Search query: quantum computing with quantum dots on quantum linear supports

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abstract: Motivated by the recently demonstrated ability to attach quantum dots to polymers at well defined locations, we propose a condensed phase analog of the ion trap quantum computer: a scheme for quantum computation using chemically assembled semiconductor nanocrystals attached to a linear support. The linear support is either a molecular string (e.g. DNA) or a nanoscale rod. The phonon modes of the linear support are used as a quantum information bus between the dots. Our scheme offers greater flexibility in optimizing material parameters than the ion trap method, but has additional complications. We discuss the relevant physical parameters, provide a detailed feasibility study, and suggest materials for which quantum computation may be possible with this approach. We find that Si is a potentially promising quantum dot material, already allowing a 5 10 qubit quantum computer to operate with an error threshold of 10 3
author_names: Kenneth R Brown

Daniel A Lidar

K Birgitta Whaley
title: Quantum computing with quantum dots on quantum linear supports
year: 2001

author_names: Andrew J Landahl
title: Universal fault tolerant adiabatic quantum computing with quantum dots or donors
year: 2016

abstract: We explore the pattern of frequency dependent linear and second non linear optical responses of repulsive impurity doped quantum dots harmonically confined in two dimensions. The dopant impurity potential chosen assumes a Gaussian form and it is doped into an on center location. The quantum dot is subject to a periodically oscillating external electric field. For some fixed values of transverse magnetic field strength (oc) and harmonic confinement potential (o0) the influence of impurity strength (V0) and impurity domain (x) on the diagonal components of the frequency dependent linear (axx and ayy) and second non linear (gxxxx and gyyyy) responses of the dot are computed through a linear variational route. The investigations reveal that the optical responses undergo enhancement with increase in both V0 and x values. However, in the limitingly small dopant strength regime one observes a drop in the optical responses with increase in V0. A time average rate of energy transfer to the system is often invoked to support the findings.
author_names: Nirmal Kumar Datta

Manas Kumar Ghosh
title: Impurity strength and impurity domain modulated frequency dependent linear and second non linear response properties of doped quantum dots
year: 2011

abstract: Abstract A two qubit quantum gate scheme is proposed in a system of semiconductor quantum dots (QDs) located in a high Q single mode optical cavity with extremely low loss. The spin degrees of freedom of the only excess conduction electron of the QD are employed as qubits. By means of properly tailored ultrafast laser pulses and Pauli blocking effects, we can perform conditional phase gate on two non neighboring spins, mediated by the cavity mode.
author_names: Mang Feng

Irene D'Amico

Paolo Zanardi

Fausto Rossi
title: A scheme of quantum computing with semiconductor quantum dots in optical cavity
year: 2003

abstract: An all carbon memristive synapse is highly desirable for hardware implementation in future wearable neuromorphic computing systems. Graphene oxide (GO) can exhibit resistive switching (RS) and may be a feasible candidate to achieve this objective. However, the digital type RS often occurring in GO based memristors restricts the biorealistic emulation of synaptic functions. Here, an all carbon memristive synapse with analog type RS behavior was demonstrated through photoreduction of GO and N doped carbon quantum dot (NCQD) nanocomposites. Ultraviolet light irradiation induced the local reduction of GO near the NCQDs, therefore forming multiple weak conductive filaments and demonstrating analog RS with a continuous conductance change. This analog RS enabled the close emulation of several essential synaptic plasticity behaviors; more importantly, the high linearity of the conductance change also facilitated the implementation of pattern recognition with high accuracy. Furthermore, the all carbon memristive synapse can be transferred onto diverse substrates, showing good flexibility and 3D conformality. Memristive potentiation/depression was stably performed at 450 K, indicating the resistance of the synapse to high temperature. The photoreduction method provides a new path for the fabrication of all carbon memristive synapses, which supports the development of wearable neuromorphic electronics. A graphene based device can help computer chips behave more like human brains by transmitting current across thread like wires. Neural synapses store memories by accessing different types of conductive states. Chinese researchers led by Haiyang Xu at Northeast Normal University in Changchun and Zhenhui Kang at Soochow University in Suzhou now demonstrate that graphene sheets with different conductivity levels caused by adding or removing oxygen atoms can also exhibit synapse like behavior. The team developed a carbon nitrogen composite to sandwich between two graphene electrodes with high and low levels of conductivity. Exposing the composite to ultraviolet light created numerous tiny filaments between the electrodes that physically restrict electron flow and provide gradual smooth transitions between the graphene electrodes' two conductive states. The organic framework of this device also provides inherent flexibility for wearable devices. All carbon memristive synapse is built through photo reduction of a nanocomposite comprised of graphene oxide and N doped carbon quantum dots. The analog type resistive switching was demonstrated, which enabled the emulation of synaptic learning and pattern recognition with high accuracy. The all carbon devices possess excellent transferability, flexibility and resistance to high temperature, showing the potential for the development of wearable neuromorphic computing system.
author_names: Y Y Lin

