Scaling of decoherence effects in quantum computers

Abstract

The scaling of decoherence rates with qubit number N is studied for a simple model of a quantum computer in the situation where N is large. The two state qubits are localized around well-separated positions via trapping potentials and vibrational centre of mass motion of the qubits occurs. Coherent one and two qubit gating processes are controlled by external classical fields and facilitated by a cavity mode ancilla. Decoherence due to qubit coupling to a bath of spontaneous modes, cavity decay modes and to the vibrational modes is treated. A non-Markovian treatment of the short time behaviour of the fidelity is presented, and expressions for the characteristic decoherence time scales obtained for the case where the qubit/cavity mode ancilla is in a pure state and the baths are in thermal states. Specific results are given for the case where the cavity mode is in the vacuum state and gating processes are absent and the qubits are in (a) the Hadamard state (b) the GHZ state.     

Transition to classical behavior in mesoscopic magnetic systems and quantum decoherence

A mesoscopic CrNi₆ system is used to demonstrate the possibility of transitions from quantum to classical behavior in thermally stimulated tunneling processes. The main mechanism responsible for these transitions is assumed to be quantum decoherence produced as a result of thermal interaction between the spin system and its neighborhood. Results are presented of calculations of the disruption probability for a mesoscopic system as a function of temperature. It is shown that the main characteristic which can reveal the decoherence effect is the nonmonotonic behavior of the disruption probability at low temperatures. Pis’ma Zh. Tekh. Fiz. 25, 1–5 (August 26, 1999)     

A molecular dynamics simulation study on trapping ions in a nanoscale Paul trap

We found by molecular dynamics simulations that a low energy ion can be trapped effectively in a nanoscale Paul trap in both vacuum and aqueous environments when appropriate AC/DC electric fields are applied to the system. Using the negatively charged chlorine ion as an example, we show that the trapped ion oscillates around the center of the nanotrap with an amplitude dependent on the parameters of the system and applied voltages. Successful trapping of the ion within nanoseconds requires an electric bias of GHz frequency, in the range of hundreds of mV. The oscillations are damped in the aqueous environment, but polarization of water molecules requires the application of a higher voltage bias to reach improved stability of the trapping. Application of a supplemental DC driving field along the trap axis can effectively drive the ion off the trap center and out of the trap, opening up the possibility of studying DNA and other charged molecules using embedded probes while achieving a full control of their translocation and localization in the trap.     

Quantum computing in the solid state: the challenge of decoherence

The current status of solid-state implementations of quantum computing is briefly described. There are numerous candidate proposals, but only comparatively recently have some of them begun to progress to the point of demonstrating coherent motion of the individual quantum bits (qubits), and the controlled coupling of more than one qubit remains a significant challenge. We also present a generalization of the fluctuation-dissipation theorem that provides a relationship between the coherent evolution entangling two spatially separated qubits, and the associated irreducible decoherence. This relationship may be used to bound the maximum attainable figures of merit in proposed two-qubit gates.     

Theoretical study on controllability of quantum state energy in an InGaAs/GaAs quantum dot buried in InGaAs

We theoretically studied the relationship between quantum energy states and structural parameters of an InGaAs/GaAs quantum dot (QD) buried in strained InGaAs, emitting at 1.1 to 1.4 em. The crystal distortion of the buried QD structure in three dimensions was computed based on the three-dimensional finite element method. Under the computed strain fields, the Schr?dinger equation was solved to obtain wavefunctions and eigenenergies. By calculating the dependence on structural parameters, we investigated the controllable range of the ground state energy and the energy separation between the ground state and the first excited state. We found that the energy separation exhibited a maximum value as a function of QD composition, enabling us to identify the composition of the QD structure. The effects of the burying layer composition and QD diameter were also investigated to expand the controllable range of the state energy. We also showed that the wavefunction symmetry was improved by burying the QD in the InGaAs layer. Our results will be useful in developing advanced devices for optical telecommunications and quantum information technology.     

