“source beam filling factor” τ = 1000 s background temp.: 300 K Towards a Broadband Q-Band VLTI Heterodyne Instrument E. A. Michael 1, L. Labadie 2, N. Honingh 2, K. Jacobs 2, U.U. Graf 2, S. Schlemmer 2, J. Stutzki 2 1 Department of Electrical Engineering, University of Chile, Av. Tupper 2007, Santiago, Chile 2 1. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany Abstract- Expanding VLTI long-baseline interferometry to some atmospheric windows in the Q-band (e.g. 20.5 or 24.5 µm) would offer strong complementarity between the Paranal instrument suite and ALMA to observe bright, massive, embedded objects but also giant stars. In this limited spectral range, the heterodyne approach is appropriate considering the high thermal background, along with recent experimental findings on a quantum-limit heterodyne detection scheme allowing to envision competitive spectral resolution. A new class of heterodyne mixers to be developed in the next years may extend the bandwidth up to hundred GHz. IF-processors could sub-divide this into por- tions manageable by next generation digital correlators. QCL telescope-LOs could be phase-locked to a central reference laser at 1550 nm. Fringe tracking information from ex- isting shorter wavelength instruments could generate a suitable phase-modulation of the LOs to compensate piston phase jitter making very long integration times possible. MOTIVATION AND GENERAL CONCEPT  Proof of Concept at 1.55µm with small telescopes on very bright stars  1.4 – 1.8 mm is an atmospheric window with >95% transmission (H-band)  High-precision phase control beneficial for largest baselines + nulling interferometry  Mobility: connect any group of existing less-used medium size optical telescopes (e.g. at La Silla, Paranal, Las Campanas, or Tololo, Chile)  Baselines of many km should be possible: try the 14” Goto-Dobsonians with the ALMA buried fibers to run a demonstration experiment ESO VLTI workshop München Garching, 17 June - 20 June, 2019 4. Visions  Migrate the near-IR prototype to mid-IR by developing the graphene mixers and tunable mid-IR LOs  Use triple-correlation signals (containing phase closure) to enable long integration times  Increase the IF-bandwidth, use faster digital correlators  proof frequency averaging mode and dispersed heterodyne receiver concept (optionally first at 1550 nm)  Use our interferometer prototype as a test bench for fast mid–infrared receivers  identify and adapt tunable sources or frequency combs in the mid-infrared for LO-purposes 3. Heterodyne versus direct detection thanks to CONICYT-ALMA funds Nº 31090018 and 31140025, QUIMAL Nº150010, CATA-Basal PFB-06, ESO Mixed-Committee 2016 + 2018 ©NASA ©ALMA ©ESO Science goals for using very large telescopes in the N– and Q-band Active Galactic Nuclei Planet Formation Stellar Formation and Evolution Two-telescope version 2. Proposal: Develop BCRs at mid-infrared wavelengths  Various options ffor universal and fast IR-materials/architectures  In synergy with SOFIA developments, try microfabrication and test, first as single mixers: 1. Graphene-based plasmonic coupling photoconductive mixers reaching >50 GHz IF: 2. Alternative 1: Traveling-wave metal-insulator-insulator-metal tunneling junctions (TW-MIIM): 1. New concept of Balanced Correlation Receivers (BCR)  De-correlation of quantum noise of local oscillator at power splitters enables to reach quantum-noise limit and even overcome it: experimental evidence at 1550 nm  Investigated with digital correlation using a ROACH-board (Xilinx FPGA-board with two 3 GSPS) 3. Alternative 2: Travelling-wave (new!) HgCdTe-photodiodes (“TW-MCT”)  Then try to combine pairs of single mixers to balanced mixers  Conducting a BCR experiment at 450 GHz to check quantum effect observed at 1550 nm Developed tip-tilt correction with magnetic fiber actuator  Plotted are pre-detection SNRs: radiation- noise limited  worsens by detection efficiency + post-