149 research outputs found
Decoherence in Nearly-Isolated Quantum Dots
Decoherence in nearly-isolated GaAs quantum dots is investigated using the
change in average Coulomb blockade peak height upon breaking time-reversal
symmetry. The normalized change in average peak height approaches the predicted
universal value of 1/4 at temperatures well below the single-particle level
spacing, but is greatly suppressed for temperature greater than the level
spacing, suggesting that inelastic scattering or other dephasing mechanisms
dominate in this regime.Comment: Significant revisions to include comparison to theory. Related papers
available at http://marcuslab.harvard.ed
Spin Degeneracy and Conductance Fluctuations in Open Quantum Dots
The dependence of mesoscopic conductance fluctuations on parallel magnetic
field is used as a probe of spin degeneracy in open GaAs quantum dots. The
variance of fluctuations at high parallel field is reduced from the low-field
variance (with broken time-reversal symmetry) by factors ranging from roughly
two in a 1 square-micron dot at low temperature, to four or greater in 8
square-micron dots. The factor of two is expected for simple Zeeman splitting
of spin degenerate channels. A possible explanation for the unexpected larger
factors in terms of field-dependent spin orbit scattering is proposed.Comment: Includes new reference to related theoretical work, cond-mat/0010064.
Other minor changes. Related papers at http://marcuslab.harvard.ed
Saturation of dephasing time in mesoscopic devices produced by a ferromagnetic state
We consider an exchange model of itinerant electrons in a Heisenberg
ferromagnet and we assume that the ferromagnet is in a fully polarized state.
Using the Holstein-Primakoff transformation we are able to obtain a
boson-fermion Hamiltonian that is well-known in the interaction between light
and matter. This model describes the spontaneous emission in two-level atoms
that is the proper decoherence mechanism when the number of modes of the
radiation field is taken increasingly large, the vacuum acting as a reservoir.
In the same way one can see that the interaction between the bosonic modes of
spin waves and an itinerant electron produces decoherence by spin flipping with
a rate proportional to the size of the system. In this way we are able to show
that the experiments on quantum dots, described in D. K. Ferry et al. [Phys.
Rev. Lett. {\bf 82}, 4687 (1999)], and nanowires, described in D. Natelson et
al. [Phys. Rev. Lett. {\bf 86}, 1821 (2001)], can be understood as the
interaction of itinerant electrons and an electron gas in a fully polarized
state.Comment: 10 pages, no figure. Changed title. Revised version accepted for
publication in Physical Review
Quantum Computation with Quantum Dots and Terahertz Cavity Quantum Electrodynamics
A quantum computer is proposed in which information is stored in the two
lowest electronic states of doped quantum dots (QDs). Many QDs are located in a
microcavity. A pair of gates controls the energy levels in each QD. A
Controlled Not (CNOT) operation involving any pair of QDs can be effected by a
sequence of gate-voltage pulses which tune the QD energy levels into resonance
with frequencies of the cavity or a laser. The duration of a CNOT operation is
estimated to be much shorter than the time for an electron to decohere by
emitting an acoustic phonon.Comment: Revtex 6 pages, 3 postscript figures, minor typos correcte
Electron transport through double quantum dots
Electron transport experiments on two lateral quantum dots coupled in series
are reviewed. An introduction to the charge stability diagram is given in terms
of the electrochemical potentials of both dots. Resonant tunneling experiments
show that the double dot geometry allows for an accurate determination of the
intrinsic lifetime of discrete energy states in quantum dots. The evolution of
discrete energy levels in magnetic field is studied. The resolution allows to
resolve avoided crossings in the spectrum of a quantum dot. With microwave
spectroscopy it is possible to probe the transition from ionic bonding (for
weak inter-dot tunnel coupling) to covalent bonding (for strong inter-dot
tunnel coupling) in a double dot artificial molecule. This review on the
present experimental status of double quantum dot studies is motivated by their
relevance for realizing solid state quantum bits.Comment: 32 pages, 31 figure
Zero-point fluctuations in the ground state of a mesoscopic normal ring
We investigate the persistent current of a ring with an in-line quantum dot
capacitively coupled to an external circuit. Of special interest is the
magnitude of the persistent current as a function of the external impedance in
the zero temperature limit when the only fluctuations in the external circuit
are zero-point fluctuations. These are time-dependent fluctuations which
polarize the ring-dot structure and we discuss in detail the contribution of
displacement currents to the persistent current. We have earlier discussed an
exact solution for the persistent current and its fluctuations based on a Bethe
ansatz. In this work, we emphasize a physically more intuitive approach using a
Langevin description of the external circuit. This approach is limited to weak
coupling between the ring and the external circuit. We show that the zero
temperature persistent current obtained in this approach is consistent with the
persistent current calculated from a Bethe ansatz solution. In the absence of
coupling our system is a two level system consisting of the ground state and
the first excited state. In the presence of coupling we investigate the
projection of the actual state on the ground state and the first exited state
of the decoupled ring. With each of these projections we can associate a phase
diffusion time. In the zero temperature limit we find that the phase diffusion
time of the excited state projection saturates, whereas the phase diffusion
time of the ground state projection diverges.Comment: 12 pages, 5 figure
Photocurrent, Rectification, and Magnetic Field Symmetry of Induced Current Through Quantum Dots
We report mesoscopic dc current generation in an open chaotic quantum dot
with ac excitation applied to one of the shape-defining gates. For excitation
frequencies large compared to the inverse dwell time of electrons in the dot
(i.e., GHz), we find mesoscopic fluctuations of induced current that are fully
asymmetric in the applied perpendicular magnetic field, as predicted by recent
theory. Conductance, measured simultaneously, is found to be symmetric in
field. In the adiabatic (i.e., MHz) regime, in contrast, the induced current is
always symmetric in field, suggesting its origin is mesoscopic rectification.Comment: related papers at http://marcuslab.harvard.ed
Coupled quantum dots as quantum gates
We consider a new quantum gate mechanism based on electron spins in coupled
semiconductor quantum dots. Such gates provide a general source of spin
entanglement and can be used for quantum computers. We determine the exchange
coupling J in the effective Heisenberg model as a function of magnetic (B) and
electric fields, and of the inter-dot distance (a) within the Heitler-London
approximation of molecular physics. This result is refined by using
sp-hybridization, and by the Hund-Mulliken molecular-orbit approach which leads
to an extended Hubbard description for the two-dot system that shows a
remarkable dependence on B and a due to the long-range Coulomb interaction. We
find that the exchange J changes sign at a finite field (leading to a
pronounced jump in the magnetization) and then decays exponentially. The
magnetization and the spin susceptibilities of the coupled dots are calculated.
We show that the dephasing due to nuclear spins in GaAs can be strongly
suppressed by dynamical nuclear spin polarization and/or by magnetic fields.Comment: 10 pages, 4 figures. v2: minor corrections, appendix added. to be
published in Phys.Rev.
Measurement of Two-Qubit States by a Two-Island Single Electron Transistor
We solve the master equations of two charged qubits measured by a
single-electron transistor (SET) consisted of two islands. We show that in the
sequential tunneling regime the SET current can be used for reading out results
of quantum calculations and providing evidences of two-qubit entanglement,
especially when the interaction between the two qubits is weak
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