Coherent quantum microwave transmission is key to realizing modular superconducting quantum computers and distributed quantum networks. However, a large number of incoherent photons are thermally generated in the microwave frequency spectrum. Hence, coherent transmission of microwave fields has long been believed to be infeasible without refrigeration. In this work, we propose a novel method for coherent microwave transmission using a typical microwave waveguide at room temperature. The proposed scheme considers two cryogenic nodes (i.e., a transmitter and a receiver) connected by a room-temperature microwave waveguide. At the receiver side, we implement a cryogenic loop antenna coupled to an LC harmonic oscillator inside the output port of the waveguide, while the LC harmonic oscillator is located outside the waveguide. The loop antenna converts the quantum microwave fields (which contain both signal and thermal noise photons) to a quantum voltage across the coupled LC harmonic oscillator. We show that by properly designing the loop antenna, the number of detected noise photons can be significantly less than one. Simultaneously, the detected signal photons can be maintained at a sufficient number greater than one by transmitting a proper number of photons at the input port of the waveguide. For example, we show that for a 10 GHz microwave signal, when using a room-temperature transmission waveguide of 5m length, 35 coherent photons are detected across the LC circuit by transmitting 32x10^4 signal photons at the input port of the waveguide. Interestingly, the number of detected noise photons is maintained as small as 6.3x10^-3. The microwave transmission scheme proposed in this work paves the way towards realizing practical modular quantum computers with a simple architecture.
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