Zhongqiang Wang

Xue Zhang

Tao Zeng

Liang Bai

Zhenhui Kang

Changhua Wang

Xiaoning Zhao

Haiyang Xu

Yichun Liu
title: Photoreduced nanocomposites of graphene oxide/N doped carbon dots toward all carbon memristive synapses
venue: NPG Asia Materials
year: 2020

abstract: Control over electron spin states, such as coherent manipulation, filtering and measurement promises access to new technologies in conventional as well as in quantum computation and quantum commu nication. We review our proposal of using electron spins in quantum confined structures as qubits and discuss the requirements for implementing a quantum computer. We describe several realizations of one and two qubit gates and of the read in and read out tasks. We discuss recently proposed schemes for using a single quantum dot as spin filter and spin memory device. Considering electronic EPR pairs needed for quantum communication we show that their spin entanglement can be detected in meso scopic transport measurements using metallic as well as superconducting leads attached to the dots.
author_names: Guido Burkard

Hans-Andreas Engel

Daniel Loss
title: Spintronics and quantum dots for quantum computing and quantum communication
year: 2000

abstract: Since the seminal work by Shor who proposed a quantum algorithm factorizing integers into prime factors, it has become manifest that the laws of quantum mechanics provide resources for computation that overpower classical physics. The computational advantages that quantum physics offers have stimulated a tremendous amount of theoretical and experimental research. In this context, spin systems have played a major role, given that the spin degree of freedom with the paradigmatic case of the spin 1/2 of electrons represents an obvious candidate for the encoding of an elementary bit of quantum information (qubit) On the other hand, however, quantum objects are very fragile entities, being very susceptible to the environment they reside in. This fragility of qubits is one of the main obstacles in the realization of a quantum computer. In this thesis, we mainly address the two following questions relevant to quantum computation. i) How is it possible to realize quantum gates both in a reliable and scalable way? ii) How can we store quantum information in a way that is resilient to the errors caused by the thermal environment? We focus on spin systems and demonstrate that long range spin spin interactions in the models considered can have beneficial effects. In their pioneering work, Loss and DiVincenzo proposed a way to perform quantum computation in a semiconductor based architecture where the spin state of an electron trapped in a quantum dot is chosen to encode the elementary qubit. In this proposal, the spins are required to lie spatially close to each other, and this might complicate the realization of a scalable architecture. In the first part of the thesis we thus propose a scheme that allows the constraint on the positioning of the qubits to be relaxed. This is achieved by introducing a ferromagnetic coupler between the distant qubits, to which it is coupled via a dipolar interaction. Most importantly, our proposal is applicable to any type of spin qubits and in particular to the technologically very relevant silicon based qubits and NV centers in diamond to which previous coupling schemes do not apply. As additional key element, a quantum computer needs a memory capable of reliably storing quantum information in the presence of thermal fluctuations. This brings us to the second part of this thesis, where we consider self correcting memories, for which the protection against thermal noise is built in at the hardware level. We propose physical models that exhibit these self correcting properties, using as a starting point the well known topologically ordered toric code. In particular, we investigate how to induce long range interactions between the spins of the toric code, since such interactions help increase the memory lifetime. As a first step, we study a honeycomb quantum spin model coupled to delocalized cavity modes. We investigate the properties of the low energy toric code Hamiltonian and show that the coupling to cavity modes prolongs the lifetime of the memory and offers a method to detect the presence of excitations. While the introduction of extended bosonic modes makes the model non local, we also propose a purely local