Narrow Results

We review the results of our current research on quantum engineering which include the theory, modeling and simulations of quantum devices for potential applications to threat reduction and homeland security. In particular, we discuss: (i) scalable solid-state quantum computation with qubits based on (a) nuclear spins of impurity atoms in solids, (b) superconducting junctions, and (c) unpaired electron spins of spin radicals in self-assembled organic materials; (ii) quantum neural devices; (iii) quantum annealing; (iv) novel magnetic memory devices based on magnetic tunneling junctions with large tunneling magnetoresistance; (v) terahertz detectors based on microcantilever as a light pressure sensor; (vi) BEC based interferometers; (vii) quantum microscopes with a single-spin resolution based on (a) a magnetic resonant force microscopy and (b) an optically detected magnetic resonance; and (viii) novel approach for suppression of fluctuations in free space high-speed optical communication. Finally, we describe the similarities between the behavior of cross sections in reactions with heavy nuclei in the regions of strongly overlapped resonances and electron conductivity in semiconductor heterostructures.

We numerically investigate the Josephson transport through ferromagnetic insulators (FIs) by taking into account its band structure. By use of the recursive Green's function method, we found the formation of the π junction in the case of the fully spin-polarized FI (FPFI), e.g., La₂BaCuO₅. Moreover, the 0-π transition is induced by increasing the thickness of FPFI. On the other hand, Josephson current through the Eu chalcogenides shows the π junction behavior in the case of the strong d-f hybridization between the conduction d and the localized f electrons of Eu. Such FI-based Josephson junctions may becomes a element in the architecture of future quantum information devices.

We deal with the problem of nanoscale superconductivity. Nanoscale superconductivity remains to be one of the most interesting research areas in condensed mater. Recent technology and experiments have fabricated high-quality superconducting MgB₂ nanoparticles. We consider the two-band superconductivity in ultrasmall grains, by extending the Richardson exact solution to two-band systems, and develop the theory of interactions between nano-scale ferromagnetic particles and superconductors. The properties of nano-sized two-gap superconductors and the Kondo effect in superconducting ultrasmall grains are investigated as well. The theory of the Josephson effect is presented, and his application to quantum computing are analyzed.

Using the two-time Green's function method, we analyze the temperature effect on the famous Cirac and Zoller model. Formulas for the average number of different mode phonons and the deviation of its internal state from its ground state are derived. The influence of the internal state energy and trap frequency on the number of phonons is also discussed. The upper limit of temperature for regular quantum computation in the Cirac and Zoller model is obtained.

Ternary quantum circuits play a significant role in future quantum computing technology because they have many advantages over binary quantum circuits. Subtraction is considered as being one of the key arithmetic operations; hence, subtractors are very essential for the construction of various computational units of quantum computers and other complex computational systems. In this paper, we have proposed the realization of a quantum reversible ternary half-subtractor circuit using a generalized ternary gate, a ternary Toffoli gate, and a ternary C²NOT gate. Based on the realization of the ternary half-subtractor, we proposed the realization of a ternary full-subtractor.

We review the q-deformed spin network approach to Topological Quantum Field Theory and apply these methods to produce unitary representations of the braid groups that are dense in the unitary groups. Our methods are rooted in the bracket state sum model for the Jones polynomial. We give our results for a large class of representations based on values for the bracket polynomial that are roots of unity. We make a separate and self-contained study of the quantum universal Fibonacci model in this framework. We apply our results to give quantum algorithms for the computation of the colored Jones polynomials for knots and links, and the Witten–Reshetikhin–Turaev invariant of three manifolds.

Quantum computers are believed to perform high-speed calculations, compared with conventional computers. However, the quantum computer solves NP (non-deterministic polynomial) problems at a high speed only when a periodic function can be used in the process of calculation. To overcome the restrictions stemming from the quantum algorithm, we are studying the emulation by a LSI (large scale integrated circuit). In this report, first, it is explained why a periodic function is required for the algorithm of a quantum computer. Then, it is shown that the LSI emulator can solve NP problems at a high speed without using a periodic function.

It is argued that the existing schemes of fault-tolerant quantum computation, designed for discrete time models and based on quantum error correction, fail for continuous time Hamiltonian models, even with Markovian noise.

This paper presents a quantum search algorithm for Automated Test Pattern Generation (ATPG) of VLSI circuits. For given digital circuits, a neural network is created that represents the digital gates with their interconnections as neurons. This neural network is characterized by an energy function E, which is the mathematics representation of neural network. The solution of the energy function gives us the test vector. The test vector is a combination of input values of digital circuits that detects a particular fault. In this paper, specific aspects of quantum theory like superposition and quantum parallelism are applied to find the solution of this energy function. The algorithm developed is so efficient that it requires only (where N is the total number of vectors) iterations to find the desired test vector whereas in classical computing, it takes N/2 iterations. At the end, a comparison is made between exhaustive, simulated annealing and quantum based techniques. Experimental results show that the quantum search algorithms are more efficient than classical algorithms and can be applied with more efficiency.

Using two-time Green's function we analyze the effect of temperature on the Mølmer and Sørensen (M–S) model of cold trapped ion quantum computer. A formula for the deviation of an internal state from its ground state is derived in the first-order approximation. The fluctuation of internal state caused by thermal motion is also discussed. The deviation of internal state when different trap parameters changes with the temperature are given too. The upper temperature limit for regular quantum computation is obtained and our result is in agreement with experiment.

