127 research outputs found
Quantum detection of electronic flying qubits
We consider a model of a detector of ballistic electrons at the edge of a
two-dimensional electron gas in the integer quantum Hall regime. The electron
is detected by capacitive coupling to a gate which is also coupled to a passive
RC circuit. Using a quantum description of this circuit, we determine the
signal over noise ratio of the detector in term of the detector
characteristics. The back-action of the detector on the incident wavepacket is
then computed using a Feynman-Vernon influence functional approach. Using
information theory, we define the appropriate notion of quantum limit for such
an "on the fly" detector. We show that our particular detector can approach the
quantum limit up to logarithms in the ratio of the measurement time over the RC
relaxation time. We argue that such a weak logarithmic effect is of no
practical significance. Finally we show that a two-electron interference
experiment can be used to probe the detector induced decoherence.Comment: 15 pages, 7 figures, published versio
Minimum Distance Estimation and Testing of DSGE Models from Structural VARs
The aim of this paper is to complement the MDE--SVAR approach when the weighting matrix is not optimal. In empirical studies, this choice is motivated by stochastic singularity or collinearity problems associated with the covariance matrix of Impulse Response Functions. Consequently, the asymptotic distribution cannot be used to test the economic model's fit. To circumvent this difficulty, we propose a simple simulation method to construct critical values for the test statistics. An empirical application with US data illustrates the proposed method.MDE, SVAR, DSGE models.
Real time decoherence of Landau and Levitov quasi-particles in quantum Hall edge channels
Quantum Hall edge channels at integer filling factor provide a unique
test-bench to understand decoherence and relaxation of single electronic
excitations in a ballistic quantum conductor. In this Letter, we obtain a full
visualization of the decoherence scenario of energy (Landau) and time (Levitov)
resolved single electron excitations at filling factor . We show that
the Landau excitation exhibits a fast relaxation followed by spin-charge
separation whereas the Levitov excitation only experiences spin-charge
separation. We finally suggest to use Hong-Ou-Mandel type experiments to probe
specific signatures of these different scenarios.Comment: 14 pages, 8 figure
Chocs d’Offre et Optimalité de la Politique Monétaire dans la Zone Euro
This article assesses monetary policy's performances in the Euro zone in the face of supply shocks. We determine the responses of output, inflation, labor share and the nominal interest rate to a supply shock as identified through a structural VAR model. We then develop a DSGE model with nominal rigidities subject to the optimal monetary policy. The model is estimated and tested on the basis of its ability to reproduce the responses drawn from the VAR model. Our results suggest that assuming optimal monetary policy allows for a satisfying fit to the data.Supply shocks ; SVAR ; Optimal Monetary Policy.
La TVA sociale : bonne ou mauvaise idée ?
The quantitative and dynamic consequence of a social VAT reform, i.e. a fiscal reform consisting in substituting VAT for social contributions, is assessed using two general equilibrium models. The first one is a Walrasian model with no other frictions than distortionary taxation of labor and capital incomes and consumption. The second one introduces in addition matching frictions in the labor market. Two alternative financing schemes are considered for the practical details of implementing the social VAT. In all cases, the fiscal reform turns out to generate a small, positive long--run effect on aggregate variables and yields a modest welfare gain. In the no--friction model, this welfare gain is substantially reduced when the reform is pre--announced six quarters prior to implementation. The effect of such a pre-announced reform are smaller when labor market frictions are taken into account.social VAT, DGE, pre-announced fiscal reform.
