Two photon metrology

Activities continued along the action lines agreed in the last meeting:

- opportunities for funding by the 6th (EC) Framework Programme Various different opportunities for EU funding of a two photon metrology project have been considered. In particular, a proposal within FET-Open (STREP) has been presented and not funded.

Research results

Main results are related to the comparison between IEN and BNM-CNAM scheduled for 2003. The quantum efficiency of detector CP-00-A of BNM-INM has been measured by both IEN and BNM-INM at = 633 nm. BNM result is = 0,7412 0,0083with a relative uncertainty of 1,1 %, while IEN result is = 0,7412 0,0052with a relative uncertainty of 0,7 %. Moreover, BNM-INM measured the quantum efficiency by the two-photon method and by comparison with reference detectors directly linked to the cryogenic radiometer of BNM-INM. Results are summarized in the following table:

  Quantum efficiency relative uncertainty
BNM-INM(1) 0,7412 ± 0,0083 1,1 %
BNM-INM(2) 0,6943 ± 0,047 6,8 %

Results agree within the declared uncertainty. Correlated photon techniques show promise in absolute radiometry. Its application have the unusual characteristics that they are intrinsincally absolute and do not rely on any external radiometric standards. In the process of parametric fluorescnce, individual photons from an UV pump beam decay in a non-linear crystal into pairs of correlated IR-visible photons under the restrictions of energy and momentum conservation. the detection in terms of energy and direction of one (visible) photon in the pair can be used to predict not only the existence of the other (IR) photon, but also its direction and energy. This is the basis for the absolute measurement of IR detector quantum efficiency through visible photons. by analogy with an atomic system, correlated photons can be thought of as spontaneous decay of the pump photons. Any additional IR imput to the pumped crystal will produce a stimulated decay of pump photons into correlated pairs. It has been showwn that the ratio between the output signals with and wothout an input to the crystal allows the unknown input radiance to be measured in the fundamental units of photon per mode.

The topics of immediat interest are :

a- quantum efficiency measurements; while current activity in this area claims accuracy at or approaching the 0.5 % level (SIS) it appears that 0.1 % uncertainty is achievable. (conventional measurements in the photon counting regime can achieve an accuracy of 0.1 %)

b- absolute measuremnts of the spectral radiance of thermal sources. Current estimates of the uncertaintiy in two-photon thermal radiance measurements are the 1-5 % level, while conventional measurements are at the 1 % level or better, and therefore our target should be at the 0.5 % or better level.

Co-operation in research should make more eficient use of resources in a shorter time scale and without unnecessary duplication of effort. The aim will be to identify and quantify the factors that currently limit the accuracy, reliability and usefulness of two photon metrology and evaluate wether alternative schemes or improved techniques or technology offer the promise of further improvement. Participating research groups will focus on different, aspects of two-photon metrology and in this way a broad range of schemes/problems could be investigated.

Further Information

Progress report available as of May 2004. Progress report available as of March 2003

Photometry and Radiometry (PR)
Coordinating Institute
INRIM (Italy)
Further Partners
NIST (United States of America)