Holonomic quantum computing in symmetry-protected ground states of spin chains

While solid-state devices offer naturally reliable hardware for modern classical computers, thus far quantum information processors resemble vacuum tube computers in being neither reliable nor scalable. Strongly correlated many body states stabilized in topologically ordered matter offer the possibility of naturally fault tolerant computing, but are both challenging to engineer and coherently control and cannot be easily adapted to different physical platforms. We propose an architecture which achieves some of the robustness properties of topological models but with a drastically simpler construction. Quantum information is stored in the symmetry-protected degenerate ground states of spin-1 chains, while quantum gates are performed by adiabatic non-Abelian holonomies using only single-site fields and nearest-neighbor couplings. Gate operations respect the symmetry, and so inherit some protection from noise and disorder from the symmetry-protected ground states.Comment: 19 pages, 4 figures. v2: published versio

Fundamental Forces Driving United States and International Financial Regulations Reform

Multiple forces create a systemic crisis of the proportions of the Global Financial Crisis of 2008. Global and domestic financial reform is a difficult and perplexing task, one that is likely to take many years, and one that will surely continue to be shaped by a diverse range of forces. Recent measures remain incomplete and in some cases are even proving to be misdirected. This article considers seven fundamental forces shaping actions on future reform, specifically the (1) long term impact of the Crisis (and all financial crises); (2) increase in the “financialization” of the global economy, seemingly disproportionate to the growth in the real economy; (3) dramatic increase in financial interconnectedness worldwide, accompanied by a growing complexity in financial institutions and the regulatory framework, all of which could be becoming unmanageable; (4) “human factor” in finance, which tends to confound our assumptions about market behavior; (5) growth in the critical yet little understood and regulated “shadow banking system;” (6) deep technology revolution, which continues to transform the dynamics of the global economy; and (7) “next convergence” between Western and “emerging” economies, which is changing the global economic profile and presenting profound new challenges to financial refor

Whither Capitalism? Financial externalities and crisis

As with global warming, so with financial crises – externalities have a lot to answer for. We look at three of them. First the financial accelerator due to ‘fire sales’ of collateral assets -- a form of pecuniary externality that leads to liquidity being undervalued. Second the ‘risk- shifting’ behaviour of highly-levered financial institutions who keep the upside of risky investment while passing the downside to others thanks to limited liability. Finally, the network externality where the structure of the financial industry helps propagate shocks around the system unless this is checked by some form of circuit breaker, or ‘ring-fence’. The contrast between crisis-induced Great Recession and its aftermath of slow growth in the West and the rapid - and (so far) sustained - growth in the East suggests that successful economic progress may depend on how well these externalities are managed

Haldane Sashes in Quantum Hall Spectra

We show that the low-temperature sash features in the lowest Landau-level (LLL) tunneling density-of-states (TDOS) recently discovered by Dial and Ashoori are intimately related to the discrete Haldane-pseudopotential interaction energy scales that govern fractional quantum Hall physics. Our analysis is based on expressions for the tunneling density-of-states which become exact at filling factors close to $\nu=0$ and $\nu=1$, where the sash structure is most prominent. We comment on other aspects of LLL correlation physics that can be revealed by accurate temperature-dependent tunneling data.Comment: Added referenc

The performance of forecast-based monetary policy rules under model uncertainty

We investigate the performance of forecast-based monetary policy rules using five macroeconomic models that reflect a wide range of views on aggregate dynamics. We identify the key characteristics of rules that are robust to model uncertainty: such rules respond to the one-year-ahead inflation forecast and to the current output gap and incorporate a substantial degree of policy inertia. In contrast, rules with longer forecast horizons are less robust and are prone to generating indeterminacy. Finally, we identify a robust benchmark rule that performs very well in all five models over a wide range of policy preferences