*Quantum Mechanics* is a physical theory that describes Nature at the length scale of [[Atom|atoms]] and molecules. For the longest time it was mostly relevant as a physical theory. Today, it forms the basis of all [[Quantum Technologies|quantum technologies]]. We invite you to join us for a short journey and visit the four most important aspects of *quantum mechanics*. ![[quantum_mechanics.excalidraw.light.svg]] Let us start our journey with the ingredient that puts the “quantum” into “quantum mechanics”. At the beginning of the 20th century, scientists like Max Planck and Albert Einstein noticed that certain observations can only be explained if one assumes that energy and other physical quantities sometimes occur in small, indivisible packets, the so-called [[Discreteness|quanta]]. This is completely different from what we know from our everyday experience: When we heat a pot of water on a stove, the water continously heats up. The thermometer does not jump from one temperature to another. In an old mercury thermometer, for example, the mercury column moves gradually upwards. It does not suddenly jump from 10°C to 20°C.  In quantum mechanics, however, there are quantities that can only take on certain, [[Discreteness|discrete values]]. We actually observe jumps from one value to the next, without a value being able to occur in between. This is where the word quantum leap originally comes from. But that's not all. Quantum mechanics allows [[Superposition|superpositions]]: a particle can be in several places at the same time, with a certain probability. Quantum mechanics introduces a new connection in a way: classical options are usually related by an “or”: The sky is blue or green. It cannot be both at the same time. In quantum mechanics, these options can be connected with an “and”. In the commonly used analogy of [[Schrödinger’s Cat]], the cat is both dead and alive. Only when we [[Measurement|observe]] the system, will we recover exactly one of the options. More fundamentally, particles like [[Electron|electrons]] have a degree of freedom called [[Spin|spin]]. [[Spin]] is an example for a [[Discreteness|discrete quantity]] and we can imagine it as an arrow. This arrow can either point up or down. Wait, actually it can point up and down if the electron’s spin state is in a superposition. In [[quantum technologies]], spin is commonly used as a [[qubit]], the quantum analogue of a [[bit]] which is used in [[Classical Information|classical computers]]. A more accurate the representation of spin is the [[Bloch Sphere|Bloch sphere]]. The third effect is the [[Heisenberg Uncertainty Principle|Heisenberg uncertainty principle]]. It states that certain quantities such as position and speed cannot be [[Measurement|measured]] simultaneously with any degree of precision. Wouldn't that be a great excuse for the next speed check when driving too fast? Since the police officer knew your position, he could not have measured your velocity precisely. Unfortunately, this excuse will not get us very far in everyday situations. The Heisenberg uncertainty principle is only noticeable at the scale of atoms and molecules. All of the phenomena described so far occur with individual particles. The fourth and final effect makes quantum physics truly special. This effect is called [[Entanglement|entanglement]] and can only occur when at least two particles come together. When two particles are entangled, we can no longer describe them independently and changes to one particle also affect the other. This correlation is stronger than any correlation that we know from our everyday experience. It represents one of the fundamental resources of [[Quantum Technologies|quantum technology]]. >[!read]- Further Reading >- [[Discreteness]] >- [[Superposition]] >- [[Heisenberg Uncertainty Principle]] >- [[Entanglement]] >- [[Interpretations of Quantum Mechanics]] >- [[Wave Function]] >- [[Quantum State]] >[!ref]- References