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.

Quantum Sensing for Dummies

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 (³A), an excited triplet state (³E) and a metastable singlet state (1A and 1E). A triplet state is characterised by the fact that, in the absence of external disturbance, three states (as indicated by the small 3 above A and E) are energetically indistinguishable. In the NV centre, these are three spin states of the electron pairs: ms=0 and the states ms=±1. The distinction between ms=±1 refers to the orientation of the spin relative to the NV axis. If an electron pair is in the ground state (ms=0) and is excited by green light, it is initially raised to the excited state (ms=0). Part of the energy of the electron pair is released in the form of lattice vibrations (phonons) until the system reaches the excited state (ms=0). When it returns to the ground state, it is highly likely to follow the path in which red light (637 nm) is emitted in the form of a photon. With a lower probability, it follows the path via the singlet state, in which no photon of wavelength 637 nm is emitted, and ends up in the ms=±1 state.

Ensemble

This refers to a large number of nitrogen vacancies within a diamond. By measuring the signal from many nitrogen vacancies, the extremely high spatial resolution of a single NV centre is lost, but the intensity of the fluorescent light increases. Such diamonds are easier to produce and are available at a lower cost as microdiamonds.

Fluorescence

Light emission at a lower energy level than that of the excitation; here, the nitrogen vacancy (shown in red) is excited optically by ultraviolet, blue or green light. In this process, the absorption of a photon (frequency f_a) raises an electron or electron pair to a higher energy level – that is, to a higher orbital, as in the atomic model. The electron or electron pair then releases energy in various ways (non-radiative transitions) and thus ends up in an excited but slightly lower-energy state. Upon returning to the original energy level, energy is emitted in the form of a photon (frequency f_e < f_a).

Color Center

There are various colour centres in crystal lattices. These are irregularities in the crystal lattice that are optically active. They may, for example, be trapped impurity atoms, vacancies or fractures in the lattice structure.

NV Center

The crystal lattice of a diamond consists primarily of carbon. A nitrogen vacancy is defined as a situation where a carbon atom is replaced by nitrogen and there is a vacancy in the immediate vicinity of the lattice. At this nitrogen vacancy, an extra electron is typically bound from the crystal lattice, creating an NV centre that can be used as a quantum system at room temperature. This absorbs green light, or light of shorter wavelengths such as blue and ultraviolet, and emits red fluorescence.

ODMR

This is a measurement technique for optically determining the energy levels of the spin states of an electron pair. The energy levels of the spin states ms=±1 of an electron pair change in the presence of a magnetic field, an electric field, temperature or pressure. When this occurs due to a magnetic field, it is referred to as the Zeeman effect and the Zeeman splitting of the ms=±1 energy levels. The ODMR is optically excited by green light and the readout (or detection) is performed using the red fluorescent light emitted by the NV centres. This distinction in the colour of the light makes such quantum systems optically and technically interesting, as it allows for a very straightforward distinction between excitation and response. A positive side effect of the optical properties of NV centres and the relevant energy levels is that, when optically excited by green light, the system is extremely likely to end up in the ground state (ms=0). This process is referred to as optical pumping or initialisation. Microwaves of the appropriate wavelength can excite the electron pairs in the NV centre between the ground-state energy levels or stimulate them to return to the ground state (stimulated emission). If the microwave frequency matches a transition between the ms=0 state and one of the ms=±1 states with sufficient precision, this creates a resonant system. In this system, the spin—and thus its magnetic moment—oscillates resonantly between the two states. To determine the correct microwave frequencies, the microwave is varied across a frequency range and the corresponding fluorescence intensity is measured. This experiment is repeated many times and the measurement results are averaged. In this way, a fluorescence spectrum is obtained with dips in the fluorescence at the points where the frequency corresponds to the energy differences between the ms=0 and ms=±1 states. The spacing of the dips then correlates, for example via the Zeeman effect, with the strength of the magnetic field.

Quenching

Fluorescence quenching describes the reduction in the intensity of fluorescent light caused by external factors. For example, excessively strong magnetic fields that are perpendicular to the NV axis in the diamond can cause such a drop in intensity. This effect can be used to make a rough estimate of the magnetic field strength.

Single NV

A single NV centre is a single nitrogen vacancy in a diamond, or several such vacancies that are sufficiently far apart so that they can be detected independently of one another and do not interact.

Zeeman Splitting

Splitting of the fluorescence peaks in the fluorescence spectrum due to the presence of a magnetic field. In the diagram of the energy levels, the energy levels move further apart depending on the strength of the applied magnetic field.

ZFS

Zero-field splitting (2.87 GHz) is the energy difference between the ms=0 state and the ms=±1 states at a temperature of approximately 27 °C and in the absence of an external magnetic field. The frequency of the ZFS varies with temperature and can therefore be used for temperature measurement.