Free-space Channel Research Articles

Tunable Violet Laser Diode System for Optical Wireless Communication

We report a tunable self-injection locked violet laser diode external cavity system exhibiting a continuous wavelength tunability of 5.15 nm (400.28 - 405.43 nm) with mean side-mode-suppression-ratio (SMSR) and linewidth of ~23 dB and ~190 pm, respectively. The effects of injection current and temperature indicate a robust system besides being cost-effective and straightforward. Moreover, a successful indoor on-off keying transmission at two different locked modes on a 0.4 m free space channel showed ~10 times improvement in the bit-error-rate (BER) with value ~8 × 10 -4 at 2 Gb/s, and better performance on 0.8 m channel length at 1.75 Gb/s compared to the free-running laser case. Our work is a potential step towards the realization of future high data capacity narrow-wavelength-spaced multiplexed optical wireless communication system wherein continuously tunable laser sources are expected to play a crucial role as transmitters.

High-capacity (2,3) threshold quantum secret sharing based on asymmetric quantum lossy channels

The main weakness of entanglement is its sensitiveness to the photon loss. In this paper, we exploit the different transmission losses of the free-space and optical fiber quantum channels, to develop a novel approach for (2,3) threshold quantum secret sharing (QSS) of classical information. To be exact, the Dealer Alice allocates W-state to three participants Bob, Charlie and David in terms of the asymmetric losses of their quantum channels, preventing any one participant from recovering the secret alone, but allowing any two of them to recover the secret. In such a way, Alice can flexibly choose the suitable degree of freedom to allocate the quantum shares with respect to the loss characteristics of different quantum channels. Our proposed scheme improves the information capacity from three bits to $$(\log _{2} m+2)$$ bits, where m denotes the dimension of orbital angular momentum, and improves the security and flexibility of quantum communication, confirming QSS as a realistic technology for safeguarding secret shares in transmission. This work opens a convenient and favorable way to perform QSS.

Phase compensation for free-space continuous-variable quantum key distribution.

Large-scale and flexible deployment of quantum networks is possible with reliable free-space quantum key distribution. However, signal fading occurs in free-space channels and causes various adverse effects. Under this circumstance, phase compensation becomes a challenging task for quantum key distribution using continuous variables. Here we investigate the feasibility of implementing phase compensation via simply computing the correlation between transmitted and received data. Demonstration and performance analysis are conducted with real transmittance of a 150-m free-space fading channel; results indicate the applicability of this compensation scheme to free-space quantum communication systems.

Multimode Time-Delay Interferometer for Free-Space Quantum Communication

Quantum communication schemes such as quantum key distribution (QKD) and superdense teleportation provide unique opportunities to communicate information securely. Increasingly, optical communication is being extended to free-space channels, but atmospheric turbulence in free-space channels requires optical receivers and measurement infrastructure to support many spatial modes. Here we present a multi-mode, Michelson-type time-delay interferometer using a field-widened design for the measurement of phase-encoded states in free-space communication schemes. The interferometer is constructed using glass beam paths to provide thermal stability, a field-widened angular tolerance, and a compact footprint. The performance of the interferometer is highlighted by measured visibilities of $99.02\pm0.05\,\%$, and $98.38\pm0.01\,\%$ for single- and multi-mode inputs, respectively. Additionally, high quality multi-mode interference is demonstrated for arbitrary spatial mode structures and for temperature changes of $\pm1.0\,^{\circ}$C. The interferometer has a measured optical path-length drift of $130\,$nm$/\,^{\circ}$C near room temperature. With this setup, we demonstrate the measurement of a two-peaked, multi-mode, single-photon state used in time-phase QKD with a visibility of $97.37\pm 0.01\,\%$.

Multiplexed terrestrial free space optic system design using dual polarized multicarrier transmission technique

A hybrid technique for terrestrial free space optic (FSO) system has been proposed in this paper. The hybrid technique is a combination of 8 × 40 Gbps dense wavelength-division multiplexing and a multicarrier transmission (MCT) scheme such as coherent optical orthogonal frequency-division multiplexing (CO-OFDM). The prime design objective is to minimize the issues related to the terrestrial FSO-based system for atmospheric turbulence, bandwidth management, and receiver sensitivity. The cyclic prefix bits have been added with the data symbols of CO-OFDM, which minimizes the effect of nonlinearity due to atmosphere and reduces intersymbol interference. The dual-polarized quadrature phase-shift keying (DP-QPSK) sequence is used to modulate the subcarriers of the MCT system. DP-QPSK is very well known for its optical signal-to-noise ratio tolerance. It has been found that, by the incorporation of a hybrid scheme in the FSO system, a propagation range of 2.2 km in atmospheric turbulence regime for the targeted bit error rate (BER) of 3.8 × 10 − 3 or log of BER of −2.42 is achieved. A contrast performance comparison has also been carried out for the proposed hybrid system with different advanced modulation schemes such as differential quaternary phase-shift keying and four-level quadrature amplitude modulation. The Gamma-Gamma-type distribution function is considered for estimating the turbulence effect of the free space channel. Mathematical modeling related to the proposed system has also been incorporated for better analysis. This type of hybrid technique for the FSO system with the incorporation of DP-QPSK modulation format will give more technical feasibility to the modern communication system.

