We present a continuous-variable (CV), quantum-key-distribution (QKD)-inspired, keyed physical-layer masking method for classical M-level pulse-amplitude-modulation (M-PAM) links. A transmitter adds a per-symbol Gaussian dither [Formula: see text], generated by a seeded pseudorandom number generator (PRNG), directly to the analog waveform, and an authorized receiver that shares the PRNG, its seed, and [Formula: see text] regenerates and subtracts the same sequence prior to slicing. Because masking and demasking act on the data-carrying optical signal, the scheme operates over conventional, amplifier-compatible fiber without reserving a separate quantum channel. First, we analyze an idealized continuous-valued baseband model in which the PAM symbols pass through an additive-white-Gaussian-noise (AWGN) channel and no extra amplitude quantization or link impairments are present. In this setting we show that, when the seed and [Formula: see text] are matched, subtractive cancellation is essentially ideal and the Gray-coded 4-PAM bit-error-rate (BER) versus signal-to-noise ratio (SNR) curve coincides with the standard AWGN benchmark, whereas seed or parameter mismatches act as additional Gaussian noise and produce several-decibel SNR penalties or high, SNR-independent BER floors. We then implement the same masking mechanism in a system-level OptiSystem model of an intensity-modulation/direct-detection (IM/DD) link and enable a [Formula: see text] double-quantization mapping, in which dithered 4-PAM symbols are passed through a mid-rise 8-level quantizer at the transmitter and a mid-tread 4-level quantizer at the receiver. In this OptiSystem realization the resulting pseudo-constellation exhibits amplitude statistics reminiscent of probabilistic shaping and an intrinsic, SNR-independent algorithmic BER floor, typically [Formula: see text] in back-to-back simulations. By choosing forward-error-correction (FEC) codes whose waterfall threshold lies near this floor and by tuning the dither variance at the transmitter and receiver, the link can be operated in a deliberately fragile edge-of-FEC regime in which small excess disturbances (for example, seed or variance mismatch) push the BER out of the decodable region and strongly obfuscate the payload. The proposed masking is modulation-agnostic and is intended as a classical protection layer rather than a full QKD protocol; keying material can be supplied over an authenticated supervisory channel or by an independent QKD system.

Quantum information processing enables secure communication, quantum teleportation, and computation. However, current protocols are limited by the narrow electronic bandwidth of standard measurement devices (megahertz to gigahertz), vastly underusing the broad optical bandwidth (10 to 100 terahertz) of readily available quantum light sources. We introduce a general framework for frequency-multiplexing of quantum channels along with methods for efficient processing of quantum information in those channels across the full optical bandwidth. Using a broadband squeezed-light source, spectral manipulation, and parametric homodyne detection, we generate, process, and measure multiple quantum channels in parallel. We demonstrate this through multiplexed protocols of both continuous-variable quantum key distribution (CV-QKD) and quantum teleportation. We experimentally demonstrate a proof-of-principle realization of multiplexed CV-QKD over 23 independent spectral channels with eavesdropping detection in each channel. These techniques pave the way for massively parallel quantum processing, potentially boosting the throughput of quantum protocols by orders of magnitude.

We provide a new characterisation of quantum supermaps in terms of an axiom that refers only to sequential and parallel composition. Consequently, we generalize quantum supermaps to arbitrary monoidal categories and operational probabilistic theories. We do so by providing a simple definition of locally-applicable transformation on a monoidal category. The definition can be rephrased in the language of category theory using the principle of naturality, and can be given an intuitive diagrammatic representation in terms of which all proofs are presented. In our main technical contribution, we use this diagrammatic representation to show that locally-applicable transformations on quantum channels are in one-to-one correspondence with deterministic quantum supermaps. This alternative characterization of quantum supermaps is proven to work for more general multiple-input supermaps such as the quantum switch and on arbitrary normal convex spaces of quantum channels such as those defined by satisfaction of signaling constraints.

