Optimal Encoding Capacity of a Linear Optical Quantum Channel

TL;DR

This paper analyzes the maximum information capacity of linear optical quantum channels, revealing that non-rail encoding and certain encoding schemes can significantly enhance data transmission without entangling operations.

Contribution

It derives an analytic formula for channel capacity with linear optical encoding and demonstrates encoding schemes that outperform traditional methods without entanglement.

Findings

Capacity is improved by non-rail encoding.

Explicit encoding scheme using dense coding achieves higher information transfer.

Potential for greater gains in larger systems.

Abstract

Here, we study the capacity of a quantum channel, assuming linear optical encoding, as a function of available photons and optical modes. First, we observe that substantial improvement is made possible by not restricting ourselves to a rail-encoded qubit basis. Then, we derive an analytic formula for general channel capacity and show that this capacity is achieved without requiring the use of entangling operations typically required for scalable universal quantum computation, e.g. KLM measurement-assisted transformations. As an example, we provide an explicit encoding scheme using the resources required of standard dense coding using two dual-rail qubits (2 photons in 4 modes). In this case, our protocol encodes one additional bit of information. Greater gains are expected for larger systems.