Multiple Access Techniques for Bipolar Optical Code Division in Wireless Optical Communications

In this paper, multiple access techniques for bipolar optical code division are proposed for application in wireless optical communications. First, several fiber Bragg gratings (FBGs) and optical circulators are used to encode optical signals. Then, the horizontal or vertical polarization state can be controlled using bipolar data. Experiments were conducted using two devices to determine the feasibility of using multiple access techniques for bipolar optical code division in free-space optical communication. The first experiment tested the first architecture, in which each user operates a mechanical optical switch by switching data bits to generate optical signals with specific polarization states through a collimator in a free-space channel. As the decoding components, several FBGs and optical circulators are used as primary devices. Decoded signals are sent to a balanced photodetector, which can reconstruct original data from the encoder. The second experiment involved a second architecture, in which an encoder with an erbium-doped fiber amplifier and a decoder with an attenuator are used to improve system stability. The third experiment was designed to test whether multiple access interference can be alleviated with the proposed architectures. In the final experiment, an ultrafast optical switch was used instead of the original optical switch for selecting optical signals with a specific polarization state to improve the overall transmission rate. The experimental results indicate that the proposed scheme could be successfully implemented in free-space optical communication systems. The architectures of the proposed scheme mitigate multiple access interference with a simple and cost-effective design.

Analysis of a single-mode waveguide at sub-terahertz frequencies as a communication channel

We study experimentally the transmission of an electromagnetic waveguide in the frequency range from 160 to 300 GHz. Photo-mixing is used to excite and detect the fundamental TE10 mode in a rectangular waveguide with two orders-of-magnitude lower impedance. The large impedance mismatch leads to a strong frequency dependence of the transmission, which we measure with a high-dynamic range of up to 80 dB and with high frequency-resolution. The modified transmission function is directly related to the information rate of the waveguide, which we estimate to be about 1 bit per photon. We suggest that the results are applicable to a Josephson junction employed as a single-photon source and coupled to a superconducting waveguide to achieve a simple on-demand narrow-bandwidth free-space number-state quantum channel.

Genuine time-bin-encoded quantum key distribution over a turbulent depolarizing free-space channel.

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.

Security Analysis of Discrete-Modulated Continuous-Variable Quantum Key Distribution over Seawater Channel

We investigate the optical absorption and scattering properties of four different kinds of seawater as the quantum channel. The models of discrete-modulated continuous-variable quantum key distribution (CV-QKD) in free-space seawater channel are briefly described, and the performance of the four-state protocol and the eight-state protocol in asymptotic and finite-size cases is analyzed in detail. Simulation results illustrate that the more complex is the seawater composition, the worse is the performance of the protocol. For different types of seawater channels, we can improve the performance of the protocol by selecting different optimal modulation variances and controlling the extra noise on the channel. Besides, we can find that the performance of the eight-state protocol is better than that of the four-state protocol, and there is little difference between homodyne detection and heterodyne detection. Although the secret key rate of the protocol that we propose is still relatively low and the maximum transmission distance is only a few hundred meters, the research on CV-QKD over the seawater channel is of great significance, which provides a new idea for the construction of global secure communication network.

Comprehensive study of orbital angular momentum shift keying systems with a CNN-based image identifier

Different possible configurations of orbital angular momentum shift keying (OAM-SK) systems with an image identifier based on convolutional neural network (CNN) are comprehensively studied in this paper. These configurations transmit information by employing different OAM modes (eigenmodes, conjugate modes or multimodes) at the transmitter side and utilize coherent or incoherent detection at the receiver side. We analyze firstly the OAM mode property, the property of free-space channel with atmospheric turbulence (AT), the CNN-based image identifier, and then discuss the feasibility and performance of all possible OAM-SK configurations. We quantitatively study and compare the available OAM-SK configurations with different AT strength and transmission distances in terms of demodulation accuracy. Results show that the coherent systems outperform their incoherent counterparts in terms of the demodulation accuracy. In addition, the OAM-SK configuration with eigenmode and coherent detection has the highest demodulation accuracy among all the possible configurations.

