Project 2 – Quantum Computing and Quantum Coherence

Single-electron effects in a coupled dot-ring system
L. Meier, A. Fuhrer, T. Ihn, K. Ensslin, W. Wegscheider, M. Bichler
Phys. Rev. B 69, 241302(R) (2004)
Aharonov-Bohm oscillations are studied in the magnetoconductance of a micron-sized open quantum ring coupled capacitively to a Coulomb-blockaded quantum dot. As the plunger gate of the dot is modulated and tuned through a conductance resonance, the amplitude of the Aharonov-Bohm oscillations in the transconductance of the ring displays a minimum. We demonstrate that the effect is due to a single-electron screening effect, rather than to dephasing. Aharonov-Bohm oscillations in a quantum ring can thus be used for the detection of single charges. ...
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Multi-terminal transport through a quantum dot in the Coulomb blockade regime
R. Leturcq, D. Graf, T. Ihn, K. Ensslin, D. D. Driscoll, A. C. Gossard
Europhys. Lett., 67 (3), pp. 439-445 (2004)
Three terminal tunnelling experiments on quantum dots in the Coulomb blockade regime allow a quantitative determination of the coupling strength of individual quantum states to the leads. Exploiting this insight we have observed independent fluctuations of the coupling strengths as a function of electron number and magnetic field due to changes in the shape of the wave function in the dot. Such a detailed understanding and control of the dot-lead coupling can be extended to more complex systems such as coupled dots, and is essential for building functional quantum electronic systems. ...
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Time-Resolved Detection of Individual Electrons in a Quantum Dot
R. Schleser, E. Ruh, T. Ihn, K. Ensslin, D. C. Driscoll, A. C. Gossard
Applied Physics Letters, Volume 85, Issue 11, pp. 2005-2007
We present measurements on a quantum dot and a nearby, capacitively coupled, quantum point contact used as a charge detector. With the dot being weakly coupled to only a single reservoir, the transfer of individual electrons onto and off the dot can be observed in real time in the current signal from the quantum point contact. From these time-dependent traces, the quantum mechanical coupling between dot and reservoir can be extracted quantitatively. A similar analysis allows the determination of the occupation probability of the dot states. ...
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Magnetic field dependent transmission phase of a double dot system in a quantum ring
M. Sigrist, A. Fuhrer, T. Ihn, K. Ensslin, S. E. Ulloa, W. Wegscheider, M. Bichler
Phys. Rev. Lett. 93, 066802 (2004)
The Aharonov-Bohm effect is measured in a four-terminal open ring geometry based on a Ga[Al]As heterostructure. Two quantum dots are embedded in the structure, one in each of the two interfering paths. The number of electrons in the two dots can be controlled independently. The transmission phase is measured as electrons are added to or taken away from the individual quantum dots. Although the measured phase shifts are in qualitative agreement with theoretical predictions, the phase evolution exhibits unexpected dependence on the magnetic field. For example, phase lapses are found only in cert ...
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Effects of dephasing on shot-noise in an electronic Mach-Zehnder interferometer
Florian Marquardt, C. Bruder
Phys. Rev. B 70, 125305 (2004)
We present a theoretical study of the influence of dephasing on shot noise in an electronic Mach-Zehnder interferometer. In contrast to phenomenological approaches, we employ a microscopic model where dephasing is induced by the fluctuations of a classical potential. This enables us to treat the influence of the environment's fluctuation spectrum on the shot noise. We compare against the results obtained from a simple classical model of incoherent transport, as well as those derived from the phenomenological dephasing terminal approach, arguing that the latter runs into a problem when applied ...
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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. ...
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Positive Cross Correlations in a Three-Terminal Quantum Dot with Ferromagnetic Contacts
A. Cottet, W. Belzig, C. Bruder
Phys. Rev. Lett. 92, 206801 (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 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 with resp ...
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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 ...
