Predicting atmospheric turbulence for secure quantum communications in free space.

In recent years, the PM2.5 air pollution has been increasingly serious, which not only affects the air quality and visibility, but also has effects on free space optical signal transmission. However, the research about the relationship between the PM2.5 air pollution and the free space quantum communication has not yet been started. To investigate this relationship, the PM2.5 distribution function and its chemical extinction should be analyzed first. According to the degree of PM2.5 atmospheric pollution and the humidity of the atmosphere, the relationships among the PM2.5 index, the humidity of the atmosphere and the channel attenuation of the free space quantum communication can then be established. According to the amplitude damping channel and the depolarizing channel, the effects of the degree of PM2.5 air pollution on channel capacity, channel average fidelity, channel bit error rate are put out and simulated finally. Simulation results show that, if the air humidity is 30% and the PM2.5 index is 50, the channel capacity, channel average fidelity and the channel bit error rate of free space quantum communication will be 0.83, 0.91 and 0.0048 respectively. While the air humidity is 30% and the PM2.5 index is 300, the above channel parameters will be respectively 0.21, 0.56 and 0.0192. Further more, the channel average fidelity has an obvious difference between the two kinds of channel, and it is also related to the probability of the value of the source characters. Thus, the degree of PM2.5 air pollution has a significant effect on the performance of free space quantum communication. And, in order to improve the reliability of quantum communication in free space, the parameters should be adjusted adaptively based on the status of PM2.5 air pollution.

Satellite-based quantum communication is critical in establishing a global quantum network for its ability to achieve thousand-kilometer-level tasks without the requirement of quantum relay. The free-space channel, however, is affected by diffraction, regular refraction, and atmospheric turbulence, which induce random wandering and broadening of the transmitted beam, resulting in the transmittance of fluctuation. In such a scenario, the ability to predict the unknown situation has become a problem worth considering. In this paper, we suggest a machine learning (ML) assisted prediction of continuous-variable quantum teleportation (CVQT) over satellite-ground channel. We apply the prediction algorithm, which involves <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$k$</tex-math></inline-formula> -nearest neighbor (KNN) algorithm and decision tree (DT) algorithm, to the squeezing parameter and the satellite altitude. Numerical analysis shows that the predicted parameter can reflect the actual derivation and calculation under certain circumstances within the allowable range of error. Moreover, we apply the prediction algorithm to the case affected by the fluctuating parameter transmittance. The results show that the ML-assisted prediction results are consistent with the actual data when zenith angles, turbulence strengths, and wavelengths are taken into account. It demonstrates the high identifying accuracy without increasing the complexity of the system.

Despite its widespread use in fiber optics, encoding quantum information in photonic time-bin states is usually considered impractical for free-space quantum communication as turbulence-induced spatial distortion impedes the analysis of time-bin states at the receiver. Here, we demonstrate quantum key distribution using time-bin photonic states distorted by turbulence and depolarization during free-space transmission. Utilizing a novel analyzer apparatus, we observe stable quantum bit error ratios of 5.32 %, suitable for generating secure keys, despite significant wavefront distortions and polarization fluctuations across a 1.2 km channel. This shows the viability of time-bin quantum communication over long-distance free-space channels, which will simplify direct fiber/free-space interfaces and enable new approaches for practical free-space quantum communication over multi-mode, turbulent, or depolarizing channels.

The vector vortex beam (VVB) has attracted significant attention due to its intrinsic diversity of information and has found great applications in both classical and quantum communications. However, a VVB is unavoidably affected by atmospheric turbulence (AT) when it propagates through the free-space optical communication environment, which results in detection errors at the receiver. In this paper, we propose a VVB classification scheme to detect VVBs with continuously changing polarization states under AT, where a diffractive deep neural network (DDNN) is designed and trained to classify the intensity distribution of the input distorted VVBs, and the horizontal direction of polarization of the input distorted beam is adopted as the feature for the classification through the DDNN. The numerical simulations and experimental results demonstrate that the proposed scheme has high accuracy in classification tasks. The energy distribution percentage remains above 95% from weak to medium AT, and the classification accuracy can remain above 95% for various strengths of turbulence. It has a faster convergence and better accuracy than that based on a convolutional neural network.

We study the channel capacity for orbital angular momentum (OAM) based quantum free-space optical communications. Inspired by recent demonstrations for OAM-based single-photon communication, we construct the quantum density operator in matrix form, based on OAM eigenkets, and determine the quantum channel model suitable for study of the quantum communication over atmospheric turbulence channels. The quantum channel model is derived from OAM eigenkets transition probabilities. By using this model we determine the OAM quantum channel capacity in the presence of atmospheric turbulence. The proposed quantum channel model is of high importance for future study of quantum error correction coding to extend the transmission distance and data rate of free-space quantum communications.

