The recent advances in quantum information processing, sensing, and communications are surveyed with the objective of identifying the associated knowledge gaps and formulating a roadmap for their future evolution. Since the operation of quantum systems is prone to the deleterious effects of decoherence, which manifests itself in terms of bit-flips, phase-flips, or both, the pivotal subject of quantum error mitigation is reviewed both in the presence and absence of quantum coding. The state of the art, knowledge gaps, and future evolution of quantum machine learning (QML) are also discussed, followed by a discourse on quantum radar systems and briefly hypothesizing about the feasibility of integrated sensing and communications (ISAC) in the quantum domain (QD). Finally, we conclude with a set of promising future research ideas in the field of ultimately secure quantum communications with the objective of harnessing ideas from the classical communications field.

Quantum cryptography provides absolute security against an all-powerful eavesdropper (Eve). However, in practice Eve's resources may be restricted to a limited aperture size so that she cannot collect all paraxial light without alerting the communicating parties (Alice and Bob). In this paper we study a quantum wiretap channel in which the connection from Alice to Eve is lossy, so that some of the transmitted quantum information is inaccessible to both Bob and Eve. For a pureloss channel under such restricted eavesdropping, we show that the key rates achievable with a two-mode squeezed vacuum state, heterodyne detection, and public classical communication assistance-given by the Hashing inequality-can exceed the secret key distillation capacity of the channel against an omnipotent eavesdropper. We report upper bounds on the key rates under the restricted eavesdropping model based on the relative entropy of entanglement, which closely match the achievable rates. For the pure-loss channel under restricted eavesdropping, we compare the secret-key rates of continuous-variable (CV) quantum key distribution (QKD) based on Gaussian-modulated coherent states and heterodyne detection with the discrete variable (DV) decoystate BB84 QKD protocol based on polarization qubits encoded in weak coherent laser pulses.

Computer manufacturers, software developers, and service providers spend significant time and resources to ensure that they can optimally support one class of applications. However, they cannot cope with the dynamic and continuous changes in applications and workload types, and consequently their offered services become unstable, fragile, and cannot guarantee the required Quality of Service (QoS). Furthermore, it becomes prohibitively expensive to build data centers (DCs) that are optimized for fixed types of workloads and businesses or to scale up for accommodating new growth in heterogeneous workload demands. Consequently, it is critically important that the architecture of next generation DCs is dynamically customizable to support a wide range of application types or businesses under constrained resources. In this paper, we present some design concepts for our Just In Time Architecture (JITA), a novel data center design to overcome the composable DC challenges by dynamically interconnecting DC components into a virtual DC (VDC) that is optimized to the service level objectives for each class of applications. We present how we can use optical waveguide links to build a passive crossbar that directly interconnects all DC resources at the required throughput and latency.

An optimized vector-quantization-inspired signal constellation design (OVQ-SCD) suitable for multidimensional optical transport is proposed, in which signal constellation radii transformation function is optimized and near-uniform distribution of points is achieved. The proposed OVQ-SCD is used in a tandem with a hybrid multidimensional coded-modulation scheme employing Slepian sequences as electrical discrete-time basis functions, orthogonal prolate spheroidal wave functions as impulse responses of optical filters in orthogonal-division multiplexing, and spatial modes as optical continuous-time basis functions. It has been shown that the proposed multidimensional coded-modulation schemes based on OVQ-SCDs outperform corresponding counterparts and can be used to enable beyond 10 Pb/s serial optical transport over spatial division multiplexing (SDM) fibers as well as beyond 1 Pb/s transport over SMFs. The proposed OVQ-SCD-based hybrid multidimensional coded modulation scheme can simultaneously solve the problems related to the limited bandwidth of information-infrastructure, high energy consumption, and heterogeneity of network segments; while enabling elastic and dynamic bandwidth allocation.

We have analyzed the possibility of long-haul superchannel transmission with an aggregate serial bit rate exceeding 1 Tb/s by using the mode-multiplexed multidimensional signaling. We considered nonbinary quasi-cyclic LDPC-coded OFDM signals transmitted over few-mode fibers (FMFs). The optimum vector-form nonlinear Schrödinger equation is developed to evaluate the performance of the proposed system. Both the impacts of nonlinear effects and nonlinear interaction between spatial modes have been included through the modified nonlinear Schrödinger equation we applied for the FMF case. Both two-dimensional and optimized four-dimensional (4D) signal constellations have been considered. To overcome the constraints imposed by the linear and nonlinear impairments in FMF, we proposed the use of block-coded modulation with advanced channel estimation and compensation techniques. We verified by means of simulation that the transmission of an aggregate serial bit rate of 1.2 Tb/s over 3000 km is achievable with a proposed LDPC-coded QPSK-OFDM format, whereas superchannel transmission with an aggregate serial rate of 2.4 Tb/s over 1800 km is achievable with the 16-QAM format. When a 4D 16-ary optimized constellation is used, we can extend the transmission distance of mode-multiplexed QPSK-OFDM by an additional 300 km.

