|
Project 2 – Quantum Computing and Quantum Coherence
Solid-State Quantum Communication With Josephson Arrays
Alessandro Romito, Rosario Fazio, C. Bruder
Phys. Rev. B 71, 100501(R) (2005)
Josephson junction arrays can be used as quantum channels to transfer quantum information between distant sites. In this paper we discuss simple protocols to realize state transfer with high fidelity. The channels do not require complicated gating but use the natural dynamics of a properly designed array. We investigate the influence of static disorder both in the Josephson energies and in the coupling to the background gate charges, as well as the effect of dynamical noise. We also analyze the readout process, and its back action on the state transfer. ...
Link to journal
Coulomb Scattering in a 2D Interacting Electron Gas and Production of EPR Pairs
D. S. Saraga, B. L. Altshuler, Daniel Loss, and R. M. Westervelt
Phys. Rev. Lett. 92, 246803 (2004)
We propose a setup to generate nonlocal spin Einstein-Podolsky-Rosen pairs via pair collisions in a 2D interacting electron gas, based on constructive two-particle interference in the spin-singlet channel at the /2 scattering angle. We calculate the scattering amplitude via the Bethe-Salpeter equation in the ladder approximation and small rs limit and find that the Fermi sea leads to a substantial renormalization of the bare scattering process. From the scattering length, we estimate the current of spin-entangled electrons and show that it is within experimental reach. ...
Link to journal
Spin Transport in Diffusive Superconductors
Jan Petter Morten, Arne Brataas, Wolfgang Belzig
Phys. Rev. B 70, 212508 (2004)
We employ the Keldysh formalism in the quasiclassical approximation to study transport in a diffusive superconductor. The resulting 4x4 transport equations describe the flow of charge and energy as well as the corresponding flow of spin and spin energy. Spin-flip scattering due to magnetic impurities is included. We find that the spin-flip length is renormalized in the superconducting case and propose an experimental system to measure the spin-accumulation in a superconductor.
...
Link to journal
DC-transport in superconducting point contacts: a full counting statistics view
J.C. Cuevas, W. Belzig
Phys. Rev. B 70, 214512 (2004)
We present a comprehensive theoretical analysis of the dc transport properties of superconducting point contacts. We determine the full counting statistics for these junctions, which allows us to calculate not only the current or the noise, but all the cumulants of the current distribution. We show how the knowledge of the statistics of charge transfer provides an unprecedented level of understanding of the different transport properties for a great variety of situations. We illustrate our results with the analysis of junctions between BCS superconductors, contacts between superconductors with ...
Link to journal
Molecular spintronics: Coherent spin transfer in coupled quantum dots
Florian Meier, Veronica Cerletti, Oliver Gywat, Daniel Loss, D. D. Awschalom
Phys. Rev. B 69, 195315 (2004)
Time-resolved Faraday rotation has recently demonstrated coherent transfer of electron spin between quantum dots coupled by conjugated molecules. Using a transfer Hamiltonian ansatz for the coupled quantum dots, we calculate the Faraday rotation signal as a function of the probe frequency in a pump-probe setup using neutral quantum dots. Additionally, we study the signal of one spin-polarized excess electron in the coupled dots. We show that, in both cases, the Faraday rotation angle is determined by the spin transfer probabilities and the Heisenberg spin exchange energy. By comparison of our ...
Link to journal
Phonon-induced decay of the electron spin in quantum dots
Vitaly N. Golovach, Alexander Khaetskii, Daniel Loss
Phys. Rev. Lett. 93, 016601 (2004)
We study spin relaxation and decoherence in a GaAs quantum dot due to spin-orbit (SO) interaction. We derive an effective Hamiltonian which couples the electron spin to phonons or any other fluctuation of the dot potential. We show that the spin decoherence time T2 is as large as the spin relaxation time T1, under realistic conditions. For the Dresselhaus and Rashba SO couplings, we find that, in leading order, the effective B field can have only fluctuations transverse to the applied B field. As a result, T2 = 2T1 for arbitrarily large Zeeman splitt ...
Link to journal
Measurement Efficiency and n-Shot Readout of Spin Qubits
Hans-Andreas Engel, Vitaly Golovach, Daniel Loss, L.M.K. Vandersypen, J.M. Elzerman, R. Hanson, L.P. Kouwenhoven
Phys. Rev. Lett. 93, 106804 (2004)
We consider electron spin qubits in quantum dots and define a measurement efficiency e to characterize reliable measurements via n-shot readouts. We propose various implementations based on a double dot and a quantum point contact (QPC) and show that the associated efficiencies e vary between 50% and 100%, allowing single-shot readout in the latter case. We model the readout microscopically and derive its time dynamics in terms of a generalized master equation, calculate the QPC current, and show that it allows spin readout under realistic conditions.
