Quantum Physics

An ok layman’s introduction to QM.

Basics

• Quantum systems are fundamentally different from classical ones; the underlying logic is not the same
• Classical waves are determined by frequency and wavelength
• Waves can be standing or travelling
• Standing waves have quantified values for allowed wavelength
• Pauli principle: no two quantum particles in a system can occupy the same state

Energy

• Molecules have a compound ground state lower than that of the separate atoms (therfore it’s stability…)
• Fusion is the release of this difference in energy between the ground states
• Fission is the use of the byproducts of the splitting of atoms into smaller fragments to extract energy
• Natural Uranium only contains about 1% $U^{235}$
• Reactor $U^{235}$ is typically enriched to 20% concentration
• Weaponised $U^{235}$ typically is of 90% concentration

Electricity

• Quantum waves in solids span the entire sample
• Current involves a net difference in the flow of these “global” electrons
• A current can flow given that there are available states around the Fermi energy
• A material’s ion lattice create a gap in the state band structure
• Given available states below the gap a material can conduct electrons

Semiconductors

• A semiconductor does like an insulator have all its states below the gap filled
• The gap in semiconductors are however in comparable size to typical thermal energies allowing some to be excited across the gap
• The excited electrons and the leftovver states below the gap; positive holes can conduct electricity
• n-type conductors have an excess of electrons
• p-type conductors have an excess of holes

Superconductors

• Cooper pairs can form by pairs of electrons by their interaction with the ion lattice causing a net attraction
• These cause superconductivity
• High-temp superconductors work at about 100K

Quantum computing

• A qubit is a (quantum) system that can exist in two states or a superposition thereof
• Qubits can ensure secure key exchange for cryptographical applications. Eavesdropping modifies the quantum system irreversibly

In total

• Copenhagen view of QM posits that hidden variables does not exist
• Subjectivie theories postulates wavefunction collapse only when observed by “a concious agent” (human mind)
• Hidden variable theores postulate the existence of some unobservable properties.
• Because of Bell’s theorem, hidden variable theories can only be non-local (spooky action at a distance) breaking with relativity
• Many-worlds theories possits that the world branches of at every point where a collapse is made.