41 research outputs found
A few-electron quadruple quantum dot in a closed loop
We report the realization of a quadruple quantum dot device in a square-like
configuration where a single electron can be transferred on a closed path free
of other electrons. By studying the stability diagrams of this system, we
demonstrate that we are able to reach the few-electron regime and to control
the electronic population of each quantum dot with gate voltages. This allows
us to control the transfer of a single electron on a closed path inside the
quadruple dot system. This work opens the route towards electron spin
manipulation using spin-orbit interaction by moving an electron on complex
paths free of electron
Ergodic vs diffusive decoherence in mesoscopic devices
We report on the measurement of phase coherence length in a high mobility
two-dimensional electron gas patterned in two different geometries, a wire and
a ring. The phase coherence length is extracted both from the weak localization
correction in long wires and from the amplitude of the Aharonov-Bohm
oscillations in a single ring, in a low temperature regime when decoherence is
dominated by electronic interactions. We show that these two measurements lead
to different phase coherence lengths, namely and . This difference
reflects the fact that the electrons winding around the ring necessarily
explore the whole sample (ergodic trajectories), while in a long wire the
electrons lose their phase coherence before reaching the edges of the sample
(diffusive regime).Comment: LaTeX, 5 pages, 4 pdf figures ; v2: revised versio
Quantum Coherence at Low Temperatures in Mesoscopic Systems: Effect of Disorder
We study the disorder dependence of the phase coherence time of quasi
one-dimensional wires and two-dimensional (2D) Hall bars fabricated from a high
mobility GaAs/AlGaAs heterostructure. Using an original ion implantation
technique, we can tune the intrinsic disorder felt by the 2D electron gas and
continuously vary the system from the semi-ballistic regime to the localized
one. In the diffusive regime, the phase coherence time follows a power law as a
function of diffusion coefficient as expected in the Fermi liquid theory,
without any sign of low temperature saturation. Surprisingly, in the
semi-ballistic regime, it becomes independent of the diffusion coefficient. In
the strongly localized regime we find a diverging phase coherence time with
decreasing temperature, however, with a smaller exponent compared to the weakly
localized regime.Comment: 21 pages, 30 figure
Magnetic dephasing in mesoscopic spin glasses
We have measured Universal Conductance Fluctuations in the metallic spin
glass Ag:Mn as a function of temperature and magnetic field. From this
measurement, we can access the phase coherence time of the electrons in the
spin glass. We show that this phase coherence time increases with both the
inverse of the temperature and the magnetic field. From this we deduce that
decoherence mechanisms are still active even deep in the spin glass phase
On the link between mechanics and thermal properties: mechanothermics
We report on the theoretical derivation of macroscopic thermal properties
(specific heat, thermal conductivity) of an electrically insulating rod
connected to two reservoirs, from the linear superposition of its mechanical
mode Brownian motions. The calculation is performed for a weak thermal
gradient, in the classical limit (high temperature). The development is kept
basic as far as geometry and experimental conditions are concerned, enabling an
almost fully analytic treatment. In the modeling, each of the modes is subject
to a specific Langevin force, which enables to produce the required temperature
profile along the rod. The theory is predictive: the temperature gradient (and
therefore energy transport) is linked to motion amplitude cross-correlations
between nearby mechanical modes. This arises because energy transport is
actually mediated by mixing between the modal waves, and not by the modes
themselves. This result can be tested on experiments, and shall extend the
concepts underlying equipartition and fluctuation-dissipation theorems. The
theory links intimately the macroscopic size of the clamping region where the
mixing occurs to the microscopic lengthscale of the problem at hand: the phonon
mean-free-path. We believe that our work should impact the domain of thermal
transport in nanostructures, with future developments of the theory toward the
quantum regime
Qubits de spin (de la manipulation et déplacement d'un spin électronique unique à son utilisation comme détecteur ultra sensible)
