Generating single photons

Single-photon sources are interesting in particular for the field of quantum radiometry, where very low optical powers in the femtowatt range (fW – i.e. approx. a billionth of the power of a laser pointer) have to be measured with very small uncertainty.

The single photons are generated by exciting only one single atom, molecule or ion which can emit exactly one photon per excitation process. The next photon is emitted only after further excitation – i.e. after an interval of time. In principle, such a single-photon source offers the possibility of realizing a new primary standard for optical power as a complement to the blackbody radiator and the synchrotron radiation source. This is possible because the optical power is directly related to the total number of photons. The photon flux and the optical power can be determined with very low measurement uncertainty by counting the photons and measuring their energy (which is given by the wavelength).

For a number of years, PTB has been working intensively on developing and metrologically characterizing single-photon sources for use in quantum radiometry. This has been done in collaboration with diverse European national metrology institutes, research institutions and universities within the scope of European metrology research programs such as EMPIR. Within the scope of the EMPIR project titled “Single Photon Sources as New Quantum Standards” (SIQUST), which is still in progress, a single-photon source based on the organic dye molecule dibenzoterrylene (DBT) has recently been developed and characterized. Compared to previous developments in radiometry, this source represents significant progress with regard to the photon flux (1.3 million photons per second), the spectral bandwidth (< 0.2 nm), and the single-photon purity, which is characterized by means of the so-called second-order correlation function g(2). The traceability of the optical radiant power, which can be adjusted between 37 fW and 334 fW, at a wavelength of 785.6 nm, allows the direct calibration of the detection efficiency of a single-photon avalanche detector (SPAD) by comparison with a calibrated analog reference silicon (Si) detector (i.e. a detector generating a photocurrent). This novel source represents a significant step toward the establishment of a standard single-photon source.