In this paper, we propose several fiber Bragg gratings (FBGs) based on both weak coherent state (WCS) and entanglement-assisted (EA) multidimensional QKD (MQKD) protocols with mutually unbiased bases (MUBs) derived from the complex Hadamard matrices. We also propose how to implement them by using FBGs with orthogonal impulse responses, derived from Slepian sequences. The proposed FBG-based protocols are capable of simultaneously achieving unprecedented spectral efficiency and secret-key rates in long-distance quantum communication. Regarding the WCS protocols, random base selection and FGB-MUB-based protocols are introduced. Regarding EA protocols, we introduce a generic class of FBG-based N-dimensional protocols and describe the corresponding FBG-based encoders and decoders. The security analysis of the proposed MQKD protocols is further performed by taking into account FBG fabrication imperfections, modeled by the L-neighbor model. It has been shown that the proposed MQKD protocols significantly outperform the conventional two-dimensional QKD protocols, and impose less stringent bandwidth requirements compared to multidimensional time-frequency and time-bin protocols.
In this paper, we propose several fiber Bragg gratings (FBGs) based on both weak coherent state (WCS) and entanglement-assisted (EA) multidimensional QKD (MQKD) protocols with mutually unbiased bases (MUBs) derived from the complex Hadamard matrices. We also propose how to implement them by using FBGs with orthogonal impulse responses, derived from Slepian sequences. The proposed FBG-based protocols are capable of simultaneously achieving unprecedented spectral efficiency and secret-key rates in long-distance quantum communication. Regarding the WCS protocols, random base selection and FGB-MUB-based protocols are introduced. Regarding EA protocols, we introduce a generic class of FBG-based N-dimensional protocols and describe the corresponding FBG-based encoders and decoders. The security analysis of the proposed MQKD protocols is further performed by taking into account FBG fabrication imperfections, modeled by the L-neighbor model. It has been shown that the proposed MQKD protocols significantly outperform the conventional two-dimensional QKD protocols, and impose less stringent bandwidth requirements compared to multidimensional time-frequency and time-bin protocols.
In this paper, we propose several FBG-based both weak coherent state and entanglement assisted multidimensional QKD (MQKD) protocols with mutually unbiased bases (MUBs) derived from the complex Hadamard matrices. We also propose how to implement them by using FBGs with orthogonal impulse responses. The proposed MQKD protocols significantly outperform the conventional two-base QKD protocols, and impose less stringent bandwidth requirements compared to multidimensional time-frequency (t–f) protocols. The proposed FBG-based protocols are capable of simultaneously achieving unprecedented spectral efficiency and secret-key rates (SKRs) in long-distance quantum communication.
In this paper, we propose several entanglement assisted QKD protocols based on time-energy encoding with the number of mutually unbiased bases (MUBs) larger than two. We describe how to implement these protocols based on: (i) optical FFT device implemented in integrated optics with the help of Franson interferometers and (ii) Weyl gate. We also describe the corresponding weak-coherent state-based protocol. By employing the N-dimensional pulse position modulation (ND-PPM) approach, the secret key rate of single photon pulse per signaling interval protocols can be improved by N/log2N times. However, the corresponding entanglement assisted protocols require the use of cavity quantum electrodynamics (CQED) principles to further entangle single photon pulse per frame state. We then analyze the security of the proposed protocols and provide the finite secret key rates in the presence of various imperfections including background errors and timing jitter, for which we propose the K-neighbor model. Finally, we provide the improvements in secret key rates of proposed protocol over conventional two-base time-energy QKD protocol.
