arXiv subjects

Daniel Loss

Publications and source records attributed to Daniel Loss.

At least 523 records · Page 29Linked to original sources

arXiv1999

Quantum Computing and Quantum Communication with Electrons in Nanostructures

If the states of spins in solids can be created, manipulated, and measured at the single-quantum level, an entirely new form of information processing, quantum computing and quantum communication, will be possible. We review our proposed spin-quantum dot architecture for a quantum computer and review some recent results on a deterministic source of entanglement generated by coupling quantum dots. Addressing the feasibility of quantum communication with entangled electrons we consider a scattering set-up with an entangler and beam splitter where the current noise exhibits bunching behavior for electronic singlet states and antibunching behavior for triplet states. We show that spin currents can produce noise even in the absence of any charge currents.

cond-mat.mes-hall↗

arXiv1999

Noise in Multiterminal Diffusive Conductors: Universality, Nonlocality and Exchange Effects

We study noise and transport in multiterminal diffusive conductors. Using a Boltzmann-Langevin equation approach we reduce the calculation of shot-noise correlators to the solution of diffusion equations. Within this approach we prove the universality of shot-noise in multiterminal diffusive conductors of arbitrary shape and dimension for purely elastic scattering as well as for hot electrons. We show that shot-noise in multiterminal conductors is a non-local quantity and that exchange effects can occur in the absence of quantum phase coherence even at zero electron temperature. It is also shown that the exchange effect measured in one contact is always negative -- in agreement with the Pauli principle. We discuss a new phenomenon in which current noise is induced by thermal transport. We propose a possible experiment to measure locally the effective noise temperature. Concrete numbers for shot-noise are given that can be tested experimentally.

cond-mat.mes-hall↗

arXiv1999

Physical Optimization of Quantum Error Correction Circuits

Quantum error correcting codes have been developed to protect a quantum computer from decoherence due to a noisy environment. In this paper, we present two methods for optimizing the physical implementation of such error correction schemes. First, we discuss an optimal quantum circuit implementation of the smallest error-correcting code (the three bit code). Quantum circuits are physically implemented by serial pulses, i.e. by switching on and off external parameters in the Hamiltonian one after another. In contrast to this, we introduce a new parallel switching method that allows faster gate operation by switching all external parameters simultaneously. These two methods are applied to electron spins in coupled quantum dots subject to a Heisenberg coupling H=J(t) S_1*S_2 which can generate the universal quantum gate `square-root-of-swap'. Using parallel pulses, the encoding for three-bit quantum error correction in a Heisenberg system can be accelerated by a factor of about two. We point out that parallel switching has potential applications for arbitrary quantum computer architectures.

cond-mat.mes-hall↗

arXiv1999

Spin relaxation in Mn12-acetate

We present a comprehensive derivation of the magnetization relaxation in a Mn12-acetate crystal based on thermally assisted spin tunneling induced by quartic anisotropy and weak transverse magnetic fields. The overall relaxation rate as function of the magnetic field is calculated and shown to agree well with data including all resonance peaks. The Lorentzian shape of the resonances is also in good agreement with recent data. A generalized master equation including resonances is derived and solved exactly. It is shown that many transition paths with comparable weight exist that contribute to the relaxation process. Previously unknown spin-phonon coupling constants are calculated explicitly.

cond-mat.mes-hall↗

arXiv1999

Nonlinear sigma Model Treatment of Quantum Antiferromagnets in a Magnetic Field

We present a theoretical analysis of the properties of low-dimensional quantum antiferromagnets in applied magnetic fields. In a nonlinear sigma model description, we use a spin stiffness analysis, a 1/N expansion, and a renormalization group approach to describe the broken-symmetry regimes of finite magnetization, and, in cases of most interest, a low-field regime where symmetry is restored by quantum fluctuations. We compute the magnetization, critical fields, spin correlation functions, and decay exponents accessible by nuclear magnetic resonance experiments. The model is relevant to many systems exhibiting Haldane physics, and provides good agreement with data for the two-chain spin ladder compound CuHpCl.

cond-mat.str-el↗

arXiv1998

Coupled quantum dots as quantum gates

We consider a new quantum gate mechanism based on electron spins in coupled semiconductor quantum dots. Such gates provide a general source of spin entanglement and can be used for quantum computers. We determine the exchange coupling J in the effective Heisenberg model as a function of magnetic (B) and electric fields, and of the inter-dot distance (a) within the Heitler-London approximation of molecular physics. This result is refined by using sp-hybridization, and by the Hund-Mulliken molecular-orbit approach which leads to an extended Hubbard description for the two-dot system that shows a remarkable dependence on B and a due to the long-range Coulomb interaction. We find that the exchange J changes sign at a finite field (leading to a pronounced jump in the magnetization) and then decays exponentially. The magnetization and the spin susceptibilities of the coupled dots are calculated. We show that the dephasing due to nuclear spins in GaAs can be strongly suppressed by dynamical nuclear spin polarization and/or by magnetic fields.

