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Semiconductor quantum dots (QDs) are one of the leading candidates for realizable qubits, as well as for many other advances in quantum computing and quantum communication. The spatial overlapping of wavefunctions describing each single electron in these nanoscale devices results in tunable spatial entanglement. In this article, we explore the case of two electrons in two-dimensional double quantum dot systems. We compute the two-particle wavefunction through a variational method combined with Hermite finite elements and study the spatial entanglement of electrons. We show that symmetry in the geometry of the double quantum dots plays a role in obtaining optimal entanglement, while a broken symmetry can lead to additional resonances in entanglement that are associated with the crossings of states. We also show that one can finely tune the level of spatial entanglement by altering the geometry of the quantum dots or by applying external fields, which corresponds to an "entanglement spectroscopy." Finally, we study how impurities in the potential profile of the QDs affect the level of entanglement. [ABSTRACT FROM AUTHOR]

The celebrated minimax principle of Yao says that for any Boolean-valued function f with finite domain, there is a distribution µ over the domain of f such that computing f to error against inputs from µ is just as hard as computing f to error on worst-case inputs. Notably, however, the distribution µ depends on the target error level1: the hard distribution which is tight for bounded error might be trivial to solve to small bias, and the hard distribution which is tight for a small bias level might be far from tight for bounded error levels. In this work, we introduce a new type of minimax theorem which can provide a hard distribution µ that works for all bias levels at once. We show that this works for randomized query complexity, randomized communication complexity, some randomized circuit models, quantum query and communication complexities, approximate polynomial degree, and approximate logrank. We also prove an improved version of Impagliazzo's hardcore lemma. Our proofs rely on two innovations over the classical approach of using Von Neumann's minimax theorem or linear programming duality. First, we use Sion's minimax theorem to prove a minimax theorem for ratios of bilinear functions representing the cost and score of algorithms. Second, we introduce a new way to analyze low-bias randomized algorithms by viewing them as "forecasting algorithms" evaluated by a certain proper scoring rule. The expected score of the forecasting version of a randomized algorithm appears to be a more fine-grained way of analyzing the bias of the algorithm. We show that such expected scores have many elegant mathematical properties--for example, they can be amplified linearly instead of quadratically. We anticipate forecasting algorithms will find use in future work in which a fine-grained analysis of small-bias algorithms is required. [ABSTRACT FROM AUTHOR]

Photochromic materials are of great interest because they enable the fabrication of photo-activated switches. In this study, an experiment is proposed in which two chromene-based photochromic layers were inserted into the arms of a Mach–Zehnder interferometer. The chromene was studied from the perspective of optical absorption to determine the wavelength-dependent complex refractive index. Impinging ultraviolet light on one of the chromene layers induces a transition from the closed to the open form of the chromene, resulting in different phase shifts in the two arms of the interferometer. This results in a change in the probability of detecting a photon by the two detectors after the second mirror of the Mach–Zehnder interferometer. The experiment may be of interest to researchers working in the fields of quantum information and quantum communications. [ABSTRACT FROM AUTHOR]

To explore the multi-directional quantum communication in a three-dimensional (3D) system, 13- and (2n + 1)-qutrit entangled states are constructed by 3D Hadamard gates and 3D controlled NOT gates in this paper. Then we propose a novel theoretical scheme to fulfill four-party double-direction cyclic controlled quantum teleportation (QT) by using a 13-qutrit entangled state as the quantum channel. In this scheme, each observer can teleport two different arbitrary unknown single-qutrit states to the other two observers under the control of the supervisor, respectively and synchronously, which realizes cyclic controlled QT in both clockwise and counterclockwise directions simultaneously. Each observer performs two 3D Bell basis measurements and the supervisor carries out a single-qutrit measurement in the 3D Z-basis. After hearing the measurement results from the other two observers and the supervisor, each observer can restore the target states by utilizing suitable 3D Weyl operators. Based on the (4n + 1)-qutrit entangled channel that we construct, we exhibit in detail how to extend the presented four-party double-direction cyclic controlled QT scheme to the situation with n (n > 3) observers. Analysis shows that the success probability of both the proposed four-party and (n + 1)-party schemes can reach 1, and these two schemes are expected to be realized experimentally in the near future. Through detailed discussion, we find that the security of our schemes and the control power of the supervisor can be guaranteed, and the intrinsic efficiency of the proposed schemes is relatively high. [ABSTRACT FROM AUTHOR]

Quantum networks, relying on the distribution of quantum entanglement between remote locations, have the potential to transform quantum computation and secure long-distance quantum communication. However, a fundamental ingredient for fibre-based implementations of such networks, namely entanglement between a single spin and a photon directly emitted at telecom wavelengths, has been unattainable so far. Here, we use a negatively charged exciton in an InAs/InP quantum dot to implement an optically active spin qubit taking advantage of the lowest-loss transmission window, the telecom C-band. We investigate the coherent interactions of the spin-qubit system under resonant excitation, demonstrating high fidelity spin initialisation and coherent control using picosecond pulses. We further use these tools to measure the coherence of a single, undisturbed electron spin in our system. Finally, we demonstrate spin-photon entanglement in a solid-state system with entanglement fidelity F = 80.07 ± 2.9%, more than 10 standard deviations above the classical limit. Quantum communication networks would greatly benefit from the possibility to have solid-state emitters being directly interfaced with telecom fibers, without the need for wavelength conversion. Here, the authors demonstrate coherent control of an InAs/InP quantum dot, as well as entanglement between its electron spin and the frequency of a telecom photon. [ABSTRACT FROM AUTHOR]

Significant progress has been achieved in leveraging atomic systems for the effective operation of quantum networks, which are essential for secure and long-distance quantum communication protocols. The key elements of such networks are quantum nodes that can store or generate both single and entangled photon pairs. The primary mechanisms leading to the production of single and entangled photon pairs revolve around established techniques such as parametric down-conversion, four-wave mixing, and stimulated Raman scattering. In contrast to solid-state platforms, atomic platforms offer a more controlled approach to the generation of single and entangled photon pairs, owing to the progress made in atom manipulation techniques such as trapping, cooling, and precise excitation schemes facilitated by the use of lasers. This review article delves into the techniques implemented for generating single and entangled photon pairs in atomic platforms, starting with a detailed discussion of the fundamental concepts associated with single and entangled photons and their characterization techniques. The aim is to evaluate the strengths and limitations of these methodologies and offer insights into potential applications. Additionally, the article will review the extent to which these atomic-based systems have been integrated into operational quantum communication networks. [ABSTRACT FROM AUTHOR]