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Dynamical spin-blockade in a quantum dot with paramagnetic leads
A. Cottet and W. Belzig
Europhys. Lett., 66 (3) , pp. 405-411 (2004)
We investigate current fluctuations in a three-terminal quantum dot in the sequential tunneling regime. Dynamical spin-blockade can be induced when the spin-degeneracy of the dot states is lifted by a magnetic field. This results in super-Poissonian shot noise and positive zero-frequency cross-correlations. Our proposed setup can be realized with semiconductor quantum dots. ...
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Spin-flip noise in a multiterminal spin valve
W. Belzig and M. Zareyan
Phys. Rev. B 69, 140407(R) (2004)
We study shot noise and cross-correlations in a four terminal spin-valve geometry using a Boltzmann-Langevin approach. The Fano factor (shot noise to current ratio) depends on the magnetic configuration of the leads and the spin-flip processes in the normal metal. In a four-terminal geometry, spin-flip processes are particularly prominent in the cross-correlations between terminals with opposite magnetization. ...
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Andreev Reflection Eigenvalue Density in Mesoscopic Conductors
P. Samuelsson, W. Belzig, and Yu. V. Nazarov
Phys. Rev. Lett. 92, 196807 (2004)
The energy-dependent Andreev reflection eigenvalues determine the transport properties of normal-superconducting systems. We evaluate the eigenvalue density to get insight into the formation of resonant electron-hole transport channels. The circuit-theory-like method developed can be applied to any generic mesoscopic conductor or combinations thereof. We present the results for experimentally relevant cases of a diffusive wire and a double tunnel junction. ...
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Quantum dissipative dynamics of the magnetic resonance force microscope in the single-spin detection limit
Hanno Gassmann , Mahn-Soo Choi, Hangmo Yi, and C. Bruder
Phys. Rev. B 69, 115419 (2004)
We study a model of a magnetic resonance force microscope (MRFM) based on the cyclic adiabatic inversion technique as a high-resolution tool to detect single electron spins. We investigate the quantum dynamics of spin and cantilever in the presence of coupling to an environment. To obtain the reduced dynamics of the combined system of spin and cantilever, we use the Feynman-Vernon influence functional and get results valid at any temperature as well as at arbitrary system-bath coupling strength. We propose that the MRFM can be used as a quantum measurement device, i.e., not only to detect the ...
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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 ...
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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. ...
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Dynamical Coulomb Blockade and Spin-Entangled Electrons
Patrik Recher and Daniel Loss
Phys. Rev. Lett. 91, 267003 (2003)
We consider the creation of mobile and nonlocal spin-entangled electrons from tunneling of a BCS-superconductor (SC) to two normal leads of finite resistivity. The resulting dynamical Coulomb blockade effect, which we describe phenomenologically in terms of an electromagnetic environment, is shown to be enhanced for tunneling of two electrons from a Cooper pair into the same lead compared to the desired pair-split process where each electron enters a different lead. Conversely, this latter process is suppressed by a finite separation between the tunneling points on the SC. ...
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Hyperfine interaction in a quantum dot: Non-Markovian electron spin dynamics
W. A. Coish, Daniel Loss,
Phys. Rev. B 70, 195340 (2004)
We have performed a systematic calculation for the non-Markovian dynamics of a localized electron spin interacting with an environment of nuclear spins via the Fermi contact hyperfine interaction. This work applies to an electron in the s -type orbital ground state of a quantum dot or bound to a donor impurity, and is valid for arbitrary polarization p of the nuclear spin system, and arbitrary nuclear spin I in high magnetic fields. In the limit of p=1 and I=1/2, the Born approximation of our perturbative theory recovers the exact electron spin dynamics. We have found the form of the generaliz ...
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Rigorous Born Approximation and beyond for the Spin-Boson Model
D. P. DiVincenzo, D. Loss
cond-mat/0405525.
