317 research outputs found
Vibration multistability and quantum switching for dispersive coupling
We investigate a resonantly modulated harmonic mode, dispersively coupled to
a nonequilibrium few-level quantum system. We focus on the regime where the
relaxation rate of the system greatly exceeds that of the mode, and develop a
quantum adiabatic approach for analyzing the dynamics. Semiclassically, the
dispersive coupling leads to a mutual tuning of the mode and system into and
out of resonance with their modulating fields, leading to multistability. In
the important case where the system has two energy levels and is excited near
resonance, the compound system can have up to three metastable states.
Nonadiabatic quantum fluctuations associated with spontaneous transitions in
the few-level system lead to switching between the metastable states. We
provide parameter estimates for currently available systems
The theory of coherent dynamic nuclear polarization in quantum dots
We consider the dynamic nuclear spin polarization (DNP) using two electrons
in a double quantum dot in presence of external magnetic field and spin-orbit
interaction, in various schemes of periodically repeated sweeps through the
S-T+ avoided crossing. By treating the problem semi-classically, we find that
generally the DNP have two distinct contributions - a geometrical polarization
and a dynamic polarization, which have different dependence on the control
parameters such as the sweep rates and waiting times in each period. Both terms
show non-trivial dependence on those control parameter. We find that even for
small spin-orbit term, the dynamical polarization dominates the DNP in presence
of a long waiting period near the S-T+ avoided crossing, of the order of the
nuclear Larmor precession periods. A detailed numerical analysis of a specific
control regime can explain the oscillations observed by Foletti et.~al.~in
arXiv:0801.3613.Comment: 22 pages, 6 figure
Detection of spin injection into a double quantum dot: Violation of magnetic-field-inversion symmetry of nuclear polarization instabilities
In mesoscopic systems with spin-orbit coupling, spin-injection into quantum
dots at zero magnetic field is expected under a wide range of conditions.
However, up to now, a viable approach for experimentally identifying such
injection has been lacking. We show that electron spin injection into a
spin-blockaded double quantum dot is dramatically manifested in the breaking of
magnetic- field-inversion symmetry of nuclear polarization instabilities. Over
a wide range of parameters, the asymmetry between positive and negative
instability fields is extremely sensitive to the injected electron spin
polarization and allows for the detection of even very weak spin injection.
This phenomenon may be used to investigate the mechanisms of spin transport,
and may hold implications for spin-based information processing
Controlled Population of Floquet-Bloch States via Coupling to Bose and Fermi Baths
External driving is emerging as a promising tool for exploring new phases in
quantum systems. The intrinsically non-equilibrium states that result, however,
are challenging to describe and control. We study the steady states of a
periodically driven one-dimensional electronic system, including the effects of
radiative recombination, electron-phonon interactions, and the coupling to an
external fermionic reservoir. Using a kinetic equation for the populations of
the Floquet eigenstates, we show that the steady-state distribution can be
controlled using the momentum and energy relaxation pathways provided by the
coupling to phonon and Fermi reservoirs. In order to utilize the latter, we
propose to couple the system and reservoir via an energy filter which
suppresses photon-assisted tunneling. Importantly, coupling to these reservoirs
yields a steady state resembling a band insulator in the Floquet basis. The
system exhibits incompressible behavior, while hosting a small density of
excitations. We discuss transport signatures, and describe the regimes where
insulating behavior is obtained. Our results give promise for realizing Floquet
topological insulators.Comment: 24 pages, 7 figures; with appendice
Spin relaxation due to deflection coupling in nanotube quantum dots
We consider relaxation of an electron spin in a nanotube quantum dot due to
its coupling to flexural phonon modes, and identify a new spin-orbit mediated
coupling between the nanotube deflection and the electron spin. This mechanism
dominates other spin relaxation mechanisms in the limit of small energy
transfers. Due to the quadratic dispersion law of long wavelength flexons,
, the density of states
diverges as . Furthermore, because here the spin couples directly
to the nanotube deflection, there is an additional enhancement by a factor of
compared to the deformation potential coupling mechanism. We show that
the deflection coupling robustly gives rise to a minimum in the magnetic field
dependence of the spin lifetime near an avoided crossing between
spin-orbit split levels in both the high and low-temperature limits. This
provides a mechanism that supports the identification of the observed
minimum with an avoided crossing in the single particle spectrum by Churchill
et al.[Phys. Rev. Lett. {\bf 102}, 166802 (2009)].Comment: Final version accepted for publication. References added
Semi-classical model for the dephasing of a two-electron spin qubit coupled to a coherently evolving nuclear spin bath
We study electron spin decoherence in a two-electron double quantum dot due
to the hyperfine interaction, under spin-echo conditions as studied in recent
experiments. We develop a semi-classical model for the interaction between the
electron and nuclear spins, in which the time-dependent Overhauser fields
induced by the nuclear spins are treated as classical vector variables.
