Search

I describe the main ideas of constructive physics and its role for the probability interpretation of quantum theory. It is shown how the explicit probability space for quantum systems gives the formal representation of entanglement and decoherence. [ABSTRACT FROM AUTHOR]

This study considers implementations of error correction in a simulation language on a classical computer. Error correction will be necessarily in quantum computing and quantum information. We will give some examples of the implementations of some error correction codes. These implementations will be made in a more general quantum simulation language on a classical computer in the language Mathematica. The intention of this research is to develop a programming language that is able to make simulations of all quantum algorithms and error corrections in the same framework. The program code implemented on a classical computer will provide a connection between the mathematical formulation of quantum mechanics and computational methods. This gives us a clear uncomplicated language for the implementations of algorithms. [ABSTRACT FROM AUTHOR]

We present realistic simulation for single and double phosphorous donors in a silicon-based quantum computer design by solving a valley-orbit coupled effective-mass equation for describing phosphorous donors in strained silicon quantum well (QW). Using a generalized unrestricted Hartree-Fock method, we solve the two-electron effective-mass equation with quantum well confinement and realistic gate potentials. The effects of QW width, gate voltages, donor separation, and donor position shift on the lowest singlet and triplet energies and their charge distributions for a neighboring donor pair in the quantum computer(QC) architecture are analyzed. The gate tunability are defined and evaluated for a typical QC design. Estimates are obtained for the duration of spin half-swap gate operation. [ABSTRACT FROM AUTHOR]

Due to the increasing computing power of the graphics processing units (GPU), they are becoming more and more popular when solving general purpose algorithms. As the simulation of quantum computers results on a problem with exponential complexity, it is advisable to perform a parallel computation, such as the one provided by the SIMD multiprocessors present in recent GPUs. In this paper, we focus on an important quantum algorithm, the quantum Fourier transform (QTF), in order to evaluate different parallelization strategies on a novel GPU architecture. Our implementation makes use of the new CUDA software/hardware architecture developed recently by NVIDIA. [ABSTRACT FROM AUTHOR]

We describe a method by which single photons may be detected at frequencies in excess of 1GHz. This is accomplished with the use of surface acoustic waves (SAWs) in undoped gate-patterned heterostructures which drag excitons into a quantum-dot detector. The structure could consist of an undoped InGaAs quantum well embedded close to the surface of a GaAs bulk. We will also discuss the plasma excitations in a double quantum well structure whose electron density is modulated by the periodic potential of the SAW. These plasmon resonances may be employed in a detector when a bio-molecule is on the surface of the heterostructure. This is especially so for two coupled parallel layers. These plasmon resonances may be employed in a detector when the plasma resonances become unstable and radiate energy. The feasibility of a quantum computer using the electron spin driven by these high-frequency sound waves is currently under investigation. One of the aspects to be considered using SAWs is its application to sensors, detectors and other devices. These devices have the potential of making significant technological advancement in material science. There have been alternative schemes for photodetectors and sensors for detecting bio-molecules. Alternative architectures for quantum computers exploiting the electron charge and spin have also been suggested. This project is providing students an opportunity to look at these devices critically. [ABSTRACT FROM AUTHOR]

