A communication channel is a physical system that transfers information from one place to another. Examples of communication channels include wires, optical fibers, and chains of spins that propagate spin waves through a medium. This Letter shows that the power-limited communication capacity of a multimode optical fiber or a set of parallel spin chains can be enhanced by introducing nonlinear couplings between the modes or chains. In particular, M coupled, entangled modes can send M bits in the same time it takes a single mode to send a single bit, and in the same time it takes M uncoupled, unentangled modes to send sqrt[M] bits.
A natural question in characterizing the information theoretic power of quantum channels is to ask at what rate entanglement is needed in order to asymptotically simulate a quantum channel in the presence of free classical communication. We call this the entanglement cost of a channel, and prove a formula describing it for all channels. We discuss two applications. Firstly, we are able to link the security in the noisy-storage model to a problem of sending quantum rather than classical information through the adversary's storage device. This not only greatly improves the range of parameters where security could be shown previously, but allows us to prove security for storage devices for which no non-trivial statements were known before. Secondly, our result has consequences for the study of the strong converse quantum capacity. Here, we show that any coding scheme that sends quantum information through a quantum channel at a rate larger than the entanglement cost of the channel has an exponentially small fidelity.
Entanglement is an important resource for various applications of quantum computation. Another important endeavor is to establish the role of entanglement in practical implementation where system of interest is affected by various kinds of noisy channels. Here, a single classical bit is used to send information under the influence of a noisy quantum channel. The entanglement content of quantum states is computed under noisy channels such as amplitude damping, phase damping, squeesed generalised amplitude damping, Pauli channels and various collective noise models on the protocols of quantum key distribution.
This thesis investigates the feasibility of utilising classical error-correcting codes in quantum communications, specifically in the entanglement-assisted communication of classical information over quantum depolarising channels. A classical-quantum communication system is presented, which relies on error-correction in the classical domain to achieve a classical information rate that approaches the entanglement-assisted capacity of a depolarising channel. Classical information is transmitted over the quantum channel by means of the superdense coding protocol. Different scenarios for the system are considered, including the noiseless and noisy distribution of initial entanglement resources and the use of higher-order entangled states. The overall transmission model corresponding to each scenario is shown to reduce to a classical, discrete and memoryless channel. The capacities of these equivalent classical channels are derived, and an inherent capacity loss when binary classical error correcting codes are employed is identified, motivating the use of non-binary codes. It is ultimately demonstrated that duo-binary turbo codes outperform binary turbo codes in all investigated scenarios, a result of the former's ability to exploit correlations between the pairs of classical bits communicated via the superdense coding protocol.
We study the advantages of quantum communication models over classical communication models that are equipped with a limited number of qubits of entanglement. In this direction, we give explicit partial functions on
n bits for which reducing the entanglement increases the classical communication complexity exponentially. Our separations are as follows. For every
k≥1:
Q∥∗ versus
R2∗: We show that quantum simultaneous protocols with
Θ~(k5log3n) qubits of entanglement can exponentially outperform two-way randomized protocols with
O(k) qubits of entanglement. This resolves an open problem from [Gav08] and improves the state-of-the-art separations between quantum simultaneous protocols with entanglement and two-way randomized protocols without entanglement [Gav19, GRT22].
R∥∗ versus
Q∥∗: We show that classical simultaneous protocols with
Θ~(klogn) qubits of entanglement can exponentially outperform quantum simultaneous protocols with
O(k) qubits of entanglement, resolving an open question from [GKRW06, Gav19]. The best result prior to our work was a relational separation against protocols without entanglement [GKRW06].
R∥∗ versus
R1∗: We show that classical simultaneous protocols with
Θ~(klogn) qubits of entanglement can exponentially outperform randomized one-way protocols with
O(k) qubits of entanglement. Prior to our work, only a relational separation was known [Gav08]. Our techniques can also be used to show advantages of quantum communication models over hybrid classical-quantum models, i.e., models that have a large amount of both classical communication and quantum simultaneous communication.
This paper investigates the physical layer security in wireless communication networks, focusing on the scenario where a base station sends confidential messages to a legitimate node while facing potential eavesdropping. The study assumes that the channel state information for both authorized and unauthorized nodes is not available. To assess both security and reliability, the paper introduces the use of bit error rate (BER) as a quantifiable security metric. The objective of the research is to evaluate the security performance improvement of the system through BER. Specifically, the impact of artificial noise (AN) power allocation ratio, and the number of transceiver antennas on BER performance is characterized. The numerical results demonstrate that artificial noise is effective against unknown passive eavesdropping attacks and leads to significant performance gains. For instance, by transmitting artificial noise power at approximately 50% of the total power to deliberately impair the eavesdropper’s reception, the BER gap between intended and unauthorized users can reach 0.4.