Une estimation de la cible implicite d’inflation dans la zone euro
Euro area countries as a whole have experienced a marked downward trend over the 1980s. Over this period, the unemployment rate has increased and economic activity has been sluggish. Changes in the implicit inflation target, viewed as low frequency movements of inflation, might possibly explain these developments. To highlight this issue, the present study estimates the dynamics of the implicit inflation target in the euro zone over the period 1970Q1-2004Q4. Based on a small macroeconometric model, the implicit target, not known by the econometrician, is identified through a minimal set of theoretical restrictions: (i) the inflation target is a non stationary process, (ii) inflation is a monetary phenomenon in the long-run, and (iii) changes in the implicit target have no long-run effects whatsoever on real variables. The model is estimated so as to match output growth, changes in inflation and the ex post real interest rate. Our main results are: (i) inflation target shocks account for the bulk of nominal fluctuations; (ii) due to monetary policy inertia and nominal stickiness, changes in the target generate large swings in the real interest rate translating into substantial short-run effects on real variables; (ii) in spite of this inflation target shocks moderately impact on output dynamics.Implicit inflation target, Macroeconometric modelling, Euro area.
Separation of neutral and charge modes in one dimensional chiral edge channels
Coulomb interactions have a major role in one-dimensional electronic
transport. They modify the nature of the elementary excitations from Landau
quasiparticles in higher dimensions to collective excitations in one dimension.
Here we report the direct observation of the collective neutral and charge
modes of the two chiral co-propagating edge channels of opposite spins of the
quantum Hall effect at filling factor 2. Generating a charge density wave at
frequency f in the outer channel, we measure the current induced by
inter-channel Coulomb interaction in the inner channel after a 3-mm propagation
length. Varying the driving frequency from 0.7 to 11 GHz, we observe damped
oscillations in the induced current that result from the phase shift between
the fast charge and slow neutral eigenmodes. We measure the dispersion relation
and dissipation of the neutral mode from which we deduce quantitative
information on the interaction range and parameters.Comment: 23 pages, 6 figure
Supercollision cooling in undoped graphene
Carrier mobility in solids is generally limited by electron-impurity or
electron-phonon scattering depending on the most frequently occurring event.
Three body collisions between carriers and both phonons and impurities are
rare; they are denoted supercollisions (SCs). Elusive in electronic transport
they should emerge in relaxation processes as they allow for large energy
transfers. As pointed out in Ref. \onlinecite{Song2012PRL}, this is the case in
undoped graphene where the small Fermi surface drastically restricts the
allowed phonon energy in ordinary collisions. Using electrical heating and
sensitive noise thermometry we report on SC-cooling in diffusive monolayer
graphene. At low carrier density and high phonon temperature the Joule power
obeys a law as a function of electronic temperature .
It overrules the linear law expected for ordinary collisions which has recently
been observed in resistivity measurements. The cubic law is characteristic of
SCs and departs from the dependence recently reported for metallic
graphene below the Bloch-Gr\"{u}neisen temperature. These supercollisions are
important for applications of graphene in bolometry and photo-detection
Conserved spin and orbital phase along carbon nanotubes connected with multiple ferromagnetic contacts
We report on spin dependent transport measurements in carbon nanotubes based
multi-terminal circuits. We observe a gate-controlled spin signal in non-local
voltages and an anomalous conductance spin signal, which reveal that both the
spin and the orbital phase can be conserved along carbon nanotubes with
multiple ferromagnetic contacts. This paves the way for spintronics devices
exploiting both these quantum mechanical degrees of freedom on the same
footing.Comment: 8 pages - minor differences with published versio
Electron quantum optics : partitioning electrons one by one
We have realized a quantum optics like Hanbury Brown and Twiss (HBT)
experiment by partitioning, on an electronic beam-splitter, single elementary
electronic excitations produced one by one by an on-demand emitter. We show
that the measurement of the output currents correlations in the HBT geometry
provides a direct counting, at the single charge level, of the elementary
excitations (electron/hole pairs) generated by the emitter at each cycle. We
observe the antibunching of low energy excitations emitted by the source with
thermal excitations of the Fermi sea already present in the input leads of the
splitter, which suppresses their contribution to the partition noise. This
effect is used to probe the energy distribution of the emitted wave-packets.Comment: 5 pages, 4 figure
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