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.
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.
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.
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.
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.
Single-photon Metrology at LENA