Qubit measurements with a double-dot detector

We propose to monitor a qubit with a double-dot (DD) resonant-tunneling detector, which can operate at higher temperatures than a single-dot detector. In order to assess the effectiveness of this device, we derive rate equations for the density matrix of the entire system. We show that the signal-to-noise ratio can be greatly improved by a proper choice of the parameters and location of the detector. We demonstrate that quantum interference effects within the DD detector play an important role in the measurement. Surprisingly, these effects produce a systematic measurement error, even when the entire system is in a stationary state.Comment: 4 pages, some explanations added, Phys. Rev. Lett., in pres

Network Overload due to Massive Attacks

We study the cascading failure of networks due to overload, using the betweenness centrality of a node as the measure of its load following the Motter and Lai model. We study the fraction of survived nodes at the end of the cascade $p_f$ as function of the strength of the initial attack, measured by the fraction of nodes $p$, which survive the initial attack for different values of tolerance $\alpha$ in random regular and Erd\"os-Renyi graphs. We find the existence of first order phase transition line $p_t(\alpha)$ on a $p-\alpha$ plane, such that if $p <p_t$ the cascade of failures lead to a very small fraction of survived nodes $p_f$ and the giant component of the network disappears, while for $p>p_t$, $p_f$ is large and the giant component of the network is still present. Exactly at $p_t$ the function $p_f(p)$ undergoes a first order discontinuity. We find that the line $p_t(\alpha)$ ends at critical point $(p_c,\alpha_c)$ ,in which the cascading failures are replaced by a second order percolation transition. We analytically find the average betweenness of nodes with different degrees before and after the initial attack, investigate their roles in the cascading failures, and find a lower bound for $p_t(\alpha)$. We also study the difference between a localized and random attacks

The stability of adaptive synchronization of chaotic systems

In past works, various schemes for adaptive synchronization of chaotic systems have been proposed. The stability of such schemes is central to their utilization. As an example addressing this issue, we consider a recently proposed adaptive scheme for maintaining the synchronized state of identical coupled chaotic systems in the presence of a priori unknown slow temporal drift in the couplings. For this illustrative example, we develop an extension of the master stability function technique to study synchronization stability with adaptive coupling. Using this formulation, we examine local stability of synchronization for typical chaotic orbits and for unstable periodic orbits within the synchronized chaotic attractor (bubbling). Numerical experiments illustrating the results are presented. We observe that the stable range of synchronism can be sensitively dependent on the adaption parameters, and we discuss the strong implication of bubbling for practically achievable adaptive synchronization.Comment: 21 pages, 6 figure

Constructing local bulk observables in interacting AdS/CFT

Local operators in the bulk of AdS can be represented as smeared operators in the dual CFT. We show how to construct these bulk observables by requiring that the bulk operators commute at spacelike separation. This extends our previous work by taking interactions into account. Large-N factorization plays a key role in the construction. We show diagrammatically how this procedure is related to bulk Feynman diagrams.Comment: 41 pages, LaTeX. v2: reference correcte

Coupled sulfur and oxygen isotope insight into bacterial sulfate reduction in the natural environment

We present new sulfur and oxygen isotope data in sulfate (δ34SSO4 and δ18OSO4 respectively), from globally distributed marine and estuary pore fluids. We use this data with a model of the biochemical steps involved in bacterial sulfate reduction (BSR) to explore how the slope on a δ18OSO4 vs. δ34SSO4 plot relates to the net sulfate reduction rate (nSRR) across a diverse range of natural environments. Our data demonstrate a correlation between the nSRR and the slope of the relative evolution of oxygen and sulfur isotopes (δ18OSO4 vs. δ34SSO4) in the residual sulfate pool, such that higher nSRR results in a lower slope (sulfur isotopes increase faster relative to oxygen isotopes). We combine these results with previously published literature data to show that this correlation scales over many orders of magnitude of nSRR. Our model of the mechanism of BSR indicates that the critical parameter for the relative evolution of oxygen and sulfur isotopes in sulfate during BSR in natural environments is the rate of intracellular sulfite oxidation. In environments where sulfate reduction is fast, such as estuaries and marginal marine environments, this sulfite reoxidation is minimal, and the δ18OSO4 increases more slowly relative to the δ34SSO4. In contrast, in environments where sulfate reduction is very slow, such as deep sea sediments, our model suggests sulfite reoxidation is far more extensive, with as much as 99% of the sulfate being thus recycled; in these environments the δ18OSO4 increases much more rapidly relative to the δ34SSO4. We speculate that the recycling of sulfite plays a physiological role during BSR, helping maintain microbial activity where the availability of the electron donor (e.g. available organic matter) is low

Local bulk operators in AdS/CFT: a boundary view of horizons and locality

We develop the representation of local bulk fields in AdS by non-local operators on the boundary, working in the semiclassical limit and using AdS_2 as our main example. In global coordinates we show that the boundary operator has support only at points which are spacelike separated from the bulk point. We construct boundary operators that represent local bulk operators inserted behind the horizon of the Poincare patch and inside the Rindler horizon of a two dimensional black hole. We show that these operators respect bulk locality and comment on the generalization of our construction to higher dimensional AdS black holes.Comment: 28 pages, 4 figures, late

Deciphering top flavor violation at the LHC with B factories

The LHC will have unprecedented sensitivity to flavor-changing neutral current (FCNC) top quark decays, whose observation would be a clear sign of physics beyond the standard model. Although many details of top flavor violation are model dependent, the standard model gauge symmetries relate top FCNCs to other processes, which are strongly constrained by existing data. We study these constraints in a model independent way, using a low energy effective theory from which the new physics is integrated out. We consider the most important operators which contribute to top FCNCs and analyze the current constraints on them. We find that the data rule out top FCNCs at a level observable at the LHC due to most of the operators comprising left-handed first or second generation quark fields, while there remains a substantial window for top decays mediated by operators with right-handed charm or up quarks. If FCNC top decays are observed at the LHC, such an analysis may help decipher the underlying physics.Comment: 17 pages, 5 figures; some typos correcte

The microbially driven formation of siderite in salt marsh sediments

We employ complementary field and laboratory‐based incubation techniques to explore the geochemical environment where siderite concretions are actively forming and growing, including solid‐phase analysis of the sediment, concretion, and associated pore fluid chemistry. These recently formed siderite concretions allow us to explore the geochemical processes that lead to the formation of this less common carbonate mineral. We conclude that there are two phases of siderite concretion growth within the sediment, as there are distinct changes in the carbon isotopic composition and mineralogy across the concretions. Incubated sediment samples allow us to explore the stability of siderite over a range of geochemical conditions. Our incubation results suggest that the formation of siderite can be very rapid (about two weeks or within 400 hr) when there is a substantial source of iron, either from microbial iron reduction or from steel material; however, a source of dissolved iron is not enough to induce siderite precipitation. We suggest that sufficient alkalinity is the limiting factor for siderite precipitation during microbial iron reduction while the lack of dissolved iron is the limiting factor for siderite formation if microbial sulfate reduction is the dominant microbial metabolism. We show that siderite can form via heated transformation (at temperature 100°C for 48 hr) of calcite and monohydrocalcite seeds in the presence of dissolved iron. Our transformation experiments suggest that the formation of siderite is promoted when carbonate seeds are present