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The security of modern cryptographic systems used today is based on unproven mathematical assumptions that could be disproved at any time. Moreover, future quantum computers will be able to break our public key cryptography. In contrast, quantum cryptography is based on fundamental laws of quantum mechanicals: the no-cloning theorem, which states that not all properties of a quantum system can be read out simultaneously. The encoding in single photons is crucial, as only then is the information protected from eavesdropping attacks.

Many implementations of quantum cryptography utilize weak coherent states. In order to reduce the amount of multi-photon pulses, a very low mean photon number is used, which implies that most pulses are actually empty and carry no information. Instead, we are using single photons emitted from fluorescent defects in solid-state crystals. By using a true single-photon source, we can enhance the data rate significantly. While our current experiments are still in a laboratory, in the near future, we will also test free-space links in the field and establish quantum links between distant buildings. 

Optical quantum technologies require sources of true photon sources. Their applications include quantum cryptography, fundamental quantum optics experiments, quantum computing as well as metrology and sensing. A suitable process is the fluorescence of a single two-level quantum system because the excitation and subsequent decay to the ground state stakes a finite time. The emitter can, therefore, only emit a single photon.

We use defects in the 2D material hexagonal boron nitride as a quantum light-emitting platform. These emitters have a high quantum efficiency and short excited-state lifetime at room temperature, which results in a high single photon luminosity. Moreover, the 2D crystal lattice leads to near-ideal outcoupling, as emitters in atomically thin materials are not surrounded by any high refractive index material. At the moment we are combining our emitters with optical systems, including microcavities and integrated waveguide circuits.

Quantum mechanics is based on certain postulates, such as Born's rule, which states that probability density is given by the absolute square of the wave function. While it is impossible to prove this without making other assumptions on the mathematical structure of the measurement process, one can make interferometric experiments and check if the results follow the distribution predicted by Born's rule. A consequence of any deviation would be higher-order interference in multi-path interferometers.

We have shown that a fundamental quantum advantage is possible when using true single photons instead of coherent states produced by a laser in interferometers. This allowed us to increase the sensitivity and find a tight upper bound to any potential deviation. Now, we are increasing the phase stability of our interferometer and using more complex multi-path interferometers to further enhance the sensitivity of our experiments.

Quantum memories are an essential building block for quantum repeaters and therefore required for long-distance quantum communication networks. A quantum memory can store and preserve a quantum state and release it at a later time. One of the key challenges is not only to build an efficient quantum memory but also the interface to the quantum communication wavelength.

We use vapors of alkali-metal atoms(such as Rubidium or Cesium) as the memory media and store photons based on the electromagnetically induced transparency(EIT) effect. Our first goal is to understand the coupling of our single photon source to reference vapor cells and then reduce the photon linewidth sufficiently for demonstrating slow light in a memory-compatible experiment.