Using Qubits to Measure Fidelity in Mesoscopic Systems

We point out the similarities in the definition of the `fidelity' of a quantum system and the generating function determining the full counting statistics of charge transport through a quantum wire and suggest to use flux- or charge qubits for their measurement. As an application we use the notion of fidelity within a first-quantized formalism in order to derive new results and insights on the generating function of the full counting statistics.Comment: 5 pages, 1 figur

Quantum gambling using mesoscopic ring qubits

Quantum Game Theory provides us with new tools for practising games and some other risk related enterprices like, for example, gambling. The two party gambling protocol presented by Goldenberg {\it et al} is one of the simplest yet still hard to implement applications of Quantum Game Theory. We propose potential physical realisation of the quantum gambling protocol with use of three mesoscopic ring qubits. We point out problems in implementation of such game.Comment: 4 pages, 1 figure, poster during XXX Intern. Conf. of Theoretical Physics, Electron correlations in nano- and microsystems, Ustron 9-14 September 2006. Minor corrections, references added; to appear in physica status solidi

Quantum measurement of a mesoscopic spin ensemble

We describe a method for precise estimation of the polarization of a mesoscopic spin ensemble by using its coupling to a single two-level system. Our approach requires a minimal number of measurements on the two-level system for a given measurement precision. We consider the application of this method to the case of nuclear spin ensemble defined by a single electron-charged quantum dot: we show that decreasing the electron spin dephasing due to nuclei and increasing the fidelity of nuclear-spin-based quantum memory could be within the reach of present day experiments.Comment: 8 pages, 2 figures; minor changes, published versio

Error-Resistant Distributed Quantum Computation in Trapped Ion Chain

We consider experimentally feasible chains of trapped ions with pseudo-spin 1/2, and find models that can potentially be used to implement error-resistant quantum computation. Similar in spirit to classical neural networks, the error-resistance of the system is achieved by encoding the qubits distributed over the whole system. We therefore call our system a ''quantum neural network'', and present a ''quantum neural network model of quantum computation''. Qubits are encoded in a few quasi-degenerated low energy levels of the whole system, separated by a large gap from the excited states, and large energy barriers between themselves. We investigate protocols for implementing a universal set of quantum logic gates in the system, by adiabatic passage of a few low-lying energy levels of the whole system. Naturally appearing and potentially dangerous distributed noise in the system leaves the fidelity of the computation virtually unchanged, if it is not too strong. The computation is also naturally resilient to local perturbations of the spins.Comment: 10 pages, 7 figures, RevTeX4; v2: another noise model analysed, published versio

High-efficiency cluster-state generation with atomic ensembles via the dipole-blockade mechanism

We demonstrate theoretically a scheme for cluster-state generation, based on atomic ensembles and the dipole-blockade mechanism. In the protocol, atomic ensembles serve as single-qubit systems. Therefore, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultracold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step Q-qubit GHZ states with success probability p(success) similar to eta(Q/2), where eta is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer

Generalized spin squeezing inequalities in NN qubit systems: theory and experiment

We present detailed derivations, various improvements and application to concrete experimental data of spin squeezing inequalities formulated recently by some of us [Phys. Rev. Lett. {\bf 95}, 120502 (2005)]. These inequalities generalize the concept of the spin squeezing parameter, and provide necessary and sufficient conditions for genuine 2-, or 3- qubit entanglement for symmetric states, and sufficient entanglement condition for general $N$-qubit states. We apply our method to theoretical study of Dicke states, and, in particular, to $W$-states of $N$ qubits. Then, we analyze the recently experimentally generated 7- and 8-ion $W$-states [Nature {\bf 438}, 643 (2005)]. We also present some novel details concerning this experiment. Finally, we improve criteria for detection of genuine tripartite entanglement based on entanglement witnesses.Comment: Final versio

Entanglement of superconducting charge qubits by homodyne measurement

We present a scheme by which projective homodyne measurement of a microwave resonator can be used to generate entanglement between two superconducting charge qubits coupled to this resonator. The non-interacting qubits are initialised in a product of their ground states, the resonator is initialised in a coherent field state, and the state of the system is allowed to evolve under a rotating wave Hamiltonian. Making a homodyne measurement on the resonator at a given time projects the qubits into an state of the form (|gg> + exp(-i phi)|ee>)/sqrt(2). This protocol can produce states with a fidelity as high as required, with a probability approaching 0.5. Although the system described is one that can be used to display revival in the qubit oscillations, we show that the entanglement procedure works at much shorter timescales.Comment: 17 pages, 7 figure