Quantum computers will perform computations at the atomic scale and operate according to the rules of quantum mechanics wherein quantum bits can exist in two or more states at once. Quantum entanglement is a remarkable property which plays a crucial role in quantum computation and can be utilized for applications like quantum teleportation. Another field, embryonics is inspired by the basic processes of molecular biology and by the embryonic development of living beings. Embryonic systems have properties unique to the living world like self-repair and self-reconstruction. Self-repair allows partial reconstruction in case of a minor fault, while self-replication allows complete reconstruction of the original device in case of a major fault. This paper presents a novel algorithm based on quantum teleportation for development of embryonic systems, called as Quantum Teleportation Embryonic Algorithm (QTEA). An example application of a robust quantum BCD to seven segment decoder is also presented. The results obtained are quite interesting and hold great promise for design of dependable systems through synthesis of quantum computing and embryonic systems. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Quantum computing is a novel paradigm that promises to be the future of computing. The performance of quantum algorithms has proved to be stunning. ANN within the context of classical computation has been used for approximation and classification tasks with some success. This paper presents an idea of quantum neural networks along with the training algorithm and its convergence property. It synergizes the unique properties of quantum bits or qubits with the various techniques in vogue in neural networks. An example application of Fisher’s Iris data set, a benchmark classification problem has also been presented. The results obtained amply demonstrate the classification capabilities of the quantum neuron and give an idea of their promising capabilities. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Grover’s database search algorithm, although discovered in the context of quantum computation, can be implemented using any physical system that allows superposition of states. A physical realization of this algorithm is described using coupled simple harmonic oscillators, which can be exactly solved in both classical and quantum domains. Classical wave algorithms are far more stable against decoherence compared to their quantum counterparts. In addition to providing convenient demonstration models, they may have a role in practical situations, such as catalysis. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Though attractive from scalability aspects, optical approaches to quantum computing are highly prone to decoherence and rapid population loss due to nonradiative processes such as vibrational redistribution. We show that such effects can be reduced by adiabatic coherent control, in which quantum interference between multiple excitation pathways is used to cancel coupling to the unwanted, non-radiative channels. We focus on experimentally demonstrated adiabatic controlled population transfer experiments wherein the details on the coherence aspects are yet to be explored theoretically but are important for quantum computation. Such quantum computing schemes also form a back-action connection to coherent control developments. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Proposed silicon-based quantum-computer architectures have attracted attention because of their promise for scalability and their potential for synergetically utilizing the available resources associated with the existing Si technology infrastructure. Electronic and nuclear spins of shallow donors (e.g. phosphorus) in Si are ideally suited candidates for qubits in such proposals, where shallow donor exchange gates are frequently invoked to perform two-qubit operations. An important potential problem in this context is that intervalley interference originating from the degeneracy in the Si conduction-band edge causes fast oscillations in donor exchange coupling, which imposes significant constraints on the Si quantum-computer architecture. We discuss the theoretical origin of such oscillations. Considering two substitutional donors in Si, we present a systematic statistical study of the correlation between relative position distributions and the resulting exchange distributions. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Our progress towards realization of the all-silicon solid-state NMR quantum computer [PRL, 89, 017901 (2002) and revised version] is reported. We show experimentally that the phase memory time TM (or phase decoherence time T2) of 29Si nuclear spins in single-crystal Si at room temperature can be extended up to 25 s (and possibly more) using RF decoupling techniques [quant-ph/0309164 (2003)]. A simple estimate shows that a T2 of 25 s will allow for 106 single-qubit and 104 two-qubit operations, by far the largest number of operations possible among existing solid-state quantum computer schemes. Recent progress in materials science needed for the realization of the all-silicon quantum computer is also discussed. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

A photon number resolving detector is a fundamental device for realizing quantum computing, to say nothing of improving quantum key distribution. It is considered possible to establish a multipurpose optical quantum computer by combining of a photon number resolving detector and non-classical light such as a single photon or squeezed light in a feedback system. We are developing a photon number resolving detector at the telecommunication wavelength of 1.5 μm, in which the attenuation ratio of optical fiber is minimum. We adopt a InGaAs pin photodiode and a GaAs JFET in a charge integration amplifier, that is cooled to 4.2 K to reduce thermal noise and leak current. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Hypothesis elimination is a special case of Bayesian updating, where each piece of new data rules out a set of prior hypotheses. We describe an algorithm to perform hypothesis elimination for a class of probability distributions encoded on a register of qubits, and establish a lower bound on the computational resources it requires. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

