The practical limits of photon communication

C. Bendjaballah

Yuhua Chen, Subhash C. Kak, Pramode K. Verma +3 more

Contemporary implementations of Quantum Key Distribution are based on BB84, first proposed in 1984, and commercially implemented for limited market applications in the early 2000s. A major limitation of BB84 is that it requires no more than a single photon per time slot. But no device can reliably generate single photons with guaranteed periodicity. In this paper, we address the practical issues of realizing quantum cryptography systems. In particular, we address the major shortcomings of quantum cryptography as practiced today - the limitation of requiring a single photon to be used in communication. This not only affects the security of the practical quantum cryptography systems, but also restricts the distance over which secure keys can be sent and their rate. Our approach is not limited to a single photon per time-slot thus making it possible to send keys faster and over longer distances. The paper discusses the innate nature of multi-photon communication protocols as a surrogate for quantum communication while giving it a cryptographic strength that would closely match that of a pure quantum communication system.

Michael Hippke

A photon can encode several bits of information based on an alphabet of its time of arrival, energy, and polarization. Heisenberg's uncertainty principle places a limit on measuring pairs of physical properties of a particle, limiting the maximal information efficiency to <59 bits per photon in practice, and <171 bits per photon at Planck energy, at a data rate of one photon per second.

Konrad Banaszek

Linying Lu, Zhihua Chen

Abstract Quantum secure direct communication (QSDC) enables the direct transmission of confidential messages without pre-establishing a secret key. Device independence (DI) can further enhance its practical security against untrusted devices. However, existing DI-QSDC protocols rely on a single Clauser–Horne–Shimony–Holt inequality, which severely limits both the achievable communication distance and the secret message capacity, while the detection loophole remains open in DI-QSDC. To overcome these limitations, we propose a one-sided DI-QSDC protocol based on single-photon sources and heralded entanglement. The protocol combines entanglement distribution, loss-tolerant steering-based security verification, and measurement-DI communication verification and decoding. We employ the Navascués–Pironio–Acín hierarchy based semidefinite programming relaxations to evaluate the secret message capacity. Numerical simulations show that our protocol extends communication distances by factors of 5.6 and 32.2, and improves the practical secure communication efficiency by factors of 1.8 and 1102.5, compared to single-photon-source-based DI-QSDC and SPDC-based DI-QSDC, respectively.

Muhammad Amad Khan, Prof Tasawar