The World of Quantum

Welcome to the fascinating world of quanta! On this page you will delve into the fundamentals and applications of quantum technology, a field that challenges the limits of our understanding of matter, energy and information. Learn more about the principles of quantum physics, its revolutionary applications in areas such as communication, computing and sensor technology, and discover how quantum technology can shape the future of our world.

What are quanta?

Quantum physics has been popular at least since films like Oppenheimer. But what are quanta actually? What sounds like an abstract phenomenon actually has a huge impact on the world we live in! The following explanatory videos will give you an insight into the exciting world of quanta and show you how quantum technologies work.

Harald Lesch explains quantum physics

Quantum 1x1 by TüftelLab

Glossary

Diamond

Diamond is the hardest naturally occurring material. It consists of 5 carbon atoms that join together in regular structures to form tetrahedron-shaped crystal lattices. A diamond in its pure form is transparent and colourless. However, impurities and defects in the crystal lattice can cause the diamond to appear in a certain colour. Through targeted doping with nitrogen vacancies (NV), as required for our quantum magnetometer, for example, the crystal takes on a pink to purple colour. Diamonds occur naturally in our earth's mantle due to high pressures and high temperatures. However, these diamonds are only available in limited quantities, making them very expensive and the impurities cannot be controlled. Fortunately, diamonds can also be produced synthetically. There are three manufacturing processes for this:

• Detonation diamonds o Here graphite is literally made to explode. This produces nanodiamonds that can be used as seed diamonds for HPHT diamonds.
• CVD - chemical vapour deposition o This process requires a starting substrate, which is introduced into the CVD chamber. The CVD chamber contains a gas (e.g. methane) in which carbon is bound. Energy input causes a plasma to form in the chamber, i.e. ions and electrons of the individual components of the gas mixture are present. The carbon contained in the plasma is then gradually deposited on the substrate. The longer the process runs, the thicker the substrate becomes. The starting substrate is extremely important for forming the crystal structure.
• HPHT - high pressure, high temperature o This process imitates the natural formation of diamonds under high pressures and high temperatures. A seed diamond (the detonation diamond) is placed in an environment with carbon (graphite), which is deposited on the diamond seed due to the ambient conditions and allows the diamond to continue to grow. It is the most widely used method for producing diamonds at low cost.

Now all that remains to be clarified is how the nitrogen atoms and imperfections get into the diamond. In the CVD process, nitrogen can be added during the manufacturing process and incorporated into the entire crystal lattice. The diamonds are then irradiated to create defects in the crystal lattice. The diamonds are then cured at high temperatures (> 700 °C), allowing these defects to move in the crystal. As the nitrogen atoms in the crystal lattice generate stresses, the probability of a defect in the crystal lattice settling next to a nitrogen atom increases. If no nitrogen is added during the CVD process, pure diamonds are created. The nitrogen defects can then be subsequently introduced by irradiation, e.g. only in the immediate surface. The same also applies to HPHT diamonds.

If you would like to delve deeper into the subject of diamonds and their properties, it is best to visit the Element Six Diamond Handbook.

Energy Levels of NV

There is a triplet ground state (ground state ³A2), an excited triplet state (excited state, ³E), and a metastable singlet state (singlet state 1A1 and 1E). In the ground state, there are three spin states of the electron pairs, ms=0 and the ms=±1 states. The ms=±1 is also described as a degenerate state because the ms=0 state is the more likely occurring state. If an electron pair is in the ms=0 ground state and is excited by green light, it is raised to the excited ms=0 state. When returning to the ground state, it most likely follows a path where red light is emitted in the form of a photon plus lattice vibration (phonon). With lower probability, it takes the path through the singlet state, where only phonons are emitted, and lands in the ms=±1 state. If an electron pair is in the ms=±1 ground state and is excited by green light, it is raised to the excited ms=±1 state. When returning to the ground state, it most likely takes the path through the singlet state, emitting invisible light as well as phonons, and lands in the ms=0 state. With lower probability, it directly falls into the ms=±1 ground state, emitting a red photon.

Ensemble

A collection of nitrogen-vacancy centers in a diamond, so close together that they interact with each other. They are simpler to manufacture and are cost-effective as microdiamonds.

Fluorescence

Light emission from the nitrogen-vacancy center in red through optical excitation with ultraviolet, blue, or green light. Here, stimulation raises an electron to a higher energy level, meaning it is promoted to a higher shell, and when it falls back to its original energy level, energy is emitted in the form of a photon.

Color Center

There are various color centers in crystal lattices; the most important for us is the NV center. This involves foreign atoms trapped in a crystal lattice, which cause absorption of specific wavelengths of light, resulting in a specific color of the crystal.

NV Center

Short for Nitrogen Vacancy. The crystal lattice of a diamond consists of carbon. Nitrogen is embedded in this lattice, leaving a vacancy. An additional electron from the crystal lattice can become trapped at this nitrogen-vacancy center, creating an NV- center. This can be used as a quantum system at room temperature. It absorbs green light and shorter wavelengths, like blue and ultraviolet light.

ODMR

Optically Detected Magnetic Resonance (ODMR) is a measurement technique used to optically read the energy levels of the spin states of an electron pair. The energy levels of the ms=±1 spin states of an electron pair change due to the presence of a magnetic field, electric field, temperature, or pressure, known as Zeeman splitting. For ODMR, optical excitation with green light and a microwave is required. Microwave excitation can increase the likelihood that electron pairs are in the ms=±1 state, while optical excitation with green light increases the likelihood that electron pairs are in the ms=0 state, allowing the system to be initialized. Initialization here means that the initial state is known with high probability. Microwave frequency is then varied, and the corresponding fluorescence intensity is measured. This experiment is repeated multiple times, and the results are averaged. This way, a fluorescence spectrum with dips (decreases in intensity) corresponding to the energies of the ms=±1 states is obtained. The distance between the dips correlates with the strength of a magnetic field, and their absolute position, relative to the zero-phonon line (ZPL), corresponds to a temperature.

Quenching

Measurement of fluorescence intensity that depends on the magnetic field strength through optical excitation with green light and optical readout.

Single NV

A single nitrogen-vacancy center in a diamond, or multiple centers with a sufficiently large distance between them, so they do not interact with each other.

Zeeman Splitting

The splitting of the fluorescence dip in the fluorescence spectrum due to the presence of a magnetic field. At frequencies below 2.87 GHz, the ms=-1 dip can be observed, and at frequencies above 2.87 GHz, the ms=+1 dip can be observed. In the energy-level diagram, the energy levels move further apart as a magnetic field is applied.

ZFS

Zero Field Splitting (2.87 GHz) is the energy difference between ms=0 and ms=±1 at a temperature of around 27°C and no external magnetic field. The frequency of ZFS changes with temperature, making it useful for temperature measurement.