Within the lowest-order Born approximation, we present an exact calculation of the time dynamics of the spin-boson model in the ohmic regime. We observe non-Markovian effects at zero temperature that scale with the system-bath coupling strength and cause qualitative changes in the evolution of coherence at intermediate times of order of the oscillation period. These changes could significantly affect the performance of these systems as qubits. In the biased case, we find a prompt loss of coherence at these intermediate times, whose decay rate is set by $\sqrt{\alpha}$, where $\alpha$ is the co ...
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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 ...
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Spin injection across magnetic/nonmagnetic interfaces with finite magnetic layers
Alexander Khaetskii, J. Carlos Egues, Daniel Loss, Charles Gould, Georg Schmidt, Laurens W. Molenkamp
Phys. Rev. B 71, 235327 (2005)
We have reconsidered the problem of spin injection across ferromagnet/nonmagnetic-semiconductor (FM/NMS) and dilute-magnetic-semiconductor/nonmagnetic-semiconductor (DMS/NMS) interfaces, for structures with finite width d of the magnetic layer (FM or DMS). By using appropriate physical boundary conditions, we find expressions for the resistances of these structures which are in general different from previous results in the literature. When the magnetoresistance of the contacts is negligible, we find that the spin-accumulation effect alone cannot account for the d dependence observed in recent ...
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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. ...
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Peralkynylated Buta-1,2,3-Trienes: Exceptionally Low Rotational Barriers of Cumulenic C=C Bonds in the Range of Those of Peptide C-N Bonds
Audrey Auffrant, Bernhard Jaun, Peter D. Jarowski, Kendall N. Houk, François Diederich
Chemistry - A European Journal, Volume 10, Issue 12 , Pages 2906 - 2911
A variety of 1,1,4,4-tetraal kynylbutatrienes and 1,4-dialkynylbutatrienes was synthezized by dimerization of the corresponding gem-dibromoolefins. Both 1H and 13C NMR spectroscopy indicated that the di- and tetraalkynylated butatrienes are formed as a mixture of cis and trans isomers. Variable temperature NMR studies evidenced a facile cis-trans isomerization, thus preventing the separation of these isomers by gravity or high-performance liquid chromatography (HPLC). For 1,1,4,4-tetraalkynylbutatrienes, the activation barrier ΔG^≠ was measured by magnetization transfer to ...
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Dissipation effects in spin-Hall transport of electrons and holes
John Schliemann, Daniel Loss
Phys. Rev. B 69, 165315 (2004)
We investigate the spin-Hall effect of both electrons and holes in semiconductors using the Kubo formula in the correct zero-frequency limit taking into account the finite momentum relaxation time of carriers in real semiconductors. This approach allows to analyze the range of validity of recent theoretical findings. In particular, the spin-Hall conductivity vanishes for vanishing spin-orbit coupling if the correct zero-frequency limit is performed. ...
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Optical Detection of Single-Electron Spin Decoherence in a Quantum Dot
Oliver Gywat, Hans-Andreas Engel, Daniel Loss, R. J. Epstein, F. Mendoza, D. D. Awschalom
Phys. Rev. B 69, 205303 (2004)
We propose a method based on optically detected magnetic resonance (ODMR) to measure the decoherence time $T_{2}$ of a single electron spin in a semiconductor quantum dot. The electron spin resonance (ESR) of a single excess electron on a quantum dot is probed by circularly polarized laser excitation. The photoluminescence is modulated due to the ESR which enables the measurement of electron spin decoherence. We study different possible schemes for such an ODMR setup. ...
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Shot noise: from Schottky's vacuum tube to present-day quantum devices
Christian Schonenberger, Stefan Oberholzer
Proc. SPIE, Vol. 5469 article 5469-32
Shot-noise in the electrical current through a 'device' is caused by random processes that determine the electron transport from source to drain. Two sources can be distinguished: on the hand, electrons may randomly emanate from the contacts (source and drain), because the relevant states in the reservoirs fluctuate. On the other hand, the transmission through the device is non-deterministic (non-classical). As we demonstrate in this article the former dominates noise in the vacuum tube, whereas the latter applies to coherent mesoscopic devices, which have been studied in great detail during t ...
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