Physical Implementation Of Quantum Information Processing In Decoherence-free Subspace

Quantum information is a new subject, which is the combination of quantum theory and information theory. Compared with classical information science, it manifests distinct advantages in many respects. For example, information capacity, information security, operates speed, and so on. These advantages are due to the coherence of quantum system. However, decoherence induced from the more or less interaction between quantum system and environment, which will interfere the quantum information processing, even spoil the quantum information. How to resist decoherence has become the aim of many physicist. There are three schemes have been proposed so far, The first is quantum information coded, the second is dynamically decoupled from the environment, and the third is topological quantum computation. In these schemes, quantum information coded has attracted widespread attention for its universal application, which prevents the quantum entanglement from noise mainly through two methods, one is error correction coded, and the other is error protection coded. The former is applicable to any quantum system, but need to introduce a large number of auxiliary bits. The latter, for its symmetry, become a very effective scheme in dealing with collective dephasing. The scheme is proposed by Duan Lu-ming and Guo Guang-can from University of Science and Technology, they use an atom pair consist of two atoms as a logic qubit, which forms a Decoherence-free subspace immune to collective dephasing. The quantum information processing can effectively resist decoherence in the Decoherence-free subspace. The paper proposes some schemes for Physical Implementation of Quantum Information Processing in Decoherence-free Subspace based on cavity quantum electrodynamics.Our main results include: (1)We propose a scheme of how to realize quantum SWAP gate in decoherence-free subspace based on cavity QED system. The scheme is built on the input-output formulation of a single-sided cavity, where atom is used as storage qubit and photon is used as flying qubit. We can implement the controlled phase gate between two atoms, the Hadamard gate of one logic qubit and the controlled phase gate between two logic qubits by means of cavity assisted interactions with single-photon pulse. Here we propose a scheme of how to realize the SWAP gate between one logic qubit and one flying qubit. The scheme is immune to dephasing and the loss of photon only affects the successful probability but has no influence on the fidelity.(2) We propose a scheme of how to implement quantum dense coding in decoherence-free subspace based on cavity QED system. One of the main obstacles for the implementation of quantum information in cavity QED is the decoherence of the cavity field. However, In our scheme, by using logic qubit to encode quantum information, the decoherence can be effectively resisted. The scheme is built on the input-output formulation of a single-sided cavity, when one party operates on the logic qubit, the other party can identify it with the help of the quantum logic qubit gate. So we can transfer the classical information by quantum channel, and information capacity increases.