Climbing Mount Scalable: Physical Resource Requirements for a Scalable Quantum Computer

The primary resource for quantum computation is Hilbert-space dimension. Whereas Hilbert space itself is an abstract construction, the number of dimensions available to a system is a physical quantity that requires physical resources. Avoiding a demand for an exponential amount of these resources places a fundamental constraint on the systems that are suitable for scalable quantum computation. To be scalable, the effective number of degrees of freedom in the computer must grow nearly linearly with the number of qubits in an equivalent qubit-based quantum computer.     

Hedged maximum likelihood quantum state estimation

This Letter proposes and analyzes a new method for quantum state estimation, called hedged maximum likelihood (HMLE). HMLE is a quantum version of Lidstone's law, also known as the "add β" rule. A straightforward modification of maximum likelihood estimation (MLE), it can be used as a plug-in replacement for MLE. The HMLE estimate is a strictly positive density matrix, slightly less likely than the ML estimate, but with much better behavior for predictive tasks. Single-qubit numerics indicate that HMLE beats MLE, according to several metrics, for nearly all "true" states. For nearly pure states, MLE does slightly better, but neither method is optimal.     

Premicellar and micelle formation behavior of aqueous anionic surfactants in the presence of triphenylmethane dyes: protonation of dye in ion pair micelles

The premicellar and micelle formation behaviors of four cationic triphenylmethane dyes, viz, Pararosaniline (RN), Crystal violet (CV), Ethyl violet (EV), and Malachite green (MG), in aqueous anionic surfactant solutions of sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), and sodium dodecyl sulfonate (SDSN) have been studied by spectral and surface tension measurements. The study was carried out within a pH range where the dyes are stable in their quinoid forms. The dyes have been found to form dye–surfactant ion pairs (DSIPs) with the surfactants, at the surfactant concentrations well below their critical micelle concentration, CMC*. The DSIPs behave like nonionic surfactants and form an air–water interfacial monolayer. The DSIPs have a lower critical micelle concentration (CMC_IP), greater efficiency, and lower effectiveness than the corresponding pure surfactants. As the surfactant concentration is increased below the CMC*, the DSIPs start forming micelles of their own where the dye gets protonated and exists as a protonated dye–surfactant ion pair (PDSIP) in the ion pair micelles. As the concentration of the surfactant exceeds the CMC* of the pure surfactant, the protonation reverses gradually with the dye remaining in the micelles in solubilized form and the DSIPs in the air–water interfacial monolayer are replaced by pure surfactants. The distorted helical isomeric form (isomer B) of the dyes is favored in the PDSIPs. Copyright © 2009 John Wiley & Sons, Ltd.     

Tensor Distributions on Signature-changing Space-times

Irregularities in the metric tensor of a signature-changing space-time suggest that field equations on such space-times might be regarded as distributional. We review the formalism of tensor distributions on differentiable manifolds, and examine to what extent rigorous meaning can be given to field equations in the presence of signature-change, in particular those involving covariant derivatives. We find that, for both continuous and discontinuous signature-change, covariant differentiation can be defined on a class of tensor distributions wide enough to be physically interesting.     

Two-dimensional Quantum Random Walk

We analyze several families of two-dimensional quantum random walks. The feasible region (the region where probabilities do not decay exponentially with time) grows linearly with time, as is the case with one-dimensional QRW. The limiting shape of the feasible region is, however, quite different. The limit region turns out to be an algebraic set, which we characterize as the rational image of a compact algebraic variety. We also compute the probability profile within the limit region, which is essentially a negative power of the Gaussian curvature of the same algebraic variety. Our methods are based on analysis of the space-time generating function, following the methods of Pemantle and Wilson (J. Comb. Theory, Ser. A 97(1):129–161, 2002).     

Helium and point defect accumulation: (i) microstructure and mechanical behaviour

Ferritic/martensitic (F/M) steels are good candidate structural materials for the future fusion reactors and spallation sources. However, irradiation of steels is known to produce hardening, loss of ductility, shift in ductile to brittle transition temperature (DBTT) and reduction of fracture toughness and creep resistance starting at low doses. Helium (He), produced by transmutation during the irradiation, also impacts mechanical properties. Numerous experimental and theoretical studies on the evolution of the microstructure of steels under irradiation have been conducted until now. We review the effect of irradiation-induced point defects and in particular of He on the mechanical properties of F/M steels. To cite this article: R. Schäublin et al., C. R. Physique 9 (2008).     

A study of energy transfer between electronically excited states using a picosecond laser pulse

By means of a novel picosecond polarization spectrometer we have determined that under conditions of normal pumping of organic systems a substantial annihilation of excited states occurs before the 6–8 ps duration of the pumping pulse. The effect is exposed by noting that prior to 6 ps the optical absorption of the S₁ transient produced by 353 nm pumping of 1,2-benzanthracene is caused to be depolarized at higher concentrations. In another case (xanthone) where no long range interactions can occur the depolarization of the spectra only occurred at low concentrations due to saturation effects.     

Current Interruption at Powers up to 1 GW with Crossed-Field Tubes

This paper describes the current interruption and voltage holdoff capabilities of a wide range of crossed-field tubes (XFTs). The XFTs investigated have active electrode areas ranging from 96 to 8000 cm2 with mean diameters from 2 to 56 cm. For the larger XFTs, currents up to 10 kA have been interrupted at voltages up to 100 kV, corresponding to a switched power of 1 GW. Experiments have shown the peak interruptible current to vary linearly with tube radius. The mechanism responsible for the limitation may be related to the self-generated magnetic field, but other factors are also shown to play a role.     

Particle production from signature change

We consider the (massless) scalar field on 2-dimensional manifolds whose metric changes signature and which admit a spacelike isometry. Choosing the wave equation so that there will be a conserved Klein-Gordon product implicitly determines the junction conditions one needs to impose in order to obtain global solutions. The resulting mix of positive and negative frequencies produced by the presence of Euclidean regions depends only on the total width of the regions, and not on the detailed form of the metric.This essay received honorable mention in the 1991 competition of the Gravity Research Foundation.