A major difficulty in realising a solid-state quantum computer is to reliably readout its qubit state at the end of computation. In this paper, we propose an efficient readout scheme making use of the resonance tunneling of a ballistic electron produced by a single electron pump. The resonance condition is perturbed when a charge qubit is placed nearby and consequently the transmission property of the ancillary readout electron reflects the qubit state to be measured. A fully quantum dynamical model for the readout of solid-state charge qubits is presented, which allows us to examine in detail the coherent evolution of the ancillary electron and the qubit system. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Ion traps are one of the leading technologies for the implementation of quantum computers in hardware. They are a vacuum technology that captures small number of laser-cooled ions in a linear trap and uses their quantum states to construct quantum circuits. We present the simulation results for quantum computing in a miniature ion trap in order to investigate electrostatics, ion motion and trapping efficiency for a given electrode geometry. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We propose and find pulse sequences for non-adiabatic two-qubit gates (CNOT, swap and controlled Z gates) for the Kane silicon-based electron mediated nuclear spin quantum computer architecture. These quantum gates are simpler, with higher fidelity and faster than existing proposals. Any two qubit gate may be implemented using similar pulse sequences. In this paper, we investigate the effect of decoherence on the operation of these quantum gates and some simple algorithms for a variety of dephasing rates. Our model shows that conservative estimates of the dephasing rates in the Kane architecture give gate fidelities comparable to that required for fault tolerant quantum computing. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The inherent resistance of geometric phase to decoherence may enable fault-tolerant quantum computation. However, as decoherence tends to affect many implementations, we must first understand the behavior of such phases for mixed states. Here we report an experimental measurement of geometric phase for polarization mixed states. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The growing interest about entanglement reflects the belief that unfactorizable states of a multipartite system provide an unreplaceable resource for quantum computing. Unavoidably, the system-environment interaction leads to decoherence, which is noxious for quantum computers, since it implies loss of the information stored in the system. Very recently the possibility of environment-induced entanglement has opened new intriguing perspectives. Here, we investigate the dynamics of a couple of spontaneously emitting two-level atoms confined within a bad single-mode cavity. Our main result is that the matter subsystem may be guided into a stationary robust entangled state. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Markovian regime decoherence effects in quantum computers are studied in terms of the fidelity for the situation where the number of qubits N becomes large. A standard model involving three-state lambda system ionic qubits is considered, with qubits localised around well-separated positions via trapping potentials. The centre of mass vibrations of the qubits act as a reservoir. Coherent one and two qubit gating processes are controlled by time dependent localised classical EM fields that address specific qubits, the two qubit gating processes being facilitated by a cavity mode ancilla, which permits state interchange between qubits. With suitably chosen parameters, the decoherence time can be made essentially independent of N. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The endohedral fullerene 15N@C60 comprises an electron spin S = 3/2 coupled to a nuclear spin I = 1/2 and is therefore ideally suited for experimental testing of basic properties of quantum mechanics. We will show that the 15N@C60 molecule represents a multi qubit system where different kinds of entangled states can be generated. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Devices that employ single atoms to store and manipulate information will be constructed in the near future. For example a solid state quantum computer device has been proposed that encodes information in the nuclear spin of shallow arrays of single 31P atoms (qubits) in a matrix of pure silicon. Construction of these devices presents formidable challenges. We have proposed a strategy that employs single ion implantation, with an energy of 10 to 20 keV, to load the qubits into prefabricated cells of the device that employs detector electrodes adjacent to the cells that can detect single keV ion strikes appropriate for the fabrication of shallow arrays. Our method utilises a pure silicon substrate with a very high resistivity, a thin (5 nm) SiO2 surface layer, biased electrodes applied to the surface and sensitive electronics that can detect the charge transient from single keV ion strikes. A key feature of these detectors is the ultra-thin surface dead layer. The charge collection efficiency of these detectors has been measured with MeV ions, keV x-rays and keV ions. We show that our detectors have a near 100% charge collection efficiency for MeV ions and keV x-rays. We show pulse height spectra from 15 keV H, He and P ion impacts allowing measurement of the pulse height defect for the keV ions. We review the role of these detectors in the construction of a two qubit device that will test many of the essential mechanisms of a revolutionary solid state quantum computer. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