After decades of development, continuous variable quantum key distribution technology still cannot meet people’s needs for secure communication over long distances. Free-space quantum communication provides a new way for secure communication over long distances. However, because the communication connected in free space is inevitably affected by atmospheric turbulence, the transmission rate of the channel changes rapidly according to the probability distribution, making it difficult for the communication personnel to determine the transmission rate of the channel. At the same time, discrete modulated continuously variable quantum key Distribution protocol (DM-CVQKD) has certain advantages over Gauss modulated continuously variable quantum key Distribution protocol (GM-CVQKD) in the communication distance. Therefore, this paper analyzes the key rate of DM-CVQKD in this case (that is, in the fast attenuation channel), proves that it can resist the collective attack, and observes the relationship between the parameters and the key rate in the fast attenuation channel by numerical simulation and control variables. Finally, through the analysis of numerical simulation results, it is found that DM-CVQKD can still maintain a high key rate under certain conditions, which proves that long-distance free space quantum communication is feasible under certain conditions. At the same time, it was found that the performance of DM-CVQKD was affected by the extreme case where Eve completely controlled the instantaneous transmittance.

Quantum information technology has been developing rapidly in last two decades. Very recently, quite a few optical quantum key distribution networks have been reported, enabling preliminary practical applications in secure information transfer. Meanwhile, a number of significant progresses based on quantum network have also been made in the research of linear optical quantum computing, simulation and metrology. However, on the way towards large-scale optical quantum network serious problems occur. On the one hand, the distance of fiber-based quantum communication is limited, due to intrinsic fiber loss and decreasing of entanglement quality caused by the noisy environment. On the other hand, the probabilistic feature of single and entangled photon sources would also cause an exponentially increasing overhead for large-scale quantum network. To solve the above problems, quantum repeaters and/or transmission of optical quantum bits over free space channel can be efficiently exploited for future wide-area realization of quantum communication. In addition, memory built-in quantum repeaters would also enable scalable linear optical quantum computing, simulation and high precision measurement. In this talk I will present some recent experiments from our group, including eight-photon entanglement, topological quantum error-correction, quantum repeater and efficient and long-lived quantum memory, and entanglement distribution and quantum teleportation over 100km-scale free-space quantum channels. These experiments show the promising future possibility towards scalable quantum network with photons and atoms.

Intermodal quantum key distribution (QKD) enables seamless integration of fiber networks and free-space channels, which are essential to create a global quantum network. Here we report on a recent field trial of an intermodal QKD system within a 3-node heterogeneous quantum network, that we presented in details in the work by F. Picciariello et al., arXiv:2310.17441. The network consists of two polarization-based transmitters and a single receiver, where the active channel is alternately switched between a 620-meter free-space link and a 17-kilometer deployed fiber link in the metropolitan area of Padova. The trial evaluates the performance of the free-space channel in relation to the atmospheric turbulence strength. Conducted over several hours during daylight, the trial confirms the interoperability between fiber and free-space channels, achieving a secret key rate on the order of kbps for both types of channels. Notably, the QKD hardware and software function consistently across both channels without requiring different strategies, despite the distinct characteristics of the links. This switching system offers a cost-effective solution for a trusted QKD network by minimizing the number of devices needed across different networks. The results underscore the feasibility of intermodal QKD systems for future quantum networks.

The orbital angular momentum (OAM) of photons is a promising degree of freedom for high-dimensional quantum key distribution (QKD). Due to the greater flexibility in applications and the lower loss, QAM QKD over the free-space channel is still significant. However, effectively mitigating the adverse effects of atmospheric turbulence is a persistent challenge. In contrast to previous works focusing on correcting static simulated turbulence, we investigate the performance of OAM QKD in real atmospheric turbulence with real-time adaptive optics (AO) correction. We show that, it is possible to mitigate the errors induced by weak turbulence and establish a secure channel under some conditions. The cross-talk induced by turbulence and the performance of AO systems are investigated in a lab-scale link with controllable turbulence. The relations between the crosstalk and AO specifications is also studied. Our experimental results suggest that an advanced AO system with fine beam tracking, reliable beam stabilization, precise wavefront sensing and accurate wavefront correction is necessary to adequately correct turbulence-induced error.

Recent advances in pointing and tracking capabilities of small satellite platforms have enabled adoption of capabilities such as high-resolution Earth Observation (EO), inter-satellite laser communications and, more recently, quantum communications. Quantum communications requires unusually narrow optical beams and tight pointing performance (on the order of ten microradians) to close an inherently brightness-limited quantum link. This limit is due to quantum communication protocols such as quantum key distribution and teleportation requiring individual quantum states to be transmitted with photon number restrictions. We examine an opportunity to combine quantum communications with laser communications in sharing an optical link. We discuss a combined quantum and laser communication terminal capable of performing space-to-ground entanglement-distribution and high data rate communications on a 12U CubeSat with a 95mm beam expander and an 60 cm aperture optical ground telescope. Photon pairs produced by the quantum terminal are entangled in polarization so the polarization must be maintained throughout the optical link. We discuss active and passive compensation methods in space and polarization reference frame correction using a polarized reference beacon at the ground station. The combined quantum and laser communication terminal approach enables secure communications over an optical channel with rates of 100 Mbps and sub-nanosecond time transfer.

