RaCS: Near‐Zero‐Error Classical Data Encoding on Photonic Quantum Processors via Redundancy‐Assisted Coherent‐State Codes

International audience

Quantum state discrimination plays a central role in quantum information and communication. For the discrimination of optical quantum states, the two most widely adopted measurement techniques are photon detection, which produces discrete outcomes, and homodyne detection, which produces continuous outcomes. While various protocols using photon detection have been proposed for optimal and near-optimal discrimination between two coherent states, homodyne detection is known to have higher error rates, with its minimum achievable error rate often referred to as the Gaussian limit. In this work, we demonstrate that, despite the fundamental differences between discretely labelled and continuously labelled measurements, continuously labelled non-Gaussian measurements can also achieve near-optimal coherent state discrimination. We design two discrimination protocols that surpass the Gaussian limit: one using non-Gaussian unitary operations with homodyne detection, and another based on orthogonal polynomials. Our results show that photon detection is not required for near-optimal coherent state discrimination and that we can achieve error rates close to the Helstrom bound at low energies with continuously labelled measurements. We also find that our schemes maintain an advantage over the photon detection-based Kennedy receiver for a moderate range of coherent state amplitudes.

We introduce a general mapping for encoding quantum communication protocols involving pure states of multiple qubits, unitary transformations, and projective measurements into another set of protocols that employ coherent states of light in a superposition of optical modes, linear optics transformations and measurements with single-photon threshold detectors. This provides a general framework for transforming a wide class of protocols in quantum communication into a form in which they can be implemented with current technology. In particular, we apply the mapping to quantum communication complexity, providing general conditions under which quantum protocols can be implemented with coherent states and linear optics while retaining exponential separations in communication complexity compared to the classical case. Finally, we make use of our results to construct a protocol for the Hidden Matching problem that retains the known exponential gap between quantum and classical one-way communication complexity.

This paper evaluates the coherent optical code-division multiple-access (OCDMA) techniques and examines the overall system performance in terms of signal-to-noise ratio (SNR) penalty as a function of simultaneous users accommodated to maintain an appropriate value of the bit-error rate (BER) for homodyne and heterodyne detections. As spreading codes, the proposed structures are utilizing a recently introduced prime code family hereby referred to as double-padded modified prime code (DPMPC). As a coherent modulation, binary phase-shift keying (BPSK) format is deployed. In homodyne detection, two different phase modulations are studied including either an external phase-modulator or injection-locking methods. The phase limitation and the performance for two methods plus multiple-access interferences (MAI) and receiver noise in a shot-noise limited regime are investigated. In heterodyne detection, BER analysis of the system based on only external phase modulator is examined. It is found that by maintaining BER = 10−9, this system can accommodate an increased number of simultaneous users to compare with systems which employ conventional bipolar codes.

The phase-coding quantum cryptographic scheme using the homodyne detection\nand weak coherent state [Hirano et al.,Phys. Rev. A 68, 042331 (2003)] provides\nthe simplest continuous-variable quantum key distribution scheme from the\nexperimental side. However, the inherent loss of the practical system will not\nonly increase the bit error rate (BER) but also affect the security of the\nfinal key. In this paper, we propose a single-photon-detection attack, and then\nthe security of the final key will be compromised in some parameter regimes.\nOur results show that the BER induced by Eve can be lower than the inherent BER\ninduced by the loss of the system in some parameter regimes. Furthermore, our\nattack gives the maximal communication distance of this scheme for given\nexperimental parameters.\n

This paper studies an efficient inter-carrier interference (ICI) Cancellation scheme. The scheme works in two very simple steps. At the transmitter side, one data symbol is modulated on to a group of adjacent sub-carriers with a group of weighting coefficients. The weighting coefficients are designed such that the ICI caused by the channel frequency errors can be minimized. At the receiver side, by linearly combining the received signal on these sub-carriers with proposed coefficients, the residual ICI contained in the received signal can then be further reduced. In this paper performance of Orthogonal frequency division multiplexing (OFDM) systems in the presence of frequency offset between the transmitter and the receiver has been simulated in terms of carrier-to-interference (CIR) ratio and bit error rate (BER) performance. In this paper BER curves were used to evaluate the performance of ICI self-cancellation scheme. It is observed from the figure that in presence of small frequency offset and binary alphabet size, self cancellation gives the best results. From figure it has been concluded that BER of standard OFDM with ICI self cancellation using binary phase shift keying (BPSK) is better than BER of standard OFDM with ICI self cancellation using quardrature phase shift keying (QPSK). ICI self cancellation scheme is used for lower frequency offset values and lower modulation schemes like BPSK But as self cancellation scheme does not require very complex hardware and software implementation this scheme gives the best result for lower frequency offset and alphabet size.