Collisional cooling of large ions in electrospray mass spectrometry

Collisional cooling of ions in the rf-only multipole guides has become a method of choice for coupling electrospray sources to various mass analyzers. Normally parameters of such ion guides (length, pressure) provide enough thermalization and focusing for ions in a wide mass range. Noncovalent complexes, however, have more compact conformations than denatured biomolecules of similar mass and, therefore may not be transmitted efficiently through standard ion guides, as demonstrated by theoretical analysis, simulations, and experiments. Several methods of improving collisional cooling for large compact ions have been developed on a quadrupole time-of-flight instrument, which include operating the ion guides at higher pressure and trapping ions to increase the cooling time. Improved transmission of heavy ions obtained with those methods is studied in experiments with proteasome 20S, an oligomeric protein noncovalent complex with molecular weight around 692,000, and a few other compounds.     

SiGe band engineering for MOS,CMOS and quantum effect devices

In this paper, we review recent progress in SiGe MOS technology. Progress in high mobility p-channel and n-channel devices will be presented as well as some of the materials and processing issues related to the fabrication of these heterostructures. In addition, we will present an outlook on the integration of these devices to complimentary MOS (CMOS) based on Si on Insulator technology (SOI). New directions of novel devices utilizing selective epitaxial growth and the integration of Si/Ge superlattices for enhanced performance in field effect transistors are described. Finally, we will examine some of the materials challenges of integrating SiGe technologies with current CMOS production processes.     

Ground state of3He-A in a sphere

We compare the free energies and currents of four trial states of³He-A in spherical geometries. The spherical boojum texture is found to be energetically stable for all container radii down to R ～46ξ, where ξ is the superfluid coherence length. Textures with line defects in the bulk will become energetically favorable below this radius.     

Radiative interface state study in CdSe/ZnS quantum dots covered by polymer

The paper presents the transformation of photoluminescence (PL) spectra of nonconjugated and bioconjugated core/shell CdSe/ZnS QDs covered by PEG polymer at the aging in ambient air. Studied QDs are characterized by the sizes: (i) 3.6-4.0 nm and color emission with the maxima at 560-565 nm (2.19-2.25 eV) and (ii) 5.2-5.3 nm and with emission at 605-610 nm (2.02-2.08 eV). The part of 565 nm CdSe/ZnS QDs has been bioconjugated to the mouse anti PSA (Prostate-Specific Antigen) antibodies and the part of 605 nm QDs has been bioconjugated to the antihuman IL10 (Interleukin 10) antibodies using the commercially available 565 nm and 605 nm QD conjugation kits. It is revealed that the aging process in ambient air has the very strong impact on PL spectra of nonconjugated core/shell CdSe/ZnS QDs covered with PEG polymer. The aging process relates to the polymer modification in ambient air that is accompanied by the three effects: (i) polymer transparency increasing for the emission of CdSe cores (2.03 or 2.20 eV), (ii) the intensity stimulation of high energy PL bands (2.37, 2.73 and 3.06 eV) related to the interface states at the ZnS/PEG polymer interface and (iii) the elastic strain modification in QD systems. The concentration of interface states at the ZnS/polymer interface increases at the aging of PEG polymer in ambient air.     

Ground state and excitations of disordered boson systems

After an introduction to the dirty bosons problem, we present a gaussian theory for the ground state and excitations. This approach is physically equivalent to the Bogoliubov approximation. We find that ODLRO can be destroyed with sufficient disorder. The density of states and localization of the elementary excitations are discussed.     

Developments in the engineering of refrigeration installations for cooling mines

The need to reduce the electrical energy consumption associated with the cooling of mines has recieved considerable attention in recent years. Various methods for reducing the amount of cooling water circulated underground in mines has also been looked at. These requirements have led to several developments such as cooling of the ventilation air on surface, the use of energy recovery devices, refrigeration machines for producing water close to 0°C and currently consideration is being given to installing ice making machines on the surface. These developments and their field of application are analysed and discussed.     

