We discuss quantum hyperdense coding (qHDC), a quantum communication protocol which is capable of transmitting up to 2.58 bits per hyperentangled photon pair (exceeding classical limits). We present current progress on experimental implementation of qHDC using polarization and timing modes of single photons.
We present a method, known as hyperdense coding, which uses photons hyperentangled in polarization and temporal mode to transmit up to 2.81 bits/photon of classical information over a two-qubit quantum channel. Furthermore, the hyperentangled photons used in this approach are much less susceptible to the influences of turbulence than spatial qubits, allowing for turbulence-resistant communication. We compare this technique to previously implemented hyperentanglement-enhanced superdense coding implementations which have a maximum theoretical channel capacity of 2 bits/photon.
We present a quantum hyperdense coding protocol with hyperentanglement in polarization and spatial-mode degrees of freedom of photons first and then give the details for a quantum secure direct communication (QSDC) protocol based on this quantum hyperdense coding protocol. This QSDC protocol has the advantage of having a higher capacity than the quantum communication protocols with a qubit system. Compared with the QSDC protocol based on superdense coding with d-dimensional systems, this QSDC protocol is more feasible as the preparation of a high-dimension quantum system is more difficult than that of a two-level quantum system at present.
We propose a two-step quantum secure direct communication (QSDC) protocol with hyperentanglement in both the spatial-mode and the polarization degrees of freedom of photon pairs which can in principle be produced with a beta barium borate crystal. The secret message can be encoded on the photon pairs with unitary operations in these two degrees of freedom independently. This QSDC protocol has a higher capacity than the original two-step QSDC protocol as each photon pair can carry 4 bits of information. Compared with the QSDC protocol based on hyperdense coding, this QSDC protocol has the immunity to Trojan horse attack strategies with the process for determining the number of the photons in each quantum signal as it is a one-way quantum communication protocol.
Superdense coding proved that entanglement-assisted quantum communications can improve the data transmission rates compared to classical systems. It allows sending 2 classical bits between the parties in exchange of 1 quantum bit and a pre-shared entangled Bell pair. This paper introduces a new protocol which is intended for distributed communication. Using a pre-shared entangled Bell pair and 1 classical bit 2,5 classical bits can be transmitted in average. This means not only valuable increase in capacity but the two-way distributed operation opens new fields of investigation.