Multicarrier transmission techniques like multicarrier code division multiple access (MC-CDMA) is the strongest contender for the future generation wireless networks which is supposed to cater to the need of very high data rate with mobility and at the same time combat multipath and intersymbol interference in a spectrally efficient manner. To enhance the spectral efficiency further, some form of adaptive modulation is being proposed (Wasantha et al. (2002)). In this paper, first of all, we simulate the performance of various single-h continuous phase modulation (CPM) schemes in an MC-CDMA system by varying the two CPM parameters - size of symbol alphabet (M) and modulation index (h). The parameter sets considered are {M=2, h=0.5}, {M=2, h=0.875}, {M=4, h=0.5} and {M=4, h=0.875}. Simulation results show that the higher order modulation schemes (M=4) need better channel conditions than the lower order schemes (M=2) for a given bit error rate (BER). Moreover, as expected (Xiong (2000)), h=0.875 (approximate optimum value) gives a better BER performance than h=0.5 for a given BER. Based on these simulation results, we then design two adaptive modulation based systems, one considering h=0.5 and the other with h=0.875, each with M=2 and 4. Results of the adaptive systems clearly indicate that the adaptive switching of the modulation formats in each of these two systems increases the system capacity per given bandwidth (spectrum efficiency) effectively without sacrificing the expected BER performance. Also, it is seen that the adaptive system with h=0.875 is superior to the h=0. 5 system, both in terms of BER performance and the number of simultaneous users, but obviously at the cost of reduced bandwidth efficiency as higher the value of h, the higher bandwidth it requires.

Quantum communication is the interdisciplinary science of quantum mechanics and telecommunication theory. It has advantages of perfect information security and high efficiency in transmission. In recent years, the theoretical and experimental results show that quantum communication systems have the superiority over the traditional communication systems. Quantum communication systems are hopeful for solving the information security problems that everyone is facing today, therefore, they possess broad application prospects and are forming a research hotspot of the telecommunications field currently. On the other hand, Voice over Internet Protocol (VoIP) is the method to transmit the digitized packet voice in Internet around the world. The advantages of VoIP are that it can carry voice, data, video, telephone conference, electronic commerce, and electronic mail economically. VoIP can realize the information storage and retransmission easily and flexibly. However, VoIP also encounters the problem of information security. We are trying to combine the quantum communications network and the VoIP system together and build a brand new network named quantum VoIP network which combines the advantages of both quantum communications and VoIP. The data packets may be delayed and lost in a queue up with a router due to the congestion and link failure during the transmission of quantum information. In order to ensure the performance of quantum VoIP system, the routing optimization strategies are proposed in the paper. The relay technology based on entanglement swapping is adopted. The multiuse quantum communications are realized by giving priority to the quantum channels with the least relay nodes. Theoretical analysis and simulation results show that when the data transmission links are fail to work properly and routers are in congestion, adopting the routing optimization strategies in M/M/m queuing system with the bit error rate (BER) of quantum bit setting to be 0.2 and the number of common channels increasing from 4 to 8,, the percentage of call failure in quantum communication network decreases from 0.25 to 0.024, and the maximum throughput of quantum networks increases from 64 kbps to 132 kbps. In comparison, when the number of common channels is set to be 4 andthe BER of the quantum bit is from 0.3 to 0.1, the maximum throughput of quantum networks increases from 41 kbps to 140 kbps. Thus it can be concluded that the routing optimization strategies proposed in this paper can improve the performance of quantum VoIP system significantly.