Quantum dense coding (QDC) provides great potential for high‐capacity quantum communication. However, it is highly demanded for practical applications to realize high‐capacity QDC with multiple coded information. Here, a high‐capacity QDC with multiple streams is reported in different channels simultaneously through frequency‐division multiplexing (FDM). The broadband entangled state is generated from a pair of degenerate optic al parametric amplifiers with short cavity lengths. Based on the resultant broadband entanglement, multiple pieces of information coded using binary phase shift keying (BPSK) are transferred with the FDM method. As an experimental demonstration, four pieces of information composed of pseudo‐random numbers are transmitted at a rate of 4 Mbit s–1 using BPSK encoding. The decoded bit error rate reaches 10−3$ 10^{-3}$, which is an average 35‐fold improvement compared with the classical scheme. Furthermore, it is possible to make full use of sideband resource for four different information by using orthogonal FDM. This scheme can be extended to more different information by directly increasing the FDM sideband subchannels, and opens an avenue to construct high‐capacity quantum communication, while minimizing the cost of quantum resource. [ABSTRACT FROM AUTHOR]

Chiral light emission plays a key role in sensing, tomography, quantum communication, among others. Whereas, achieving highly pure, tunable chirality emission across a broad spectrum currently presents significant challenges. Free‐electron radiation emerges as a promising solution to surpass these barriers, especially in hard‐to‐reach regimes. Here, chiral free‐electron radiation is presented by exploiting the spin‐momentum locking (SML) property of spoof surface plasmons (SSPs). When the phase velocity of free electrons matches that of the SSPs, the SSPs can be excited. By implementing wavenumber compensation through perturbations, the confined SSPs are transformed into free‐space free‐electron radiation. Owing to the law of angular momentum conservation, this process converts the transverse spin angular momentum of SSPs into the longitudinal spin angular momentum of free‐electron radiation during the process, producing pure, tunable, and chiral free‐electron radiation across a broad spectrum. This method achieves an optimal degree of circular polarization approaching −1. The innovative methodology can be adapted to SML‐enabled guided states or silicon photonics platforms, offering new avenues for achieving chiral emission. [ABSTRACT FROM AUTHOR]

The quantum key distribution (QKD) technique provided a promising resolution to the current security threats in Quantum Communication. However, the conventional QKD approach is vulnerable to hacker attacks and the Quantum particles used in QKD lose their energy during long-distance communication. To increase the distance coverage of quantum communication, our system proposed a Multi-layer Proxy Encryption Scheme (MPES) using entangled quantum particles. The main advantage of MPES over other conventional communication techniques is that each quantum repeater in the network acts as a different source of encryption with both sender and receiver nodes. For effective long-distance communication, this system adopted a trust-based short-distance protocol to find the path of photon transfer. The entangled photon that is used in communication is done through normal fibre optic cable. The presence of an eavesdropper can be measured with the help of an error correction protocol and a public key is sent through the normal communication channel. This pattern is checked throughout the communication path and malicious nodes that change its pattern will be eliminated from the network. This multiple encoding enhances the security level and reduces the end decryption time effectiveness. Quantum repeaters are used in the QKD protocol to extend the transmission distance by implementing quantum correlations. Unlike the conventional QR, our proposed structure performs one-way communication by encoding and decoding the data within a single node. The reader can decode the information from the actual sender and the writer transforms this decoded data to another node of quantum repeaters. The qubits that are decoded will be in a bell state, and qubits transfer the data in the form of polarization.Moreover, the shortest path algorithm-based photon transfer is done in this approach which increased the execution period of the proposed approach and also turned into the enhanced cost-effective technique. The obtained key error rate of the proposed system is compared with the conventional BB84 protocol and the comparison result proved an increase of 30% in error reduction and reduced energy consumption. [ABSTRACT FROM AUTHOR]

Hybrid W entangled states are essential in quantum information processing, quantum communication, and quantum technology. In this Letter, we propose a simple method to prepare hybrid W entangled states between n superconducting (SC) qubits and n microwave resonators (MRs) in circuit QED. Only two basic operations are needed for the preparation of hybrid W states. The operational time decreases as the number of qubits increases. Since no ancillary cavity is required, the hardware resources for the state preparation are minimized. Because the state preparation does not involve any measurements, the hybrid W entangled states are generated in a deterministic way. Moreover, during the entire preparation, the high-energy levels of the SC qutrits remain unexcited, which greatly reduces decoherence of the SC qutrits. As an example, our numerical simulation demonstrates that high-fidelity preparation of the hybrid W entangled state of three SC qubits and three MRs is feasible within the current circuit QED technique. This method is universal and can be applied to generate hybrid W states of n matter qubits (e.g., atomic qubits, NV center qubits, quantum dot qubits, and magnon qubits) and n photonic qubits in various physical systems. [ABSTRACT FROM AUTHOR]

Efficient and compact single photon emission platforms operating at room temperature with ultrafast speed and high brightness will be fundamental components of the emerging quantum communications and computing fields. However, so far, it is very challenging to design practical deterministic single photon emitters based on nanoscale solid‐state materials that meet the fast emission rate and strong brightness demands. Here, a solution is provided to this longstanding problem by using metallic nanocavities integrated with hexagonal boron nitride (hBN) flakes with defects acting as nanoscale single photon emitters (SPEs) at room temperature. The presented hybrid nanophotonic structure creates a rapid speedup and large enhancement in single photon emission at room temperature. Hence, the nonclassical light emission performance is substantially improved compared to plain hBN flakes and hBN on gold‐layered structures without nanocavity. Extensive theoretical calculations are also performed to accurately model the new hybrid nanophotonic system and prove that the incorporation of plasmonic nanocavity is key to efficient SPE performance. The proposed quantum nanocavity single photon source is expected to be an element of paramount importance to the envisioned room‐temperature integrated quantum photonic networks. [ABSTRACT FROM AUTHOR]

