A universal quantum circuit for two-qubit transformations with three CNOT gates

We consider the implementation of two-qubit unitary transformations by means of CNOT gates and single-qubit unitary gates. We show, by means of an explicit quantum circuit, that together with local gates three CNOT gates are necessary and sufficient in order to implement an arbitrary unitary transformation of two qubits. We also identify the subset of two-qubit gates that can be performed with only two CNOT gates.Comment: 3 pages, 7 figures. One theorem, one author and references added. Change of notational conventions. Minor correction in Theorem

Error models for mode-mismatch in linear optics quantum computing

One of the most significant challenges facing the development of linear optics quantum computing (LOQC) is mode-mismatch, whereby photon distinguishability is introduced within circuits, undermining quantum interference effects. We examine the effects of mode-mismatch on the parity (or fusion) gate, the fundamental building block in several recent LOQC schemes. We derive simple error models for the effects of mode-mismatch on its operation, and relate these error models to current fault tolerant threshold estimates.Comment: 6 pages, 7 figure

Exact relaxation in a class of non-equilibrium quantum lattice systems

A reasonable physical intuition in the study of interacting quantum systems says that, independent of the initial state, the system will tend to equilibrate. In this work we study a setting where relaxation to a steady state is exact, namely for the Bose-Hubbard model where the system is quenched from a Mott quantum phase to the strong superfluid regime. We find that the evolving state locally relaxes to a steady state with maximum entropy constrained by second moments, maximizing the entanglement, to a state which is different from the thermal state of the new Hamiltonian. Remarkably, in the infinite system limit this relaxation is true for all large times, and no time average is necessary. For large but finite system size we give a time interval for which the system locally "looks relaxed" up to a prescribed error. Our argument includes a central limit theorem for harmonic systems and exploits the finite speed of sound. Additionally, we show that for all periodic initial configurations, reminiscent of charge density waves, the system relaxes locally. We sketch experimentally accessible signatures in optical lattices as well as implications for the foundations of quantum statistical mechanics.Comment: 8 pages, 3 figures, replaced with final versio

Chiral-logarithmic Corrections to the S and T Parameters in Higgsless Models

Recently, Higgsless models have proven to be viable alternatives to the Standard Model (SM) and supersymmetric models in describing the breaking of the electroweak symmetry. Whether extra-dimensional in nature or their deconstructed counterparts, the physical spectrum of these models typically consists of ``towers'' of massive vector gauge bosons which carry the same quantum numbers as the SM W and Z. In this paper, we calculate the one-loop, chiral-logarithmic corrections to the S and T parameters from the lightest (i.e. SM) and the next-to-lightest gauge bosons using a novel application of the Pinch Technique. We perform our calculation using generic Feynman rules with generic couplings such that our results can be applied to various models. To demonstrate how to use our results, we calculate the leading chiral-logarithmic corrections to the S and T parameters in the deconstructed three site Higgsless model. As we point out, however, our results are not exclusive to Higgsless models and may, in fact, be used to calculate the one-loop corrections from additional gauge bosons in models with fundamental (or composite) Higgs bosons.Comment: 45 pages, 15 figures, added references, analysis of three site model expanded to include delocalized fermion

Reliability considerations in the design, assembly, and testing of the mariner iv power system

Reliability considerations in design, assembly, and testing of Mariner IV power syste

A Complete Atlas of HI Absorption toward HII Regions in the Southern Galactic Plane Survey (SGPS1)

We present a complete catalog of H I emission and absorption spectrum pairs, toward H II regions, detectable within the boundaries of the Southern Galactic Plane Survey (SGPS I), a total of 252 regions. The catalog is presented in graphical, numerical and summary formats. We demonstrate an application of this new dataset through an investigation of the locus of the Near 3kpc Arm.Comment: Accepted for publication by ApJS Feb 6, 2014. Data files and Figure Set (252 images) to appear in the on-line version of the journa

One-loop Corrections to the S Parameter in the Four-site Model

We compute the leading chiral-logarithmic corrections to the S parameter in the four-site Higgsless model. In addition to the usual electroweak gauge bosons of the Standard Model, this model contains two sets of heavy charged and neutral gauge bosons. In the continuum limit, the latter gauge bosons can be identified with the first excited Kaluza-Klein states of the W^\pm and Z bosons of a warped extra-dimensional model with an SU(2)_L \times SU(2)_R \times U(1)_X bulk gauge symmetry. We consider delocalized fermions and show that the delocalization parameter must be considerably tuned from its tree-level ideal value in order to reconcile experimental constraints with the one-loop results. Hence, the delocalization of fermions does not solve the problem of large contributions to the S parameter in this class of theories and significant contributions to S can potentially occur at one-loop.Comment: 28 pages, 7 figure