model consisting of a toric code embedded in a three dimensional cubic lattice of hopping bosons; the low energy sector of a toric code coupled to a three dimensional Heisenberg ferromagnet in a broken symmetry state realizes this model. Our analysis leads to an energy penalty for the creation of defects that grows linearly with the size L of the memory and thus to a lifetime increasing exponentially with L. In the third part of this thesis, we study spin systems that support anyons, i.e. particles with fractional statistics. Similar to the toric code, such systems are topologically ordered: they are immune to local perturbations and quantum gates are implemented by non local operations, namely the exchange of anyons, whose outcomes depend only on the topology of the exchange. Here again the fault tolerance is achieved at the level of the hardware and physical systems supporting non abelian anyons are thus promising platforms for quantum computation. We focus on spin systems that exhibit some of these properties and specifically on variations of the honeycomb quantum spin model. We first investigate the exact solution of the honeycomb model in detail and derive an explicit formula for the projector onto the physical subspace. We use this result to study inhomogeneous open spin ladders, related to the honeycomb model, which can be tuned between topological and non topological phases. We test the robustness of Majorana end states (MES) which emerge at the boundary between sections in different topological phases. Furthermore, we present a trijunction setup where MES can be braided. This is of interest since MES in these spin ladders potentially follow non abelian braiding statistics. Finally, we study the ground states of the aforementioned ladders and show that they are free of vortices when the signs of the spin couplings are all positive or negative. To prove this, we use exact reflection positivity based methods as well as approximate methods. In the last part of the thesis, we provide an extension of the Mermin Wagner theorem to a system of lattice spins that are spin coupled to itinerant and interacting charge carriers. We prove that neither (anti ferromagnetic nor helical long range order is possible in one and two dimensions at any finite temperature (in the absence of spin orbit) The fundamental question whether spontaneous ordering of the lattice spins occurs in these systems is of interest in the context of quantum computation; the polarization of nuclear spins coupled to a two dimensional electron gas is a possible route towards the reduction of decoherence induced by the fluctuating Overhauser field in gate defined quantum dots.
author_names: Fabio L Pedrocchi
title: Spin systems and long range interactions for quantum memories and quantum computing
year: 2013

abstract: The support by the ISEN Booster Award has significantly advanced our group's research, resulting in 4 publications in Journal of Chemical Physics and Journal of Chemical Theory and Computation. In addition, the support by ISEN has allowed us to obtain two federal grants: a regular grant from Department of Energy Basic Energy Sciences and the National Science Foundation CAREER Award (to be annouced) I have presented 6 invited talks in international conferences, including the Sanibel Symposium, Telluride meetings, and the ACS 2013 Fall meeting. In the following, I will summarize our research achievements during the support by ISEN, and future directions. We have developed a wave function ansatz based on tensor decompositions, Ps i ui+ i ui2 i ui 1/4 i ui2 in order to apply rigorous wave function methods to nanoscale materials including quantum dots. We decided to test our hypothesis using a simple system, a molecular dimer, which itself is interesting in solar energy conversoin. In the article that we published in J. Chem. Phys. we demonstrated that the active space wave functions of molecular dimers can be compactly represented by a linear combination of monomer wave functions as Ps i ui+ " i ui 1/4 i ui 1/4 This expansion becomes exact when a large number of states are included in the summation. Figure 1 shows the numerical proof of potential exactness using a benzene dimer. Importantly we have found that the Hamiltonian matrix elements between product basis functions can be computed without forming dimer basis functions explicitly:
author_names: Tamar Seideman