Free-space continuous-variable quantum key distribution of unidimensional Gaussian modulation using polarized coherent states in an urban environment

We use single homodyne detector to accomplish Continuous-Variable quantum key distribution(CV QKD) in a laboratory and urban environment free-space channel. This is based on Gaussian modulation with coherent-states in the polarization degree of freedom. We achieved a QKD distance at 460m, at the repetition rate of 10 kHz. We give the security of this protocol against collective attack in the asymptotic regime. The secure key rate is 0.152 kbps at the typical reconciliation efficiency of 0.95. The experiment setup of this scheme is simplified and the difficulty to realize has been remarkably reduced compared to traditional symmetric modulation ones, for example, GG02 protocol. The influence of security key rate brought by asymmetric modulation is small in a relative low channel loss condition in the free-space environment. This scheme is expected to be significance meaning to the future practically utilize.

Quantum hacking of free-space continuous-variable quantum key distribution by using a machine-learning technique

The transmission coefficient of the free-space channel is fluctuating due to the occurrence of random fluctuations of the refractive index and noise. The Gaussian-modulated quantum states of light may degrade into a non-Gaussian mixture at a certain probability after being transmitted through the free-space channel. In such a situation, Eve can apply an entanglement-distillation attack to steal secret information without it being revealed if Alice and Bob regard the practical fluctuation losses as constant. However, the entanglement-distillation attack is available only when the transmitting states are a non-Gaussian mixture; otherwise, Eve will expose herself. Exploiting this loophole and considering the limitations of the attack, we enhance Eve's eavesdropping ability by integrating a machine-learning technique with the entanglement-distillation attack. With the help of convolutional neural network (CNN, one of the most popular machine leaning tools), Eve can choose the best opportunity to launch the entanglement-distillation attack. Therefore, Eve has a chance to bias the estimation of the parameters by improving the entanglement resource of the mixed states, which allows Eve to render the final keys shared between the legitimate parties insecure. Our proposed CNN-based entanglement-distillation attack does not require additional resources and can be implemented using simple means based on the present technologies.

A Voltage Pulse Generator for Measurement-Device-Independent Quantum Key Distribution

Measurement-device-independent quantum key distribution (MDIQKD) protocol has been proven to be unaffected by all hacking attacks and ensures the security of information theory even when the performance of single-photon detectors is not ideal. Single-mode fiber and free space are the two main channel choices in MDIQKD experiments. In this paper, we present a voltage pulse generator (VPG) for the MDIQKD, focusing on the realization of pulse control using a wideband amplifier that offers a high dynamic range, on the prevention of waveform distortion and insufficient drive capacity. In order to implement MDIQKD experiments, either fiber channel or free-space channel, a pulse modulation module is needed to modulate the wide pulse chopping, decoy state, normalization, phase encoding, and time encoding. The test results of the joint test with fiber channel MDIQKD show that the quantum bit error ratio (QBER) in the time state is around 0.034%, and the QBER in the phase state is around 28%. The VPG has fulfilled the requirements of system indicator and has been put into use in actual single-mode fiber channel MDIQKD experiments.

Research Progress Of Quantum Repeaters

At present, the main components of quantum communication channels are optical fibers and free space. However, whether it is a fiber channel or a free-space channel, the channel in which it propagates is affected by noise, resulting in loss or absorption of photons. Experimental studies have shown that the quantum key can basically meet the requirements of the local area network, and the safety distance is about 100km. However, if you want to achieve longer distance distribution, you can’t do without quantum repeaters. The quantum repeater can divide the long-distance communication distance into several segments, and then perform quantum key distribution in several segments respectively; then establish a longer-distance EPR pair between adjacent nodes, and establish an EPR pair to utilize entanglement switching and Entanglement purification technology; finally repeat the above steps, and finally achieve long-distance information transmission. The development of quantum repeaters can solve the communication problems facing the world and promote the replacement of global communication methods.

Quantum error rejection for faithful quantum communication over noise channels

Quantum state transmission is a prerequisite for various quantum communication networks. The channel noise inevitably introduces distortion of quantum states passing through either a free-space channel or a fibre channel, which leads to errors or decreases the security of a practical quantum communication network. Quantum error rejection is a useful technology to faithfully transmit quantum states over large-scale quantum channels. It provides the communication parties with an uncorrupted quantum state by rejecting error states. Usually, additional photons or degrees of freedom are required to overcome the adverse effects of channel noise. As quantum error rejection method consumes less quantum resource than other anti-noise methods, it is more convenient to perform error-rejection quantum state transmission with current technology. In this review, several typical quantum error-rejection schemes for single-photon state transmission are introduced in brief and some error-rejection schemes for entanglement distribution are also briefly presented.

