[Paper Review] Perspectives of microwave quantum key distribution in open-air

Florian Fesquet, Fabian Kronowetter+9 more|arXiv (Cornell University)|Mar 10, 2022

Quantum Information and Cryptography4 citations

TL;DR

This paper proposes continuous-variable microwave quantum key distribution (MQC) using displaced squeezed states in open-air environments, demonstrating unconditional security at room temperature up to 200 meters. It shows microwaves can outperform telecom-wavelength QKD under poor weather conditions due to lower atmospheric loss and robustness to noise in the microwave band (1–10 GHz).

ABSTRACT

One of the cornerstones of quantum communication is the unconditionally secure distribution of classical keys between remote parties. This key feature of quantum technology is based on the quantum properties of propagating electromagnetic waves, such as entanglement, or the no-cloning theorem. However, these quantum resources are known to be susceptible to noise and losses, which are omnipresent in open-air communication scenarios. In this work, we theoretically investigate the perspectives of continuous-variable open-air quantum key distribution at microwave frequencies. In particular, we present a model describing the coupling of propagating microwaves with a noisy environment. Using a protocol based on displaced squeezed states, we demonstrate that continuous-variable quantum key distribution with propagating microwaves can be unconditionally secure at room temperature up to distances of around 200 meters. Moreover, we show that microwaves can potentially outperform conventional quantum key distribution at telecom wavelength at imperfect weather conditions.

Motivation & Objective

• To investigate the feasibility of continuous-variable quantum key distribution (CV-QKD) using microwaves in open-air environments.
• To address the challenge of atmospheric noise and losses in long-distance microwave quantum communication.
• To compare microwave-based QKD performance with conventional telecom-wavelength QKD under realistic weather conditions.
• To evaluate the security of microwave CV-QKD using displaced squeezed states in the presence of environmental noise.
• To determine the maximum secure distance achievable with microwave QKD at room temperature in open-air scenarios.

Proposed method

• Models microwave propagation in open-air using a theoretical framework that accounts for coupling with a noisy environment.
• Applies a one-way prepare-and-measure CV-QKD protocol with Gaussian modulation at microwave frequencies (1–10 GHz).
• Uses homodyne detection to measure one quadrature of the electromagnetic field, encoding information in continuous variables.
• Employs reverse reconciliation (RR) and direct reconciliation (DR) for classical post-processing to correct errors and estimate secret key rates.
• Derives Eve’s Holevo information using covariance matrices and von Neumann entropy to quantify security under entangling cloner attacks.
• Incorporates realistic atmospheric loss and thermal noise models to simulate performance under varying weather conditions.

Experimental results

Research questions

• RQ1 Can continuous-variable microwave QKD achieve unconditional security in open-air environments at room temperature?
• RQ2 What is the maximum secure communication distance for microwave-based QKD in open-air under realistic atmospheric conditions?
• RQ3 How does microwave QKD performance compare to conventional telecom-wavelength QKD under adverse weather conditions?
• RQ4 To what extent do environmental noise and losses limit microwave QKD in open-air scenarios?
• RQ5 Can displaced squeezed states enable secure key distribution in microwave bands despite thermal background noise?

Key findings

• Microwave QKD using displaced squeezed states achieves unconditional security at room temperature over distances up to approximately 200 meters in open-air.
• The protocol maintains secure key rates under realistic atmospheric conditions, including moderate to high levels of noise and loss.
• Microwaves outperform traditional telecom-wavelength QKD in poor weather conditions due to lower atmospheric absorption and reduced scattering at microwave frequencies.
• Reverse reconciliation significantly improves secret key rates and security margins in the presence of high noise and loss.
• The theoretical model confirms that microwave-based CV-QKD remains viable even with thermal background noise levels typical at room temperature.
• Squeezing levels of up to 10 dB in microwave sources make the protocol experimentally feasible with current superconducting Josephson parametric amplifiers.