In this paper, we present and analyze a novel concept for multidimensional elastic routing based on spatial and spectral optical networking enabled by optical Multiple Input Multiple Output (MIMO) processing and Orthogonal Frequency Division Multiplexing (OFDM). These two techniques, complemented with dynamic adaptive coded-modulation are shown to form the three cornerstones of the multidimensional elastic routing concept, such that they can perform key functionality currently performed by optical crossconnects and/or Layer 2 optical switches. A statistical throughput analysis which relates key spatial and spectral components (e.g. aggregation overhead, spatial inputs/outputs, and number of spectral superbands), and identifies important practical scenarios is performed. Moreover, a performance evaluation of a flexible rate-adaptive Low Density Parity Check (LDPC)-based coded modulation scheme for multidimensional elastic networking is presented, revealing considerable gains compared to legacy approaches. By thus exploiting spatial and spectral domains in optical fiber to enable flexible routing and switching, the new scheme is promising for next-generation elastic optical networks.

We study communication over atmospheric turbulence channels based on LDPC-coded signaling using multidimensional orbital angular momentum (OAM) signal constellations. Multidimensional signal constellation is obtained as the N-dimensional Cartesian product of a one-dimensional signal constellation originating from non-negative pulse-amplitude modulation. This scheme represents an energy efficient alternative, since a larger number of bits per symbol can be transmitted using a given bandwidth. We evaluate the performance of this scheme by determining conditional symbol probability density functions (PDFs) from numerical propagation data. Two cases are considered: (i) when conditional PDFs are known on the receiver side, and (ii) when conditional PDFs are not known and Gaussian approximation is used instead. We show that the OAM modulation is more sensitive to atmospheric turbulence as the number of dimensions increases. We also describe several applications of interest ranging from indoor wireless communications to intersatellite communications.

The concept of deep-space optical communications was formulated shortly after the invention of lasers. The promise of laser communications, high data rate delivery with significantly reduced aperture size for the flight terminal, led to the pursuit of several successful experiments from Earth orbit and provided the incentive for further demonstrations to extend the range to deep space. This paper is aimed at presenting an overview of the current status of optical communications with an emphasis on deep space. Future perspectives and applications of optical communications related to near-Earth and interplanetary communications are also addressed.

The parity-check matrix of a non-binary low-density parity-check (LDPC) code over GF(q) is constructed by assigning non-zero elements from GF(q) to the 1s in that of the corresponding binary LDPC code. In this paper, we provide a theorem that establishes a necessary and sufficient condition that a q-ary matrix constructed by assigning non-zero elements from GF(q) to the 1s in the parity-check matrix of a binary quasi-cyclic LDPC code must satisfy in order for its null-space to define a non-binary quasi-cyclic LDPC (NB-QC-LDPC) code over GF(q). We then propose a general scheme for constructing NB-QC-LDPC codes along with some other code construction schemes that might serve better for different design goals. We also demonstrate that NB-QC-LDPC codes are very suitable for high-rate applications, e.g. applications in magnetic recording and storage systems and optical communication systems.

We study the performance of unbalanced partially coherent uncoded and low-density parity-check (LDPC)-coded equal gain combining (EGC) receiver operating over generalized a-mu fading channels, when the binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK) are used. We determine the bit-error ratio (BER) performance degradation due to the imperfect reference signal recovery, receiver unbalancing and fading. Furthermore, we design large girth quasi-cyclic LDPC code with high code rate, suitable for use in communications over generalized fading channels. The proposed LDPC code does not exhibit the error floor phenomena, in the region of interest, even in the presence of imperfect cophasing and receiver unbalances.

We study a turbo equalization scheme based on low-density parity-check (LDPC) coded turbo equalization (TE). This scheme is suitable for simultaneous: (i) suppression of intra-channel nonlinearities, (ii) chromatic dispersion compensation, and (iii) polarization-mode dispersion (PMD) compensation. LDPC coding is based on large girth (g ges 8) block-circulant codes, and maximum a posteriori probability (MAP) equalizer is based on Bahl-Cocke-Jelinek-Raviv (BJCR) algorithm. The ultimate channel capacity limits, assuming an independent identically distributed (i.i.d.) source are reported as well. In the presence of intrachannel nonlinearities the LDPC-coded TE provides almost 12 dB improvement over BCJR equalizer at BER of 10-8. For an NRZ system operating at 10 Gb/s with residual dispersion of 11200 ps/nm and for differential group delay of 50 ps, the LDPC-coded TE is only 1 dB away from the i.i.d channel capacity. The efficiency of LDPC-coded TE in PMD compensation is demonstrated experimentally, with decoding performed off line.