Satellite-based quantum communications including quantum key distribution (QKD) represent one of the most promising approaches toward global-scale quantum communications. To determine the viability of transmitting quantum signals through the atmosphere, it is essential to conduct atmospheric simulations for both uplink and downlink quantum communications. In the case of the uplink scenario, the initial phase of the beam's propagation involves interaction with the atmosphere, making simulation particularly critical. To analyze the atmosphere over the Indian subcontinent, we begin by validating our approach by utilizing atmospheric data obtained from the experiments carried out in the Canary Islands within the framework of Quantum Communication (QC). We also verify our simulation methodology by reproducing simulation outcomes from diverse Canadian locations, taking into account both uplink and downlink scenarios in Low Earth Orbit (LEO). In this manuscript, we explore the practicality of utilizing three different ground station locations in India for uplink-based QC, while also considering beacon signals for both uplink and downlink scenarios. The atmospheric conditions of various geographical regions in India are simulated, and a dedicated link budget analysis is performed for each location, specifically focusing on three renowned observatories: IAO Hanle, Aries Nainital, and Mount Abu. The analysis involves computing the overall losses of the signal and beacon beams. The findings indicate that the IAO Hanle site is a more suitable choice for uplink-based QC when compared to the other two sites. [ABSTRACT FROM AUTHOR]

When operating in space, on-board Silicon Geiger-Mode Avalanche Photodiodes (GM-APDs) are exposed to radiation damage that result in an increase in dark count rates. Thermal annealing has been found to mitigate the damage, although prior studies have largely focused on annealing following a single session of proton irradiation. This work reports that thermal annealing can be performed simply with the built-in thermo-electric coolers of the GM-APDs. Annealing was also done in 10 min intervals for a clearer view of the recovery curve. Mitigation of damage from repeated γ radiation was observed from the: (1) halving of the increase in dark count rates on average and (2) outperformance of room temperature annealing ( 25 ∘ C) by 33 % . Additionally, we show that heavy doses of γ radiation (21 krad) have a probability of causing Random Telegraph Signals in GM-APDs that can be suppressed by lowering the operating temperature. [ABSTRACT FROM AUTHOR]

As a result of the development of digital and internet technologies, digital content theft has become a major problem for the multimedia industry. Piracy is controlled by digital rights management (DRM) system. Multimedia on mobile devices is a novel idea for businesses where security is crucial. Numerous anonymous authentication schemes for mobile-DRM (M-DRM) systems have been developed during the past decade using the assumptions of factorization and discrete logarithms, which are proved insecure in the presence of scalable quantum computers by Shor's algorithm. Therefore, a quantum secure communication mechanism must be developed for M-DRM systems. To create a quantum-safe environment, this paper designs a lattice-based three-factor authentication session establishment technique for content key sharing in the DRM system, which provides forward secrecy and user anonymity and resists off-line password guessing, replay, impersonation, insider and signal leakage attacks. The random oracle model (ROM) is adopted to design the scheme's proof. Further, a comparison of existing schemes with the proposed scheme in terms of execution and communication costs is provided to demonstrate the efficiency of designed protocol. [ABSTRACT FROM AUTHOR]

This study introduces a cutting-edge image encryption algorithm aimed at elevating security standards. The Pan-Tompkins Algorithm (PTA) for key generation is proposed for the first time in this study. Additionally, employing steganography through the Least Significant Bit (LSB) method for embedding keys within the encrypted image enhances secure key distribution, thereby fortifying the encryption process. On the other hand, the integration of advanced algorithms, such as Zigzag scanning, the Affine Image Encryption Algorithm (AA), and the Vigenere Image Encryption Algorithm (VA), constitutes the fundamental innovation of the proposed image encryption algorithm. The proposed algorithm is named PanAAVA:Affine Algorithm and Vigenere Algorithm Encryption with PTA-Based Key Generation. The PanAAVA algorithm ensures unparalleled security by encrypting the positions and values of pixels using AA and VA. Notably, using PTA for key generation marks a distinctive and new key generation method feature of the algorithm. To assess the effectiveness of the PanAAVA, a comprehensive comparative analysis is conducted against well-established encryption methodologies, including Lena, Baboon, Airplane, and Pepper. The PanAAVA demonstrates exceptional proficiency in histogram analysis. The PanAAVA demonstrates a Unified Average Changing Intensity (UACI) of 33.4044%. Additionally, the Number of Pixels Change Rate (NPCR) is measured at 99.7442%, showcasing the algorithm's effectiveness in inducing significant pixel changes. The proposed algorithm's Mean Square Error (MSE) is calculated at 3.20679E5%. The proposed algorithm's Peak Signal to Noise Ratio (PSNR) is recorded at 9.512475. The Key Space Size of the proposed algorithm is measured at 2209. Regarding correlation analysis, the PanAAVA achieves a high correlation score of 7.9996. The proposed algorithm successfully passes the National Institute of Standards and Technology (NIST) analysis, demonstrating a remarkably strong correlation close to 0 and a Structural Similarity Index Measure (SSIM) of 0.9977. Furthermore, regarding quantum communication, the proposed algorithm maintains stable key rates of 47.5 ± 0.8 kHz during the day and 50.9 ± 0.7 kHz at night. Additionally, PanAAVA achieves low Quantum Bit Error Rate (QBER) values of 4.77 ± 0.02, ensuring reliable and secure communication. The PanAAVA also demonstrates robust asymmetries at 49.81 ± 0.02 and 50.14 ± 0.03 for a crystal length of 20 mm. highlighting PanAAVA's adaptability and effectiveness in different scenarios. PanAAVA outperforms other encryption algorithms concerning performance measurements and comparisons. In conclusion, the PanAAVA emerges as a beacon of superior security capabilities and innovation in image encryption, showcasing the potential to redefine standards in the field. [ABSTRACT FROM AUTHOR]

A few years ago Hong et al. (Quantum Inf Process 16:236, 2017) proposed a quantum identity authentication protocol using single photons and executable on currently available quantum hardware. Zawadzki later published two attacks on this protocol, and suggested a mitigation in the same work. In this comment we point out an additional vulnerability that causes the prover Alice to leak a percentage of her secret key at every authentication attempt. The latter is due to a problematic policy in the generation and management of decoy states. We conclude by showing a simple mitigation that addresses the issue. [ABSTRACT FROM AUTHOR]