The possibility of a complicated internal structure of an elementary particle was analyzed. In this case a particle may represent a quantum computer with many degrees of freedom. The evolution process of the genome was considered. The probability of formation of new species by means of random mutations was shown to be negligibly small. This behavior of the genome of an organism might result from the operation of elementary particle quantum computer. Deterministic model of evolution is considered. [ABSTRACT FROM AUTHOR]

While classical computing science has developed a variety of methods and programming languages around the concept of the universal computer, the typical description of quantum algorithms still uses a purely mathematical, non-constructive formalism which makes no difference between a hydrogen atom and a quantum computer. This paper investigates, how the concept of procedural programming languages, the most widely used classical formalism for describing and implementing algorithms, can be adopted to the field of quantum computing, and how non-classical features like the reversibility of unitary transformations, the non-observability of quantum states or the lack of copy and erase operations can be reflected semantically. It introduces the key concepts of procedural quantum programming (hybrid target architecture, operator hierarchy, quantum data types, memory management, etc.) and presents the experimental language QCL, which implements these principles. [ABSTRACT FROM AUTHOR]

A Fourier-based quantum computational learning algorithm with similarities to classical basis function networks is developed. Instead of a Gaussian basis, the quantum algorithm uses a discrete Fourier basis with the output being a linear combination of the basis. A set of examples is considered as a quantum system that undergoes unitary transformations to produce learning. The main result of the work is a quantum computational learning algorithm that is unique among quantum algorithms as it does not assume a priori knowledge of a function &fnof. [ABSTRACT FROM AUTHOR]

The quantum 1/f effect is a fundamental new aspect of quantum mechanics, quantum electrodynamics, and quantum field theory in general, with practical importance in most high-technology applications. It is based on the reaction of material currents to their spontaneous emission of infra-quanta such as photons, gravitons, transversal phonons, spin waves, etc. It is the result of decoherence of entangled states of particles and their spontaneous bremsstrahlung, a consequence of infrared-divergent interactions between particles and their field. It is the quantum manifestation of classical turbulence and it represents the most fundamental form of quantum chaos. It is described by the simple universal formula of conventional and coherent quantum 1/f noise, important in engineering, science and technology. It provides a new physical meaning to the notion of ‘constant current’, in time and space, similar to the 1937 definition of elastic processes by Bloch and Nordsieck. Finally, it is an interesting aspect of the concrete way in which matter generates its forms of existence, for instance time and space. Quantum 1/f spin decoherence rates, known to severely limit the performance of quantum computers, are shown here to be also affected by the quantum 1/f effect. Indeed, the elementary spin-flip process has a bremsstrahlung amplitude, leading to a non-stationary state with 1/f quantum fluctuations, and a disentangled system of non-localized low-frequency photons with negative conditional entropy. Thus, decoherence is due to the entangled system's interaction with the rest of the world, as is its quantum 1/f fluctuation which can be expressed in qubits. Increasing the spin-excess n is one way to reduce these fluctuations. In general, we find that both decoherence and its quantum 1/f noise could be controlled by better insulating the system in a new way. [ABSTRACT FROM AUTHOR]

We consider experimental issues relevant to quantum computing, and discuss the best way to achieve the essential requirements of reliable quantum memory and gate operations. Nuclear spins in trapped ions or atoms are a very promising candidate for the qubits. We estimate the parameters required to couple atoms using light via cavity QED in order to achieve quantum gates. We briefly comment on recent improvements to the Cirac-Zoller method for coupling trapped ions via their vibrational degree of freedom. Error processes result in a trade-off between quantum gate speed and failure probability. A useful quantum computer does appear to be feasible using a combination of ion trap and optical methods. The best understood method to stabilise a large computer relies on quantum error correction. The essential ideas of this are discussed, and recent estimates of the noise requirements in a quantum computing device are given. [ABSTRACT FROM AUTHOR]

