87 research outputs found
Shaping charge excitations in chiral edge states with a time-dependent gate voltage
We study a coherent conductor supporting a single edge channel in which
alternating current pulses are created by local time-dependent gating and sent
on a beam-splitter realized by a quantum point contact. The current response to
the gate voltage in this setup is intrinsically linear. Based on a fully
self-consistent treatment employing a Floquet scattering theory, we analyze the
effect of different voltage shapes and frequencies, as well as the role of the
gate geometry on the injected signal. In particular, we highlight the impact of
frequency-dependent screening on the process of shaping the current signal. The
feasibility of creating true single-particle excitations with this method is
confirmed by investigating the suppression of excess noise, which is otherwise
created by additional electron-hole pair excitations in the current signal
Decoherence and relaxation of single electron excitations in quantum Hall edge channels
A unified approach to decoherence and relaxation of energy resolved single
electron excitations in Integer Quantum Hall edge channels is presented. Within
the bosonization framework, relaxation and decoherence induced by interactions
and capacitive coupling to an external linear circuit are computed. An explicit
connexion with high frequency transport properties of a two terminal device
formed by the edge channel on one side and the linear circuit on the other side
is established.Comment: 4 pages, 3 figure
A Coherent RC Circuit
We review the first experiment on dynamic transport in a phase-coherent
quantum conductor. In our discussion, we highlight the use of time-dependent
transport as a means of gaining insight into charge relaxation on a mesoscopic
scale. For this purpose, we studied the ac conductance of a model quantum
conductor, i.e. the quantum RC circuit. Prior to our experimental work, M.
B\"{u}ttiker, H. Thomas and A. Pr\^{e}tre first worked on dynamic mesoscopic
transport in the 1990s. They predicted that the mesoscopic RC circuit can be
described by a quantum capacitance related to the density of states in the
capacitor and a constant charge relaxation resistance value equal to half of
the resistance quantum h/2e^2, when a single mode is transmitted between the
capacitance and a reservoir. By applying a microwave excitation to a gate
located on top of a coherent submicronic quantum dot that is coupled to a
reservoir, we validate this theoretical prediction on the ac conductance of the
quantum RC circuit. Our study demonstrates that the ac conductance is directly
related to the dwell time of electrons in the capacitor. Thereby, we observed a
counterintuitive behavior of a quantum origin: as the transmission of the
single conducting mode decreases, the resistance of the quantum RC circuit
remains constant while the capacitance oscillates.Comment: 30 page
Electron quantum optics in quantum Hall edge channels
In this paper, we review recent developments in the emerging field of
electron quantum optics, stressing analogies and differences with the usual
case of photon quantum optics. Electron quantum optics aims at preparing,
manipulating and measuring coherent single electron excitations propagating in
ballistic conductors such as the edge channels of a 2DEG in the integer quantum
Hall regime. Because of the Fermi statistics and the presence of strong
interactions, electron quantum optics exhibits new features compared to the
usual case of photon quantum optics. In particular, it provides a natural
playground to understand decoherence and relaxation effects in quantum
transport.Comment: 13 pages, 6 figures. To appear in the proceedings of StatPhys 24
satellite conference on "International Conference on Frustrated Spin Systems,
Cold Atoms and Nanomaterials" held in Hanoi (14-16 July 2010
Plasmon scattering approach to energy exchange and high frequency noise in nu=2 quantum Hall edge channels
Inter-edge channel interactions in the quantum Hall regime at filling factor
nu= 2 are analyzed within a plasmon scattering formalism. We derive analytical
expressions for energy redistribution amongst edge channels and for high
frequency noise, which are shown to fully characterize the low energy plasmon
scattering. In the strong interaction limit, the predictions for energy
redistribution are compared with recent experimental data and found to
reproduce most of the observed features. Quantitative agreement can be achieved
by assuming 25 % of the injected energy is lost towards other degrees of
freedom, possibly the additional gapless excitations predicted for smooth edge
potentials.Comment: 4 pages, 4 figure
A high sensitivity ultra-low temperature RF conductance and noise measurement setup
We report on the realization of a high sensitivity RF noise measurement
scheme to study small current fluctuations of mesoscopic systems at milliKelvin
temperatures. The setup relies on the combination of an interferometric ampli-
fication scheme and a quarter-wave impedance transformer, allowing the mea-
surement of noise power spectral densities with GHz bandwith up to five orders
of magnitude below the amplifier noise floor. We simultaneously measure the
high frequency conductance of the sample by derivating a portion of the signal
to a microwave homodyne detection. We describe the principle of the setup, as
well as its implementation and calibration. Finally, we show that our setup
allows to fully characterize a subnanosecond on-demand single electron source.
