Category: Quantum Physics FAQs

It’s designing a system where quantum states constructively interfere to produce the right answer. SMBC’s “The Talk” is an astonishingly good introduction. I heard that quantum computers try all the possible answers at the same time. That’s only part of how quantum algorithms work. You can certainly put a quantum computer into a uniform superposition of inputs…

As humans, we never perceive superpositions of matter waves. There are lots of different ideas about why that should be. One of the oldest, called “the Copenhagen interpretation” after a conference where lots of famous physicists met to talk about quantum physics, is that somehow when we measure a quantum system, the wave function undergoes…

A wave function is a function from classical configurations to complex numbers. You can think of it as an infinite list of complex numbers, where the index into the list is given by the configuration. The Schrödinger equation describes a single spinless particle, where a configuration is an element of ℝ³, a set of coordinates…

An observer is any quantum system separate from the system being observed that becomes entangled with it during the measurement process. An observer can be as small or as large as you like, from an electron to a human, to a galactic cluster. See this comment for an analysis of the double slit experiment with a single…

At a very basic level, we could say that quantum physics is used to describe the microworld, while the macroworld is governed by classical physics. However, many macroscopic objects can be described with quantum physics – you just have to know where to look. 

In order to make a measurement, we need a quantum system X to be measured and a quantum system Y (“the observer”) to serve as the record of the measurement. The measurement itself is any physical process that makes the state of Y depend on X. If the state of X is not an eigenstate…

Spin is a kind of angular momentum that fundamental particles have. It doesn’t have a classical analogue. It is an intrinsic property of elementary particles on one hand, and a quantized observable which behaves like the angular momentum from classical mechanics on the other. Similarly to how mass is the energy associated to some particles just by their existence, spin…

Quantum mechanics is a branch of physics that deals with the behaviour of matter and light on the atomic and subatomic scales. Classical Physics could not explain the dual behaviour of matter and Heisenberg’s uncertainty principle. Quantum physics was successful in explaining the dual behaviour of matter.