This paper introduces a bidirectional quantum controlled teleportation (BQCT) protocol within a multi-hop communication network, designed to teleport an arbitrary n -qubit state through an m -hop network framework. Utilizing the IBM Quantum (IBMQ) Experience simulation framework and the Qiskit library, we empirically substantiate the protocol's efficacy. Our findings indicate consistent teleportation across varying hop counts, though the precision of the output state diminishes with an increase in hops. This research further delves into the impact of quantum noise—namely amplitude-damping, phase-damping, bit-flip, and phase-flip—on the protocol's performance. A significant finding is that the detrimental effects of quantum noise escalate with the number of hops, with noise influence showing independence from the input state and causing an exponential decrease in output state fidelity. Thus, our analysis suggests a potential for optimizing real quantum communication systems through a balance between error reduction strategies and the maximum tolerable noise level. [ABSTRACT FROM AUTHOR]

In free-space communication, partially coherent fields are more resistant to atmospheric turbulence, resulting in reduced beam loss and less scintillation at the detector. In this article, we used the Glauber–Sudarshan P function analogy to study the nonclassical properties of the m-photon-subtracted (or added) squeezed vacuum state generated from a partially coherent Gaussian Schell model pump. Utilizing a novel formulation, we examine the Wigner distribution function and decoherence of the Wigner distribution function of these partially coherent multi-photon-subtracted (or added) states in the amplitude decay model. Further, the nonclassicality and non-Gaussianity of these states, along with the purity of the photon-added two-mode squeezed vacuum state, is analyzed. Our findings show that by optimizing the squeezing parameter and pump parameters, the partially coherent pump can produce multi-photon-subtracted (or added) squeezed vacuum states with partially coherent properties while preserving the nonclassicality and non-Gaussianity in these states. We expect these results to be useful for long-distance quantum communication, quantum metrology and quantum sensing. [ABSTRACT FROM AUTHOR]

In this study, we investigate the effects of chromatic dispersion on single-photon temporal wave functions (TWFs) in the context of quantum communications. Departing from classical beam analysis, we focus on the temporal shape of single photons, specifically exploring generalized Gaussian modes. From this foundation, we introduce chirped and unchirped Gaussian TWFs, demonstrating the impact of the chirp parameter in mitigating chromatic dispersion effects. Furthermore, we extend our investigation to time-bin qubits, a topic of ongoing research relevance. By exploring the interplay of dispersion effects on qubit interference patterns, we contribute essential insights to quantum information processing. This comprehensive analysis considers various parameters, introducing a level of complexity not previously explored in the context of dispersion management. We demonstrate the relationships between different quantities and their impact on the spreading of TWFs. Our results not only deepen the theoretical understanding of single-photon TWFs but also offer practical guidelines for system designers to optimize symbol rates in quantum communications. [ABSTRACT FROM AUTHOR]

Quantum mechanics emerged a century ago. It abandoned the deterministic worldview that underlies classical mechanics for a more general theoretical system. It is now believed that everything should be described by quantum mechanics. Furthermore, in our efforts to rethink the theoretical system for information processing based on the classical worldview within the framework of quantum mechanics, it has become clear that even information processing tasks that were previously considered intractable can now be performed within the framework of quantum mechanics. For this issue, we interviewed NTT Distinguished Researcher Koji Azuma, a leader in the field of quantum network research at NTT, about his current research and his mindset as a researcher. [ABSTRACT FROM AUTHOR]

Quantum information and quantum foundations are becoming popular topics for advanced undergraduate courses. Many of the fundamental concepts and applications in these two fields, such as delayed choice experiments and quantum encryption, are comprehensible to undergraduates with basic knowledge of quantum mechanics. In this paper, we show that the quantum eraser, usually used to study the duality between wave and particle properties, can also serve as a generic platform for quantum key distribution. We present a pedagogical example of an algorithm to securely share random keys using the quantum eraser platform and propose its implementation with quantum circuits. [ABSTRACT FROM AUTHOR]

We show that any Boolean function with approximate rank r can be computed by bounded-error quantum protocols without prior entanglement of complexity O (r log r) . In addition, we show that any Boolean function with approximate rank r and discrepancy δ can be computed by deterministic protocols of complexity O(r), and private coin bounded-error randomized protocols of complexity O ((1 δ) 2 + log r) . Our deterministic upper bound in terms of approximate rank is tight up to constant factors, and the dependence on discrepancy in our randomized upper bound is tight up to taking square-roots. Our results can be used to obtain lower bounds on approximate rank. We also obtain a strengthening of Newman's theorem with respect to approximate rank. [ABSTRACT FROM AUTHOR]

The interconversion of different types of entangled states not only can realize the information transmission but also play a significant role in quantum information technologies, including increasing scalability and computational power, and reducing error rates. Here, two protocols for achieving a complete interconversion between W state and Knill–Laflamme–Milburn state assisted by the quantum dot (QD)‐cavity systems and common quantum control gates are proposed. In particular, the protocols employ a heralded approach strategically designed to predict potential failures and facilitate seamless interaction between the QD‐cavity system and photons with the help of a single photon detectors, enhancing experimental accessibility. Through extensive analyzes and evaluations of two protocols, the proposed two protocols achieve remarkable utilization rates of photons (i.e., unit in principle) and achieve near‐unit fidelities and high efficiencies in principle. [ABSTRACT FROM AUTHOR]

Quantum communication is a key to information security, which has a wide range of applications in military, financial, and social development. Understanding the technological emergence in advance would contribute to gain full socio-economic benefits and reducing risk. This research attempts to explore the emergence of quantum communication from the bibliometric perspective. By analyzing the patent from the Derwent Innovation Index, this research analyzes the development phase, the technology emergence, the main contributors' network, the distribution of regions, and technology categories of quantum communication. The results show that quantum communication is in the growth phase according to the theory of the technology life cycle. China, the United States, and Japan are the leading countries. This research also identifies the main patent assignees and their collaboration networks. By introducing the technological emergence indicators, this research identifies four emerging topics in quantum communication and analyzes the layout of main patent assignees within these topics. This research would contribute to the endeavour of scientific communities toward the development of quantum communication. [ABSTRACT FROM AUTHOR]