Coordination and valence state of transition metal ions in alkali-borate glasses

Borate glasses of the 20R₂O·80B₂O₃ type, where R = Li, Na and K, were colored by doping with transition metal ions (Co, Ni, Cr and Mn). The glasses were obtained by melting at the temperature of 1150 °C. For these glasses optical absorption in UV–VIS–NIR range were recorded. Analysis of the spectra allows to be determined the coordination and oxidation states of the doping transition metal ions. Changes of their coordination or oxidation are presented as a function of the optical basicity Λ after Duffy. Cobalt and nickel are present in examined borate glasses as divalent ions (Co²⁺, Ni²⁺) in octahedral coordination mainly, but the tetrahedral coordination state of cobalt is also possible. Chromium and manganese are present in the borate glasses in various oxidation state, though Cr³⁺ and Mn³⁺ ions in the octahedral coordination are probably dominant. A decrease of the electronegativity of the modifiers (Li → Na → K) and an increase of the glass matrix basicity cause a shift of the oxidation/reduction equilibrium towards higher valences of the transition metals (Cr⁶⁺, Mn³⁺).     

Ground state structures and properties of small hydrogenated silicon clusters

We present results for ground state structures and properties of small hydrogenated silicon clusters using the Car-Parrinello molecular dynamics with simulated annealing. We discuss the nature of bonding of hydrogen in these clusters. We find that hydrogen can form a bridge like Si-H-Si bond connecting two silicon atoms. We find that in the case of a compact and closed silicon cluster hydrogen bonds to the silicon cluster from outside. To understand the structural evolutions and properties of silicon cluster due to hydrogenation, we have studied the cohesive energy and first excited electronic level gap of clusters as a function of hydrogenation. We find that first excited electronic level gap of Sin and SinH fluctuates as function of size and this may provide a first principle basis for the short-range potential fluctuations in hydrogenated amorphous silicon. The stability of hydrogenated silicon clusters is also discussed.     

Ground state properties and structural phase transition of beryllium chalcogenides

The ground state properties and the structural phase transformation of beryllium chalcogenides (BeS, BeSe, and BeTe) have been investigated using first principle full potential-linearized augmented plane wave method (FP-LAPW) within density functional theory. We used local density approximation with and without generalized gradient correction as well as the Engel Vosko’s GGA formalism to find band gap. From the obtained band structures, the electron (hole) valence and conduction effective masses are deduced. We have determined the full set of first-order elastic constant, which have not been established experimentally. We have also calculated the energy–volume relations for these compounds in the zinc-blende (B3) and the NiAs (B8) phases. Hence we have obtained the lattice parameters, bulk modulus, pressure derivative of bulk modulus and cohesive energy as well as structural transition pressure. The calculated ionicity parameter which expected from the charge density behavior compared well with the Phillips’ ionicity scale.     

ESR and TSL study of hole and electron traps in LuAG:Ce,Mg ceramic scintillator

The process of hole and electron localization in LuAG:Ce,Mg ceramics is studied by electron spin resonance (ESR) and thermally stimulated luminescence (TSL). Hole traps, which are created by UV irradiation, are detected in the form of O⁻ centers. Mg-perturbed variants of O⁻ centers are proposed to exist. The thermal stability of such defects is studied, proving that they are stable up to room temperature. The interaction between O⁻ centers and shallow electron traps is studied by thermally stimulated luminescence (TSL) and phosphorescence experiments, which reveal the occurrence of an a-thermal tunneling process between trapped electrons and randomly distributed Ce centers. By correlating the TSL-derived trap parameters and temperature dependent ESR intensities, it is found that O⁻ centers compete with Ce centers in free electron capture.     

Simulating quantum transport in nanoscale MOSFETs: ballistic hole transport, subband engineering and boundary conditions

We present a modeling scheme for simulating ballistic hole transport in thin-body fully depleted silicon-on-insulator pMOSFETs. The scheme includes all of the quantum effects associated with hole confinement and also accounts for valence band nonparabolicity approximately. This simulator is used to examine the effects of hole quantization on device performance by simulating a thin (1.5-nm) and thick (5-nm) body double-gated pMOSFET in the ballistic limit. Two-dimensional electrostatic effects such as drain-induced barrier lowering (DIBL) and off-equilibrium transport are emphasized as part of this study. The effect of channel orientation on the device performance is examined by simulating pMOSFETs with channels directed along <100> and <110>. Simulated device characteristics for identical nMOSFETs and pMOSFETs are compared in order to explore the effects of subband engineering on CMOS technology. Novel floating boundary conditions used in simulating ballistic transport are highlighted and discussed.