Optimal control of a leaking qubit

Physical implementations of quantum bits can contain coherent transitions to energetically close non-qubit states. In particular, for anharmonic oscillator systems such as the superconducting phase qubit and the transmon a two-level approximation is insufficient. We apply optimal control theory to the envelope of a resonant Rabi pulse in a qubit in the presence of a single, weakly off-resonant leakage level. The gate error of a spin flip operation reduces by orders of magnitude compared to simple pulse shapes. Near-perfect gates can be achieved for any pulse duration longer than an intrinsic limit given by the nonlinearity. The pulses can be understood as composite sequences that refocus the leakage transition. We also discuss ways to improve the pulse shapes.Comment: 4 pages, 2 figure

On the energy dependence of the D^+/D^- production asymmetry

In this paper we discuss the origin of the asymmetry present in D meson production and its energy dependence. In particular, we have applied the meson cloud model to calculate the asymmetries in D^-/D^+ meson production in high energy p-p collisions and find a good agreement with recent LHCb data. Although small, this non-vanishing asymmetry may shed light on the role played by the charm meson cloud of the proton.Comment: 8 pages, 8 figures. arXiv admin note: text overlap with arXiv:hep-ph/000927

On the absorption and production cross sections of KK and K∗K^*

We have computed the isospin and spin averaged cross sections of the processes $\pi K^*\to \rho K$ and $\rho K^*\to \pi K$, which are crucial in the determination of the abundances of $K^*$ and $K$ in heavy ion collisions. Improving previous calculations, we have considered several mechanisms which were missing, such as the exchange of axial and vector resonances ($K_1(1270)$, $K^*_2(1430)$, $h_1(1170)$, etc...) and also other processes such as $\pi K^*\to \omega K, \phi K$ and $\omega K^*,\,\phi K^*\to \pi K$. We find that some of these mechanisms give important contributions to the cross section. Our results also suggest that, in a hadron gas, $K^*$ production might be more important than its absorption

Magic state distillation with low overhead

We propose a new family of error detecting stabilizer codes with an encoding rate 1/3 that permit a transversal implementation of the pi/8-rotation $T$ on all logical qubits. The new codes are used to construct protocols for distilling high-quality `magic' states $T|+>$ by Clifford group gates and Pauli measurements. The distillation overhead has a poly-logarithmic scaling as a function of the output accuracy, where the degree of the polynomial is $\log_2{3}\approx 1.6$. To construct the desired family of codes, we introduce the notion of a triorthogonal matrix --- a binary matrix in which any pair and any triple of rows have even overlap. Any triorthogonal matrix gives rise to a stabilizer code with a transversal $T$-gate on all logical qubits, possibly augmented by Clifford gates. A powerful numerical method for generating triorthogonal matrices is proposed. Our techniques lead to a two-fold overhead reduction for distilling magic states with output accuracy $10^{-12}$ compared with the best previously known protocol.Comment: 11 pages, 3 figure

X(3872)X(3872) production and absorption in a hot hadron gas

We calculate the time evolution of the $X(3872)$ abundance in the hot hadron gas produced in the late stage of heavy ion collisions. We use effective field Lagrangians to obtain the production and dissociation cross sections of $X(3872)$. In this evaluation we include diagrams involving the anomalous couplings $\pi D^*\bar{D}^*$ and $X \bar{D}^{\ast} D^{\ast}$ and also the couplings of the $X(3872)$ with charged $D$ and $D^*$ mesons. With these new terms the $X(3872)$ interaction cross sections are much larger than those found in previous works. Using these cross sections as input in rate equations, we conclude that during the expansion and cooling of the hadronic gas, the number of $X(3872)$, originally produced at the end of the mixed QGP/hadron gas phase, is reduced by a factor of 4.Comment: 8 pages, 4 figure