An optical communication system suitable for voice, data retrieval from remote sensors and identification is described. The system design allows operation at ranges of several hundred meters. The heart of the system is a modulated MEMS mirror that is electrostatically actuated and changes between a flat reflective state and a corrugated diffractive state. A process for mass producing these mirrors at low cost was developed and is described. The mirror was incorporated as a facet in a hollow retro-reflector, allowing temporal modulation of an interrogating beam and the return of the modulated beam to the interrogator. This system thus consists of a low power, small and light communication node with large (about 60&deg;) angular extent. The system's range and pointing are determined by the interrogator /detector/demodulator (Transceiver) unit. The transceiver is comprised of an optical channel to establish line of sight communication, an interrogating laser at 1550nm, an avalanche photo diode to detect the return signal and electronics to drive the laser and demodulate the detected signal and convert it to an audio signal. A functional prototype system was built using a modified compact optical sight as the transceiver. Voice communication in free space was demonstrated. The design and test of major components and the complete system are discussed.

An optical communication system suitable for voice communication, data retrieval from remote sensors and identification had been designed, built and tested. The system design allows operation at ranges of several hundred meters. The heart of the system is a modulated MEMS mirror that is electrostatically actuated and changes between a flat reflective state and a corrugated diffractive state. A process for mass producing these mirrors at low cost was developed and implemented. The mirror was incorporated as a facet in a hollow retro-reflector, allowing temporal modulation of an interrogating beam and the return of the modulated beam to the interrogator. This modulator unit thus consists of a low power, small and light communication node with large (about 60&deg;) angular extent. The system's range and pointing are determined by the interrogator /detector / demodulator unit (the transceiver), whereas the communicating node remains small, low power and low cost. This transceiver is comprised of a magnified optical channel to establish line of sight communication, an interrogating laser at 1550nm, an avalanche photo diode to detect the return signal and electronics to drive the laser and demodulate the returned signal and convert it to an audio signal. Voice communication in free space was demonstrated at ranges larger than 200 meters. A new retro-reflector design, incorporating more modulated mirrors had been constructed. This configuration was built and tested. Its performance and advantages as compared to the single mirror retro-reflector are discussed. An alternative system design that allows higher bandwidth data transmission is described

Beam wander caused by atmospheric turbulence can significantly degrade the performance of horizontal free-space quantum communication links. Classical beam wander correction techniques cannot be applied due to the stronger requirements of transmitting single photons. One strategy to overcome this limitation consists in using a separate wavelength from that of the quantum signal to analyze the beam wander and use this information for its correction. For this strategy to work adequately, both wavelengths should be affected equally by atmospheric turbulence, i.e. no chromatic effects should be originated from beam wander. In this letter, a series of experiments are performed to prove that this is the case for {\lambda} ~ 850 nm as the quantum signal and {\lambda} ~ 1550 nm as the tracking signal of a quantum communication system.

We propose and experimentally demonstrate a turbulence-resilient free space chaotic optical communication system based on vector optical field manipulation. In this scheme, the utilization of vector beam multiplexing technology not only enhances transmission fidelity in the turbulence link but also increases the transmission capacity of the free-space chaotic communication system. A 22 Gbps non-return-zero on-off keying spatial multiplexed free-space chaotic optical communication that is resistant to turbulence is successfully demonstrated. We investigate and compare the effect of using different optical field modes on the chaotic synchronization quality and communication performance under different turbulence strengths, which includes the scalar Gaussian modes with X and Y polarizations, and V-point 1st-order cylindrical vector modes with azimuthal and radial types. The experimental results reveal that employing vector mode leads to a 10% ∼ 13% improvement in the probability of high-quality synchronization and an 8% ∼ 12% reduction in the communication interruption probability compared to the Gaussian mode. Passive turbulence-resilient technique based on vector optical field manipulation is expected to provide new solutions for developing future FSO chaotic communication systems with large capacity and high transmission reliability.

The orbital angular momentum (OAM) of light is an infinite-dimensional degree of freedom of light with several applications in both classical and quantum optics. However, to fully take advantage of the potential of OAM states, reliable detection platforms to characterize generated states in experimental conditions are needed. Here, we present an approach to reconstruct input OAM states from measurements of the spatial intensity distributions they produce. To obviate issues arising from intrinsic symmetry of Laguerre-Gauss modes, we employ a pair of intensity profiles per state projecting it only on two distinct bases, showing how this allows to uniquely recover input states from the collected data. Our approach is based on a combined application of dimensionality reduction via principal component analysis, and linear regression, and thus has a low computational cost during both training and testing stages. We showcase our approach in a real photonic setup, generating up-to-four-dimensional OAM states through quantum walk dynamics. The high performance and versatility of the demonstrated approach make it an ideal tool to characterize high-dimensional states in quantum information protocols.