Quantum error rejection and fault tolerant quantum communication

Quantum communication utilizes the quantum state as information carrier. The transmission of quantum states is therefore a precondition for various quantum communication protocols. Photons play a central role in quantum communication since they are fast, cheap, easy to control and interact weakly with the environment. However, the widely used polarization degree of freedom of photons is vulnerable to the noise during the transmission. In this article, we review two main methods to deal with the channel noise, i.e., the quantum error rejection scheme and fault tolerant quantum communication. To transmit an arbitrary single-photon state, Li and Deng proposed two faithful state transmission schemes only by resorting to passive linear optics. The success probability can be (2N+1-1)/2N+1 by introducing a wave splitter composed of N unbalance interferometers. Compared with other quantum error rejection schemes, these two scheme are practical both in maneuverability and resource consumption. They are not only suitable for single-photon pure state transmission but also able to be used for transmitting mixed state, which makes them useful for one-way quantum communication. The success probability of error rejection is usually less than 100% since some error cases are rejected. To realize complete fault tolerant quantum communication, decoherence free subspace can be used to encode quantum information. In 2008, Li et al. proposed two efficient quantum key distribution schemes over two different collective-noise channels. The noiseless subspaces are made up of two Bell states and the spatial degree of freedom is introduced to form two nonorthogonal bases. Although entangled states are employed, only single-photon measurements are required to read the information. Later, the scheme is generalized to an efficient one which transmits n-1 bits information via n Einstein-Podolsky-Rosen pairs and many fault tolerant quantum communication schemes were proposed. We compare the practicality of different anti-noise schemes based on maneuverability and resource consumption and a perspective of these two research directions is given in the last section.