# What is quantum physics?

In Latin the word ‘quantum? stands for the question ‘How much?’. In our everyday life it signifies a small portion. Originally, quantum physics got its name because of the small amounts of energy that are exchanged in the interaction of light (photons) and atoms.  Since its beginnings in year 1900, quantum physics has developed into a comprehensive theory which has been confirmed in numerous experiments and with highest precision.

Modern quantum physics comprises many phenomena which surprise our common sense since we do not observe them directly in our daily lives:

Quantum superpositions
the coexistence of states that would be regarded as mutually exclusive in our macroscopic world. This includes wave functions that describe a particle as ‘being’ in several positions or flying in several directions, a magnet vector pointing in different directions, or electrons populating different energies ‘at once’.
Quantum jumps
The discontinuous, discrete nature of energies and state transitions in confined systems, such as electrons in atom, molecules or quantum dots.
Quantum spin
An angular momentum even of point like particles which would not exist in a mechanistic world view.
Quantum tunneling
the penetration of energetically forbidden barriers
Quantum indistinguishability and quantum statistics
if two objects agree in all their quantum numbers, they are fundamentally indistinguishable and lose their identity. This leads to important consequences in their statistical behavior, which depends also on their spin. It even rules the material stability of our world (Pauli principle).
Quantum randomness
the choice of measurement outcomes – without any reason, but within statistical laws that are determined by quantum equations.
Quantum entanglement
the inseparable correlation of properties in objects – even when they are separated to a distance that they can no longer communicate by any known means.

This learning environment focuses on the first aspect: the matter-wave nature of massive particles, here in particular large and complex molecules.

## The birth of matter waves

In 1923 Louis de Broglie formulated the idea that all material particles should also be described by a wave-particle duality, similar to quanta of light. His hypothesis was based on the energy-mass equivalence of Einstein‘s theory of special relativity ($$E=mc^2$$) as well as on the relation between energy and frequency that had been found to hold for photons in quantum physics ($$E=h \nu$$). These two equations can be combined to predict that every material object is associated with a wave-like phenomenon, whose wavelength $$\lambda_dB=h/mv$$ is defined by Planck’s quantum of action h, the particle’s mass $$m$$ and its velocity $$v$$.

Even electrons which had previously been considered as point-like particles with a well-defined mass, location and momentum should thus be assigned a delocalized wave function, which we interpret as a probability amplitude today. Diffraction and interference of electrons was soon observed by Clinton Davisson and Lester Germer at Bell Labs as well as by George Paget Thomson and A. R. Reid at the University of Aberdeen.

De Broglie’s idea inspired Erwin Schrödinger 1926 to introduce what has become known as the Schrödinger equation, the key formula of quantum wave mechanics. It has become a corner stone of modern quantum science and the basis for a highly precise theory of nature.

Meanwhile the physics of matter waves has found its way into many modern technologies, including electron microscopy, superconducting interference devices (SQUIDS) with applications as ultra-sensitive magnetic field sensors, neutron scattering in the materials sciences or atom interferometers as very precise sensors for gravitational and rotational acceleration.

Our platform offers an interactive approach to ongoing matter wave experiments with complex molecules at the University of Vienna. They are driven by two motivations:

1. Can we understand why we seem to observe different phenomena in the micro world and in the macro world? What governs the transition? Why can electrons and large molecules be delocalized but not you and me?
2. If we understand how to scale quantum phenomena to more complex bodies: can we use that in advanced technologies? For refined measurements of forces and particle properties?