Teleportation and Entanglement Swapping of Continuous Quantum Variables of Microwave Radiation

Abstract: Quantum communication is needed to build powerful quantum computers and establish reliable quantum networks. At its basis lies the ability to generate and distribute entanglement to separate quantum systems, which can be used to run remote quantum operations on them or teleport quantum states from one system to another with the help of classical channels. To this end, it is useful to harness the resource of continuous-variable (CV) entanglement since it can be efficiently and unconditionally produced by squeezing light in a nonlinear medium and can be easily manipulated, distributed, and measured using standard components. While various aspects of CV-based quantum communication have been successfully demonstrated in the optical domain, some key capabilities, such as entanglement swapping, have been lacking in the microwave domain. Here, we demonstrate three key elements of CV-based microwave quantum communication, (1) a Josephson mixer operating as nondegenerate two-mode entangler with maximum measured logarithmic negativity E_N=1.5, (2) a quantum teleportation apparatus, capable of teleporting vacuum and coherent states with a maximum fidelity of 73%, which exceeds the 50% classical limit and is mainly limited by intermediate losses in the setup, and (3) an entanglement swapping system which generates entanglement between two remote noninteracting modes via entanglement swapping operations applied to input vacuum and coherent states with maximum measured logarithmic negativity E_N=0.53. Such hardware-efficient CV entanglement building blocks that are based on nondegenerate Josephson mixers could enable wide-ranging applications in modular quantum computation, quantum cryptography, and quantum communication.

• The paper demonstrates continuous-variable quantum teleportation of both vacuum and coherent states using a Josephson mixer, achieving 73% fidelity which surpasses the classical limit.
• The research details the use of a Josephson mixer for entanglement generation, reaching a maximum logarithmic negativity of 1.5 and highlighting robust nonclassical correlations.
• The study introduces an entanglement swapping system that connects noninteracting modes with a demonstrated logarithmic negativity of 0.53, paving the way for scalable quantum networks.

Teleportation and Entanglement Swapping of Continuous Quantum Variables of Microwave Radiation

The paper "Teleportation and Entanglement Swapping of Continuous Quantum Variables of Microwave Radiation" presents a significant advancement in the domain of quantum communication utilizing microwave radiation. Focusing on the critical components of quantum networks—entanglement generation, teleportation, and entanglement swapping—this study explores how these can be implemented using continuous-variable (CV) entanglement of microwave fields, offering a more hardware-efficient approach compared to discrete-variable (DV) methods.

Key Contributions

The authors demonstrate three pivotal components of CV-based microwave quantum communication:

1. Josephson Mixer as a Two-Mode Entangler: The paper introduces a Josephson mixer that functions as a nondegenerate two-mode entangler. This device achieves a maximum logarithmic negativity $E_N=1.5$, showcasing its capability to generate significant entanglement.
2. Quantum Teleportation Apparatus: The research details the development of a quantum teleportation system for vacuum and coherent states with a fidelity reaching up to 73%, surpassing the classical limit of 50%. This fidelity is primarily curtailed by intermediate setup losses.
3. Entanglement Swapping System: A novel entanglement swapping structure is realized, which yields remote entanglement between two noninteracting modes via entanglement swapping operations. The system achieves a measured maximum logarithmic negativity $E_N=0.53$.

Implications and Theoretical Insights

The ability to effectively produce and manipulate CV entanglement at microwave frequencies opens doors for applications in scalable quantum computation and robust quantum communication. The Josephson mixers used in this study offer significant advantages due to their efficient hardware integration, potentially reducing the complexity and overhead typical in optical systems.

The study's results are noteworthy as they point toward a new frontier in quantum network building, especially relevant for medium-range communication (tens of meters). In these setups, where scalability and minimization of intermediate losses are critical, CV systems using Josephson mixers could serve as fundamental components.

Numerical Analysis and Theoretical Validation

The reported maximum logarithmic negativity values testify to the successful creation of entangled states with substantial non-classical correlations. Teleportation experiments achieving 73% fidelity underscore the efficacy of the proposed setups in surpassing classical limitations, albeit the culmination of losses indicates room for optimization in system design.

Future Directions

This work sets the stage for further research aimed at minimizing losses in Josephson mixer devices and improving their saturation power to handle larger input states. Additionally, the transition from localized entanglement setups to broader, city-wide networks would benefit from devising CV to DV conversion protocols, as these would allow for greater integration with existing quantum communication infrastructures.

Furthermore, the scalability advantage posited by the CV framework could encourage its application in quantum cryptographic protocols and illumination schemes, providing a versatile platform for diverse quantum informational tasks.

Summary

The exploration of microwave CV quantum communication presented in this paper offers a promising avenue for advancing quantum network technology. Through efficient entanglement generation and manipulation using Josephson mixers, the study provides a compelling argument for transitioning towards more scalable and less resource-intensive quantum communication schemes. Thus, this research holds substantive implications for both foundational theory and practical implementations in the field of quantum information science.