10 January 2022

### Quantum teleportation goes microwave

**An international team headed by physicists from the Walther-Meißner-Institute (WMI) of the Bavarian Academy of Sciences and Humanities and Technical University of Munich (TUM) has, for the first time ever, experimentally implemented quantum teleportation based on propagating microwave signals between distant locations, while preserving the fragile quantum nature of teleported states. This key achievement opens the avenue towards distributed quantum computing in networks of superconducting quantum computers. The work has been performed in collaboration with Japanese scientists from the RIKEN Center for Quantum Computing.**

The communication of quantum signals over extended distances is one the cornerstones in quantum communication technologies and, on the long run, is expected to decisively contribute to the development of scalable quantum supercomputers. To this end, an international team led by the physicists Rudolf Gross, Frank Deppe, and Kirill Fedorov has made a leap forward by successfully implementing quantum teleportation of microwave signals over a 42 cm long superconducting cable.

"We are enthusiastic about the first experimental demonstration of a cornerstone quantum communication protocol in the technologically important microwave frequency regime," says Rudolf Gross, professor of Technical Physics at the TUM, scientific director at WMI, and spokesperson of the Excellence Cluster Munich Center for Quantum Science and Technology (MCQST). "In combination with the ongoing efforts on microwave quantum local area networks at WMI, our work constitutes another key step towards a future quantum internet architecture and microwave quantum cryptography applications," comments Kirill Fedorov, leading WMI scientist in the field of quantum communication.

##### Quantum teleportation

Quantum teleportation is defined as a disembodied transfer of quantum states between distant locations, or communicating parties. Historically, this protocol has been developed in order to address potential losses and imperfections of communication channels, such as noise, for an efficient transfer of fragile quantum states. The fundamental possibility for quantum teleportation is enabled by quantum correlations in the form of entanglement between the communicating parties. Nowadays, quantum teleportation is a fundamental benchmark protocol for communication of quantum states between two parties.

The performance of quantum teleportation is typically quantified using a measure called “fidelity”. Intuitively, fidelity describes similarity between quantum states before and after the teleportation procedure. A fidelity equal to unity reflects the fact that these two states are fully identical. It is known that by using classical communication protocols, it is impossible to reach unit fidelity while trying to communicate unknown quantum states over lossy and noisy communication channel. However, by exploiting quantum correlations, one can bypass classical limitations and achieve unit fidelity with the quantum teleportation protocol. More specifically, the transition between quantum and classical communication realms can be characterized by the ability to exceed a specific value of the teleportation fidelity, known as the no-cloning fidelity threshold.

In its seminal work, the WMI quantum communication team now managed to exceed this threshold in the teleportation of coherent microwave states. This result proves that the essential part of the quantum nature of propagating microwave states is preserved and that, in future, these quantum states can be used to transfer information between remote superconducting quantum systems, such as superconducting quantum bits.

The no-cloning threshold also has another important consequence. It literally means that one cannot clone, or copy, quantum states with a precision, or fidelity, better than the aforementioned threshold. This result is a very general consequence of the laws of quantum mechanics. In the field of quantum key distribution (QKD), this fundamental limitation can be exploited to create communication protocols which are unconditionally secure towards any possible eavesdropping. In combination with the ability of quantum teleportation to withstand noise and losses in the communication channel between distant parties, this opens exciting perspectives for near-term microwave QKD applications.

##### Squeezed microwaves and path entanglement

Quantum entanglement is the key resource required for the quantum teleportation protocol. It must be locally generated and then distributed between communicating parties. At microwave frequencies, we generate such quantum entanglement in the form of two-mode squeezed states. To this end, we suppress vacuum fluctuations in a propagating electromagnetic field in one quadrature, and simultaneously amplify another quadrature, as demanded by the Heisenberg uncertainty relation. This squeezing task can be implemented with a tailored superconducting circuit called Josephson parametric amplifier. Then, by sending suitably squeezed microwave states to a beam splitter, one obtains the desired path-entangled propagating signals at respective outputs. These signals can be naturally routed via superconducting coaxial cables to the communicating parties thanks to their propagating nature.

##### Quantum local area networks

It is evident that for the successful implementation of practically useful quantum computers, one has to bind millions, potentially billions, of physical qubits together. This gargantuan task can hardly be implemented within one chip, or even within a standalone experimental machine. Rather, by following the experience in building classical supercomputers, one has to link many smaller quantum processors to form a quantum local area network (QLAN), or even a quantum supercomputer. In this context, our teleportation results represent a first step towards quantum state distribution between distant superconducting quantum chips. At WMI, physicists are now actively testing a novel superconducting cryogenic link, operating at microwave frequencies and spanning up to 7 meters distance. This link, which is a two-node QLAN prototype, enables us to test entanglement distribution between remote superconducting processors under realistic conditions. The experimental success of microwave quantum teleportation pinpoints the future impact of such QLAN prototypes in the framework of building scalable superconducting quantum computers.

These experimental results could trigger a revolutionary development. "The experimental microwave quantum teleportation is an important step towards distributed quantum computing with superconducting circuits," says Frank Deppe, Coordinator of the European flagship project Quantum Microwave Communication and Sensing (QMiCS). “It is exciting to see enormous progress in quantum technologies during the last decade,” says Yasunobu Nakamura, Director of the RIKEN Center for Quantum Computing (RQC).

##### Publication

**Experimental quantum teleportation of propagating microwaves**

K. G. Fedorov, Michael Renger, Stefan Pogorzalek, R. Di Candia, Q. Chen, Y. Nojiri, K. Inomata, Y. Nakamura, M. Partanen, A. Marx, R. Gross, F. Deppe,**
Science Advances 7, eabk0891 (2021)
**, DOI:
10.1126/sciadv.abk0891

##### Contact

**Prof. Dr. Rudolf Gross**

Department of Physics, Technical University of Munich, and

Walther-Meißner-Institute, Bavarian Academy of Sciences and Humanities

Walther-Meißner-Str. 8, 85748 Garching, Germany

Phone: +49 89 289 14249

E-Mail: Rudolf.Gross(at)wmi.badw.de

**Dr. Kirill Fedorov**

Walther-Meißner-Institute, Bavarian Academy of Sciences and Humanities

Walther-Meißner-Str. 8, 85748 Garching, Germany

Phone: +49 89 289 14222

E-Mail: Kirill.Fedorov(at)wmi.badw.de

**Source**:
Walther-Meißner-Institut