Abstract This article presents an analysis of the practical challenges and implementation perspectives of point-to-point continuous-variable quantum key distribution (CV-QKD) systems over optical fiber. The study addresses the physical layer, including the design of transmitters, quantum channels, and receivers, with emphasis on impairments such as attenuation, chromatic dispersion, polarization fluctuations, and coexistence with classical channels. We further examine the role of digital signal processing (DSP) as the bridge between quantum state transmission and classical post-processing, highlighting its impact on excess noise mitigation, covariance matrix estimation, and reconciliation efficiency. The post-processing pipeline is detailed with a focus on parameter estimation in the finite-size regime, information reconciliation using LDPC-based codes optimized for low-SNR conditions, and privacy amplification employing large-block universal hashing. From a hardware perspective, we discuss modular digital architectures that integrate dedicated accelerators with programmable processors, supported by a reference software framework ( CV-QKD-ModSim ) for algorithm validation and hardware co-design. Finally, we outline perspectives for the deployment of CV-QKD in Brazil, starting from metropolitan testbeds and extending toward hybrid fiber/FSO and space-based infrastructures. The work establishes the foundations for the first point-to-point CV-QKD system in Brazil, while providing a roadmap for scalable and interoperable quantum communication networks.

Evaluating the maximum amount of work extractable from a nanoscale quantum system is one of the central problems in quantum thermodynamics. Previous works identified the free energy of the input state as the optimal rate of extractable work under the crucial assumption: experimenters know the description of the given quantum state, which restricts the applicability to significantly limited settings. Here, we show that this optimal extractable work can be achieved without knowing the input states at all, removing the aforementioned fundamental operational restrictions. We achieve this by presenting the construction of a quantum channel whose description does not depend on input states but nevertheless extracts work quantified by the free energy of the unknown input state. Remarkably, our result partially encompasses the case of infinite-dimensional systems, for which optimal extractable work has not been known even for the standard state-aware setting. Our results clarify that, even though whether the description of the given state is provided at the beginning of the protocol generally makes the operational setting fundamentally different in accomplishing information-theoretic tasks, not knowing the input state does not influence the optimal performance of the asymptotic work extraction.

Quantum teleportation and entanglement via quantum channels in bimetallic Fe2Cu2 based on spin-1/2 Ising-Heisenberg chain model

Abstract I will investigate the capacities of noisy quantum channels through a combined analytical
and numerical approach. First, I introduce novel flagged extension techniques that embed
a channel into a higher-dimensional space, enabling single-letter upper bounds on
quantum and private capacities. My results refine previous bounds and clarify noise
thresholds beyond which quantum transmission vanishes. Second, I present a simulation
framework that uses coherent information to estimate channel capacities in practice,
focusing on two canonical examples: the amplitude damping channel (which we confirm is
degradable and thus single-letter) and the depolarizing channel (whose capacity requires
multi-letter superadditivity). By parameterizing input qubit states on the Bloch sphere, I
numerically pinpoint the maximum coherent information for each channel and validate the
flagged extension bounds. Notably, I capture the abrupt transition to zero capacity at high
noise and observe superadditivity for moderate noise levels.

In this paper, drawing upon quantum state masking theory, we first propose a method that utilizes d-dimensional orthogonal arrays to encode quantum states into a multi-party quantum system. Building upon this foundation, we then design a quantum secret sharing scheme whose security is rooted in the principles of quantum information masking. This scheme is designed based on the hyperstar structure to define the access structure. Additionally, the protocol enables the transmission of arbitrary d-dimensional quantum states through quantum channels constructed using orthogonal arrays, ensuring that only participants within the access structure can recover the secret. We have thoroughly analyzed the security of this scheme against major quantum attacks, including intercept-resend attacks, entangle-measure attacks, and dishonest participant attack. In conclusion, our analysis examines the impact of four common types of noise on this protocol, confirming its immunity to dit-flip noise. Furthermore, we present a comparative analysis of the proposed scheme against existing quantum secret sharing protocols, highlighting its respective advantages.