The minimal time, T^Shor, in which a one-way quantum computer can execute Shor's algorithm is derived. In the absence of an external magnetic field, this quantity diverges at very small temperatures. This result coincides with that of Anders et al. obtained simultaneously to ours but using thermodynamical arguments. Such divergence contradicts the common belief that it is possible to do quantum computation at low temperatures. It is shown that in the presence of a weak external magnetic field, T^Shor becomes a quantized quantity which vanishes at zero temperature. Decoherence is not a problem because T^Shor/τ_dec < 10^-9, where τ_dec is decoherence time.

We study the dynamical properties of a cavity field coupling to a Cooper pair box (CPB). We assumed that the CPB is prepared initially in a mixed state with a coherent state for the field. By solving the time-dependent equations using the evolution operator, it shows that mean numbers of Cooper pairs is affected by the detuning. The mean number of Cooper pairs is further enhanced by the multi-photon processes in commonly used cavity field.

We discussed the entanglement generated by the quantum annealing processor in the thermal state. The quantum annealing processor is modeled using the spin-chain model. The system is analytically solved using the unitary operator method and generated correlations (Von Neuman, Shanonn entropies and Purity) are discussed. The effect of the system parameters such as coupling constant, strength coupling and bias parameter, on the dynamics of the generated entanglement is studied. It is shown that the system parameters can be used as a controller of the entanglement.

It is argued that the existing schemes of fault-tolerant quantum computation, designed for discrete time models and based on quantum error correction, fail for continuous time Hamiltonian models, even with Markovian noise.

We report low temperature electron-transport measurements on two single-electron transistors operated simultaneously and coupled capacitively to two isolated double quantum-dots in silicon. The gate voltage dependencies and microwave induced responses are presented. A gate-voltage dependent anti-crossing type response with effective Q-value ~10⁵ and splitting energy 91 neV is observed. The sharp resonance peaks are attributed to population of charge states due to photon absorption from the incident microwave irradiation, and the peak splitting is attributed to the coupling between the isolated double quantum-dots.

We review the results of our current research on quantum engineering which include the theory, modeling and simulations of quantum devices for potential applications to threat reduction and homeland security. In particular, we discuss: (i) scalable solid-state quantum computation with qubits based on (a) nuclear spins of impurity atoms in solids, (b) superconducting junctions, and (c) unpaired electron spins of spin radicals in self-assembled organic materials; (ii) quantum neural devices; (iii) quantum annealing; (iv) novel magnetic memory devices based on magnetic tunneling junctions with large tunneling magnetoresistance; (v) terahertz detectors based on microcantilever as a light pressure sensor; (vi) BEC based interferometers; (vii) quantum microscopes with a single-spin resolution based on (a) a magnetic resonant force microscopy and (b) an optically detected magnetic resonance; and (viii) novel approach for suppression of fluctuations in free space high-speed optical communication. Finally, we describe the similarities between the behavior of cross sections in reactions with heavy nuclei in the regions of strongly overlapped resonances and electron conductivity in semiconductor heterostructures.

Research on neural and neurofuzzy architectures indicates the need for the introduction of a more general concept than that of the neural unit, ornode, introduced in the pioneering work by McCulloch and Pitts¹. The neural unit that is widely used today in artificial neural networks can be considered as a nonlinear filter. This unit collects all the recently novelty in the single neuron performance as the active dendrite, timing counting and synaptic plasticity. In order to do research on such neural architectures a description language is needed. On the basis of these considerations we propose in the present paper a generalisation of the concept of neural unit, which will be denoted as morphogenetic neuron. The name "neuron" was adopted because the activation function of such a device is characterized, in the same way as in classical neural units, by a bias potential and by a weighted sum of suitable, in general nonlinear, functions of morphogenetic fields. The attribute "morphogenetic" was chosen because the data determine the weights, hologram, of the elementary source fields (WRITE OPRATOR) which generate the morphogenetic field by the linear operation of superposition (READ OPERATOR). With the morphogenetic neuron it becomes possible implementing a given input-output transfer function, without the need for resorting to laborious methods of synthesis, such as supervised training. It becomes possible to create categorical judgements of data (filter) with the use of prototype data base. We can also generate quantum gate and general transformations. The quantum computer and morphogenetic neuron computer are based on the same formal structure given by the quantum mechanics. This open the possibility to connect quantum computation with the neuron computation at a deeper level of quantum mechanics.

We review the working principle of a liquid-state Nuclear Magnetic Resonance (NMR) quantum computer where spins of nuclei in isolated molecules dissolved in a solvent work as qubits and they are controlled by radio frequency magnetic field pulses. Then, we show some of our recent experimental results, "generation and suppression of artificial decoherence" and "quantum teleportation without irreversible detection".

We numerically investigate the Josephson transport through ferromagnetic insulators (FIs) by taking into account its band structure. By use of the recursive Green's function method, we found the formation of the π junction in the case of the fully spin-polarized FI (FPFI), e.g., La₂BaCuO₅. Moreover, the 0-π transition is induced by increasing the thickness of FPFI. On the other hand, Josephson current through the Eu chalcogenides shows the π junction behavior in the case of the strong d-f hybridization between the conduction d and the localized f electrons of Eu. Such FI-based Josephson junctions may becomes a element in the architecture of future quantum information devices.