...
Link to journal
Asymmetric Quantum Shot Noise in Quantum Dots
Hans-Andreas Engel, Daniel Loss
Phys. Rev. Lett. 93, 136602 (2004)
We analyze the frequency-dependent noise of a current through a quantum dot which is coupled to Fermi leads and which is in the Coulomb blockade regime. We show that the asymmetric shot noise as function of frequency shows steps and becomes super-Poissonian. This provides experimental access to the quantum fluctuations of the current. We present an exact calculation for a single dot level and a perturbative evaluation of the noise in Born approximation (sequential tunneling regime but without Markov approximation) for the general case of many levels with charging interaction.
...
Link to journal
Controlling Spin Qubits in Quantum Dots
Hans-Andreas Engel, L. P. Kouwenhoven, Daniel Loss, C. M. Marcus
Quantum Information Processing, Volume 3, Numbers 1-5, 115 - 132
We review progress on the spintronics proposal for quantum computing where the quantum bits (qubits) are implemented with electron spins. We calculate the exchange interaction of coupled quantum dots and present experiments, where the exchange coupling is measured via transport. Then, experiments on single spins on dots are described, where long spin relaxation times, on the order of a millisecond, are observed. We consider spin-orbit interaction as sources of spin decoherence and find theoretically that also long decoherence times are expected. Further, we describe the concept of spin filteri ...
Link to journal
Electron spins in dots and rings: coherence, read out, and transport
Hans-Andreas Engel
ISBN 3-8322-2770-9, 2004
Link to journal
Influence of Dephasing on Shot Noise in an Electronic Mach-Zehnder Interferometer
Florian Marquardt and C. Bruder
Phys. Rev. Lett. 92, 056805 (2004)
We analyze shot noise under the influence of dephasing in an electronic Mach-Zehnder interferometer, of the type that was realized recently [Yang Ji et al., Nature (London) 422, 415 (2003)]. Using a model of dephasing by a fluctuating classical field, we show how the usual partition noise expression (1–) is modified. We study the dependence on the power spectrum of the field, which is impossible in simpler approaches such as the dephasing terminal, against which we compare. We remark on shot noise as a tool to distinguish thermal smearing from genuine dephasing. ...
Link to journal
Kondo Effect in a Many-Electron Quantum Ring
A. Fuhrer, T. Ihn, K. Ensslin, W. Wegscheider, and M. Bichler
Phys. Rev. Lett. 93, 176803 (2004)
The Kondo effect is investigated in a many-electron quantum ring as a function of the magnetic field. For fields applied perpendicular to the plane of the ring a modulation of the Kondo effect with the Aharonov-Bohm period is observed. This effect is discussed in terms of the energy spectrum of the ring and the parametrically changing tunnel coupling. In addition, we use gate voltages to modify the ground-state spin of the ring. The observed splitting of the Kondo-related zero-bias anomaly in this configuration is tuned with an in-plane magnetic field. ...
Link to journal
Positive cross-correlations due to dynamical channel blockade in a three-terminal quantum dot
A. Cottet, W. Belzig, C. Bruder
Phys. Rev. B 70, 115315 (2004)
We study current fluctuations in an interacting three-terminal quantum dot with ferromagnetic leads. For appropriately polarized contacts, the transport through the dot is governed by a novel dynamical spin blockade, i.e., a spin-dependent bunching of tunneling events not present in the paramagnetic case. This leads for instance to positive zero-frequency cross-correlations of the currents in the output leads even in the absence of spin accumulation on the dot. We include the influence of spin-flip scattering and identify favorable conditions for the experimental observation of this effect wit ...
Link to journal
Relaxation and Dephasing in a Many-Fermion Generalization of the Caldeira-Leggett Model
Florian Marquardt and D. S. Golubev
Phys. Rev. Lett. 93, 130404 (2004)
We analyze a model system of fermions in a harmonic oscillator potential under the influence of a fluctuating force generated by a bath of harmonic oscillators. This represents an extension of the well-known Caldeira-Leggett model to the case of many fermions. Using the method of bosonization, we calculate Green's functions and discuss relaxation and dephasing of a single extra particle added above the Fermi sea. We also extend our analysis to a more generic coupling between system and bath that results in complete thermalization of the system.
...
Link to journal
Full list of publications
|