Asymmetric error characteristics in spin-transfer torque magnetic random-access memory (STT-MRAM), particularly the imbalance between logical '0' and '1' error probabilities, can significantly degrade system reliability under conventional modulation and error-correcting schemes. This issue is especially critical in sensor network applications, where STT-MRAM is widely adopted for its non-volatility, low standby power, and robustness under energy-constrained and intermittently active operation. Existing approaches typically optimize the detection threshold under the assumption of a fixed or equiprobable bit distribution, while sparse coding techniques impose a predefined imbalance without explicitly accounting for its interaction with threshold detection. In this paper, we formulate the bit error rate (BER) minimization problem as a joint optimization of the codeword bit distribution and the detection threshold over an asymmetric cascaded STT-MRAM channel. Analytical results reveal that the minimum BER is achieved when the error probabilities associated with transmitted '0' and '1' bits are balanced, which induces an intrinsic coupling between the optimal detection threshold and the codeword composition. Motivated by this insight, we propose a new family of threshold-matched probability codes (TMPCs), in which the proportion of logical '1's in each codeword is explicitly designed to match the optimal detection threshold of the underlying channel. The proposed coding framework generalizes conventional sparse modulation by enabling adjustable bit distributions while preserving low-complexity linear encoding and syndrome-based decoding. Numerical evaluations demonstrate that the TMPC achieves consistently lower BERs than existing sparse and fixed-distribution coding schemes across a wide range of STT-MRAM operating conditions, particularly under severe write asymmetry and resistance variation. These results indicate that the proposed joint design offers a principled and flexible approach for improving reliability in STT-MRAM-based sensor networks and non-volatile memory systems.

Quantum communication allows us to share information by using the quantum states of qubits. Superdense coding is a very popular protocol or scheme for quantum communication, which uses entangled qubits. Entangled qubits can also be used to share information using an ALOHA based protocol. Performance evaluation of these protocols is essential for their implementation in quantum communication. In this paper, we calculate the bit error rates (BERs) of these two entanglement-based quantum communication schemes. It has been noticed that the BER performance is improved in an ALOHA based scheme compared to the Superdense coding scheme. However, without using any error correction schemes, the BERs are still very high in both the cases compared to the classical communication methods. The BER performance can be further improved by using the appropriate error correction schemes, which are under investigation.

This manuscript presents a secure and spectrally efficient Spatial Modulation (SM)-based Non-Orthogonal Multiple Access (NOMA) system for two-user downlink 5G wireless communication. The proposed SM-NOMA shows significant improvements in spectral efficiency and energy savings by activating only one transmit antenna per symbol duration and all other remaining antennas remain silent, reducing hardware complexity and RF chain usage compared to traditional Multiple Input-Multiple Output (MIMO) systems. In terms of detection strategy, the weak user employs Maximum Ratio Combining (MRC) to directly decode its signal, as it receives higher power and does not need interference cancellation. Conversely, the strong user first applies MRC to detect the weak user’s signal, performs Successive Interference Cancellation (SIC) to remove the weak user’s contribution, and then applies MRC again to decode its own signal. This approach balances complexity and performance, offering practical implementation benefits. Additionally, the system includes optimized power allocation to manage interference and maintain user fairness. The proposed SM-NOMA system performance is evaluated using Bit Error Rate (BER), outage probability, and the spectral efficiency under imperfect CSI and imperfect SIC conditions. SM-NOMA outperforms conventional NOMA by significantly reducing BER, lowering outage probability and enhancing spectral efficiency. At a transmit power of 25 dBm, in the case of User 1 the BER is improved from 10−2 (NOMA) to 10−3 (SM-NOMA). Similarly, for User 2, there is an improvement in BER from 10−2 (NOMA) to 10−4 (SM-NOMA). Also, the outage probability values for both the users improved significantly with User 1 dropping from the previous value of 10−1 (NOMA) to 10−5 (SM-NOMA) at 20 dBm transmit power and for User 2 it drops from 10−4 to 8.39233 × 10−5. The overall spectral efficiency improved from 2.57 bps/Hz (NOMA) to 2.74 bps/Hz (SM-NOMA). Wireless systems are easily prone to attacks by intruders. To enhance transmission security, the system integrates Quantum-Enhanced Genetic Encryption (QGE2) a hybrid encryption framework combining quantum gate-generated keys and genetic algorithm-based encryption. The sequence generated using quantum gates is used as a seed value for the genetic algorithm. It is used to encrypt a 256 × 256 8-bit grayscale image. The randomness of the proposed quantum key is tested through the NIST test. The proposed encryption scheme passed all 17 NIST randomness evaluations (15 core + 2 subsections) with p-values ≥ 0.01. Additional metrics such as correlation, entropy, and histogram analysis validate the strength of the encryption against both classical and quantum threats. The integration of spatial modulation NOMA with QGE2 ensures the safe and secure transfer of information through the Rayleigh faded wireless channel.

