High-Fidelity Quantum Information Transmission using a Nonrefrigerated Lossy Microwave Waveguide at Room Temperature.

Quantum microwave transmission is key to realizing modular superconducting quantum computers [1] and distributed quantum networks [2]. However, a large number of incoherent photons are thermally generated in the microwave frequency spectrum. Hence, high fidelity quantum microwave transmission has long been believed to be infeasible without refrigeration [3,4]. In this work, we propose a novel method for high fidelity quantum microwave transmission using a lossy waveguide at room temperature. The proposed scheme considers two cryogenic nodes (i.e., a transmitter and a receiver) connected by a room-temperature lossy microwave waveguide. First, a cryogenic pre-amplification is provided before transmission. Second, at the receiver side, a cryogenic loop antenna coupled to an LC harmonic oscillator inside the output port of the waveguide is implemented, while the LC harmonic oscillator is located outside the waveguide. The loop antenna converts the quantum microwave fields (which contain both signal and noise photons) to a quantum voltage across the coupled LC harmonic oscillator. The induced noise photons across the LC oscillator include the amplification noise, the thermal occupation of the waveguide, and the fluctuation-dissipation noise. The loop antenna detector at the receiver is designed to extensively suppress the induced noise photons across the LC oscillator to achieve effectivly cooled waveguide. We show that by properly designing the pre-amplification gain, along with the loop antenna at the receiver, a high fidelity quantum transmission (i.e., more than 0.97) can be achieved for transmission distances reaching 100 m.