## 4 Instrument Design and Key Technologies
### Solar Spectral Irradiance Traceable to Quantum Benchmark (SITQ)  
We propose an approach to trace the solar spectral irradiance to Planck's constant and photon count number by means of SPDC (Spontaneous Parametric Down-Conversion), a quantum optical effect occurring in a non-linear crystal pumped by a laser beam [26]. In an SPDC process, a one pump photon decays into a pair of photons, usually called correlated or entangled photons, since  
Figure 4: Schematic diagram of Total Solar Irradiance (TSI) calibration on satellite.  
Figure 3: Schematic diagram of hyperspectral calibration on satellite.  
they have definite relations between their frequencies, directions and polarizations. SPDC satisfies energy conservation \(1/\lambda_{p}=1/\lambda_{1}+1/\lambda_{2}\) and momentum conservation \(\mathbf{k}_{p}=\mathbf{k}_{1}+\mathbf{k}_{2}\), where \(\lambda\)s are photon wavelengths, \(\mathbf{k}\)s are wave vectors, and the subscript \(p\), 1 and 2 denote pumping and correlated photons, respectively.  
Detector calibration with SPDC is intrinsically absolute and independent of traceability to external radiometric standards. Theoretically, the accuracy of measured quantum efficiency is solely determined by how accurately the photon number is counted. This distinctive feature makes SPDC a promising candidate for an SI primary standard [27], as well as a space benchmark. With known quantum efficiency \(\eta\), the photon detector can measure photon rate with \(N=M/\eta\), or flux with \(\Phi=N\hbar v\), or irradiance with \(E=N\hbar v/A\), where \(A\) is the area of receiving aperture of the system. This forms the physical basis for tracing solar irradiance measurements to Planck's constant and the photon rate.  
Figure 5 shows the concept of a solar spectral irradiance radiometer working in the visible to shortwave infrared spectrum and incorporating internal SPDC calibration. The radiometer has two working modes, i.e., a calibration mode for traceability to SI and an observation mode for solar irradiance measurement.  
In the calibration mode, correlated photons are directed by a rotating mirror into the middle optics, and then, are spectrally separated by two monochromators, before finally reaching avalanche photodiodes. The middle optics suppress residual pump photons and the monochromators ensure the wavelength correlation \(1/\lambda_{p}=1/\lambda_{1}+1/\lambda_{2}\). The calibration mode gives the quantum efficiency or absolute responsivity of the whole system and naturally accounts for any degradation of components of the system. This approach is especially useful as a reference or benchmark in space, and it appears advantageous over the endeavor to maintain a 'highly stable' onboard standard in space.  
In the observation mode, solar radiation passes an entrance aperture and propagates along the same optical path before it is received by analogue photodiodes. The prototype uses analogue photodiodes, instead of single photon detectors, in the observation mode, since currently, no compact single photon detector is commercially available beyond 1700 nm, and solar photon rate is about five orders of magnitude higher than the maximum count rate of single photon detectors below 1700 nm under our current configuration. The absolute responsivities of the analogue photodiodes are determined in calibration mode by receiving a photon flux measured by avalanche photodiodes. Photon flux beyond 1700 nm is measured at their correlated wavelength below 1700 nm, given \(N_{1}=N_{2}\) at a pair of correlated wavelengths. The solar irradiance will be obtained as \(E=N\hbar v/A\) with the known area A of entrance aperture.