Robust phase stabilization of a dark quantum channel over a 120 km deployed fiber link

In terahertz (THz) applications, Field-effect transistors (FETs) have emerged as prime candidates for the next generation of THz and sub-THz electronics. One of the main advantages of TeraFETs over state-of-the-art commercial THz electronics based on Schottky diodes is their ability to tune the plasma frequency over a wide range via gate voltage, which adjusts the sheet carrier density (electrons or holes) in the device channel. In this paper, we demonstrate that employing a Quantum Channel (QC) design, where the back barrier is positioned in close proximity to the front barrier, resulting in a channel thickness between 1 and 10 Bohr radii, offers significant advantages for TeraFET applications. This configuration enables increasing the maximum achievable density. In electro-optics applications, the QC design provides a unique opportunity for implementing an opto-FET, where optical absorption is modulated by the gate voltage. The key advantage of the QC-HEMT is the ability to shift the absorption edge by a large energy on the order of the Fermi level by changing the sheet carrier concentration in the device channel by modulating the gate voltage and, thereby, modulating the Moss–Burstein shift, effectively altering the optical band gap sensed by incoming radiation optical radiation by an electric field. A large sheet carrier concentration achieved in QC-HEMTs allows for a much broader modulation range of plasma frequencies and facilitates frequency multiplication via nonlinear plasmonic resonance (approximately 3 times higher). A large sheet carrier concentration also facilitates frequency multiplication via nonlinear plasmonic resonance. Such nonlinear frequency rectification dramatically increasing the frequency range (up to 1 THz or even higher) at much higher achievable powers, since the power is introduced via uniform gate voltage pumping by large-area grating gate structures. For an optical QC-HEMT, a large Moss-Burstein shift makes the optical QC-HEMT design superior as an optical modulator and optical spectrometer.

Private remote quantum sensing aims at estimating a parameter at a distant location by transmitting quantum states on an insecure quantum channel, limiting information leakage and disruption of the estimation itself from an adversary. Previous results highlighted that one can bound the estimation performance in terms of the observed noise. However, if no assumptions are placed on the channel model, such bounds are very loose and severely limit the estimation. We propose and analyze a private remote sensing protocol using-for the first time, to our knowledge-continuous-variable states in the single-user setting. Assuming a typical class of lossy attacks and employing tools from quantum communication, we calculate the true estimation error and privacy of our protocol, both in the asymptotic limit of many channel uses and in the finite-size regime. Our results show that a realistic channel-model assumption, which can be validated with measurement data, allows for a much tighter quantification of the estimation error and privacy for all practical purposes.

Unambiguous discrimination of the change point for quantum channels

Quantum secure direct communication (QSDC) enables the direct and secure transmission of messages without the need for prior key distribution, offering a groundbreaking paradigm in quantum communication. This paper presents a novel symmetric QSDC protocol that ensures immunity to collective-phase and collective-rotation noises while enabling efficient and two-way message transfer. The proposed protocol features an innovative approach where qubits are transmitted only during the initial entanglement sharing phase, eliminating the need to send message carrying qubits thereafter. By leveraging this design, the protocol minimizes vulnerability to external noise and eavesdropping attempts. The symmetry of the protocol allows both parties to exchange messages seamlessly and securely, ensuring equal communication procedure. Analytical analysis and theoretical proofs demonstrate the robustness and security of the protocol under non-secure quantum channel, marking a significant advancement in practical quantum communication systems. The proposed scheme follows an entanglement-based QSDC paradigm, where confidential information is embedded into quantum correlations rather than being directly transmitted over the quantum channel.