In this study, we report a transmitter system for free‐space quantum key distribution (QKD) using the BB84 protocol, which does not require an internal alignment process, by using a wavelength‐division multiplexing (WDM) filter and polarization‐encoding module. With a custom‐made WDM filter, the signals required for QKD can be integrated by simply connecting fibers, thus avoiding the laborious internal alignment required for free‐space QKD systems using conventional bulk‐optic setups. The WDM filter is designed to multiplex the single‐mode signals from 785‐nm quantum and 1550‐nm synchronization channels for spatial‐mode matching while maintaining the polarization relations. The measured insertion loss and isolation are 1.8 dB and 32.6 dB for 785 nm and 0.7 dB and 28.3 dB for 1550 nm, respectively. We also evaluate the QKD performance of the proposed system. The sifted key rate and quantum bit error rate are 1.6 Mbps and 0.62%, respectively, at an operating speed of 100 MHz, rendering our system comparable to conventional systems using bulk‐optic devices for channel integration. [ABSTRACT FROM AUTHOR]

Quantum networks provide a prospective paradigm to connect separated quantum nodes, which relies on the distribution of long-distance entanglement and active feedforward control of qubits between remote nodes. Such approaches can be utilized to construct nonlocal quantum gates, forming building blocks for distributed quantum computing and other novel quantum applications. However, these gates have only been realized within single nodes or between nodes separated by a few tens of meters, limiting the ability to harness computing resources in large-scale quantum networks. Here, we demonstrate nonlocal photonic quantum gates between two nodes spatially separated by 7.0 km using stationary qubits based on multiplexed quantum memories, flying qubits at telecom wavelengths, and active feedforward control based on field-deployed fibers. Furthermore, we illustrate quantum parallelism by implementing the Deutsch-Jozsa algorithm and the quantum phase estimation algorithm between the two remote nodes. These results represent a proof-of-principle demonstration of quantum gates over metropolitan-scale distances and lay the foundation for the construction of large-scale distributed quantum networks relying on existing fiber channels. Full-fledged quantum communication networks would allow distributed quantum computing across multiple remote nodes, but typical implementations are stuck at meters-scale distances. Here the authors demonstrate teleportation-based nonlocal CNOT gates, distributed Deutsch-Jozsa algorithm and quantum phase estimation across 7 km space separation. [ABSTRACT FROM AUTHOR]

Quantum emissions, such as single photon emission (SPE) and superradiance (SR), are fundamental ingredients of quantum optical technology. The quest for efficient, controllable, and scalable quantum emitters is crucial for successfully implementing various quantum technologies, such as quantum computing, secure quantum communication and high-precision metrology. Recently, perovskite quantum dots (PQDs) have emerged as highly efficient sources of quantum emission due to their excellent optical properties, including near 100% quantum yield, high optical absorbance, and tunable bandgaps covering the entire visible range. This Prospective introduces the principles of quantum emissions, including SPE and SR, and summarizes recent progress in exploring quantum emissions in PQDs. We focus on the prospects, advantages, and challenges associated with the quantum emissions from PQDs. This Prospective concludes with an outlook on PQDs in advancing future quantum technologies. [ABSTRACT FROM AUTHOR]

Crystal Phase Quantum Dots (CPQDs) offer promising properties for quantum communication. How CPQDs can be formed in Au‐catalyzed GaAs nanowires using different precursor flows and temperatures by in situ environmental transmission electron microscopy (ETEM) experiments is studied. A III‐V gas supply system controls the precursor flow and custom‐built micro electro‐mechanical system (MEMS) chips with monocrystalline Si‐cantilevers are used for temperature control, forming a micrometer‐scale metal–organic vapor phase epitaxy (µMOVPE) system. The preferentially formed crystal phases are mapped at different precursor flows and temperatures to determine optimal growth parameters for either crystal phase. To control the position and length of CPQDs, the time scale for crystal phase change is investigated. The micrometer size of the cantilevers allows temperature shifts of more than 100 °C within 0.1 s at the nanowire growth temperature, which can be much faster than the growth time for a single lattice layer. For controlling the crystal phase, the temperature change is found to be superior to precursor flow, which takes tens of seconds for the crystal phase formation to react. This µMOVPE approach may ultimately provide faster temperature control than bulk MOVPE systems and hence enable engineering sequences of CPQDs with quantum dot lengths and positions defined with atomic precision. [ABSTRACT FROM AUTHOR]

This document is an editorial from the International Journal of Molecular Sciences titled "Application of Nanostructures in Biology and Medicine." It discusses the wide range of applications for nanomaterials in various fields, including energy, medicine, and industry. The document highlights the potential of nanoparticles and nanodevices in areas such as synthetic biology, drug delivery platforms, brain-computer interfaces, nanoelectronics, nanophotonics, and quantum communications technologies. The editorial also provides an overview of recent research in applied nanoscience, including materials science, chemistry, molecular and cell biology, and biotechnology. It features specific studies on the use of microparticles in pharmaceutics, metal-based nanoparticles in cancer therapy, nanoparticles in combating animal diseases, nanoparticles in wastewater treatment, chemical applications of mesoporous nanomaterials, and a new approach to assessing cell viability for implant development. [Extracted from the article]

We introduce a class of circuits that solve a particular case of the Bernstein–Vazirani recursive problem for second-level recursion. This class of circuits allows for the implementation of the oracle using a number of T -gates that grows linearly with the number of qubits in the problem. We find an application of this scheme to quantum homomorphic encryption (QHE), which is an important cryptographic technology useful for delegated quantum computing, allowing a remote server to perform quantum computations on encrypted quantum data, so that the server cannot know anything about the client's data. Liang's QHE schemes are suitable for circuits with a polynomial number of gates T / T † . Thus, the simplified circuits we have constructed can be evaluated homomorphically in an efficient manner. [ABSTRACT FROM AUTHOR]