Entangled states are a crucial component in quantum computers, and are of great interest in their own right, highlighting the inherent nonlocality of quantum mechanics. As part of the drive toward larger entangled states for quantum computing, we have engineered the most complex entangled state so far in a collection of four trapped atomic ions. Notably, we employ a technique which is readily scalable to much larger numbers of atoms. Limits to the current experiment and plans to circumvent these limitations are presented. [ABSTRACT FROM AUTHOR]

Nitrogen in C[sub 60] (N@C[sub 60]) has properties which could be favourable for spin quantum computing. The spin system has an electron spin S = 3/2 and a nuclear spin I = 1. Due to the good shielding of the nitrogen atom from its environment, the spin relaxation times are relatively long compared to other spin systems. Therefore, the decoherence time (i.e. time available for computing) is fairly long. It has been proposed that endohedral fullerenes can be arranged on the surface of a substrate in such a way that the spins of different N@C[sub 60] interact with one another and thus the number of bits can be augmented. Specific arrangements and possibilities of addressing individual spins will be discussed. [ABSTRACT FROM AUTHOR]

The requirements that the candidate technologies have to fulfill in order to build a quantum computer are discussed with particular reference to a newly proposed implementation based on high capacitance coupled Josephson junctions. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ability to distinguish between coherent states optimally plays in important role in the efficient usage of quantum resources for classical communication and sensing applications. While it has been known since the early 1970's how to optimally distinguish between two coherent states, generalizations to larger sets of coherent states have so far failed to reach optimality. In this work we outline how optimality can be achieved by using a small quantum computer, building on recent proposals for optimal qubit state discrimination with multiple copies [ABSTRACT FROM AUTHOR]

Quantum versions of random walks have diverse applications that are motivating experimental implementations as well as theoretical studies. Recent results showing quantum walks are "universal for quantum computation" relate to algorithms, to be run on quantum computers. We consider whether an experimental implementation of a quantum walk could provide useful computation before we have a universal quantum computer. [ABSTRACT FROM AUTHOR]

We have integrated a microfabricated phase Fresnel lens (PFL) with an ion trap and used it to image a 174Yb+ ion. The observed collection efficiency was 4.1± 1.3%, in agreement with a predicted performance of 4.6% based on optical characterization and suitable for use in quantum computing. A maximum signal to background scatter noise of 23±4 was measured near saturation intensity (s = 0.7). The depth of focus was 11 μm and the field of view in excess of 100 μm across. [ABSTRACT FROM AUTHOR]

Teleportation-based quantum information processing is reviewed. [ABSTRACT FROM AUTHOR]

This paper gives a coarse overview of the historical development of algorithms used for information security. It is shown that the development of these encryption algorithms has been made in small incremental steps for almost 2000 years until the latter part of the last century when three revolutionary inventions were made. The main properties of these new technologies, the public key encryption method, quantum cryptography and quantum computing are explained and demonstrates why they can be seen as quantum leaps in the development of information security. [ABSTRACT FROM AUTHOR]

In the Dimension Embedded in Unified Symmetry (D.E.U.S.; [1]) we made a qualitative description of the way in which we can construct the Large Scale Structure of the Universe from the knot-particle equivalence. Even that we are limited by the lack of computational power implemented on a nonlinear computational architecture needed to conduct this study to its finish, we are still able to give the algorithm to be used in a future simulation, on a, let say, quantum computer. [ABSTRACT FROM AUTHOR]

The article focuses on the goal of the Grover quantum algorithm which is to find an item in a randomly ordered database. The advantage of Grover algorithm is rooted in apply a cleverly constructed Grover gate to a convenient initial entangled state of n-orbit registers. Entangled states however are very sensitive to decoherence originating from the interaction of quantum computer and its environment.

