How do you explain quantum physics?

 There are three simple, but important aspects.

1. What are the odds that every star in the universe (that has a planet) has that planet in the same exact orbit.

Now what about the “orbits” of electrons around a nucleus of an atom.

If I burn a Copper salt in a flame every atom will glow green.

If I burn Strontium salt in a flame every atom will glow red.

The colour is an indication of the energy of the electron’s energy jump.

How can every atom have a specific energy jump?

And how can that energy jump be identical for every atom of that type?

Quantum mechanics explains all that.

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2. The double slit experiment works with light.

It also works with electrons and protons and every other thing that we thought were “particles.”

Quantum mechanics explains all that.

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Lastly,

3. How can particles properties be correlated even at very large distances?

The animation on the left is entangled, on the right classical correlation.

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(This one may not be simple. Time for homework...)

File:Quantum entanglement vs classical correlation video.gif - Wikimedia Commons

From Wikimedia Commons, the free media repository No higher resolution available. Note: Due to technical limitations , thumbnails of high resolution GIF images such as this one will not be animated. The limit on Wikimedia Commons is width × height × number of frames ≤ 100 million.

https://commons.wikimedia.org/wiki/File:Quantum_entanglement_vs_classical_correlation_video.gif

This video demonstrates the difference between entangled and classically correlated quantum states when the polarization of photons is considered. In the scene on the left, the source produces photon pairs in a singlet state, which is maximally entangled. In the scene on the right, the photon pairs are created in a dephased singlet state, which is mixed and only classically correlated. In both scenes, there is a source of photon pairs in the center. One photon of each pair propagates to the detection station on the left and its partner photon propagates to the detection station on the right. Each detection station consists of a polarizing beam splitter and two detection screens. The detection stations can measure the polarization of incoming photons in different linearly-polarized bases. The video comprises three parts. In the first part, the photons are measured in the H/V basis. Both entangled and classically correlated states give rise to the same measurement results (up to random fluctuations that are intrinsic to the quantum measurements). In the second part, the measurements are performed in different bases, where the difference between the two states becomes apparent. In the third part, only the probabilities of photon detections are plotted and the detection stations are rotated smoothly over the entire range of linear polarizations. Even though the probabilities for the classically correlated state vary as the rotation angle increases, the probabilities for the entangled singlet state remain constant.

Why do we study quantum mechanics if nobody understands it?

The underlying equations are fairly simple equations on wave functions. They describe linear wave equations that evolve the wave functions over time. The dynamics is both deterministic and time reversible (with the appropriate CPT symmetry).

Maybe you are worried about it using complex numbers. But classical waves like ocean waves can be described with complex numbers, e.g. using the velocity as the Imaginary component.

Maybe you are worried about it using spinors. But classical waves like light polarisation can also be modelled with spinors.

Maybe you are concerned about objects that return to the same configuration after 720${}^o$. But classical anti-twister mechanisms do this too:

Maybe you find it weird that everything is discrete. But it isn’t really, discreteness only arises from bounded states, like electrons bounded to a nucleus. Classical physics has this too, in standing waves.

Maybe you are worried about the uncertainty principle. But this isn’t a case of inaccessible information, it is a case of using inappropriate variables (position and momentum) to describe something that isn’t a point but a wave.

Maybe you are worried about entanglement, action at a distance and Bell’s inequalities. But these are issues for theories that have realism and separability. Several interpretations (including Everettian/Many Worlds) do not have separability, so nothing is non-local.

Maybe you are worried about spins being either up or down but nothing in between. But this is not quite right, you can get any superposition in a continuous range, and the spin-up and spin-down represent two different types of wave. You get the equivalent in classical physics with light polarisation.

It is in fact the simplicity (linearity) of quantum physics that leads to the superposition and the many worlds effects that cause headaches for physicists that try to insist on there only ever being a single branch of the universal wave function.

Why is it difficult to understand quantum computing?

 

It's not difficult if you know a little bit of math, physics and electronics and can solve the Schrödinger equation that describes quantum systems, at least for a few simple systems, to understand the foundation, strength and limits of these systems. The Schrödinger equation in quantum systems is similar to Maxwell's equations for electrical systems or, more applied, similar to Ohm's and Kirchhoff's laws for digital systems, without which the essence of digital computers cannot be understood. The apparent difficulty of quantum computing comes from excessive marketing, and by excessive and unrealistic (unphysical) mathematical extrapolation of quantum systems, without building a real device to confirm or disprove those extrapolations. Without experimental confirmation, any description, even if is mathematical or algorithmic, is not scientific and implicitly is not technologically feasible. There are currently many mathematical and algorithmic extrapolations assumed to be related to quantum computing, but which in the absence of experimental confirmation represent only extrapolations/utopias or even technological noise, noise that contributes to the apparent difficulty to understand quantum computing for a beginner.

Essentially, after someone plays a little bit with basic quantum systems and solves the Schrödinger equation related to them and experiments a little bit with real physical systems confirming the theoretical solutions, he finds out that systems that have at least two quantum states can be used as units of quantum information, i.e. qubits (quantum bits): which can be: an electron's spin pointing up for 1 noted ∣1⟩ or down for 0 noted ∣0⟩ or vice versa, a photon's polarization, a microscopic magnetic state, etc. The quantum system thus identified with the two states ∣0⟩ and ∣1⟩ can be described in general through a state vector ∣ ⟩ which represents the quantum superposition of the two states, mathematically described through ∣ ⟩ = α∣0⟩ + β∣1⟩ which represents an informational "and" of the two states.

Quantum information processing means any transformation performed on quantum information unit (qubit) ∣ ⟩, through various quantum transformations (quantum gates) such as: as spin rotations, photon polarization changes, energy level transitions, phase shifts, etc. These transformations essentially modify at the single bit level the values ​​of α and β which establish the probabilistic weight of the basic states ∣0⟩ and ∣1⟩ between which there is the normative constraint ∣𝛼∣² + ∣𝛽∣² = 1.

To extract information from a quantum system, in this case from a processed qubit, we must apply a macroscopic measuring device that operates according to digital logic which describes digital (classical) bits b in which the states 0 and 1 are not superposed but can be exclusively either 0 or 1.

The measurement process (i.e. extracting information from a quantum system and using it at the macroscopic level, representing the transformation of ∣ ⟩ into b) involves probabilistic collapsing of the quantum system ∣ ⟩ into one of its basic states ∣0⟩ or ∣1⟩ (which now become real digital/macroscopic bits 0 or 1 ) with the respective probabilities ∣𝛼∣² or ∣𝛽∣² due to the macroscopic interaction with the quantum system and represents the greatest limitation/challenge of quantum computers.