Quantum spin chains may be used to transfer quantum states between elements of a quantum information processing device. A scheme discovered recently [1] was shown to have favorable transfer properties for single-qubit states even in the presence of built-in static disorder caused by manufacturing errors. We extend that scheme in a way suggested already in [1] and study the transfer of the four Bell states which form a maximally entangled basis in the two-qubit Hilbert space. We show that perfect transfer of all four Bell states separately and of arbitrary linear combinations may be achieved for chains with hundreds of spins. For simplicity we restrict ourselves to systems without disorder.PACS numbers: 03.67. Hk, 75.10.Pq, 75.40.Gb Quantum information processing [2] relies on a number of key elements of technology, among them quantum bits and quantum gates. Since any quantum computer will contain a large number of different quantum gates and registers, information must be transferred between these elements of the computer. One possibility for that information transfer is offered by quantum spin chains, linear arrays of suitably coupled qubits. Research on quantum information transfer by spin chains started roughly a decade ago [3] and quickly developed into an active field with many contributors (see, for example, the reviews in [4]). However, most of the research up to now has focused on the transfer of single-qubit states, although the handling of entangled multi-qubit states is of primary importance in all known algorithms of quantum information processing. In this Brief Report we show how a natural extension of a single-qubit state transfer protocol [1] can be used to achieve high-fidelity transmission of arbitrary pure two-qubit states along spin chains with up to hundreds of sites.Spin chains for quantum information transfer mostly fall into one of two classes distinguished by the degree of "engineering", or fine-tuning, of the nearest-neighbor couplings along the chain. Perfect state transfer (PST) may be achieved if all transition frequencies generated by the spin chain Hamiltonian are commensurate and hence the time evolution of arbitrary initial states becomes periodic. In combination with spatial symmetry this enables perfect "mirroring" of initial states located at one end of the chain [5][6][7][8][9]. To achieve this goal, all nearest-neighbor couplings must be tuned to specific values, hence this class of chains may be called "fully engineered". A much simpler route to good (but not perfect) state transfer is opened by modifying only the boundary couplings affecting the very first and last spins of the chain, respectively, leaving all other couplings at one and the same value [10][11][12]. In the limit of weak boundary couplings the system then possesses nearly degenerate (symmetric and antisymmetric) eigenstates concentrated on the boundary spins and the dynamics of these states may be exploited for the transfer of quantum information. That class of systems may be called boundarydominated or op...

We study the influence of narrowed distributions of the nuclear Overhauser field on the decoherence of a central electron spin in quantum dots. We describe the spin dynamics in quantum dots by the central spin model. We use analytic solutions for uniform couplings and the time dependent density-matrix renormalization group (tDMRG) for nonuniform couplings. With these tools we calculate the dynamics of the central spin for large baths of nuclear spins with or without external magnetic field applied to the central spin. The focus of our study is the influence of initial mixtures with narrowed distributions of the Overhauser field and of applied magnetic fields on the decoherence of the central spin.Comment: 20 pages, 15 figure