Cette thèse décrit une série de travaux réalisés dans le contexte des qubits de spins, allant de l'utilisation de ces qubits pour stocker de l'information à leur utilisation comme détecteurs ultra-sensibles. Nous utilisons des hétérostructures semi-conductrices d'arséniure de gallium dans lesquelles un électron unique peut être isolé au sein d'un piège électrostatique, une boîte quantique. Le spin de cet électron peut être utilisé pour encoder de l'information, et la boîte quantique contenant ce spin unique est alors vue comme un qubit (quantum bit). Au cours de cette thèse nous démontrons la réalisation expérimentale du transport d'un électron unique le long d'un circuit fermé au sein d'un système composé de quatre boîtes quantiques couplées. En considérant l'interaction spin-orbite, cette expérience ouvre la voie vers des manipulations cohérentes de spins utilisant des effets topologiques. Dans le contexte de l'ordinateur quantique et des qubits de spins, nous étudions les portes logiques à deux qubits. Dans le cadre de deux boîtes quantiques couplées par une barrière tunnel, nous démontrons qu'en contrôlant localement le champ magnétique, la porte logique à deux qubits évoluent de la porte SWAP à la porte C-phase. Nous démontrons ainsi la faisabilité d'une porte C-phase. Finalement nous montrons l'utilisation d'un qubit de spin comme un détecteur de charge ultrasensible. Un singlet-triplet qubit est un système quantique qui peut être réglé de manière à être extrêmement sensible à l'environnement électrostatique. Nous démontrons la faisabilité d'un tel détecteur, et nous montrons qu'il peut être utilisé pour détecter un électron unique.In this thesis we described a series of experimental works, which have been realized in the context of spin qubits, going from their use as information carriers to their use as very sensitive detectors. We use AlGaAs semiconducting heterostructures in which a single electron can be isolated in an electrostatic trap, the so-called quantum dot. The electron spin can be used in order to encode information, and the quantum dot containing this electron can therefore be seen as a qubit (quantum bit). During this thesis we demonstrate the first experimental realization of a single electron transport along a closed path inside a system composed of four coupled quantum dots. By considering spin-orbit interaction, this experiment opens the way toward coherent topological spin manipulations. In the context of quantum computing and spin qubits, we study the two-qubit gates. By considering two tunnel coupled quantum dots, we demonstrate by controlling the local Zeeman splitting that the natural two-qubit gate for spin qubits evolves from the SWAP gate to the C-phase gate. This work demonstrates the feasibility of the C-phase gate. Finally we use spin qubits as very sensitive detectors. A singlet-triplet qubit is a quantum system which can be tuned in order to be very sensistive to the electrostatic environment. Here we report the use of such a qubit to detect a single electron transported next to the detector.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF
Experimental Test of the Numerical Renormalization Group Theory for Inelastic Scattering from Magnetic Impurities
We present measurements of the phase coherence time \tauphi in quasi
one-dimensional Au/Fe Kondo wires and compare the temperature dependence of
\tauphi with a recent theory of inelastic scattering from magnetic impurities
(Phys. Rev. Lett. 93, 107204 (2004)). A very good agreement is obtained for
temperatures down to 0.2 . Below the Kondo temperature , the inverse
of the phase coherence time varies linearly with temperature over almost one
decade in temperature.Comment: 5 pages, 3 figure
Efficient Radio Frequency filters for space constrained cryogenic set-ups
Noise filtering is an essential part for measurement of quantum phenomena at
extremely low temperatures. Here, we present the design of a filter which can
be installed in space constrained cryogenic environment containing a large
number of signal carrying lines. Our filters have a -3db point of 65kHz and its
performance at GHz frequencies are comparable to the best available RF filters.Comment: 9 pages, 4 figures, The capacitor reference in the first version was
wrong and has been changed to the right on
Low temperature dephasing in irradiated metallic wires
We present phase coherence time measurements in quasi-one-dimensional Ag
wires implanted with Ag ions with an energy of . The
measurements have been carried out in the temperature range from up to
; this has to be compared with the Kondo temperature of iron in silver,
i.e. , used in recent experiments on dephasing in
Kondo systems\cite{mallet_prl_06,birge_prl_06}. We show that the phase
coherence time is not affected by the implantation procedure, clearly proving
that ion implantation process by itself \emph{does not lead to any extra
dephasing} at low temperature.Comment: 4 pages, 4figure
Fast end efficient single electron transfer between distant quantum dots
International audienceLateral quantum dots are a promising system for quantum information processing devices. The required basic manipulations of a single electron spin have indeed been demonstrated. However, a stringent requirement is the ability to transfer quantum information from place to place within one sample. In this work, we explore and demonstrate the possibility to transfer a single electron between two distant quantum dots in a fast and reliable manner