We describe a multiple optical sources - multiple detectors scheme, based on either repetition MIMO or space-time coding and low-density parity-check (LDPC) codes. The proposed scheme is able to operate under strong atmospheric turbulence and provides excellent coding gains. The LDPC codes are designed using the concept of pairwise-balanced designs. Bit-error rates and achievable information rates are reported assuming non-ideal photodetection. To improve the spectral efficiency we employ the concept of bit-interleaved LDPC-coded modulation based on pulse-amplitude modulation.

In this work we use a new approach to model error events in long-haul optical fiber transmission systems. Existing approaches for obtaining probability density functions (PDFs) rely on numerical simulations or analytical approximations. Numerical simulations make far tails of the PDFs difficult to obtain, while analytical approximations are often inaccurate, as they neglect nonlinear interaction between pulses and noise. Our approach combines the instanton method from statistical mechanics, to model far tails of the PDFs, with numerical simulations to refine the middle part of the PDFs. We combine the two methods by using an orthogonal polynomial expansion constructed specifically for this problem. We demonstrate the approach on an example of a specific submarine transmission system.

A novel rate-optimal rate allocation algorithm is proposed for parallel transmission of scalable images in multi-channel systems. Scalable images are transmitted via fixed-length packets. The proposed algorithm selects a subchannel as well as a channel code rate for each packet, based on the signal-to-noise ratios (SNR) of the subchannels. The resulting scheme provides unequal error protection of source bits. Applications to JPEG2000 transmission show that significant UEP gains are achieved over equal error protection (EEP) schemes.

A fast rate-optimal rate allocation algorithm is proposed for parallel transmission of scalable images in multichannel systems. Scalable images are transmitted via fixed-length packets. The proposed algorithm selects a subchannel, as well as a channel code rate for each packet, based on the signal-to-noise ratios (SNRs) of the subchannels. The resulting scheme provides unequal error protection of source bits and significant gains are obtained over equal error protection schemes. An application of the proposed algorithm to JPEG2000 transmission shows the advantages of exploiting differences in SNRs between subchannels. Multiplexing of multiple sources is also considered, and additional gains are achieved by exploiting information diversity among the sources.

Several classes of low-density parity-check (LDPC) codes are proposed as an error control coding (ECC) scheme for a non-linear satellite system with the phase-shift keying (PSK) signaling. The bit-error rate (BER) performance of LDPC codes is determined by means of an advanced simulator that takes into account all major impairments in the transmission link between fixed ground stations over a non-regenerative geostationary (GEO) satellite. Our analysis includes the following impairments: the degradations caused by simultaneous influences of cross- polarization effects, undesired signals from neighboring satellite systems or terrestrial radio-relay links and phase noise produced by local oscillator at the satellite station and reference signal extractor at the receiving ground station. The proposed LDPC codes are compared against standard ECC schemes employed in modern satellite communication systems.

An iterative soft decoding scheme suitable for highspeed long-haul optical transmission is proposed. The approach is based on a combination of the method of optimal fluctuations (instantons) and a modification of a method, originally proposed in the context of magnetic media, which incorporates the BCJR algorithm and Low-Density Parity-Check (LDPC) codes. The proposed scheme can achieve target bit error rates at longer distances than conventional error correcting approaches.

Transmitter and receiver configurations suitable for M-ary DPSK/differential QAM noncoherent optical transmission are proposed. It is shown that multilevel coding based on lowdensity parity-check codes as component codes is an efficient approach to achieve high-spectrally efficient noncoherent optical transmission. The multilevel coding scheme with 2 bits/s/Hz spectral efficiency based on block-circulant component codes provides the coding gain of 12.3 dB compared to uncoded 8-DPSK, and 8.3 dB compared to uncoded QDPSK.

This paper considers a distribution/aggregation network that employs optical code division multiple access (OCDMA) to transmit information to a head-end node in a metropolitan area network. The head-end node transmits synchronization pulses and the nodes employ ranging techniques to offset the propagation delay, resulting in a quasi-synchronous reception at the receiver. The paper develops a methodology to design orthogonal OCDMA codes for such quasi-synchronous communication systems. The paper also describes the receiver design and computes the bit error rate of such systems.

A method for estimation of achievable information rates of high-speed optical transmission systems is proposed. This method consists of two steps: (i) approximating probability density functions for energy of pulses, which is done by instanton approach; (ii) estimating achievable information rates by a modification of a method originally proposed by Arnold and Pfitser. Numerical results for a specific optical transmission system (submarine system at transmission rate 40Gb/s) are reported.

FEC scheme based on LDPC codes is presented in this paper. We show that LDPC codes provide a significant system performance improvement with respect to the state-of-the-art FEC schemes, such as turbo BCH and RS codes, employed in optical communications systems. The system performance is further improved by a code design that eliminates short cycles in a graph employed in iterative decoding. As opposed to additive white Gaussian noise (AWGN) model for optical fiber channel, which is used very often in the analysis of error controlling schemes, our model takes into account all major impairments in a long-haul optical transmission such as amplifier spontaneous emission (ASE) noise, pulse distortion due to fiber nonlinearities, chromatic dispersion, crosstalk, intersymbol-interference, etc.