Quantum noise contribution to NbN hot electron bolometer receiver

Abstract— Superconducting NbN hot electron bolometer (HEB) mixers are so far the most sensitive detectors for\nheterodyne spectroscopy in the frequency range between 1.5 THz and 5 THz. To reach the ultimate receiver noise\ntemperatures in the high end of the THz range (3-6 THz), it is crucial to understand their fundamental noise contribution\nfrom different origins. With increasing frequency, the classical output noise contribution should remain unchanged, but\nthe quantum noise contribution is expected to play an increasing role [1].\nThis paper reports the first dedicated experiment using a single NbN HEB mixer at a number of local oscillator\nfrequencies between 1.6 to 4.3 THz to address and quantify the contribution of the quantum noise to the receiver noise\ntemperature.\nWe used a spiral antenna coupled NbN HEB mixer with a bolometer size of 2 μm×0.2 μm. In order to minimize\nuncertainties in the corrections of the optical losses, we use a vacuum hot/cold load setup [2] to eliminate the air loss, and\nan uncoated elliptical Si lens. Although other components, a 3 μm Mylar beam splitter and a QMC heat filter, also\nintroduce frequency dependent optical losses, they can be accurately calibrated. Furthermore, to reduce uncertainties in\nthe data, we measure Y-factors responding to the hot/cold load by fixing the voltage, but varying the LO power [2]. As\nLO, we use a FIR gas laser.\nWe measure the Y-factor at the optimal point at different frequencies by only varying LO frequencies, but keeping\nthe rest exactly the same. We obtain DSB receiver noise temperatures, which are 842 K (at 1.6 THz), 845 K (1.9 THz), 974\nK (2.5 THz) and 1372 K (4.3 THz). After the correction for the losses of the QMC filter and the beam splitter, the noise\ndata show a linear increase with increasing frequency.\nUsing a quantum noise model [1] for HEB mixers and using a criterion for which the classical output noise must be\nconstant at different frequencies, we analyze the results and find the excess quantum noise factor β to be around 2 and\nthat 24 % of the total receiver noise temperature at 4.3 THz (at the input of the entire receiver) can be ascribed to\nquantum noise. Clearly the quantum noise has a small but measurable effect on the receiver noise temperature at this\nfrequency.\nWe are still analyzing different alternatives of interpretation for the mismatch loss between the bolometer and\nthe spiral antenna.\n[1] E. L. Kollberg and K. S. Yngvesson, “Quantum-noise theory for terahertz hot electron bolometer mixers,” IEEE Trans.\nMicrowave Theory and Techniques, 54, 2077, 2006.\n[2] P. Khosropanah, J.R. Gao, W.M. Laauwen, M. Hajenius and T.M. Klapwijk, “Low noise NbN hot-electron bolometer mixer\nat 4.3 THz,” Appl. Phys. Lett., 91, 221111, 2007.\n