Abstract Shared entanglement can significantly amplify classical correlations between systems interacting over a limited quantum channel. A natural avenue is to use entanglement of the same dimension as the channel because this allows for unitary encodings, which preserve global coherence until a measurement is performed. Contrasting this, a distributed task based on a qubit channel is demonstrated, for which irreversible encoding operations can outperform any possible coherence‐preserving protocol. This corresponds to using high‐dimensional entanglement and encoding information by compressing one of the subsystems into a qubit. Demonstrating this phenomenon requires the preparation of a 4D maximally entangled state, the compression of two qubits into one and joint qubit‐ququart entangled measurements, with all modules executed at near‐optimal fidelity. A proof‐of‐principle experiment is reported that achieves the advantage by realizing separate systems in distinct and independently controlled paths of a single photon. This result demonstrates the relevance of high‐dimensional entanglement and non‐unitary operations for enhancing the communication capabilities of standard qubit transmissions.
Abstract Shared entanglement can significantly amplify classical correlations between systems interacting over a limited quantum channel. A natural avenue is to use entanglement of the same dimension as the channel because this allows for unitary encodings, which preserve global coherence until a measurement is performed. Contrasting this, a distributed task based on a qubit channel is demonstrated, for which irreversible encoding operations can outperform any possible coherence‐preserving protocol. This corresponds to using high‐dimensional entanglement and encoding information by compressing one of the subsystems into a qubit. Demonstrating this phenomenon requires the preparation of a 4D maximally entangled state, the compression of two qubits into one and joint qubit‐ququart entangled measurements, with all modules executed at near‐optimal fidelity. A proof‐of‐principle experiment is reported that achieves the advantage by realizing separate systems in distinct and independently controlled paths of a single photon. This result demonstrates the relevance of high‐dimensional entanglement and non‐unitary operations for enhancing the communication capabilities of standard qubit transmissions.
Quantum entanglement plays a central role in many quantum communication protocols. It allows distant particles to share correlations beyond the limits of classical interactions. Entanglement is essential for superdense coding, quantum teleportation, and secure cryptographic key distribution in quantum communication. It is also a pillar for developing quantum networks, where the management and distribution of entanglement are crucial for connecting distant nodes. Although perfect entanglement is a sought-after ideal, experimental imperatives, including entanglement distribution over long distances, often limit the quality of entangled states. An important question is whether weaker entanglement still offers advantages. First, we study more general tasks than dense coding to show that simpler measurements, combined with entanglement, allow advantageous and sometimes even optimal qubit communication protocols to be obtained. We also demonstrate that simple measurements can generate quantum correlations that cannot be modelled by two classical communication bits and can constitute an optimal protocol with quantum resources. We implement a novel Bell-type inequality tailored for certifying full network non-locality (FNN) to develop certification methods guaranteeing security on an entanglement-based network. Our experiment uses two pairs of polarised entangled photons in a network configuration with three nodes, briefly referred to as a bilocal network scenario. Finally, we show that weakly entangled states can improve communication over a qubit channel using only separate (local) measurements on isotropic non-steerable two-qubit states, without interference, of individual photons, across two communication tasks: secret sharing and its stochastic variant.
This paper investigates a novel approach to quantum communication leveraging the unique properties of multi-body quantum entanglement for enhanced data compression and encoding. The core claim centers on utilizing entangled states to mitigate the detrimental effects of channel loss, a significant limitation in traditional quantum communication. The proposed mechanism involves constructing multi-body entanglement sources, employing measurement and post-processing techniques to eliminate redundant information and extract critical data. Furthermore, we design efficient quantum encoding schemes exploiting the inherent correlations within entangled states, aiming to minimize redundancy and maximize information transmission efficiency. The research demonstrates a pathway towards achieving high-fidelity, long-distance quantum information compression and encoding, representing a significant advancement over single-photon encoding approaches. The theoretical framework and key concepts are presented, outlining the potential for robust quantum communication networks.
Dense coding is the seminal example of how entanglement can boost qubit communication, from sending one bit to sending two bits. This is made possible by projecting separate particles onto a maximally entangled basis. We investigate more general communication tasks, in both theory and experiment, and show that simpler measurements enable strong and sometimes even optimal entanglement-assisted qubit communication protocols. Using only partial Bell state analysers for two qubits, we demonstrate quantum correlations that cannot be simulated with two bits of classical communication. Then, we show that there exists an established and operationally meaningful task for which product measurements are sufficient for the strongest possible quantum predictions based on a maximally entangled two-qubit state. Our results reveal that there are scenarios in which the power of entanglement in enhancing quantum communication can be harvested in simple and scalable optical experiments.
Abstract High‐dimensional quantum states, or ‘qudits’, provide significant advantages over traditional qubits in quantum communication, such as increased information capacity, enhanced noise resilience, and reduced information loss. Despite these benefits, their implementation has been constrained by challenges in generation, transmission, and detection. This paper presents a novel theoretical framework for transmitting quantum information using qudit entanglement distribution over a superposition of causal orders in two quantum channels. Using this model, a quantum switch operation for 2‐qudit systems is introduced, which facilitates enhanced fidelity of entanglement distribution and quantum teleportation. The results demonstrate that the use of qudits in entanglement distribution achieves a fidelity improvement from 0.5 (for qubit‐based systems) to 0.94 for 20‐dimensional qudits, even under noisy channel conditions. This enhancement is achieved by exploiting the increased Hilbert space of high‐dimensional states and the inherent noise‐resilience properties of quantum switches operating in superpositions of causal orders. The findings underscore the potential of qudit‐based quantum systems in achieving robust and high‐fidelity communication in environments where traditional qubit‐based systems face limitations.
One of the primary goals of information theory is to provide limits on the amount of information one can send through communication channels, and to understand encoding methods that achieve such limits. The discovery of dense coding showed that quantum channels can transmit twice as much classical information than is possible with a classical channel. This optimal rate requires using quantum systems maximally entangled between sender and receiver, and then the sender can encode her messages using unitary operations. The use of nonmaximally entangled systems has been intensively studied in recent years, and it has appeared as though unitary encoding might well be optimal in all cases. Indeed, this optimality of unitary operations has been found to hold under far more general conditions. Nonetheless, we here present numerical evidence supported by analytical arguments that there exist circumstances where more classical information can be encoded using nonunitary, rather than unitary, operations.
This paper introduces a protocol for quantum communication that employs entangled and separable pairs of particles, enabling classical information transmission without the need for classical communication between the parties. In this system, one party, Alice, manipulates the entanglement degree of photon pairs within two distinct channels, |ψ⟩ 1 and |ψ⟩ 2. The other party, Bob, subsequently measures the state of these photons. This study uses the entanglement status of each photon pair to encode classical bits: entangled pairs |00⟩ + |11⟩ represent the bit '1', while separable pairs |00⟩ represent '0'. The proposed protocol incorporates a secondary quantum channel that enhances entanglement determination accuracy through additional measurements and phase shifts. The proposed approach stands apart from conventional quantum teleportation, which requires inter-party classical communication, by only requiring an internal classical link at the receiver's end for entanglement status resolution. The proposed method offers benefits when classical communication implementation is impractical or costly. The research highlights the potential of a fully quantum-based communication system, bridging the gap between quantum entanglement and quantum teleportation in a novel, pragmatic manner. While there is a need for improvements to enhance efficiency, security, and reliability, the proposed protocol offers a promising path for future explorations in quantum channel communication.