Optical pulse shaping stands as a formidable technique in ultrafast optics, radio-frequency photonics, and quantum communications. While existing systems rely on bulk optics or integrated platforms with planar waveguide sections for spatial dispersion, they face limitations in achieving finer (few- or sub-GHz) spectrum control. These methods either demand considerable space or suffer from pronounced phase errors and optical losses when assembled to achieve fine resolution. Addressing these challenges, we present a foundry-fabricated six-channel silicon photonic shaper using microresonator filter banks with inline phase control and high spectral resolution. Leveraging existing comb-based spectroscopic techniques, we devise a system to mitigate thermal crosstalk and enable the versatile use of our on-chip shaper. Our results demonstrate the shaper's ability to phase-compensate six comb lines at tunable channel spacings of 3, 4, and 5 GHz. Specifically, at a 3 GHz channel spacing, we showcase the generation of arbitrary waveforms in the time domain. This scalable design and control scheme holds promise in meeting future demands for high-precision spectral shaping capabilities. High-resolution pulse shaping is highly sought after, but existing systems are impractical outside of laboratory settings. Here, the authors introduce a chip-scale spectral shaper that uses high-Q microresonator filter banks and inline phase control to manipulate lines at GHz-level spacing. [ABSTRACT FROM AUTHOR]

Quantum private information retrieval (QPIR) for quantum messages is a quantum communication task, in which a user retrieves one of the multiple quantum states from the server without revealing which state is retrieved. In the one-server setting, we find an exponential gap in the communication complexities between the presence and absence of prior entanglement in this problem with the one-server setting. To achieve this aim, as the first step, we prove that the trivial solution of downloading all messages is optimal under QPIR for quantum messages, which is a similar result to that of classical PIR but different from QPIR for classical messages. As the second step, we propose an efficient one-server one-round QPIR protocol with prior entanglement by constructing a reduction from a QPIR protocol for classical messages to a QPIR protocol for quantum messages in the presence of prior entanglement. [ABSTRACT FROM AUTHOR]

Quantum computing, quantum communication, and quantum networks rely on hybrid quantum systems operating in different frequency ranges. For instance, the superconducting qubits work in the gigahertz range, while the optical photons used in communication are in the range of hundreds of terahertz. Due to the large frequency mismatch, achieving the direct coupling and information exchange between different information carriers is generally difficult. Accordingly, a quantum interface is demanded, which serves as a bridge to establish information linkage between different quantum systems operating at distinct frequencies. Recently, the magnon mode in ferromagnetic spin systems has received significant attention. While the inherent weak optomagnonic coupling strength restricts the microwave‐to‐optical photon conversion efficiency using magnons, the versatility of the magnon modes, together with their readily achievable strong coupling with other quantum systems, endow them with many distinct advantages. Here, the magnon‐based microwave‐light interface is realized by adopting an optical cavity with adjustable free spectrum range and different kinds of magnetostatic modes in two microwave cavity configurations. By optimizing the parameters, a conversion efficiency of 1.75×10−8$1.75\times 10^{-8}$ with bandwidth of 24 MHz is achieved. The impact of various parameters on the microwave‐to‐optics conversion is analyzed. The study provides useful guidance and insights to further enhancing the microwave‐to‐optics conversion efficiency using magnons. [ABSTRACT FROM AUTHOR]

Quantum networks that can perform user-defined protocols beyond quantum key distribution will require fully controllable entangled quantum states. To expand the available space of generated time-bin entangled states, we demonstrate a time-bin entangled photon source that produces qubit states | ψ 〉 = α | 00 〉 + β | 11 〉 with fully controllable phase and amplitudes. Eight different two-photon states have been selected and prepared from arbitrary states on the reduced two-qubit Bloch sphere. The photon pairs encoded in the time-bin scheme were generated at 2.4 MHz with a visibility of V = 0.9475 ± 0.0016 , with a violation of the CHSH Bell's inequality by 197 standard deviations. After entanglement distribution over 100 km of single-mode fibers, we obtained a visibility of V = 0.9541 ± 0.0113 with a violation of the CHSH Bell's inequality by 6 standard deviations. The prepared states had an average fidelity of 0.9540 ± 0.0016 at the source and an average fidelity of 0.9353 − 0.0209 + 0.0100 after entanglement distribution, which shows that the quantum states generated by our time-bin entangled photon source can be fully controlled potentially to a level applicable to long-distance advanced quantum network systems. [ABSTRACT FROM AUTHOR]

Current quantum key distribution (QKD) networks focus almost exclusively on transporting secret keys at the highest possible rate. Consequently, they are built as mostly fixed, ad hoc, logically, and physically isolated infrastructures designed to avoid any penalty to the quantum channel. This architecture is neither scalable nor cost-effective and future, real-world deployments will differ considerably. The structure of the MadQCI QKD network presented here is based on disaggregated components and modern paradigms especially designed for flexibility, upgradability, and facilitating the integration of QKD in the security and telecommunications-networks ecosystem. These underlying ideas have been tested by deploying many QKD systems from several manufacturers in a real-world, multi-tenant telecommunications network, installed in production facilities and sharing the infrastructure with commercial traffic. Different technologies have been used in different links to address the variety of situations and needs that arise in real networks, exploring a wide range of possibilities. Finally, a set of realistic use cases has been implemented to demonstrate the validity and performance of the network. The testing took place during a period close to three years, where most of the nodes were continuously active. [ABSTRACT FROM AUTHOR]

Summary: The mobile satellite communication (MSC) system represents a crucial means of communication, facilitating connections between mobile users and network control centres through satellites. Given the wireless nature of satellite communication, ensuring communication security is paramount for maintaining accountability. Key exchange and authentication (KEA) mechanisms are commonly employed to secure data transmitted over insecure or open channels. Developing a lattice‐based KEA protocol for MSC systems is imperative to achieve both quantum security and efficient communication. In this direction, Yadav et al. devised ring learning with an error‐based KEA technique to ensure a quantum‐safe environment. Their scheme is quantum‐safe and satisfies desirable security attributes but has low efficiency in computation. Moreover, we have identified that their scheme is susceptible to denial of service attack and session key established at user and NCC ends is different. To overcome the flaws of Yadav et al., we propose a lattice‐based KEA protocol for the MSC system to improve computation efficiency, and get resistance against key mismatch attack. The security analysis of the proposed scheme is presented in the random oracle model. Comparative study is also presented to observe advantages in computation efficiency. [ABSTRACT FROM AUTHOR]