With the rapid advancement of quantum computing, traditional cryptographic techniques are at risk of devolution, necessitating quantum-resilient alternatives for future communication networks. This systematic literature review evaluates the role of Quantum Key Distribution (QKD) in enhancing the security of sixth-generation (6G) wireless communications. Employing the PRISMA methodology, 48 peer-reviewed studies published between 2016 and May 2025 were identified and analyzed. The review addresses three key research questions: the identification of QKD protocols applicable to 6G, challenges in their integration, and proposed solutions for seamless deployment. Findings reveal that protocols such as BB84, E91, CV-QKD, and MDI-QKD, transmitted via optical fiber and satellite channels, offer promising security guarantees. This review concludes that while QKD can significantly strengthen 6G communications against quantum threats, further interdisciplinary efforts in hardware development, standardization, and pilot implementations are essential. The study offers valuable insights for researchers, engineers, and policymakers working toward secure, quantum-resistant future networks. The study follows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology to ensure transparency, rigor, and reproducibility. A comprehensive search was conducted across major scientific databases, including IEEE Xplore, SpringerLink, ScienceDirect, and arXiv, using well-defined keywords and Boolean search strategies related to QKD, 6G networks, and quantum communication security. After removing duplicates and applying predefined inclusion and exclusion criteria, a total of 48 peer-reviewed studies published between 2016 and May 2025 were selected for detailed analysis. The selected literature was systematically classified to address three primary research questions: (i) identification of QKD protocols and technologies applicable to 6G networks, (ii) challenges hindering the integration of QKD into 6G architectures, and (iii) solutions and frameworks proposed to facilitate practical deployment. The findings reveal that prominent QKD protocols, including BB84, E91, Continuous-Variable QKD (CV-QKD), and Measurement-Device-Independent QKD (MDI-QKD), demonstrate strong potential for securing 6G communications when deployed over optical fiber and satellite-based channels. However, practical integration faces significant challenges such as scalability limitations, synchronization issues, quantum channel coexistence with classical networks, hardware complexity, and high deployment costs. The review further highlights emerging solutions that leverage Software-Defined Networking (SDN), Network Function Virtualization (NFV), blockchain-based key management, and hybrid classical-quantum security architectures to overcome these obstacles. Ongoing standardization efforts by organizations such as NIST, ETSI, and ITU-T are also identified as critical enablers for real-world adoption .

Tasks involving black boxes appear frequently in the theory of quantum information, with quantum channel discrimination as a central example that has been deeply studied. In this work, we experimentally study the discrimination between two unitary quantum channels in the multiple-shot scenario. We challenge the theoretical results concerning the probability of correct discrimination with the results collected from experiments performed on the IBM Brisbane. Our analysis shows that neither too deep quantum circuits nor circuits that create too much entanglement are suitable for the discrimination task. We conclude that circuit architectures which minimize entanglement overhead while preserving discrimination power are significantly more resilient to hardware noise if their depth does not exceed a threshold value. Consequently, our findings necessitate a paradigm shift: for execution on noisy hardware, the theoretically suboptimal circuit is, counterintuitively, often the superior choice.

Electron transport through a single quantum channel is fundamentally limited by the conductance quantum (G0 = 2e2/h ≈ 77.5 μS), achievable only in fully transparent systems without interfacial scattering. However, realizing this quantum limit in metal-molecule-metal junctions has long been hindered by intrinsic electronic mismatches at heterogeneous interfaces. Here, we report a carbon nanobelt single-molecule junction over 1 nm in length, whose conductance reaches G0, driven by the saturation of a single transport channel under ambient conditions. This unprecedented performance arises from electric-field-induced formation of covalent C-Au-C bonds at both contacts, creating atomically fused interfaces that seamlessly merge the nanobelt's π system with Au d orbitals. The resulting d-π conjugation establishes a single, transparent electronic resonance aligned with the Fermi level, suppressing backscattering and enabling near ideal quantum transport. By eliminating heterogeneous interfacial resistance at the atomic scale, this strategy offers a general blueprint for engineering atomically precise, energy-efficient nanoelectronic and optoelectronic devices.