Mark A Ratner

Toru Shiozaki
title: Year End Report Accurate Electronic Structure Theory of Quantum Dots

abstract: for an Invited Paper for the MAR05 Meeting of The American Physical Society Transport Spectroscopy of Coupled Quantum Dots in Conditions of the Kondo Effect LEONID GLAZMAN, University of Minnesota We develop electron transport theory for novel devices [1,2] which are interesting in the context of correlated electrons physics. The device proposed in Ref. [1] is designed for an observation of a non Fermi liquid behavior of itinerant electrons. The device measured in Ref. [2] may serve a similar purpose, and also may become important for quantum computing. In the case of Ref. [1] our theory [3] provides a strategy for tuning to the non Fermi liquid fixed point a quantum critical point in the space of device parameters. We explore the corresponding quantum phase transition, and make explicit predictions for the behavior of differential conductance in the vicinity of the quantum critical point. Motivated by the measurements [2] we developed a theory of conductance of Kondo quantum dots coupled by the RKKY interaction [4] Investigation of the differential conductance at fixed interaction strength may allow one to distinguish between the possible ground states of the system. Transition between the ground states is achieved by tuning the interaction strength; the nature of the transition (which includes a possibility of a non Fermi liquid point) can be extracted from the temperature dependence of the linear conductance. This research is supported by NSF grants DMR02 37296 and EIA0210736. 1. Y. Oreg and D. Goldhaber Gordon, Phys. Rev. Lett. 90, p. 136602 (2003) 2. N.J. Craig J.M. Taylor, E.A. Lester, C.M. Marcus, M.P. Hanson, and A.C. Gossard, Science 304, 565 (2004) 3. M.G. Vavilov and L.I. Glazman, preprint cond mat/0404366. 4. M. Pustilnik, L. Borda, L.I. Glazman, and J. von Delft, Phys. Rev. B 69, 115316 (2004)
author_names: Leonid I Glazman
title: Transport Spectroscopy of Coupled Quantum Dots in Conditions of the Kondo Effect
year: 2005

