Phonon-induced dephasing of singlet-triplet superpositions in double quantum dots without spin-orbit coupling

We show that singlet-triplet superpositions of two-electron spin states in a double quantum dot undergo a phonon-induced pure dephasing which relies only on the tunnel coupling between the dots and on the Pauli exclusion principle. As such, this dephasing process is independent of spin-orbit coupling or hyperfine interactions. The physical mechanism behind the dephasing is elastic phonon scattering, which persists to much lower temperatures than real phonon-induced transitions. Quantitative calculations performed for a lateral GaAs/AlGaAs gate-defined double quantum dot yield micro-second dephasing times at sub-Kelvin temperatures, which is consistent with experimental observations.Comment: Extended versio

Structure of rabbit liver fructose 1,6-bisphosphatase at 2.3 Å resolution

The three-dimensional structure of the R form of rabbit liver fructose 1,6-bisphosphatase (Fru-1,6-Pase; E.C. 3.1.3.11) has been determined by a combination of heavy-atom and molecular-replacement methods. A model, which includes 2394 protein atoms and 86 water molecules, has been refined at 2.3 Å resolution to a crystallographic R factor of 0.177. The root-mean-square deviations of bond distances and angles from standard geometry are 0.012 Å and 1.7°, respectively. This structural result, in conjunction with recently redetermined amino-acid sequence data, unequivocally establishes that the rabbit liver enzyme is not an aberrant bisphosphatase as once believed, but is indeed homologous to other Fru-1,6-Pases. The root-mean-square deviation of the C atoms in the rabbit liver structure from the homologous atoms in the pig kidney structure complexed with the product, fructose 6-phosphate, is 0.7 Å. Fru-1,6-Pases are homotetramers, and the rabbit liver protein crystallizes in space group I222 with one monomer in the asymmetric unit. The structure contains a single endogenous Mg<sup>2+</sup> ion coordinated by Glu97, Asp118, Asp121 and Glu280 at the site designated metal site 1 in pig kidney Fru-1,6-Pase R-form complexes. In addition, two sulfate ions, which are found at the positions normally occupied by the 6-phosphate group of the substrate, as well as the phosphate of the allosteric inhibitor AMP appear to provide stability. Met177, which has hydrophobic contacts with the adenine moiety of AMP in pig kidney T-form complexes, is replaced by glycine. Binding of a non-hydrolyzable substrate analog,<sup> β</sup>-methyl-fructose 1,6-bisphosphate, at the catalytic site is also examined

Sudden death of effective entanglement

Sudden death of entanglement is a well-known effect resulting from the finite volume of separable states. We study the case when the observer has a limited measurement capability and analyse the effective entanglement, i.e. entanglement minimized over the output data. We show that in the well defined system of two quantum dots monitored by single electron transistors, one may observe a sudden death of effective entanglement when real, physical entanglement is still alive. For certain measurement setups, this occurs even for initial states for which sudden death of physical entanglement is not possible at all. The principles of the analysis may be applied to other analogous scenarios, such as etimation of the parameters arising from quantum process tomography.Comment: final version, 5 pages, 3 figure

Complete disentanglement by partial pure dephasing

We study the effect of pure dephasing on the entanglement of a pair of two-level subsystems (qubits). We show that partial dephasing induced by a super-Ohmic reservoir, corresponding to well-established properties of confined charge states and phonons in semiconductors, may lead to complete disentanglement. We show also that the disentanglement effect increases with growing distance between the two subsystems.Comment: Final, considerably extended version, 6 pages, 4 figure

Structure of protease-cleaved escherichia coliα-2-macroglobulin reveals a putative mechanism of conformational activation for protease entrapment

Bacterial -2-macroglobulins have been suggested to function in defence as broad-spectrum inhibitors of host proteases that breach the outer membrane. Here, the X-ray structure of protease-cleaved Escherichia coli -2-macroglobulin is described, which reveals a putative mechanism of activation and conformational change essential for protease inhibition. In this competitive mechanism, protease cleavage of the bait-region domain results in the untethering of an intrinsically disordered region of this domain which disrupts native interdomain interactions that maintain E. coli -2-macroglobulin in the inactivated form. The resulting global conformational change results in entrapment of the protease and activation of the thioester bond that covalently links to the attacking protease. Owing to the similarity in structure and domain architecture of Escherichia coli -2-macroglobulin and human -2-macro­globulin, this protease-activation mechanism is likely to operate across the diverse members of this group

Structures and functions of carotenoids bound to reaction centers from purple photosynthetic bacteria

The photoprotective function of 15,15'-cis-carotenoids bound to the photosynthetic reaction centers (RCs) of purple bacteria has been studied using carotenoids reconstituted into carotenoidless RCs from Rhodobacter sphaeroides strain R26.1. The triplet-energy level of the carotenoid has been proposed to affect the quenching of the triplet state of special-pair bacteriochlorophyll (P). This was investigated using microsecond flash photolysis to detect the carotenoid triplets as a function of the number of conjugated double bonds, n. The carotenoid triplet signals were extracted by using singular-value decomposition (SVD) of the huge matrices data, and were confirmed for those having n = 8 to 11. This interpretation assumes that the reconstituted carotenoids occupy the same binding site in the RC. We have been able to confirm this assumption using X-ray crystallography to determine the structures of carotenoidless, wild-type carotenoid-containing, and 3,4-dihydro-spheroidene-reconstituted RCs. The X-ray study also emphasized the importance of the methoxy group of the carotenoids for binding to the RCs. Electroabsorption (Stark) spectroscopy was used to investigate the effect of the carotenoid on the electrostatic field around P. This electrostatic field changed by 10 % in the presence of the carotenoid

Phonon-induced decoherence for a quantum dot spin qubit operated by Raman passage

We study single-qubit gates performed via stimulated Raman adiabatic passage (STIRAP) on a spin qubit implemented in a quantum dot system in the presence of phonons. We analyze the interplay of various kinds of errors resulting from the carrier-phonon interaction as well as from quantum jumps related to nonadiabaticity and calculate the fidelity as a function of the pulse parameters. We give quantitative estimates for an InAs/GaAs system and identify the parameter values for which the error is considerably minimized, even to values below $10^{-4}$ per operation.Comment: Final version; considerable extensions; 18 pages, 7 figure

Adsorption of CO on a Platinum (111) surface - a study within a four-component relativistic density functional approach

We report on results of a theoretical study of the adsorption process of a single carbon oxide molecule on a Platinum (111) surface. A four-component relativistic density functional method was applied to account for a proper description of the strong relativistic effects. A limited number of atoms in the framework of a cluster approach is used to describe the surface. Different adsorption sites are investigated. We found that CO is preferably adsorbed at the top position.Comment: 23 Pages with 4 figure

Robots, Everlasting? A Framework for Classifying CS Educational Robots

Educational robots are an exciting and growing field. While some (Lego Mindstorms, for example) have been around for decades, most are only a few years old and their durability is untested; exacerbating this are those only usable with apps, that may become suddenly unavailable. This has created a nascent but significant problem: schools investing significant time and money for educational robots with little ability to know if they will work for years or just days. Other fields in science, technology, education, and math (STEM) beyond computer science also encounter this issue as more educational robots and apps for those disciplines permeate the market. While this chapter analyzes this issue from a CS perspective, the lessons learned can be applied to other STEM areas. This chapter explores the history of the problem, documents several examples of devices that have succumbed, details the unique and specific needs of school customers, and introduces the Computer Science Risk Analysis Framework for Toys (CS RAFT) to help teachers and schools evaluate a device purchase based on a holistic understanding of device longevity. This study will also provide recommendations for CS and STEM educational robot designers