Certain Investigation on Visible Light Communication with OFDM Modulated White LED Using Optisystem Simulation

Visible light communication (VLC) has drawn more attention in recent years. It is due to the reason that the existing light sources for lighting application in home, office and streets have huge amount of bandwidth and they can deliver data communication at higher bit rates. Orthogonal frequency division multiplexing (OFDM) modulation has also significant impact on communication sector now-a-days due to its healthiness to the signal distortion and support for higher data rate. In this paper we have studied the VLC system using Optisystem simulation tool for an already practically measured free space channel. White LED is used as source for our VLC system and it is driven by OFDM modulated signal. The arrangement of this system is analysed for various bit rates and link ranges. The proposed system can support 10 Gbps data rate up to 2 m link range. We have also studied the OFDM subcarrier impact on system performance and link range. The performance of designed system is compared based on its BER and constellation diagram.

Experimental investigation of quantum key distribution over a water channel.

Quantum key distribution (QKD) has undergone significant development in recent decades, particularly with respect to free-space (air) and optical fiber channels. Here, we report a proof-of-principle experiment for the BB84 protocol QKD over a water channel. First, we demonstrate again the polarization preservation properties of the water channel in optical transmission according to the measured Mueller matrix, which is close to the unit matrix. The reason for the polarization preservation, revealed by Monte Carlo simulation, is that almost all the received photons are unscattered. Then, we performed the first polarization encoding BB84 protocol QKD over a 2.37 m water channel. The results show that QKD can be performed with a low quantum bit error rate, less than 3.5%, with different attenuation coefficients.

Integrating machine learning techniques in quantum communication to characterize the quantum channel

Free-space quantum communication technology has made significant advancements over the years. However, to achieve a global quantum network there are still obstacles to overcome. To date, free-space quantum communication channels have been faced with challenges related to losses in the quantum channel, security, and low data rates. Classical machine learning techniques provide a resourceful method for determining the properties of the free-space quantum channel. In this paper, we used supervised machine learning to predict the atmospheric strength of a free-space quantum channel in the form of the Strehl ratio. We show that using the ensemble technique called random forest we could make predictions of the Strehl ratio of the quantum channel with a mean absolute percentage error of 4.44%. By adding the feature of fidelity to the training set before and after the quantum channel, we were able to predict the Strehl ratio with an improved mean absolute percentage error of 3.86%. For comparison we used other machine learning techniques such as linear regression and support vector machine with various kernels.

Phase Corrections With Adaptive Optics and Gerchberg-Saxton Iteration: A Comparison

Free space optical communications technique is the promising alternative for the access/fronthual networks to relieve the traffic congestion. The turbulence in the free space channel, however, could cause the phase distortions in optical signals, thus degrading the performance strikingly. The air turbulence effect poses a challenge for such networks deployment. To this regard, we study the phase corrections with the adaptive optics (AO) and Gerchberg-Saxton (GS) iteration to mitigate the air turbulence effect under various turbulence conditions. The improvement in performance is comprehensively compared between the both schemes. The numerical results reveal that both AO system and GS iteration enable to effectively compensate for the phase distortions resulting from the air turbulence. Under weak turbulence, the GS iteration may outperform the AO in phase correction. On the other side, as the air turbulence is into strong regime, the AO would be superior to the GS iteration.

Measurement-device-independent quantum key distribution under K-distributed strong atmospheric turbulence

Free-space quantum key distribution (QKD) allows two distant parties to share secret keys with information-theoretic security, which can pave the way for satellite-ground quantum communication to set up a global network for sharing secret message. However, free-space channels in the presence of atmospheric turbulence are affected by losses and fluctuating transmissivity which further affect the quantum bit error rate and the secure key rate. To implement free-space QKD, it is indispensable to study the effect of atmospheric turbulence. Different models have been used to describe the probability distribution for channel transmission coefficient under atmospheric turbulence, including the log-normal distribution and K distribution. In this paper, we focus on free space measurement-device-independent quantum key distribution (MDI-QKD) under K-distributed strong atmospheric turbulence. The MDI-QKD can close all loopholes on detection and achieve a similar performance to QKD, relying on time-reversed version of entanglement-based QKD protocol. Threshold post-selection method is adopted to restrain detrimental effects of the atmospheric turbulence, which is based on the selection of the intervals with higher channel transmissivity. By combining the general MDI-QKD system model with this method, we present a framework for the optimal choice of threshold. Our simulation result shows that the optimal threshold is dependent on the turbulence intensity and expected channel loss. Furthermore, compared with the original MDI-QKD protocols, the proposed protocol with threshold post-selection method can acquire a considerable better performance in key rate, especially in regions of high turbulence and high loss. What is more, this is instructive to the building of a practical free-space MDI-QKD system with better performance.