Entanglement, quantum discord, and the power of the quantum computer

Alexander Streltsov, Hermann Kampermann, Dagmar Bruß

Masoud Abolfathiyar, Entekhabi Sina

We present entangling and discording power of a gate that is made on strong contender for the basic element of a quantum computer. We present the effect gate on an experimental scheme of a two-qubit tomography. We propose a sufficient and implementable condition for the existence of nonzero quantum discord and entanglement in the considered experimental scheme.

胡要花 HU Yao-hua, 燕慧英 YAN Hui-ying

Fernando Galve, Francesco Plastina, Matteo G. A. Paris +1 more

We introduce the discording power of a unitary transformation, which assesses its capability to produce quantum discord, and analyze in detail the generation of discord by relevant classes of two-qubit gates. Our measure is based on the Cartan decomposition of two-qubit unitaries and on evaluating the maximum discord achievable by a unitary upon acting on classical-classical states at fixed purity. We find that there exist gates which are perfect discorders for any value of purity μ, and that they belong to a class of operators that includes the sqrt[SWAP]. Other gates, even those universal for quantum computation, do not possess the same property: the CNOT, for example, is a perfect discorder only for states with low or unit purity, but not for intermediate values. The discording power of a two-qubit unitary also provides a generalization of the corresponding measure defined for entanglement to any value of the purity.

Rafael Chaves, Fernando de Melo

Animesh Datta, Anil Shaji, Carlton M. Caves

We use quantum discord to characterize the correlations present in the model called deterministic quantum computation with one quantum bit (DQC1), introduced by Knill and Laflamme [Phys. Rev. Lett. 81, 5672 (1998)10.1103/PhysRevLett.81.5672]. The model involves a collection of qubits in the completely mixed state coupled to a single control qubit that has nonzero purity. The initial state, operations, and measurements in the model all point to a natural bipartite split between the control qubit and the mixed ones. Although there is no entanglement between these two parts, we show that the quantum discord across this split is nonzero for typical instances of the DQC1 ciruit. Nonzero values of discord indicate the presence of nonclassical correlations. We propose quantum discord as figure of merit for characterizing the resources present in this computational model.