cond-mat.mes-hall↗

arXiv1998

Universality of Shot-Noise in Multiterminal Diffusive Conductors

We prove the universality of shot-noise in multiterminal diffusive conductors of arbitrary shape and dimension for purely elastic scattering as well as for hot electrons. Using a Boltzmann-Langevin approach we reduce the calculation of shot-noise correlators to the solution of a diffusion equation. We show that shot-noise in multiterminal conductors is a non-local quantity and that exchange effects can occur without quantum phase coherence even at zero electron temperature. Concrete numbers for shot-noise are given that can be tested experimentally.

cond-mat.mes-hall↗

arXiv1998

Observing the Berry phase in diffusive conductors: Necessary conditions for adiabaticity

In a recent preprint (cond-mat/9803170), van~Langen, Knops, Paasschens and Beenakker attempt to re-analyze the proposal of Loss, Schoeller and Goldbart (LSG) [Phys. Rev. B~48, 15218 (1993)] concerning Berry phase effects in the magnetoconductance of diffusive systems. Van Langen et al. claim that the adiabatic approximation for the Cooperon previously derived by LSG is not valid in the adiabatic regime identified by LSG. It is shown that the claim of van~Langen et al. is not correct, and that, on the contrary, the magnetoconductance does exhibit the Berry phase effect within the LSG regime of adiabaticity. The conclusion reached by van~Langen et al. is based on a misinterpretation of field-induced dephasing effects, which can mask the Berry phase (and any other phase coherent phenomena) for certain parameter values.

cond-mat.mes-hall↗

arXiv1998

Quantum Antiferromagnets in a Magnetic Field

Motivated by recent experiments on low-dimensional quantum magnets in applied magnetic fields, we present a theoretical analysis of their properties based on the nonlinear sigma model. The spin stiffness and a 1/N expansion are used to map the regimes of spin-gap behavior, predominantly linear magnetization, and spin saturation. A two-parameter renormalization-group study gives the characteristic properties over the entire parameter range. The model is relevant to many systems exhibiting Haldane physics, and is applied here to the two-chain spin ladder compound CuHpCl.

cond-mat.str-el↗

arXiv1998

Bloch oscillations of magnetic solitons in anisotropic spin-1/2 chains

We study the quantum dynamics of soliton-like domain walls in anisotropic spin-1/2 chains in the presence of magnetic fields. In the absence of fields, domain walls form a Bloch band of delocalized quantum states while a static field applied along the easy axis localizes them into Wannier wave packets and causes them to execute Bloch oscillations, i.e. the domain walls oscillate along the chain with a finite Bloch frequency and amplitude. In the presence of the field, the Bloch band, with a continuum of extended states, breaks up into the Wannier-Zeeman ladder -- a discrete set of equally spaced energy levels. We calculate the dynamical structure factor in the one-soliton sector at finite frequency, wave vector, and temperature, and find sharp peaks at frequencies which are integer multiples of the Bloch frequency. We further calculate the uniform magnetic susceptibility and find that it too exhibits peaks at the Bloch frequency. We identify several candidate materials where these Bloch oscillations should be observable, for example, via neutron scattering measurements. For the particular compound CoCl_2.2H_2O we estimate the Bloch amplitude to be on the order of a few lattice constants, and the Bloch frequency on the order of 100 GHz for magnetic fields in the Tesla range and at temperatures of about 18 Kelvin.

cond-mat.mes-hall↗

arXiv1998

Quantum Information is Physical

We discuss a few current developments in the use of quantum mechanically coherent systems for information processing. In each of these developments, Rolf Landauer has played a crucial role in nudging us and other workers in the field into asking the right questions, some of which we have been lucky enough to answer. A general overview of the key ideas of quantum error correction is given. We discuss how quantum entanglement is the key to protecting quantum states from decoherence in a manner which, in a theoretical sense, is as effective as the protection of digital data from bit noise. We also discuss five general criteria which must be satisfied to implement a quantum computer in the laboratory, and we illustrate the application of these criteria by discussing our ideas for creating a quantum computer out of the spin states of coupled quantum dots.