Comparison of the model with experimentally-obtained echo signals allows us to
quantify the contributions of various processes such as coherent Larmor
precession and spin diffusion to the nuclear spin evolution.Comment: 14 Pages, some equations were corrected; Published July 27, 201
Modulated Floquet parametric driving and non-equilibrium crystalline electron states
We propose a method to parametrically excite low frequency collective modes
in an interacting many body system using an amplitude modulated Floquet drive
at optical frequencies. We illustrate this method on the example of terahertz
plasmons in a two dimensional electronic system. In the presence of a
sufficiently strong drive, plasmons resonant with the modulation frequency
become unstable and arrange themselves in a crystal-like structure stabilized
by interactions and nonlinearities. The new state breaks the discrete time
translational symmetry of the drive as well as the translational and rotational
spatial symmetries of the system and exhibits soft, Goldstone-like phononic
excitations
Self-Polarization and Dynamical Cooling of Nuclear Spins in Double Quantum Dots
Spontaneous nuclear polarization is predicted in double quantum dots in the
spin-blocked electron transport regime. The polarization results from an
instability of the zero-polarization state when singlet and triplet electron
energy levels are brought into resonance by the effective hyperfine field of
the nuclei on the electrons. The nuclear spins, once polarized, serve as a cold
bath for cooling electrons below the lattice (phonon) temperature. We estimate
the relevant time scales and discuss the conditions necessary to observe these
phenomena.Comment: 4 pages, 3 figures, updated journal versio
Steady state of interacting Floquet insulators
Floquet engineering offers tantalizing opportunities for controlling the dynamics of quantum many-body systems and realizing new nonequilibrium phases of matter. However, this approach faces a major challenge: generic interacting Floquet systems absorb energy from the drive, leading to uncontrolled heating which washes away the sought-after behavior. How to achieve and control a nontrivial nonequilibrium steady state is therefore of crucial importance. In this work, we study the dynamics of an interacting one-dimensional periodically driven electronic system coupled to a phonon heat bath. Using the Floquet-Boltzmann equation (FBE) we show that the electronic populations of the Floquet eigenstates can be controlled by the dissipation. We find the regime in which the steady state features an insulator-like filling of the Floquet bands, with a low density of additional excitations. Furthermore, we develop a simple rate equation model for the steady state excitation density that captures the behavior obtained from the numerical solution of the FBE over a wide range of parameters
Large-amplitude driving of a superconducting artificial atom: Interferometry, cooling, and amplitude spectroscopy
Superconducting persistent-current qubits are quantum-coherent artificial
atoms with multiple, tunable energy levels. In the presence of large-amplitude
harmonic excitation, the qubit state can be driven through one or more of the
constituent energy-level avoided crossings. The resulting
Landau-Zener-Stueckelberg (LZS) transitions mediate a rich array of
quantum-coherent phenomena. We review here three experimental works based on
LZS transitions: Mach-Zehnder-type interferometry between repeated LZS
transitions, microwave-induced cooling, and amplitude spectroscopy. These
experiments exhibit a remarkable agreement with theory, and are extensible to
other solid-state and atomic qubit modalities. We anticipate they will find
application to qubit state-preparation and control methods for quantum
information science and technology.Comment: 13 pages, 5 figure
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