The quantum internet is within reach

Secure communication between quantum computers implemented.


Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. More specifically, remote state preparation aims at the creation of a desired and known quantum state at a remote location using classical communication and quantum entanglement.

In a new study by the international team of scientists led by the Technical University of Munich (TUM), scientists have implemented secure quantum communication in the microwave band in a local quantum network. This is for the first time, scientists have reported new architecture that represents a crucial step on the road to distributed quantum computing.

Rudolf Gross, professor of technical physics at the Technical University of Munich and director of the Walther Meißner Institute (WMI), said, “We have thus laid the foundation for implementing quantum communication systems in the pervasive microwave range. This is a milestone. This puts the quantum internet, based on superconducting circuits and microwave communications, within arm’s reach.”

Scientists have been pioneering the propagation of quantum microwaves for over ten years. To begin with, they needed to demonstrate that microwave radiation even has quantum mechanical properties. Unlike with visible light, this was amazingly challenging from a specialized perspective, because of the low energy of the microwave photons.

To wipe out obstructions, the experiments were done at absolute-zero temperatures. Utilizing special cooling devices, the physicists ultimately succeeded in demonstrating the principle of entanglement in the microwave extent, a significant essential for reliable quantum correspondence.

This work enabled scientists to take a step forward towards actual application: Quantum Remote State Preparation, as they call their communication protocol. A quantum state can be set at a remote location without sending anything directly.

During the study, scientists used a so-called squeezed microwave state as the quantum state. This is a special manifestation of an electromagnetic wave that can only be explained with quantum mechanics.

Here, a wave’s vacuum fluctuations are suppressed in one plane and amplified in the plane perpendicular to the first. Two such squeezed states can be used to produce an entangled state.

Quantum circuit, developed at the Walther-Meissner-Institut (WMI), which can be used to produce restricted microwave states. Image: A. Battenberg / TUM
Quantum circuit, developed at the Walther-Meissner-Institut (WMI), which can be used to produce restricted microwave states.
Image: A. Battenberg / TUM

Frank Deppe, the WMI coordinator of the European flagship project Quantum Microwave Communication and Sensing (QMiCS), said, “The new concept could trigger a revolutionary development. The experimental implementation of secure quantum communication in the microwave domain is an important step towards distributed quantum computing.”

According to scientists, significantly longer distances are possible between quantum computers.

Gross said, “Here, one challenge will be to develop and measure several meters of cooled quantum cables. In the context of QMiCS, we are already working on extending the distance to seven meters. This puts networking of superconducting quantum computers within reach.”

The study is published in the journal Nature Communications.