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Less light! The key to quantum cryptography

In particles: Light as a stream of individual photons
In particles: Light as a stream of individual photons

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.

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Color centers in diamonds and hexagonal boron nitride (hBN) are examples of emitters based on lattice defects. Using a confocal microscope, micro-photoluminescence spectroscopy is applied to lattice defect emitters at room temperature.

At PTB, a source based on a nitrogen lattice defect in a nanodiamond has been characterized absolutely with regard to its spectral photon flux and spectral radiant flux for the first time worldwide.

Opens external link in new windowSingle-photon Metrology at LENA 

Opens external link in new windowLaser and Quantum Radiometry

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Within the scope of European research projects, PTB is characterizing single-photon sources based on InGaAs quantum dots. A measuring facility that allows the micro-photoluminescence spectroscopy of quantum dots in the near infrared range at temperatures of up to 4 K by means of a confocal microscope is available for this purpose.

Single-photon sources based on semiconducting quantum dots are particularly interesting for metrology due to their narrow-band emission, among other things. They can be combined with the determination of the absolute photon flux to realize defined powers. The objective of this work consists in developing absolute radiation standard sources, e.g. to calibrate single-photon detectors efficiently in optical quantum technology and possibly to realize the candela, the SI base unit of luminous intensity, based on quantum metrology.

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PTB is dealing with the development of high-precision measurement procedures for the calibration of semiconductor-based single-photon detectors (Si and InGaAs/InP-SPAD) with regard to their detection efficiency. A measurement facility based on the "double attenuator principle" is available for this purpose. Moreover, further parameters of the detectors that are metrologically relevant to optical quantum technologies are characterized (e.g. dead time, dark count rate and after-pulsing).

The aim is to develop a service which offers the accurate and traceable calibration of single-photon detectors for optical quantum technologies.

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Superconducting detectors such as nanowire resistors (SNSPDs) and transition-edge sensors (TESs) are particularly well suited for optical quantum high-tech applications. The reasons for their suitability are detection efficiencies > 85 % at telecom wavelength (1550 nm), count rates in the MHz range at extremely low dark count rates, and, depending on the type of detector, dead-time-free measurements and the measurement of the number/energy of detected photons.

The construction of a measurement facility for the operation of quantum-optical detectors at ultralow temperatures down to 50 mK is planned based on a magnetic cooling system (ADR cryostat). This would allow superconducting detectors to be characterized and compared, and the photon number distribution of ultraweak sources to be characterized.

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PTB is actively participating in developing technical standards in optical quantum technology, for instance in the field of quantum cryptography, within the scope of the  Working Group ETSI GS QKD of the European Telecommunications Standards Institute (ETSI). These activities are currently focusing on developing standards with a view to the specifications of optical components in systems for quantum key distribution as well as measurement methods for their characterization. From a metrological view point, parameters such as the traceability of the measurements and the correct consideration of measurement uncertainties are particularly important.

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Working Group

Opens internal link in current windowWorking Group 4.54:

Metrology for single photon sources based on lattice defects

Metrology for single photon sources based on quantum dots

Charakterization/Calibration of semiconductor-based single photon detctors

Construction of a measuring device for the characterizaton of  superconducting detectors and the photon statstics of ultra-low signals

Contribution to normative work in the field of optic quantum technologies