In quantum key distribution, secret randomness is extracted quantum-mechanically from two-sided local random choices of measurement bases. Subsequently, the public announcement of basis information is necessary to perform a security check and establish the key. Recent studies have demonstrated that, provided the basis information is accessible, even adversaries with limited computational power can readily compromise the key through side-channel attacks. In this paper, we propose a quantum key distribution scheme using entangled coherent states. The present scheme is based on the secure exchange of one-sided quantum randomness, thus obviating the necessity for basis-information announcement. This effectively closes the security loophole associated with access to basis information during side-channel attacks. The security of the present protocol has been verified against both local and global quantum attacks. Furthermore, the impact of high photon loss and an authentication scheme has been discussed. [ABSTRACT FROM AUTHOR]

It is well-known that maximally entangled GHZ states can achieve perfect teleportation and superdense coding, whereas maximally entangled W states cannot. However, it has been demonstrated that there exists a special class of non-maximally entangled W states, called as W-like states, which can overcome this limitation. Therefore, it is of great significance to prepare such W-like states for efficient quantum communication. Here, we propose two kinds of novel and efficient fusion schemes for atomic W-like states based on the large-detuning interactions between several atoms and a single-mode cavity field, with which large-scale atomic | W N + M - 1 ⟩ and | W N + M + T - 2 ⟩ states can be prepared, respectively, from two small-scale atomic | W N ⟩ and | W M ⟩ states and three small-scale atomic | W N ⟩ , | W M ⟩ and | W T ⟩ states, by detecting the states of one or two of the fused atoms. Particularly, although the fusion process of our scheme involves particle loss, the corresponding success probability is high and fixed, which may induce high fusion efficiency. Furthermore, through the investigation of the resource cost and feasibility analysis, our protocol is simple and feasible under the current experimental conditions. All these suggest that it provides an alternative strategy for preparing large-scale atomic W-like states for perfect teleportation and superdense coding. [ABSTRACT FROM AUTHOR]

Bell state measurement plays a pivotal role in the realm of quantum communication, yet a definitive solution for achieving complete Bell state measurement remains unclear. In this paper, we propose a theoretical scheme for four-dimensional orbital angular momentum Bell state measurement, harnessing the polarization and frequency degree of freedom as auxiliary tools. Within this scheme, the input state traverses a hyperentangled state analyzer comprised of quantum logic gates, ultimately enabling the realization of Bell state measurement in a four-dimensional mode. Simultaneously, serving as a novel avenue to enhance the capacity of quantum communication, this scheme has potential applications in realizing large-capacity quantum communication. [ABSTRACT FROM AUTHOR]

The way a new type of state called a hybrid state, which contains more than one degree of freedom, is used in many practical applications of quantum communication tasks with lesser amount of resources. Similarly, our aim is here to perform multi-quantum communication tasks in a protocol to approach quantum information in multi-purpose and multi-directional. We propose a hybrid multi-directional six-party scheme of implementing quantum teleportation and joint remote state preparation under the supervision of a controller via a multi-qubit entangled state as a quantum channel with 100 % success probability. Moreover, we analytically derive the average fidelities of this hybrid scheme under the amplitude-damping and the phase-damping noise. [ABSTRACT FROM AUTHOR]

As one of the most important branches of quantum information science, quantum communication is known for its unconditional security and efficiency. Nevertheless, the practical security of quantum key distribution protocols and quantum secure direct communication protocols is challenged due to the imperfections in experimental devices. Despite significant progress in theoretical and experimental research on the MDI-QSDC Protocol, challenges and unresolved issues remain. For example, further enhancing the scalability and system complexity of the protocol to meet the demands of large-scale quantum networks is necessary. In this paper, we propose a multi-party MDI-QSDC scheme based on multi-degree-of-freedom hyperentangled photons. Compared to the original MDI-QSDC protocol, our protocol allows multiple parties to participate in the information transmission process. For example, for four communicating parties, we can encode the information of three independent degrees of freedom so that each photon of each degree of freedom can transmit 2 bits of information. Moreover, all measurement tasks are performed by the fifth party, which can be untrusted or even completely controlled by eavesdroppers. The protocol is resistant to all possible attacks from imperfect measurement devices. It can eventually be extended to arbitrary degrees of freedom, allowing multiple parties to participate. [ABSTRACT FROM AUTHOR]

Quantum secure direct communication (QSDC) is a quantum communication paradigm that transmits confidential messages directly using quantum states. Measurement-device-independent (MDI) QSDC protocols can eliminate the security loopholes associated with measurement devices. To enhance the practicality and performance of MDI-QSDC protocols, we propose a one-photon-interference MDI QSDC (OPI-QSDC) protocol which transcends the need for quantum memory, ideal single-photon sources, or entangled light sources. The security of our OPI-QSDC protocol has also been analyzed using quantum wiretap channel theory. Furthermore, our protocol could double the distance of usual prepare-and-measure protocols, since quantum states sending from adjacent nodes are connected with single-photon interference, which demonstrates its potential to extend the communication distance for point-to-point QSDC. [ABSTRACT FROM AUTHOR]

Matrices are built and designed by applying procedures from lower order matrices. Matrix tensor products, direct sums or multiplication of matrices are such procedures and a matrix built from these is said to be a separable matrix. A non-separable matrix is a matrix which is not separable and is often referred to as an entangled 'matrix. The matrices built may retain properties of the lower order matrices or may also acquire new desired properties not inherent in the constituents. Here design methods for non-separable matrices of required types are derived. These can retain properties of lower order matrices or have new desirable properties. Infinite series of required type non-separable matrices are constructible by the general methods. Non-separable matrices of required types are required for applications and other uses; they can capture the structure in a unique way and thus perform much better than separable matrices. General new methods are developed with which to construct multidimensional entangled paraunitary matrices; these have applications for wavelet and filter bank design. The constructions are used to design new systems of non-separable unitary matrices; these have applications in quantum information theory. Some consequences include the design of full diversity constellations of unitary matrices, which are used in MIMO systems, and methods to design infinite series of special types of Hadamard matrices. [ABSTRACT FROM AUTHOR]

