TOPICAL REVIEW Quantum radiometry

Conventionally defined, radiometry is the field that studies the measurement of electromagnetic radiation in terms of its power, spectral characteristics, and other parameters. The term applies to electromagnetic radiation characterization in a wavelength range from nanometers to tens of microns and at all optical power levels. Because radiometry is defined so broadly, a wide variety of measurement devices, or radiometers, with a variety of physical characteristics are used. It is therefore necessary to maintain a common scale for all radiometric measurements such that every family of radiometers can be traced to that scale. Related to radiometry, the fundamental International system of units (SI) reserves a base unit for luminous intensity, called the candela. The art of luminous intensity measurement has evolved from comparison of various standardized candles and lamps prior to 1948 to the measurement of optical power suitable for cryogenic radiometers after 1979. Although measurement techniques are constantly refined and improved, the uncertainty of state of the art luminous intensity measurements is only 0.1% and is in the order of 0.01 for radiometric measurements. This is the poorest accuracy of any SI base unit measurements. Hence, the search for methods to perform measurements with higher accuracy continues. Advances in quantum optics, namely, in singleand bi-photon sources and single-photon detectors, have opened a new method for radiometry, which we will call ‘quantum radiometry’. As we shall see, this designation is somewhat artificial and hence needs clarification. For the purposes of this review, quantum radiometry is defined as the measurement of electromagnetic radiation with the help of single-photon and photon-pair sources and single-photon or photonnumber-resolving photodetectors. As such, these measurements are based on quantum mechanical laws that guarantee certain properties of photon statistics and rely on our ability to reliably detect single photons. Ideally, the accuracy of measurements based on photon counting scales as 1=N, where N is the number of detected photons. Therefore, to match the state-of-the-art accuracy of conventional radiometry one needs to collect 410 single photon detections. It is feasible to acquire such a number in 5100 s of measurement time, given the typical performance of modern single-photon detectors. It is also possible to achieve even lower uncertainties by measuring longer. Thus, ‘quantum radiometry’ can be considered as an alternative metrological technique to advance the accuracy of measurements of electromagnetic radiation.