More generally, its sensitivity and bandwith make it suitable for applications
manipulating single charges at GHz frequencies.Comment: The following article has been submitted to Review of Scientific
Instrument
Violation of Kirchhoff's Laws for a Coherent RC Circuit
What is the complex impedance of a fully coherent quantum
resistance-capacitance (RC) circuit at GHz frequencies in which a resistor and
a capacitor are connected in series? While Kirchhoff's laws predict addition of
capacitor and resistor impedances, we report on observation of a different
behavior. The resistance, here associated with charge relaxation, differs from
the usual transport resistance given by the Landauer formula. In particular,
for a single mode conductor, the charge relaxation resistance is half the
resistance quantum, regardless of the transmission of the mode. The new
mesoscopic effect reported here is relevant for the dynamical regime of all
quantum devices
The relaxation time of a chiral quantum R-L circuit
We report on the GHz complex admittance of a chiral one dimensional ballistic
conductor formed by edge states in the quantum Hall regime. The circuit
consists of a wide Hall bar (the inductor L) in series with a tunable resistor
(R) formed by a quantum point contact. Electron interactions between edges are
screened by a pair of side gates. Conductance steps are observed on both real
and imaginary parts of the admittance. Remarkably, the phase of the admittance
is transmission-independent. This shows that the relaxation time of a chiral
R-L circuit is resistance independent. A current and charge conserving
scattering theory is presented that accounts for this observation with a
relaxation time given by the electronic transit time in the c cuit
Klein-tunneling transistor with ballistic graphene
Today the availability of high mobility graphene up to room temperature makes
ballistic transport in nanodevices achievable. In particular, p-n-p transistor
in the ballistic regime gives access to the Klein tunneling physics and allows
the realization of devices exploiting the optics-like behavior of Dirac
Fermions (DF) as in the Vesalego lens or the Fabry P\'erot cavity. Here we
propose a Klein tunneling transistor based on geometrical optics of DF. We
consider the case of a prismatic active region delimited by a triangular gate,
where total internal reflection may occur, which leads to the tunable
suppression of the transistor transmission. We calculate the transmission and
the current by means of scattering theory and the finite bias properties using
Non Equilibrium Green's Function(NEGF) simulation.Comment: 4 pages, 5 figure
Coherence and Indistinguishability of Single Electrons Emitted by Independent Sources
The on-demand emission of coherent and indistinguishable electrons by
independent synchronized sources is a challenging task of quantum electronics,
in particular regarding its application for quantum information processing.
Using two independent on-demand electron sources, we trigger the emission of
two single-electron wavepackets at different inputs of an electronic
beamsplitter. Whereas classical particles would be randomly partitioned by the
splitter, we observe two-particle interferences resulting from quantum
exchange. Both electrons, emitted in indistinguishable wavepackets with
synchronized arrival time on the splitter, exit in different outputs as
recorded by the low frequency current noise. The demonstration of two-electron
interference provides the possibility to manipulate coherent and
indistinguishable single-electron wavepackets in quantum conductors.Comment: Science Express of January 24 201
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