Background: Quantum Key Distribution (QKD( is a promising technology for secure communication, leveraging the principles of quantum mechanics to provide theoretically unbreakable encryption. With the exponential growth in data traffic and the increasing need for secure communication in backbone fiber networks, integrating highbit- rate multiplexing techniques into QKD systems can enhance their efficiency and scalability. Problem: Traditional QKD systems face limitations in terms of data rate and network scalability, particularly in high-capacity optical communication networks. As data demands increase, there is a critical need for methods that can support high-bitrate multiplexing while maintaining the security and performance of QKD. Method: This study proposes a novel QKD approach using highbit- rate multiplexing in backbone fiber networks. The method involves encoding quantum keys using multiple optical channels simultaneously to increase the data throughput of the QKD system. We employ a combination of time-division multiplexing (TDM( and wavelength-division multiplexing (WDM( to optimize the use of fiber resources and enhance key distribution rates. Results: Simulation results demonstrate that the proposed method achieves a key distribution rate of 10 Mbps over a 200 km fiber link with a quantum bit error rate (QBER( of 1.5%. This represents a 50% improvement in key rate compared to conventional QKD systems without multiplexing. Additionally, the method shows enhanced scalability and network utilization, supporting up to 16 multiplexed channels with minimal impact on security. [ABSTRACT FROM AUTHOR]

Integrated quantum sources are proving to be the most effective technology of sources for scalable quantum applications. A platform that satisfies all the requirements has not prevailed yet. In this framework, we report, to the best of our knowledge, the first demonstration of a photon pair source in a silicon core fiber. The fiber, only 58 mm long, works at room temperature and shows an intrinsic brightness of 570 kHz/nm/mW2 and a coincidence-to-accidental ratio of 133 ± 2 at around 1.55 μm wavelength. The low propagation losses of the platform, ∼ 0.3 dB/cm in our source, pave the way for effective fiber-based quantum sources in the telecom band. A comparison with state-of-the-art further confirms the potential of this platform for future applications, especially in the field of quantum communication. [ABSTRACT FROM AUTHOR]

Quantum network coding (QNC) aims at alleviating quantum communication congestion in quantum networks. Although several QNC protocols have been presented, they cannot meet the practical requirements that part of source nodes intend to transmit their quantum states with same or different qubit numbers via the bottleneck network simultaneously. Here, the study presents a flexible QNC protocol by using quantum multiplexing. First, the entangled pairs are generated between adjacent nodes in a heralded way by using quantum multiplexing. Then the quantum memories of the source nodes and the ones of the corresponding target nodes are entangled when the intermediate nodes execute multiple rounds of entanglement swapping operations on their quantum memories. Finally, the quantum states are transmitted from the source nodes to their corresponding target nodes by means of quantum teleportation. Compared with the existing protocols, the protocol allows an arbitrary part of the source nodes to transmit their quantum states with same or different qubit numbers via the bottleneck network simultaneously, thereby exhibiting its flexibility. [ABSTRACT FROM AUTHOR]

Higher-dimensional communications are of interest for multiple reasons, including increasing the classical transmission capacity and, more recently, the quantum state transfer through fibers using the many modes within the fiber. For quantum communications, this enables an increase in the number of bits per photon, increasing quantum fidelity, increasing error thresholds and enabling hyperentanglement transfer, among other possibilities. A high-dimensional quantum state transfer can be transported through multimode fiber using the many modes available. However, this transfer of information through multimode optical fiber is limited by attenuation and mode coupling among the various spatial and polarization modes. Here, we consider how this mode coupling impacts the transfer process. We consider the fiber's modal properties, including orbital angular momentum, modal group numbers, and principal modes. We also investigate and propose input and output optical components, as well as fiber properties, which better mitigate the deleterious effects of mode coupling. We use the WKB approximation to the scaler wave equation as a guidance to quantify this coupling and then implement corrections to this approximation using exact solutions to the scaler wave equation. We consider methods to circumvent this mode coupling using optical fiber designs, holographic optical components and devices that are commercially available today. Some of these components, such as the holographic gratings and lenses, could be implemented using flat optics. [ABSTRACT FROM AUTHOR]

Abstract Artificial intelligence (AI) and classical machine learning (ML) techniques have revolutionised numerous fields, including quantum communication. Quantum communication technologies rely heavily on quantum resources, which can be challenging to produce, control, and maintain effectively to ensure optimum performance. ML has recently been applied to quantum communication and networks to mitigate noise‐induced errors and analyse quantum protocols. The authors systematically review state‐of‐the‐art ML applications to advance theoretical and experimental central quantum communication protocols, specifically quantum key distribution, quantum teleportation, quantum secret sharing, and quantum networks. Specifically, the authors survey the progress on how ML and, more broadly, AI techniques have been applied to optimise various components of a quantum communication system. This has resulted in ultra‐secure quantum communication protocols with optimised key generation rates as well as efficient and robust quantum networks. Integrating AI and ML techniques opens intriguing prospects for securing and facilitating efficient and reliable large‐scale communication between multiple parties. Most significantly, large‐scale communication networks have the potential to gradually develop the maturity of a future quantum internet.

Abstract Quantum key distribution (QKD) offers information‐theoretic security by leveraging the principles of quantum mechanics. This means the security is independent of all future advances in algorithm or computational power. However, due to the non‐availability of single‐photon sources, most traditional QKD protocols are vulnerable to various attacks, such as photon number‐splitting (PNS) attacks. Also, the imperfections in the measuring devices open a loophole for side channels that an eavesdropper may exploit to launch attacks such as large‐pulse attacks. As a result, this compromises the security of transmitted information. To address these challenges, the authors present a QKD protocol that is secure against both large‐pulse attacks and PNS attacks at zero‐error, in which the eavesdropper does not introduce any error, but still, the legitimate users of the system cannot distil a secure key. A notable feature of the proposed protocol is that it promotes greater robustness against both attacks than the Bennett‐Brassard 1984 (BB84) protocol or the Scarani‐Acin‐Ribordy‐Gisin 2004 (SARG04) protocol.

Abstract Quantum technology harnesses the principles of quantum mechanics to accomplish tasks in a different way as compared to the classical technologies. This includes quantum computing, which uses qubits for parallel information processing, greatly enhancing computation speed and entanglement and empowering problem‐solving abilities. Quantum communication provides secure data transmission through quantum cryptography, while quantum sensing offers improved measurement precision, benefiting areas such as cryptography, material science, and pharmaceuticals. Additionally, the implementation and commercialisation of quantum technology involve transitioning theoretical quantum concepts into practical applications and marketable products. To achieve widespread adoption of quantum industry, significant research efforts are crucial among academia, industry, and government.