Intracity Quantum Communication via Thermal Microwave Networks

Abstract

Communication over proven-secure quantum channels is potentially one of the most wide-ranging applications of currently developed quantum technologies. It is generally envisioned that in future quantum networks, separated nodes containing stationary solid-state or atomic qubits are connected via the exchange of optical photons over large distances. In this work, we explore an intriguing alternative for quantum communication via all-microwave networks. To make this possible, we describe a general protocol for sending quantum states through thermal channels, even when the number of thermal photons in the channel is much larger than 1. The protocol can be implemented with state-of-the-art superconducting circuits and enables the transfer of quantum states over distances of about 100 m via microwave transmission lines cooled to only T=4K . This opens up new possibilities for quantum communication within and across buildings and, consequently, for the implementation of intracity quantum networks based on microwave technology only.

Alternate abstract:

Plain Language Summary

Communication over secure quantum channels is potentially one of the most wide-ranging applications of quantum technologies. The security stems from the encoding of information in fragile superpositions of individual photons, which are destroyed by any attempt to extract information from them. This fragility generally restricts quantum communication to the optical domain, where photons can travel over large distances without being absorbed or distorted. Also at optical frequencies, the detrimental effect of thermal radiation, which would overwhelm the weak quantum signal at microwave or radio frequencies, is negligible. We show that, surprisingly, this common point of view is not as restrictive as is usually assumed, and we describe a method for sending individual quantum states through thermal networks that could enable microwave-based quantum communication across buildings or even cities.

The key ingredient in our protocol is the addition of a high-quality microwave oscillator that connects a qubit (the quantum equivalent of a digital bit) to the communication channel. The oscillator is used to coherently cancel thermal noise, and it reduces residual imperfections with quantum error correction. Our protocol can be implemented with state-of-the-art superconducting circuits, which are currently one of the most advanced systems for quantum information processing but must be operated at millikelvin temperatures that are achievable only with expensive refrigerators. With our new method, quantum signals could be exchanged between two refrigerators located in different rooms, or even different buildings, by connecting them via microwave transmission lines cooled to a more convenient temperature of 4 K.

Even when realistic imperfections are taken into account, our method considerably extends the range of temperatures and frequencies over which quantum communication becomes possible. In the long run, this could open up completely new ways for distributing quantum information over medium and large distances using microwave technologies.