Superconducting single-photon detectors
PTB has been developing highly sensitive SQUID-based (Superconducting QUantum Interference Device) current sensors for a range of precision measurement applications. Recently, PTB's SQUID current sensors in combination with superconducting microcalorimeters have been used successfully to characterize novel single-photon emitters and microlasers for quantum optics.
Single-photon sources in the optical and the near-infrared wavelength ranges are essential components for procedures of optical quantum communication. A promising method of manufacturing sources for single photons or for few photons is based on quantum dot micro-resonator structures. The better the emission statistics of these sources are known, the more precisely is it possible to understand their behavior in different configurations and under varying operating conditions.
Within the scope of an ongoing European research project on optical single-photon metrology, PTB is collaborating with the Technische Universität Berlin where single-photon emitters and microlasers are being developed on the basis of self-assembled InAs/GaAs quantum dots. At PTB, a measuring system has been set up to characterize such sources that emit single photons or few photos with wavelengths in the range from 800 nm to 1000 nm; it is based on the calorimetric detection of the absorption of the photons emitted by the sources. Superconducting microcalorimeters (so-called "Transition Edge Sensors" (TESs)) developed by the National Institute of Standards and Technology (NIST) are employed as detectors. The number of single photons absorbed by the TESs can be determined with great accuracy when, for the readout, they are combined with PTB's highly sensitive SQUID current sensors.
The TESs and the SQUIDs are operated inside a compact low-temperature cooler at less than 100 mK. Optical fibers ensure that the light of the sources is effectively coupled to the detectors. The new measuring system is the enhanced form of a setup which has already been used successfully at the Austrian Academy of Sciences for a so-called "loophole-free Bell test experiment" with entangled optical photons. Due to the use of TES/SQUID detectors, it is possible to directly determine the photon number distribution of the InAs/GaAs quantum dot sources in a range of up to approx. 20 photons. From the measured distributions, the proportions of thermal and coherent light can then be determined and from these, in turn, the time-dependent higher-order correlation functions. This enables the characterization of quantum dot microlasers under varying operating conditions (e.g., the identification of so-called mode-switching or of collective, spontaneous emissions). With this measuring system , it has also been possible to directly detect one- and two-photon states of a single-quantum dot emitter.
J. Beyer, 7.21, E-Mail: Joern.Beyer@ptb.de