Quantum key distribution (QKD) leverages underlying principles of quantum mechanics to realize distribution of keys with verifiable security. Despite appealing features of QKD, there are some fundamental and technical challenges that need to be solved prior to its widespread applications. First, QKD secret-key rate (SKR) is fundamentally limited by channel loss, as dictated by the rate-loss tradeoff. Quantum repeaters would be an ultimate solution to overcome this problem; however, they are well beyond the reach. The second challenge lies in the scalability and cost. Future's QKD systems must be suitable for mass production with low cost, reliable realignment-free operations, and small power consumption. To solve for these problems in a simultaneous manner, we propose to encode information in the orthogonal Slepian sequences' bases. Such an approach is highly robust against turbulence effects in free-space optical links and dispersion effects/fiber nonlinearities in fiber-optics channels, thereby improving QKD distance. Moreover, exploiting multidimensional encoding space enables high spectral efficiency QKD so that the SKR can be significantly improved. Critically, generation, processing, and detection of Slepian states can be reliably implemented in an integrated quantum photonics platform, based on both reflective and transmissive waveguide Bragg gratings (WBGs). Proposed reflective/transmissive WBG-based Slepian states are applicable to both discrete variable and continuous variable QKD systems.
It has been widely recognized that the underlying principles of quantum mechanics could be used to enable secure communications, without any additional conventional cryptographic systems, while utilizing the properties of the photons. Considerable research efforts have been invested to develop an efficient quantum key distribution (QKD) scheme over either free-space optical or fiber-optics channels. Most of these research efforts have been focused on a two-dimensional QKD, commonly realized by the use of the polarization state of photons. However, the data rates for the quantum key exchange in two-dimensional QKD are still low, while transmission distance is limited. On the other hand, it is well known that photons can carry both the spin angular momentum (SAM) and the orbital angular momentum (OAM), associated with the polarization and azimuthal phase of the complex electric field, respectively. Accordingly, we can define the combined-OAM-SAM state of a photon as |l, σ〉, where l and σ correspond to OAM and SAM indexes, respectively. Since the OAM eigenstates are orthogonal, an arbitrary number of bits per single photon can be transmitted, which could considerably increase the total secure bit rate. To improve secure data rates, we propose two types of protocols, namely, nonentangled-based (such as weak coherent state) and entanglement-assisted protocols, both employing the photon combined-OAM-SAM state, which can be used for secure key distribution over free-space optical and few-mode fiber channels. Two types of entanglement-assisted protocols are described, namely, two-basis and (D + 1)-basis protocols ( D is dimensionality of corresponding Hilbert space). We further describe how to implement the qudit gates required for implementation of these protocols. Finally, we discuss the security issues of the proposed protocols and determine both infinite and finite secret key fraction rates.
Superstructured fiber Bragg grating (FBG) technology has emerged as an attractive means to produce compact, and potentially low-cost in-fiber devices for a variety of lightwave applications. In this paper, a secure optical communication system based on tunable FBG arrays is described. Utilizing the inherent characteristics of FBG array to decompose broadband pulses simultaneously in wavelength and time, one broadband light pulse reaching a series of different Bragg gratings may be split into a number of separated pulses, each with different wavelength and is positioned at different time. To recover the original pulse, an inverted grating array is needed at the receiver side, i.e., the transmission is secure in case that the exact structure of the FBG array is unknown. Some of the factors affecting the process of encryption/decryption are indicated. To further increase the security of the system, theories about bent functions are presented and applied to generate bent sequences so as to charge the modes of FBG arrays because of the bent functions' excellent cryptographic properties. Simulations results are presented and analyzed to show the feasibility of the system. The paper is concluded with a discussion of the advantages for such an optical encryption/decryption system.
Entanglement is an essential ingredient in many quantum communication protocols. In particular, entanglement can be exploited in quantum key distribution (QKD) to generate two correlated random bit strings whose randomness is guaranteed by the non-local property of quantum mechanics. While QKD protocols based on polarization and time-bin entanglement have been widely investigated, secure key distribution based on frequency-bin entangled photon sources have received only minor attentions.Here we experimentally implement the BBM92 protocol using a frequency-bin encoded entangled state, generated by an integrated source of photon pairs in a silicon photonic chip. We execute a passive basis selection scheme and simultaneously record sixteen projective measurements over two mutually unbiased basis sets. An adaptive rotation of the measurement base is introduced to compensate the random relative phase fluctuation between the two 15-GHz-spaced frequency bins in a fiber spool.Stable transmission over a 26 km fiber spool is demonstrated with a secret key rate ≥ 4.5 bit/s.