abstract: This paper describes the design of different type of combinational such as adder subtractor preceding NAND, NOR, XOR, XNOR based on QCA design, with comparatively less number of cells and area. It is efficient for its faster speed ,smaller size and low power consumption than transistor technology.QCA provides for nano level computations using molecular component .QCA Designer gives the designer the ability to quickly layout a QCA design by providing an extensive set of CAD tools.By taking full advantage of the unique features of this technology, we are able to create complete circuits on a single layer of QCA. Keywords component, QCA, combinational circuits, majority gates, QCA designer. I.INTRODUCTION The size of CMOS transistors are decreasing day by day and it will eventually hits its limitations hence it become necessary to develop an alternate to continually improve the development of electronics devices .Devices based on quantum mechanical principles hold the promise of faster speeds and greatly reduced sizes. Most quantum device designs examined have been similar to classical device implementations in that they use currents and voltages to encode information.QCA is a potential device That can be used to implement the sequential and the combinational circuits. It is a different type of technology that will replace MOSFET in future. QCAs were introduced in 1993 by lent et al, and experimentally verified in 1997. It is expected to achieve high device density, extremely low power consumption and very high switching speed .The QCA offers a new transistorless computing paradigm in nanotechnology. QCA can be used to create s smaller electronics device with high computation switching speed and low power consumption. this make research on QCA popular and important to replace CMOS transistors in Nano scale technology fabrication. II.QCA BASICS A QCA consists of an array of quantum dot cells connected locally by the interactions of the electrons contained in each cell. The scheme is non conventional in that the quantum state of each cell is used to encode binary information. The Coulombic interaction connects the state of one cell to the state of its neighbors. Thus the problems associated with small output currents and parasitic capacitances of connecting wires do not occur. "Binary wires" composed of linear arrays of cells are effective in transmitting information, coded in the cell states, from one place to another. By applying these layouts, the hardware requirements for a QCA design can be reduced. QCA is based on electrons confining in dots and each cell has four quantum Dots The four dots are located in the corners of squares structure as shown in fig 1.The cell contains two extra mobile electrons, which are allowed to tunnel between neighboring sites of the cell. Coulombic repulsion causes the electrons to occupy antipodal sites within the cell. These two bistable states result in cell polarizations of P +1(binary 1) and P 1(binary 0) and one Null state P=0 as shown below in Fig.1. Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE 2014) 28 29 March 2014 2 QCA cells transfer information by the propagation of polarized charge instead of flow of current thus, QCA circuits have lower energy and higher processing speed. The power needed to perform the polarization changes in cells (that support logic operations) is supplied by the clock signal. A.QCA WIRE In a QCA wire, the binary signal propagates from input to output because of the electrostatic interactions between cells. Since the polarization of each cell tends to align with that of its neighbors, a linear arrangement of standard cells can be used to transmit binary information from one point to another. A QCA wire is needed to transmit signals. The wire consists of chain of cells where the cells are coupled to each other. Logic values are passed from cell to cell due to the coloumbic interactions. The polarization of input cell is propagated down the wire .Any cell along the wire that are anti polarized to the input would be at a higher energy level and would soon settle to the correct ground state. The propagation in a 90deg QCA wire is shown in Fig. 2. Another is 45deg QCA wire, in Fig.3. In this case, the propagation of the binary signal alternates between the two polarizations. B.QCA INVERTER Two standard cells in a diagonal orientation are geometrically similar to two rotated cells in a horizontal orientation. For this reason, standard cells in a diagonal orientation tend to align in opposite polarization directions as in the inverter chain. This anti aligning behavior can be used in designing a QCA inverter. The electrostatic interaction is inverted, as the quantum dots corresponding to different polarizations are misaligned between the cells as in Fig. 4(a) There are several inverter types. Inverter of two cells is shown in Fig. 4(b) Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE 2014) 28 29 March 2014 3 The "signal" comes in from the left on a binary and splits into two parallel wires which are offset from the original. Because the incoming wire extends one cell beyond the beginning of the offset wires, aligning effects dominate at the branch point. Since the horizontal and vertical interactions are dominant, the signal in the two offset wires always matches that of the incoming wire. At the right end of the inverter, the offset wires rejoin into one. However, there are no horizontal or vertical interactions at this end, so the diagonal interactions cause the signal to be inverted. C.PROGRAMMABLE LOGIC GATE The fundamental QCA logical device is a 3 input majority gate, shown in Fig.5. from which more complex circuits can be built. The central cell, labeled the device cell, has three fixed inputs, labeled A, B, and C. The device cell has its lowest energy state if it assumes the polarization of the majority of the three input cells. The inputs to a particular device can come from previous calculations or be directly fed in from array edges. Figure 5(a) shows an arrangement of five standard cells. The states of the cells on the top, left, and bottom are fixed, while the center and right cells are free to react to the fixed charge. In an actual implementation, the three neighbors would not be fixed; they would be driven by results of previous calculations or by inputs at the edge of the QCA. In the particular case shown, two of the inputs are in the "one" state, and the other is in the "zero" state. When we solve for the ground state of the free cells, we find that they both match the state of the majority of the fixed neighbors. We refer to this result, which is true for all combinations of the three inputs, as majority voting logic. To represent this new logical function, we will use the symbol shown in Fig. 5(b) As this symbol demonstrates, there are three inputs and one output for such a logic gate. Figure 5(c) shows the truth table for a majority voting logic gate. This is a summary of our simulation of all eight combinations of the three inputs. The majority voting logic function can be expressed in terms of fundamental Boolean operators as M(A,B,C) =AB+ BC+AC. Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE 2014) 28 29 March 2014 4 Figure 5 Qca Majority Gate
author_names: Aditi Dhingra

Aprana Goel

G Verma

Rashmi Chawla
title: Combinational Circuit Design using Advanced Quantum Dot Cellular Automata
year: 2014