Teleporting Single Qutrit using Symmetric-anti Symmetric Two-qutrit Basis States as Quantum Channels

Quantum information science has altered the concept of the method of processing and transporting information through the laws of quantum mechanics. Quantum entanglement is one of its basic properties, which is instrumental to non-classical correlations between spatially separate particles. New quantum communication networks and quantum computing systems distributed are based on these interrelations. In this paper, quantum information flow dynamics of multi-particle entangled networks with particular emphasis to the processes governing information transmission, redistribution and maintenance of coupled quantum nodes are discussed. The paper discloses the way in which the entangled structures comprising multiple entities can help in facilitating the sharing of information beyond the capabilities of the classical communication systems. Complex entangled structures are analyzed in terms of the channels through which the flow of quantum information is analyzed in respect to the theoretical models of multi-qubit systems and networked quantum channels. The way the communication between the particles influences the coherence, strength of correlation, as well as the stability of the whole network in general, is given particular attention. The effect of environmental noise and decoherence which can cause a break of entanglement and the loss of efficiency in quantum information exchange is also considered in the analysis. Moreover, the article discusses the application of network topology in order to establish the behaviour of entangled systems. The impact of the structural changes on information transfer reliability and speed is determined by the evaluation of the various forms of quantum nodes and connections. These findings suggest that entangled networks structured appropriately will be able to enhance stability of quantum communication and assist in distributing quantum resources more efficiently. Overall, the present research can be deemed as part of the bigger initiative of scaling up quantum communication infrastructures. The research builds knowledge in the sense that it provides an insight into the behavior of information in multi-particle entangled networks with important considerations being made as to the advancement of robust quantum networks which can be used in future to support the realization of future applications in the area of secure communication, distributed computing and the future developments in information processing technologies.

Abstract Quantum communication enables secure information transmission and entanglement distribution, but these tasks are fundamentally limited by the capacities of quantum channels. While quantum repeaters can mitigate losses and noise, entanglement swapping via a central node is ineffective against the Pauli dephasing channel due to degradation from Bell-state measurements. This suggests that purifying distributed Bell states before entanglement swapping is necessary. Although one-way hashing codes are known to saturate the dephasing channel capacity, no explicit two-way purification protocol has previously been shown to achieve this bound. In this work, we present a two-way entanglement purification protocol with an explicit, scalable circuit that asymptotically achieves the dephasing channel capacity. With each iteration, the fidelity of Bell states increases. At the final round, the residual dephasing error is suppressed doubly-exponentially, scaling as Θ(p 2 n ), enabling near-perfect Bell pairs for any fixed number of purification rounds n. The explicit circuit we propose is versatile and applicable to any number of Bell pairs, offering a practical solution for mitigating decoherence in quantum networks and distributed quantum computing.

Abstract The purpose of this paper is to further investigate the correlated and memory effects in 3D-Pauli-like noisy channels for controlled teleportation of an arbitrary unknown single-qutrit state. We first construct a three-qutrit maximally entangled state in a three-dimensional (3D) Hilbert space and subsequently propose a 3D controlled quantum teleportation (CQT) scheme for an arbitrary unknown single-qutrit state using the constructed three-qutrit maximally entangled state as the quantum channel. In this ideal case, through a few simple 3D unitary operations, the two communicating parties can perfectly accomplish the teleportation task under the supervision of the controller. Subsequently, we examine the performance of this CQT scheme under two successive uses of a 3D-Pauli-like noisy channel with memory, where the noise errors are categorized into four types: trit-flip, t-phase-flip, trit-phase-flip, and t-depolarizing. For each noise type, an analytical expression of the average fidelity is derived as a function of both the noise and memory parameters. Specifically, we find that, regardless of the noise strength, memory enhances the average fidelity for trit-flip and t-depolarizing noises, whereas for t-phase-flip and trit-phase-flip noises, memory reduces the average fidelity once the noise intensity exceeds a certain threshold. This indicates that in the first two noisy channels, memory can improve the communication efficiency of CQT, while in the latter two, excessive noise causes memory to diminish the teleportation performance.