A combination of optical frequency division multiplexing (FDM) and phase-shift-keying (PSK) homodyne detection can increase transmission capacity. However, phase sensitive transmission systems, especially repeatered ones, suffer from data-dependent optical amplitude fluctuation that is converted to phase fluctuation by fiber nonlinearity. The authors discuss how this data-dependent amplitude fluctuation affects the error rate performance of optical FDM PSK homodyne detection systems. If only the optical amplitude fluctuation induced by phase modulators is taken into account, the allowable power fluctuation to keep the power penalty at 0.5 dB at a bit error rate (BER) of 10/sup -10/ is below 0.17 mW for BPSK homodyne detection and 0.09 mW for QPSK homodyne detection. However, if only the amplitude fluctuation induced by the fiber chromatic dispersion is taken into account, the allowable number of repeaters to keep a 0.5-dB power penalty due to XPM at a BER of 10/sup -10/ is 1 for BPSK homodyne detection and below 5 for QPSK homodyne detection.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The work in this paper focuses on the system quality of direct and coherent communication system for two computers. A system quality is represented by Signal to Noise ratio (SNR) and Bit Error Rate (BER). First part of the work includes implementation of direct optical fiber communication system and measure the system quality .The second part of the work include implementation both the( homodyne and heterodyne)coherent optical fiber communication system and measure the system quality . Laser diode 1310 nm wavelength with its drive circuit used in the transmitter circuit . A single mode of 62.11 km optical fiber is selected as transmission medium . A PIN photo detector is used in the receiver circuit. The optical D-coupler was used to combine the optical signal that come from transmitter laser source with optical signal of laser local oscillator at 1310/1550 nm to obtain coherent detection . Results show that for direct detection the SNR and the BER (28.5 dB, 9.64x10-8,) respectively, while for homodyne and heterodyne coherent detection , the SNR(94.36,97.71)dB and the BER are (1.32x10-22,2.43x10-23) at maximum optical fiber length at 62.11 km. Results show that the homodyne and heterodyne detection are better than direct detection because the large output SNR and low BER of the received signal.

The work in this paper focuses on the system quality of direct and coherent communication system for two computers. A system quality is represented by Signal to Noise ratio (SNR) and Bit Error Rate (BER). First part of the work includes implementation of direct optical fiber communication system and measure the system quality .The second part of the work include implementation both the( homodyne and heterodyne)coherent optical fiber communication system and measure the system quality . Laser diode 1310 nm wavelength with its drive circuit used in the transmitter circuit . A single mode of 62.11 km optical fiber is selected as transmission medium . A PIN photo detector is used in the receiver circuit. The optical D-coupler was used to combine the optical signal that come from transmitter laser source with optical signal of laser local oscillator at 1310/1550 nm to obtain coherent detection . Results show that for direct detection the SNR and the BER (28.5 dB, 9.64x10-8,) respectively, while for homodyne and heterodyne coherent detection , the SNR(94.36,97.71)dB and the BER are (1.32x10-22,2.43x10-23) at maximum optical fiber length at 62.11 km. Results show that the homodyne and heterodyne detection are better than direct detection because the large output SNR and low BER of the received signal.

The receiver design for a high-speed free-space (wireless) optics (FSO) signal is necessarily highly complex when channel state information (CSI) is not available. Currently, although most approaches provide high detection performance in terms of bit error, receiver design is difficult to implement. This paper proposes two practical thresholding-based detection schemes, which offer significant improvement to receiver throughput on a computational load basis when CSI is not available. The first is based on a simple maximum likelihood (ML) function where the bit error rate (BER) is the same as conventional symbol-by-symbol detection. This method, however, causes a loss of BER performance. The second uses the aid of pilot-symbol-assisted modulation (PSAM) to modify the ML function when channel coefficients are temporally correlated. While numerical analysis based on this method shows that the BER performance in a log-normally distributed fading channel is very close to detection achieved with perfect CSI, the receiver suffers from increased complexity. If random processes for fading and noise are assumed as stationary and given that the detection threshold is quickly calculated and applied during a given period, such complexity of PSAM-based and symbol-by-symbol detection methods can be reduced.