distance between two ridges is as small as possible and the position of the particle is as accurate as possible. However, according to Planck's quantum hypothesis, in order to use a small amount of light a quantum must be used. This quantum will disrupt the speed of the particle. Thus, the more we want to measure its position precisely the smaller the wavelength of light must be, the greater the energy released by the quantum and the less precisely the speed of the particle will be measured. Thus Heisenberg showed that uncertainty(particle position) x uncertainty(particle velocity) x mass(particle) > Planck's constant
This principle is still controversial today, especially among philosophers, and we do not yet measure all the effects of this uncertainty. The Universe is therefore not entirely predictable since we cannot measure our present universe with excatety. This theory thus participated in the creation of quantum mechanics since in 1920, Schrödinger, Dirac, and Heisenberg defined the state of particles as a combination (or probability) of their position and velocity and no longer the precise measurement of its two quantities.
All of those theories combined under the name of quantum mechanics lead counter-intuitive properties of matter thta we do not experience at our scale:
matter seems to interact with itself. We can therefor explain the double split experiement (and the interference pattern) with electrons or light
Eventually the whole infinitly small scale models or theories had to the improved considering the new implications of quantum mechanics. For example, Bohr's model of the atom did not provide a general scheme of why systems are quantized, nor explained the wave-particle duality and was limited in predicting the emission and absorption in atoms (and the energy levels of the atom)
Quantum computing: what impacts on our daily lives?
Since the 2000s, researchers around the world have been trying to develop the quantum computer which is based on important aspects of quantum mechanics such as superposition and entanglement. A classical computer program is a binary system (i.e. bits that can take the value of 0 or 1), whereas qubits (quantum and bits) would be the component of the quantum computer programs. Qubits can have multiple values simultaneously (for 2 qubits, it is a superposition of 00, 01, 10, 11). Instead of performing one calculation at a time, the quantum computer processes all the hypotheses at the same time. This speed is what makes quantum computing a very promising field in many different areas (not only in science but also in business…). When a classical computer would have taken ten million years to find the solution, a quantum computer will perform the calculation in just a few hours.
While it seems unrealistic to develop quantum computers as a personal tool in the foreseeable future for financial reasons, because of their fragility (to manipulate qubits, the temperature must be -273 ° C), but also because they would be useless when performing our daily tasks, many other applications are possible. This quantum technology could, for example, allow the optimization of logistics, weather forecasting (to build better climate models by studying more parameters) and plot the evolution of financial market. All of which would require calculations unachievable with conventional technologies. According to Bernard Ourghanglian, quantum computing is likely going to improve the AI applications as we know them today. Indeed, the results obtained by a team of physicists in California using quantum technologies regarding satellite image recognition are promising and better than with conventional technologies. In the longer term, the quantum computer could revolutionize our modes of communication by establishing the quantum internet.
However, our current data encryption technique, which is based on the "RSA" technique named after its three founders (R. Rivest, A. Shamir, L. Adleman) and has been effective for more than 40 years, could be inefficient against quantum computing. Indeed the mode of encryption, as we know it today, is asymmetric meaning that the public encryption key consists of the product of two huge prime numbers and is different from the private decryption key which consists of these two numbers, unknown to the rest of the world (if you want to know more of this subject, I strongly recommend the video on quantum computing by veritasium or the course on cryptocurrency by Brilliant). Public keys as well as encrypted messages could be intercepted. Using the public key, the quantum computer would perform the necessary computation to find the two prime numbers, the data could be decrypted easily therefor accessible by huge corporation like google or Microsoft who have invested in such promising technology. Today, some nations or independent organizations store encrypted data (such as passwords, search results, or other secret government information) in order to decrypt it in a decade or so during a procedure called "Store Now Decrypt Later". This is why NIST in the United States is working on the development of a post-quantum cryptography norm that could be integrated into the HTTPS protocol.
The main powers have already invested in this quantum race such as China with "its colossal investments", the GAFA which invest in "research centers that cost them tens of millions per year" as well as the European Union, not to mention the many renowned physicists renoun for their discovery in this field such as the French Alain Aspect, the American John F. Clauser and the Austrian Anton Zeilinger awarded the Nobel Prize in Physics for their discoveries on quantum entanglement.
However, this quantum race, which will revolutionize the internet of tomorrow, is far from over. Today, most quantum computers contain between twenty and a thousand qubits at most which are not necessarily all operational. Once the superposition of all possible results has been obtained, it must be possible to extract only the desired information; which for the moment is only feasible in some very specific cases, using for example the Fourier algorithm. Lately Microsoft announced a “ground-breaking discovery”, on the storage of qubits which would be step in the right direction towards the “quantum super-computer” (you can read this article for more information: “Microsoft achieves first milestone towards a quantum supercomputer”)
