  Quantum microwave transmission is key to realizing modular superconducting
quantum computers and distributed quantum networks. A large number of
incoherent photons are thermally generated within the microwave frequency
spectrum. The closeness of the transmitted quantum state to the
source-generated quantum state at the input of the transmission link (measured
by the transmission fidelity) degrades due to the presence of the incoherent
photons. Hence, high-fidelity quantum microwave transmission has long been
considered to be infeasible without refrigeration [3,4]. In this study, we
propose a novel method for high-fidelity quantum microwave transmission using a
room-temperature lossy waveguide. The proposed scheme consists of connecting
two cryogenic nodes (i.e., a transmitter and a receiver) by the
room-temperature lossy microwave waveguide. First, cryogenic preamplification
is implemented prior to transmission. Second, at the receiver side, a cryogenic
loop antenna is placed inside the output port of the waveguide and coupled to
an LC harmonic oscillator located outside the waveguide. The loop antenna
converts quantum microwave fields (which contain both signal and noise photons)
to a quantum voltage across the coupled LC harmonic oscillator. The loop
antenna detector at the receiver is designed to extensively suppress the
induced photons across the LC oscillator. The signal transmittance is
maintained intact by providing significant preamplification gain. Our
calculations show that high-fidelity quantum transmission (i.e., more than 95%)
is realized based on the proposed scheme for transmission distances reaching
100 m.
