6,477 research outputs found
Towards Realistic Time-Resolved Simulations of Quantum Devices
We report on our recent efforts to perform realistic simulations of large
quantum devices in the time domain. In contrast to d.c. transport where the
calculations are explicitly performed at the Fermi level, the presence of
time-dependent terms in the Hamiltonian makes the system inelastic so that it
is necessary to explicitly enforce the Pauli principle in the simulations. We
illustrate our approach with calculations for a flying qubit interferometer, a
nanoelectronic device that is currently under experimental investigation. Our
calculations illustrate the fact that many degrees of freedom (16,700
tight-binding sites in the scattering region) and long simulation times (80,000
times the inverse Bandwidth of the tight-binding model) can be easily achieved
on a local computer.Comment: 8 pages, 6 figure
Manipulating Andreev and Majorana Bound States with microwaves
We study the interplay between Andreev (Majorana) bound states that form at
the boundary of a (topological) superconductor and a train of microwave pulses.
We find that the extra dynamical phase coming from the pulses can shift the
phase of the Andreev reflection, resulting in the appear- ance of dynamical
Andreev states. As an application we study the presence of the zero bias peak
in the differential conductance of a normal-topological superconductor junction
- the simplest, yet somehow ambiguous, experimental signature for Majorana
states. Adding microwave radiation to the measuring electrodes provides an
unambiguous probe of the Andreev nature of the zero bias peak.Comment: 4 pages, 4 figure
Stopping electrons with radio-frequency pulses in the quantum Hall regime
Most functionalities of modern electronic circuits rely on the possibility to
modify the path fol- lowed by the electrons using, e.g. field effect
transistors. Here we discuss the interplay between the modification of this
path and the quantum dynamics of the electronic flow. Specifically, we study
the propagation of charge pulses through the edge states of a two-dimensional
electron gas in the quantum Hall regime. By sending radio-frequency (RF)
excitations on a top gate capacitively coupled to the electron gas, we
manipulate these edge state dynamically. We find that a fast RF change of the
gate voltage can stop the propagation of the charge pulse inside the sample.
This effect is intimately linked to the vanishing velocity of bulk states in
the quantum Hall regime and the peculiar connection between momentum and
transverse confinement of Landau levels. Our findings suggest new possibilities
for stopping, releasing and switching the trajectory of charge pulses in
quantum Hall systems.Comment: 5 pages, 4 figure
Classical and quantum spreading of a charge pulse
With the technical progress of radio-frequency setups, high frequency quantum
transport experiments have moved from theory to the lab. So far the standard
theoretical approach used to treat such problems numerically--known as Keldysh
or NEGF (Non Equilibrium Green's Functions) formalism--has not been very
successful mainly because of a prohibitive computational cost. We propose a
reformulation of the non-equilibrium Green's function technique in terms of the
electronic wave functions of the system in an energy-time representation. The
numerical algorithm we obtain scales now linearly with the simulated time and
the volume of the system, and makes simulation of systems with 10^5 - 10^6
atoms/sites feasible. We illustrate our method with the propagation and
spreading of a charge pulse in the quantum Hall regime. We identify a classical
and a quantum regime for the spreading, depending on the number of particles
contained in the pulse. This numerical experiment is the condensed matter
analogue to the spreading of a Gaussian wavepacket discussed in quantum
mechanics textbooks.Comment: 4 pages, 5 figures; to be published in IEEE Xplore, in Proceedings to
IEEE 17th International Workshop on Computational Electronics 2014, June 3 -
6, 2014, Paris, France. Correction of typographic mistakes and update of ref.
1
The stochastic background: scaling laws and time to detection for pulsar timing arrays
We derive scaling laws for the signal-to-noise ratio of the optimal
cross-correlation statistic, and show that the large power-law increase of the
signal-to-noise ratio as a function of the the observation time that is
usually assumed holds only at early times. After enough time has elapsed,
pulsar timing arrays enter a new regime where the signal to noise only scales
as . In addition, in this regime the quality of the pulsar timing
data and the cadence become relatively un-important. This occurs because the
lowest frequencies of the pulsar timing residuals become gravitational-wave
dominated. Pulsar timing arrays enter this regime more quickly than one might
naively suspect. For T=10 yr observations and typical stochastic background
amplitudes, pulsars with residual RMSs of less than about s are already
in that regime. The best strategy to increase the detectability of the
background in this regime is to increase the number of pulsars in the array. We
also perform realistic simulations of the NANOGrav pulsar timing array, which
through an aggressive pulsar survey campaign adds new millisecond pulsars
regularly to its array, and show that a detection is possible within a decade,
and could occur as early as 2016.Comment: Submitted to Classical and Quantum Gravity for Focus Issue on Pulsar
Timing Arrays. 15 pages, 5 figure
- …