Information is the most important resource of our time. Enormous amounts of data are collected, processed in computers and exchanged via glass fibers, the air and satellites. We are caught up in information flows that never break and that race around the length and breadth of the globe at the speed of light. Much of this data has to be exchanged between the sender and the receiver in a safe way, as not everything that is communicated is allowed or supposed to be in the public eye. This includes patients’ data in the field of medicine as well as financial data that is communicated with and between banks and highly sensitive data from the fields of politics and the economy. Forms of communication that are protected from unauthorized access are necessary for all these data transfers.

• Quantum communication and quantum cryptography: Data is encrypted so that it can be transported securely. The encryption technology used today is based on mathematical algorithms, in which the prime factorization of large numbers plays a crucial role – a mathematical problem that is time-consuming but can, in principle, be solved by each computer. This means that encryption technology is competing in a never-ending race against unlawfully accessing data. In addition, sensitive stored data might be decoded at a later time when computers with higher performance are available. Transporting data in an inherently secure way, for example with single photons, is however promised by the principles of the quantum world. With quantum cryptography, which is based on the laws of nature rather than mathematical algorithms, it is physically impossible to “listen in” without being detected.
• Quantum radiometry: Ultraweak light signals, which may even be single photons, play a central role in various fields of fundamental research. These fields range from astronomy and experiments about the foundations of quantum physics to the life sciences. Furthermore, the candela (the base unit of luminous intensity) and its derived units in photometry and radiometry can in principle be expressed in the form of a known amount of photons with a known wavelength.

Those of us who want to look at light precisely inevitably turn to PTB. With the smallest measurement uncertainties in the world, PTB is able to create and detect even ultraweak optical signals. To use quantum cryptography in practice, the careful measurement of all the properties of the underlying hardware is absolutely necessary, so that the “quantum secure” properties can in fact be guaranteed. The range of work here encompasses metrological fundamental research and development along with setting up special calibration services. Single-photon sources currently being studied at PTB are based, for instance, on lattice defects (e.g. color centers in diamonds) and on semiconducting quantum dots made of indium gallium arsenide.