cond-mat.mes-hall↗

arXiv1997

Macroscopic Quantum Tunneling of Ferromagnetic Domain Walls

Quantum tunneling of domain walls out of an impurity potential in a mesoscopic ferromagnetic sample is investigated. Using improved expressions for the domain wall mass and for the pinning potential, we find that the cross-over temperature between thermal activation and quantum tunneling is of a different functional form than found previously. In materials like Ni or YIG, the crossover temperatures are around 5 mK. We also find that the WKB exponent is typically two orders of magnitude larger than current estimates. The sources for these discrepancies are discussed, and precise estimates for the transition from three-dimensional to one-dimensional magnetic behavior of a wire are given. The cross-over temperatures from thermal to quantum transitions and tunneling rates are calculated for various materials and sample sizes.

cond-mat.mes-hall↗

arXiv1997

Mesoscopic Effects in the Fractional Quantum Hall Regime

Edge states of the quantum Hall fluid provide an opportunity to study mesoscopic effects in a highly correlated electron system that is both experimentally accessible and theoretically tractable. In this paper we review recent work on the persistent current and Aharonov-Bohm magnetoconductance oscillations in the fractional quantum Hall effect regime.

cond-mat.mes-hall↗

arXiv1997

Macroscopic Quantum Coherence in Molecular Magnets

We study macroscopic quantum coherence in antiferromagnetic molecular magnets in the presence of magnetic fields. Such fields generate artificial tunnel barriers with externally tunable strength. We give detailed semi-classical predictions for the tunnel splitting in various regimes for low and high magnetic fields. We show that the tunneling dynamics of the Neel vector can be directly measured via the static magnetization and the specific heat. We also report on a new quantum phase arising from fluctuations. The analytic results are complemented by numerical simulations.

cond-mat.mes-hall↗

arXiv1997

Quantum Computation with Quantum Dots

We propose a new implementation of a universal set of one- and two-qubit gates for quantum computation using the spin states of coupled single-electron quantum dots. Desired operations are effected by the gating of the tunneling barrier between neighboring dots. Several measures of the gate quality are computed within a newly derived spin master equation incorporating decoherence caused by a prototypical magnetic environment. Dot-array experiments which would provide an initial demonstration of the desired non-equilibrium spin dynamics are proposed.

cond-mat.mes-hall↗

arXiv1997

Macroscopic Quantum Coherence in Ferrimagnets

We study macroscopic quantum coherence (MQC) in small ferrimagnets. Through semi-classical calculations we show that even a small uncompensated moment has a drastic effect on MQC. In particular, there is a rapid crossover to a regime where the MQC tunnel splitting is equal to that obtained for a ferromagnet, even though the system is still an antiferromagnet for all other aspects. We calculate this tunnel splitting via instanton methods and compare it with numerical evaluations. As an application we re-examine the experimental evidence for MQC in ferritin and show that even though the uncompensated moment of ferritin is small it greatly modifies the MQC behavior. We also discuss the implications of our results for MQC in molecular magnets.

cond-mat.mes-hall↗

arXiv1996

Mesoscopic Effects in the Fractional Quantum Hall Regime: Chiral Luttinger versus Fermi Liquid

We study tunneling through an edge state formed around an antidot in the fractional quantum Hall regime using Wen's chiral Luttinger liquid theory extended to include mesoscopic effects. We identify a new regime where the Aharonov-Bohm oscillation amplitude exhibits a distinctive crossover from Luttinger liquid power-law behavior to Fermi-liquid-like behavior as the temperature is increased. Near the crossover temperature the amplitude has a pronounced maximum. This non-monotonic behavior and novel high-temperature nonlinear phenomena that we also predict provide new ways to distinguish experimentally between Luttinger and Fermi liquids.

cond-mat↗

arXiv1996

Berry's phase and Quantum Dynamics of Ferromagnetic Solitons

We study spin parity effects and the quantum propagation of solitons (Bloch walls) in quasi-one dimensional ferromagnets. Within a coherent state path integral approach we derive a quantum field theory for nonuniform spin configurations. The effective action for the soliton position is shown to contain a gauge potential due to the Berry phase and a damping term caused by the interaction between soliton and spin waves. For temperatures below the anisotropy gap this dissipation reduces to a pure soliton mass renormalization. The gauge potential strongly affects the quantum dynamics of the soliton in a periodic lattice or pinning potential. For half-integer spin, destructive interference between soliton states of opposite chirality suppresses nearest neighbor hopping. Thus the Brillouin zone is halved, and for small mixing of the chiralities the dispersion reveals a surprising dynamical correlation: Two subsequent band minima belong to different chirality states of the soliton. For integer spin, the Berry phase is inoperative and a simple tight-binding dispersion is obtained. Finally it is shown that external fields can be used to interpolate continuously between the Bloch wall dispersions for half-integer and integer spin.

cond-mat↗