Free‐Space Time‐Bin Encoded Quantum Key Distribution from Near‐ to Mid‐Infrared Wavelengths
Claudia De Lazzari, Tecla Gabbrielli, Natalia Bruno, Francesco Cappelli, Domenico Ribezzo, Nicola Biagi, Nicola Corrias, Simone Borri, Mario Siciliani de Cumis, Paolo De Natale, Davide Bacco, Alessandro Zavatta
ABSTRACT Quantum technologies play a central role in establishing new ways of quantum‐secured communication. This work investigates free‐space quantum communication and explores the advantage of implementing quantum key distribution (QKD) with weak coherent states produced by a light source in the mid‐infrared (). Time‐bin encoded quantum key distribution is simulated, and it is demonstrated that a free‐space QKD link operating in the mid‐infrared outperforms configurations based on conventional near‐infrared wavelengths under various weather scenarios, with a particular significance in adverse meteorological conditions.
ABSTRACT Quantum technologies play a central role in establishing new ways of quantum‐secured communication. This work investigates free‐space quantum communication and explores the advantage of implementing quantum key distribution (QKD) with weak coherent states produced by a light source in the mid‐infrared (). Time‐bin encoded quantum key distribution is simulated, and it is demonstrated that a free‐space QKD link operating in the mid‐infrared outperforms configurations based on conventional near‐infrared wavelengths under various weather scenarios, with a particular significance in adverse meteorological conditions.
Free-space quantum key distribution (QKD) represents a groundbreaking advancement in secure communication, enabling secure key exchange over vast distances and offering robust encryption for the future quantum internet. However, the compatibility between fiber and free-space infrastructures continues to pose challenges for QKD protocols. Indeed, free-space and fiber-based networks commonly use different wavelengths and qubits encoding schemes. On the one hand, free-space QKD typically exploits visible light for its beneficial beam divergence compared to longer wavelengths, and polarization encoding for its robustness against turbulence. On the other hand, fiber-based QKD employs infrared light, particularly the C-band, because it shows the minimum losses with silica fibers, and time-bin encoding, due to polarization instability in optical fibers. In our study, we demonstrate the viability of a time-bin encoded QKD protocol operating in the C-band through horizontal turbulent free-space channels. We test the setup into a 50 m and a 500 m long links, achieving an average secure key rate of, respectively, 793 kbps and 40 kbps over several hours of measurements. The results encourage further exploration of the interoperability between free-space and fiber-based infrastructures, opening new possibilities for connecting terminal users with satellites in hybrid infrastructures.
Free-space quantum key distribution (QKD) represents a groundbreaking advancement in secure communication, enabling secure key exchange over vast distances and offering robust encryption for the future quantum internet. However, the compatibility between fiber and free-space infrastructures continues to pose challenges for QKD protocols. Indeed, free-space and fiber-based networks commonly use different wavelengths and qubits encoding schemes. On the one hand, free-space QKD typically exploits visible light for its beneficial beam divergence compared with longer wavelengths, and polarization encoding for its robustness against turbulence. On the other hand, fiber-based QKD employs infrared light, particularly the C-band, because it shows the minimum losses with silica fibers, and time-bin encoding, due to polarization instability in optical fibers. In our study, we demonstrate the viability of a time-bin encoded QKD protocol operating in the C-band through horizontal turbulent free-space channels. We test the setup into a 50-m- and a 500-m-long link, respectively achieving an average secure key rate of 793 kbps and 40 kbps over several hours of measurements. The results encourage further exploration of the interoperability between free-space and fiber-based infrastructures, opening new possibilities for connecting terminal users with satellites in hybrid infrastructures.
Abstract Satellite quantum key distribution technology has developed rapidly using near-infrared wavelengths and is expected to enable global quantum communication. However, link availability is still hampered by detrimental effects in the free-space channel, such as background noise from solar radiation and attenuation from turbulence and weather such as haze and fog. One potential mitigation technique is to move to the mid-infrared atmospheric transmission window (3–5 µ ms) where background noise and turbulence effects are significantly reduced. While mid-infrared quantum technology is not as well developed, advancements in mid-infrared entangled photon pair generation and nonlinear upconversion single-photon detectors could be poised to enable daytime satellite downlinks with increased reliability. This review compares the state of the art for quantum transmitters and receivers in the mid-infrared to the more established near-infrared technology. The goal is to identify gaps in transmitter and/or receiver technology in the mid-infrared, and to determine if the mid-infrared can offer significant advantages over the near infrared for quantum communication.
Quantum key distribution (QKD) is a promising technology to enable secure cryptography after quantum computers have been developed. It allows for a key growing protocol that permits creating absolutely random keys to be used in the onetime pad codification scheme. Enabling a global QKD network is one of the final goals of the field. However, to do this with conventional optical fibres presents a fundamental limitation due to their intrinsic loss. Free-space, and specifically satellite links, have been proposed as an alternative and have gathered a lot of interest in recent years. They are considered one of the best candidates to enable a global network. Free-space QKD implementations are dominated by polarisation encoding protocols due to the relative transparency of the atmosphere to polarization. Nonetheless, time-bin and phase codifications offer some advantages and can be practical thanks to new passive interferometer designs. In this paper, the first free-space Coherent One-Way (COW) implementation is reported, some design considerations are commented, and the results of the experiment are shown. These show how time-bin/phase codifications are interesting candidates for free-space QKD.
Abstract Due to the high noise caused by solar background radiation, the existing satellite‐based free‐space quantum communication experiments are mainly carried out at night, hindering the establishment of a practical, all‐day, real‐time global‐scale quantum network. Given that the 2–5 µm mid‐infrared (MIR) band has extremely low solar background radiation and strong scattering resistance, it is one of the ideal bands for free‐space quantum communication. Here, first, on the preparation of a high‐quality MIR (3370 nm) and telecom band (1555 nm) two‐color polarization‐entangled photon source is reported, and the effective detection of MIR photons through a frequency upconversion process. Then, the source is used to realize a principle quantum key distribution based on free‐space and fiber hybrid channels in a laboratory. This work represents a significant step toward developing all‐day global‐scale quantum communication networks.
Time‐bin encoding is an attractive method for transmitting photonic qubits over long distances with minimal decoherence. It allows a simple receiver for quantum key distribution (QKD) that extracts a key by measuring time of arrival of photons and detects eavesdropping by measuring interference of pulses in different time bins. In the past, coherent pulses have been generated using a CW laser and an intensity modulator. A greatly simplified transmitter is proposed and demonstrated here that works by directly modulating the laser diode. Coherence between pulses is maintained by a weak seed laser. The modulator‐free source creates time‐bin encoded pulses with a high extinction ratio (29.4 dB) and an interference visibility above 97 %. The resulting QKD transmitter gives estimated secure key rates up to 4.57 Mbit/s, the highest yet reported for coherent‐one‐way QKD, and can be programmed for all protocols using weak coherent pulses.