As a promising physical realization of a quantum computer, we discuss a system with stacked quantum dots buried in adjacent to the channel of a spin field-effect transistor. In this scheme, one can measure only the edge qubit (nearest to the channel) via the spin-blockade measurement, but still it is possible to know the state of the whole qubits. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Quantum computation in its binary concept requires a set of two different quantum states, a quantum two-level system that physically realizes the quantum bit (qubit). Amongst the many other existing proposals, we discuss qubits based on the charge distribution of a few electrons. Especially the role of higher lying states is investigated with respect to their role as source of decoherence for the two-level system of the qubit constructed in the ground state. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

In this paper a quantum computer based on the recombination processes happening in semiconductor devices is presented. A “data element” and a “computational element” are derived based on Schokley-Read-Hall statistics and they can later be used in order to manifest a simple and known quantum algorithm. Such a paradigm is shown by the application of the proposed technology onto the Shor’s period-finding algorithm. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We have been developing a nuclear spin polarizer for solid-state NMR quantum computers (QCs). The scheme utilizes the methods of optical pumping and polarization transfer, and enables initialization of the qubits in the QC materials where the optical pumping is not directly applicable. The current status of the development is described in both the instrumentation and the material aspects. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Recently, the quantum computer (QC) using the nano-devices have significantly attracted attention, because a large-scale extention of the qubits could be easily realized in the nano-devices. However, some problems for the realization of the QC with nano-devices arise from the short decoherence time and the interaction of qubits only between nearest-neighbor qubits. Therefore, we try to design the optimal quantum circuit of the quantum Fourier transform in various network system by means of the genetic algorithm (GA). © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The possible creation of architectures of quantum cellular automata formed by simple molecules opens a very promising and interesting area of research due to the possibility of going beyond the current limits of miniaturization and integration of devices. In this research we theoretically study the electronic properties of a quan- tum dot array in graphene nanoribbons and in an array of molecules with graphane structures. The role of quantum dots in the ribbons and in the mole- cules is played by oxide reduction centers that can trap or release electrons. With the knowledge about these properties we design cellular automata archi- tectures with nanoribbons and molecular arrays, with this it will be feasible to store and process logic information at room temperature. The stability of the proposed graphene structures are studied using quan- tum methods of geometric optimization [1]. The electronic properties of the nanoribbons are obtained from first-principle calculations based on pseudo- potentials by using the generalized gradient approximation (GGA) of Perdew- Burke-Ernzerhof [2-3]. With the parameters obtained from the study of the electronic properties of the cells that make up the automata, we can make a simulation of the dynamical response of the system. To do this, we use a set of accelerated algorithms for discrete systems [4] based on the Glauber dynamic [5]. Our results show that the studied system can be scaled so that the propagation of digital information throughout the automata is possible at room temperature. [ABSTRACT FROM AUTHOR]

We analyze transport properties and develop schemes to control transport behavior in low-dimensional quantum many-body systems. We provide evidence to refute the widely accepted conjecture that integrable quantum many-body systems exhibit ballistic transport, whereas chaotic models show diffusive transport. Spin-1/2 Heisenberg chains are considered, as they are prototype quantum many-body systems and may show both integrable and chaotic limits according to appropriate adjustments of parameters in the Hamiltonian. In the absence of disorder and with only nearest neighbor interactions, spin-1/2 chains are integrable models. We verify that in this case the transport of local magnetization is indeed ballistic. On the other hand, diffusive transport is observed only when chaos is induced by on-site disorder, whereas for chaos caused by next-nearest neighbor interactions, the transport remains ballistic. In addition, methods of coherent quantum control to induce a transition from diffusive to ballistic transport are proposed. Our studies are likely to find applications in the design of devices with controllable conductivity, the mitigation of the effects of local heating, spintronics, and transport of information in quantum computers. [ABSTRACT FROM AUTHOR]